STATION PAPER NO. 85
FOREST SERVICE
NORTHEASTERN FOREST EXPERIMENT STATION
U. S. DEPARTMENT OF AGRICULTURE
1956
UPPER DARBY, PA.
RALPH W. MARQUIS, DIRECTOR
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MULL A
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The role of
FOREST HUMUS
watershed management
New Enghnd
G . R. Trimble, Jr.
Howard W. Lull
PREFACE
This paper is a part of a problem analysis
for watershed-management research to be conducted
by the Northeastern Forest Experiment Station at
the Hubbard Brook Experimental Forest in the
White Mountains of New Hampshire.
CONTENTS
Page
...................
HUMUS DEVELOPMENT . . . . . . . . . . . . . . . .
HUMUS CLASSIFICATION . . . . . . . . . . . . . . .
INTRODUCTION
............
...............
WATER STORAGE . . . . . . . . . . . . . . . . . .
Storage c a p a c i t i e s . . . . . . . . . . . . . . .
Factors t h a t a f f e c t humus depth and type . . . .
Climate . . . . . . . . . . . . . . . . . . .
Topography . . . . . . . . . . . . . . . . . .
Stand composition. age, and s i t e q u a l i t y . . .
Induced f a c t o r s . . . . . . . . . . . . . . .
Suggested research . . . . . . . . . . . . . . .
INFILTRATION & PERCOLATION
Suggested research
. . . . .. .. .. .. .. .. .. .. .. .. .. ........
EVAPORATION
Suggested research
GENERAL SUMMARY
.................
APPENDIX
Key f o r c l a s s i f i c a t i o n of f o r e s t humus types
Explanatorynotes
..
..............
LITERATURE CITED . . . . . . . . . . . . . . . . .
1
2
4
8
12
13
13
17
17
19
19
21
22
23
24
25
27
28
31
The role of
FOREST HUMUS
in watershed management
in New England
by
George R. Trimble J r . and Howard W. ~ u l l '
N o r t h e a s t e r n F o r e s t Experiment S t a t i o n
F o r e s t S e r v i c e , U.S. Dept. A g r i c u l t u r e
INTRODUCTION
FOREST HUMUS i s one o f t h e most i n t e r e s t i n g comI t s surface serves a s a
ponents o f t h e f o r e s t environment.
d e p o s i t o r y f o r l e a f f a l l and n e e d l e f a l l , w i t h s u c c e s s i v e
d e p t h s marking s t a g e s of t r a n s m u t a t i o n from t h e f r e s h l y
f a l l e n t o t h e decomposed.
And humus i s r e s p o n s i v e :
humus
t y p e and depth a r e i n d i c a t o r s of f o r e s t t r e a t m e n t and, t o
some e x t e n t , of s i t e q u a l i t y . I n t r i g u e d by t h e s e c h a r a c t e r i s t i c s , f o r e s t e r s and s o i l s men have examined them i n d e t a i l
and have p u b l i s h e d a w e a l t h of s c i e n t i f i c p a p e r s on t h e
p h y s i c a l p r o p e r t i e s of humus, i t s c l a s s i f i c a t i o n , development, and n u t r i e n t c o n t e n t . Most o f t h e s e p a p e r s were about
b a s i c s t u d i e s ; l i t t l e p r a c t i c a l a p p l i c a t i o n i s involved. To
d a t e , f o r e s t managers have made l i t t l e use of what i s known
about humus.
I n watershed management, a n u n d e r s t a n d i n g o f t h e hydrology o f humus mag o f f e r more immediate p r a c t i c a l a p p l i cation.
F i r s t of a l l , humus s e r v e s a d u a l f u n c t i o n :
it
s t o r e s and t r a n s m i t s m o i s t u r e ; and i t h e l p s s h i e l d t h e s o i l
a g a i n s t t h e s o i l - e r o d i n g f o r c e o f r a i n f a l l . These f u n c t i o n s
'Mr. Trimble i s research f o r e s t e r i n charge o f watershed-management s t u d i e s a t
the Experiment S t a t i o n ' s research c e n t e r a t Laconia, N.H.
Mr. L u l l i s c h i e f o f
the S t a t i o n ' s D i v i s i o n o f Watershed Management Research.
Like o t h e r
a r e w e l l recognized and v e r y n e a r l y axiomatic.
f a c e t s of v a t e r s hed management, however, q u e n t i t a t i v e i n t e r p r e t a t i o n l a g s behind q u a l i t a t i v e recognition.
The primary o b j e c t i v e of t h i s paper i s t o b r i n g together e x i s t e n t d a t a t h a t w i l l serve t o c l a r i f y t h e f u n c t i o n
and importance of f o r e s t humus t o watershed management i n
Nev~ England.
Since two-thirds of New England i s f o r e s t
land, t h i s r e g i o n ' s water problems and t h e i r s o l u t i o n a r e
c l o s e l y t i e d up with t h e use made of t h i s land.
I n view of
t h i s , i f forest-humus conditions have important functions i n
watershed management, t h e n i t i s important t o know what t h e y
a r e ; i t i s &.porta.nt t o study t h e f a c t o r s t h a t a f f e c t humus:
and it i s important t o know i f t h e s e f a c t o r s can be manipul a t e d by f o r e s t managers; and, i f so, how.
A secondary o b j e c t i v e i s t o n o t e where information i s
lacking. Throughout t h i s paper, a t t e n t i o n w i l l be centered
on t h e hydrological p r o p e r t i e s of f o r e s t humus and changes
i n t h e s e p r o p e r t i e s a s s o c i a t e d with t h e response of humus t o
f o r e s t s i t e and treatment.
HUMUS
DEVELOPMENT
The term lthumusll a s used by f o r e s t e r s and a s used i n
t h i s paper r e f e r s t o t h e upper s o i l l a y e r s whose characteri s t i c s most r e f l e c t t h e e f f e c t of organic matter; it i n cludes both organic m a t e r i a l and, when p r e s e n t , intermixed
mineral matter.
The hydrological c h a r a c t e r i s t i c s of t h e
humus-dominated l a y e r s o r horizons a r e governed by t h e physi c a l p r o p e r t i e s of t h e organic matter, t h e i r p r i n c i p a l
constituent.
The organic c o n s t i t u e n t of t h e humus l a y e r s c o n s i s t s
of t h e more o r l e s s decomposed residues of t h e f o r e s t f l o r a
and fauna. It i s composed of p l a n t p a r t s , t h e leaves, twigs,
limbs, bark, f r u i t , flowers, stem, roots--all
of t h e p a r t s
t h a t drop t o t h e f o r e s t f l o o r o r t h a t d i e and r o t w i t h i n t h e
s o i l . Added t o t h i s a r e t h e remains of s o i l f l o r a and fauna
and f o r e s t m i l d l i f e . F r o m t h i s potpourri, humus decays i n t o
an organic end-product t h a t i s unrecognizable a s t o source
and has considerable uniformity.
Varying i n i t s source, t h i s organic m a t e r i a l a l s o
v a r i e s i n i t s degree of decomposition, ranging from m a t e r i a l
only p a r t i a l l y decayed and possessing many of i t s o r i g i n a l
f e a t u r e s t o t o t a l l y decayed m a t e r i a l s .
This transformation
A t i t s e a r l i e s t , humus
involves numerous r e c u r r e n t cycles.
formation can conceivably begin when a l e a f f a l l s on bare
s o i l and decay begins.
Before t h e l e a f i s t o t a l l y decom-
posed, a second-year l e a f f a l l s , i n i t i a t i n g a second cycle
of decomposition.
Before t h e f i r s t cycle i s completed,
s e v e r a l c y c l e s of d e c a y m a y b e i n p r o c e s s .
Notall ofthe
p l a n t and animal d e b r i s decay a t t h e same r a t e , so t h e cycle
becomes
complex--wheels
w i t h i n wheels,
so t o speak.
Nikiforoff d e s c r i b e s t h e cycles and end-product a s follows
(24)2 :
Hence, each y e a r t h e beginning of a new cycle superimposes i t s e l f upon t h e more advanced s t a g e s of a l l preceding cycles t h a t have not y e t reached f i n a l s t a g e s .
Consequently, t h e s o i l organic matter i n any year cons i s t s of various m a t e r i a l s which represent a l l s t a g e s
of t h e l o n g cycle from t h e i n i t i a l t o t h e l a s t .
Because of t h i s , t h e general composition of t h e s o i l organic matter does not change from year t o year, and
such a steady s t a t e conceals t h e c y c l i c i t y of i t s for-mation.
Humus accumulates when annual. a d d i t i o n s a r e g r e a t e r
than annual decay. The gradual development of a humus l a y e r
provides t h e h a b i t a t t o encourage decay--the seeds f o r i t s
own destruction--so t h a t gradually t h e annual r a t e of decay
equals t h e r a t e of deposition of new m a t e r i a l s .
As
Nikiforoff puts i t (23):
From t h a t time t h e trvo processes--formation and mineralization--proceed a t an equal r a t e , and t h e s o i l may
be s a i d t o have reached a s t a t e of maturity o r one of
equilibrium with i t s n a t u r a l environment. The average
content of humus i n t h e mature s o i l remains r e l a t i v e l y
constant a s long a s no change i n n a t u r a l condition occurs. Any change i n t h e n a t u r a l condition t h a t upsets
t h e equilibrium w i l l be followed by a correspanding
change i n t h e humus content of t h e s o i l .
Changes i n n a t u r a l conditions a r e frequent a s f o r e s t
stands become t h e prey of f i r e s o r d i s e a s e s o r a r e s u b j e c t
t o various degrees of c u t t i n g ; so it follows t h a t only i n
r a r e i n s t a n c e s w i l l humus depth be i n equilibrjum with i t s
environment. More o f t e n t h a n not, h m u s depth i s l e s s t h a n
normal; f o r processes l e a d i n g t o reduction of humus a r e much
more rapid t h a n t h o s e d i r e c t e d toward i t s r e s t o r a t i o n .
2 ~ d e r l i n e dnumbers i n parentheses r e f e r t o Literature Cited, page 31.
HUMUS
CLASSIFICATION
The system of c l a s s i f y i n g f o r e s t humus g e n e r a l l y used
i n t h e United S t a t e s
i s based on t h e arrangement and
nature of t h e humus l a y e r s ( s e e ~ p p n d i x ) .
These a r e defined a s follows:
(z)
F
-
Fermentation l a y e r . P a r t i a l l y decomposed l i t t e r .
Origin of m a t e r i a l s t i l l recognizable.
H
-
Well-decomposed,
dark, amorphous organic matter,
unrecognizable a s t o o r i g i n .
Lower boundary
abrupt.
A1
-
Top l a y e r of mineral s o i l . Intimate mixture
of
organic and mineral f r a c t i o n s .
Lower boundary
diffuse.
I n t h e mor type t h i s horizon i s absent
o r i s present only a s a very weakly developed
l a y e r w i t h s l i g h t admixture o r organic matter.
Based on t h e degree of incorporation of organic mater i a l i n t o t h e s o i l , f o r e s t humus has g e n e r a l l y been broadly
c l a s s i f i e d a s e i t h e r MOR o r MULL.
The term MOR i s used t o
describe t h e condition where t h e H l a y e r r e s t s on t h e surI n c o n t r a s t , a MULL
f a c e s o i l with p r a c t i c a l l y no mixing.
type has no H l a y e r , and t h e organic m a t e r i a l i s w e l l mixed
with t h e upper p o r t i o n of t h e mineral s o i l .
I n t h e most
recent c l a s s i f i c a t i o n t h e r e appeared an intermediate type, a
DUFF MULL; t h i s combines f e a t u r e s of both mors and mulls.
Each of t h e s e t h r e e types has two t o six subtypes
based on i d e n t i f i a b l e d i f f e r e n c e s i n organic-matter content,
s t r u c t u r e , and thickness ( I ) .
This c l a s s i f i c a t i o n , based
s t r i c t l y on morphological f e a t u r e s , has been c r i t i c i z e d by
Wilde (2)
i n t h a t it i s not c o r r e l a t e d with f o r e s t r y pract i c e o r physical r e l a t i o n s h i p s .
FITilde suggests a c l a s s i f i c a t i o n based on I f . . .an a n a l y s i s of t h e underlying causes of
humus formation and establishment of t h e major genetic types
of humus development . . . I t
C e r t a i n f a c t o r s have been observed t h a t tend t o be
a s s o c i a t e d with t h e development of mors over mulls i n t h e
Northeast. These a r e :
Podsols versus brown podzolic s o i l s .
Coniferous cover versus hardwood cover.
A low versus a high s o i l pH.
These r e l a t i o n s h i p s have been noted and discussed i n general
(not s p e c i f i c a l l y f o r t h e Northeast) by numerous observers,
among whom were Lutz and Chandler (20) and Rome11 and
Heiberg (26).
However, t h e r e a l cause-and-effect r e l a t i o n s h i p s among t h e s e f a c t o r s and t h e humus type have never
been s a t i s f a c t o r i l y e s t a b l i s h e d .
I n t h i s connection, W. R. C . Handley has r e c e n t l y reported r e s u l t s from an exhaustive i n v e s t i g a t i o n on t h e
reasons f o r t h e d i f f e r e n t i a l formation of mull and mor (2).
He concluded t h a t s t a b i l i z e d l e a f p r o t e i n s a r e an important
f a c t o r i n t h e processes of mor formation.
These p r o t e i n s ,
s t a b i l i z e d i n t h e dying l e a f by tannin-like m a t e r i a l s , a r e
so r e s i s t a n t t o decomposition t h a t t h e t i s s u e s i n which they
occur accumulate on t h e surface of t h e mineral s o i l .
Withholding of supplies of a v a i l a b l e nitrogen i n these p r o t e i n s
may a l s o delay decomposition of o t h e r m a t e r i a l .
Proteins
found i n mull-producing l i t t e r , on t h e o t h e r hand, a r e not
so r e s i s t a n t t o decomposition, probably because of d i f f e r ences i n molecular composition and s t r u c t u r e .
Their ready
decomposition i s l i k e l y due t o adequate supplies of more
r e a d i l y a v a i l a b l e nitrogen.
I n c o l l a t i n g l i t e r a t u r e on t h e physical p r o p e r t i e s
and moisture r e l a t i o n s h i p s of f o r e s t humus, one finds a
c e r t a i n amount of ambiguity a s t o whether o r not f r e s h l y
The accepted clasf a l l e n l i t t e r i s an i n t e g r a l component.
s i f i c a t i o n system, described above, does not consider t h i s
m a t e r i a l because of i t s t r a n s i t o r y nature. Blow has reported, f o r example, t h a t i n an upland oak stand i n Tennessee
4 percent of t h e leaves f e l l by l a t e August, an a d d i t i o n a l
42 percent by mid-October, and t h e balance of 54 percent by
e a r l y December.
T o t a l weight of t h e f o r e s t f l o o r increased
By t h e following August
by December t o 5.5 tons p e r acre.
Annual deit decreased by decomposition t o 4.2 tons
p o s i t i o n of longleaf pine l i t t e r , on t h e o t h e r hand, has
shown two maxima, 1 9 percent of annual f a l l i n October and
40 percent i n May, June, and J u l y (12).
(a.
Because of i t s evanescence, many i n v e s t i g a t o r s have
not included l i t t e r i n t h e i r measurement of t o t a l accumulat i o n of organic matter.
Exclusion o r i n c l u s i o n of l i t t e r
w i l l a l s o a f f e c t water-storage measurements o r estimates
even though t h e storage capacity of l i t t e r i s much l e s s t h a n
t h a t of underlying more decomposed m a t e r i a l s .
Generally,
p l o t measurements of i n f i l t r a t i o n o r s u r f a c e runoff a r e made
withlitterintact.
Conifer l i t t e r m a y a f f e c t s t o r a g e a n d
i n f i l t r a t i o n measurements more t h a n hardwood because most
s t u d i e s of t h i s kind a r e made during t h e growing season when
hardwood l i t t e r i s a t a minimum.
I n t h e majority
of t h e western
and southern s t u d i e s
c i t e d i n t h i s p a p e r , h y d r o l o g i c f u n c t i o n s of t h e t o t a l
accumulated o r g a n i c l a y e r have been stuciied. I n s t u d i e s o f
h > m ~isn t h e N o r t h e a s t , e x c e p t f o r i n f i l t r a t i o n s t u d i e s , t h e
l i t t e r e f f e c t h a s g e n e r a l l y been excluded.
There a r e good
reasons f o r t h e d i f f e r e n c e s i n approach among t h e b'est,
and
South, and t h e Northeast.
F i r s t , s o u t h e r n and w e s t e r n
s t u d i e s have been more g e n e r a l l y concerned w i t h c o n i f e r s and
n o r t h e a s t e r n s t u d i e s w i t h hardwoods; and t h e amount o f
c o n i f e r l i t t e r depends much l e s s on a n n u a l l e a f f a l l t h a n
hardwood l i t t e r does.
Second, t h e importance of l i t t e r i n
r e s p e c t t o t h e g r e a t e r humus d e p t h i n t h e Northeast i s
r e l a t i v e l y much l e s s t h a n i n t h e South and West, where
l i t t e r may form t h e bulk o f t h e t o t a l o r g a n i c accumulation.
Throughout t h i s p a p e r a n a t t e m p t w i l l be made t o use
d e s i g n a t i o n s t h a t w i l l i n d i c a t e whether t h e d a t a under d i s c u s s i o n i n c l u d e s o r excludes l i t t e r .
A l l tabular material
w i l l b e d e s i g n a t e d c l e a r l y e i t h e r as f o r e s t f l o o r , which i n c l u d e s l i t t e r , o r as humus e x c l u s i v e o f l i t t e r , o r by F and
H humus d e s i g n a t i o n s .
The t e r m l l l i t t e r l l w i l l b e used o n l y
t o d e s i g n a t e c u r r e n t annual nr.cumulations.
PHYSICAL
PROPERTIES
The watershed f u n c t i o n s o f f o r e s t humus p e r t a i n t o
i t s e f f e c t on i n f i l t r a t i o n and p e r c o l a t i o n ( o r t h e movement
o f water- i n t o a n d through t h e s o i l ) , on w a t e r s t o r a g e , and
on e v a p o r a t i o n .
The manner of performance o f t h e s e t h r e e
f u n c t i o n s i s dependent on p h y s i c a l p r o p e r t i e s o f humus which
a r e r e l a t e d , namely, i t s l i g h t w e i g h t , p o r o s i t y , and g r e a t
water-holding capacity.
I t s l i g h t weight i s i l l u s t r a t e d i n t h e t a b u l a t i o n of
bulk d e n s i t i e s determined by v a r i o u s i n v e s t i g a t o r s ( t a b l e
1).
D e n s i t i e s r a n g e from 0.07 t o 1 , 0 9 .
The average
d e n s i t y f o r t h e H l a y e r i s t w i c e t h a t o f t h e F l a y e r : 0.22
Bulk d e n s i t i e s a r e l e s s f o r mor humus
compared t o 0.11.
t h a n f o r m u l l humus (which c o n t a i n s m i n e r a l s o i l ) ; and t h e y
v a r y a l s o w i t h i n t h e m u l l s w i t h f i r m m u l l having a h i g h e r
bulk d e n s i t y t h a n t h e o t h e r mulls.
(a)
t h e bulk d e n s i t y
A c c o r d i n g . t o Lutz and Chandler
o f t h e A. h o r i z o n o f f o r e s t s o i l s ( t h e FH l a y e r ) i s about
0.2; t h e A h o r i z o n i s commonly l e s s t h a n 1.0; and v a l u e s o f
1 . 5 o r more a r e c h a r a c t e r i s t i c o f deeper h o r i z o n s . A s g i v e n
3 ~ u l kdensity i s the r a t i o between the oven-dry weight of a given volume of
soil and the weight o f an equal volume of water.
i n t a b l e 1, t h e weight p e r u n i t volume of mor humus i s about
one-fourth t h a t o f mull humus and would be about one-seventh
t h a t o f deeper m i n e r a l horizons o f about 1.5 bulk d e n s i t y .
Associated w i t h t h e comparatively low d e n s i t y i s high
The bulk d e n s i t y of 0.22 f o r t h e H l a y e r o f mor
porosity.
humus, w i t h a n e s t i m a t e d s p e c i f i c g r a v i t y of 1.5,
gives a
t o t a l p o r o s i t y of 85 p e r c e n t of t h e volume; t h e average bulk
d e n s i t y of 0.11 f o r t h e F l a y e r g i v e s a t o t a l p o r o s i t y of 93
percent.
I n c o n t r a s t , a m i n e r a l s o i l w i t h bulk d e n s i t y of
1 . 5 and a s p e c i f i c g r a v i t y of 2.65 w i l l have a t o t a l porosi t y of 43 percent--about one h a l f t h a t of mor humus.
Mull
humus has p o r o s i t y v a l u e s t h a t l i e between t h o s e of mor
humus and m i n e r a l s o i l . K i t h i n t h e m u l l t y p e s , f i r m m u l l i s
t h e l e a s t porous.
Table 1.--Bulk
Humus
designation
densities of hmus
Location of
study
Humus layer
F
Greasy mor
New Hampshire
llor
Connecticut
H
Literature
reference
A1
-
(W
0.09
0.18
.07
.18
-
.07
.18
---
1.09
(*)
.87
(22)
--
(19)
(u)
Hull
Connecticut
Finn mull
Finn mull
Ifass., N.
Pa., N m York
--
--
Coarse mull
Maas., N. H.
Pa., New York
.13
--
.94
(")
-
.51
(3.2)
H.
Finn mull
Mass.,
1 3
.87
(*)
Granular mor
Mass., Ti. H.
.ll
---
Fibrous mor
Mass.,
N. A.
.09
.27
.22
---
N. H.
.15
.22
--
("1
(*I
("1
.31
Coarse, fine, and medium mull
N. H.
Greasy nor
Mass.,
Mor
Pa., New York
Fibrous duff
N m York
---
Greasy duff
New York (upper H)
--
.18
Greasy duff
New York (lower H )
--
.2ir
.U
--
-
(3.3)
(25
(25)
(2.5)
*~orey, H. F. The relationship of humus t o the vegetation-soil ccmplex and i t s application t o the
flood control problem. Allegheny Forest Expt. Sta. (now Northeast. Forest Fkpt. ~ t a . ) . 35 pp., unpublished. 1 9 0 .
Along w i t h high p o r o s i t y and l i g h t weight p e r u n i t
volume, humus p o s s e s s e s a g r e a t water-holding c a p a c i t y . I t s
f i e l d c a p a c i t y ( t h e amount o f moisture it w i l l r e t a i n
a g a i n s t t h e p u l l of g r a v i t y ) ranges between 1 0 0 and 200 perc e n t of i t s oven-dry weight. Lowdemilk r e p o r t s 180 p e r c e n t
f o r f o r e s t f l o o r s o f p i n e - f i r and pine-fir-cedar s t a n d s i n
C a l i f o r n i a (15). I n a r e c e n t study, Blow found 135 p e r c e n t
c a p a c i t y f o r upland oak f o r e s t humus i n Tennessee (2).
Under C a l i f o r n i a c h a p a r r a l , K i t t r e d g e found f o r e s t - f l o o r
.
v a l u e s r a n g i n g from 115 t o 205 p e r c e n t (Q)
I n another
s t u d y , K i t t r e d g e r e p o r t e d comparable v a l u e s of 150, 182, and
186 p e r c e n t s f o r Douglas-fir, Canary p i n e , and I4onterey p i n e
r e s p e c t i v e l y ; f o r e s t f l o o r s o f w h i t e f i r on t h e west s l o p e
o f t h e S i e r r a had f i e l d c a p a c i t i e s o f 1 6 1 t o 183 p e r c e n t
(14)
These a r e a v e r a g e v a l u e s f o r f o r e s t f l o o r s t h a t i n c l u d e t h e l i t t e r and humus l a y e r s .
L i t t e r i s known t o have
a l e s s e r water-holding c a p a c i t y t h a n t h e humus l a g e r s (12);
q u a n t i t a t i v e d a t a on t h i s p o i n t and on t h e r e l a t i o n s h i p o f
water-holding c a p a c i t y t o s t a g e o f decomposition a p p e a r t o
be few.
Minimum f i e l d - m o i s t u r e c o n t e n t s range from 20 t o 40
p e r c e n t by weight.
Blow found a v a l u e of 20 p e r c e n t i n h i s
Tennessee s t u d y ( 2 ) .
Hale and Trimble r e p o r t e d permanent
w i l t i n g p e r c e n t a g e s o f 37.5 f o r mor and 19.3 f o r m u l l a s
determined from samples t a k e n d u r i n g a drought i n t h e Upper
Susquehanna r i v e r w a t e r s h e d ; t h e s o i l s were g r a v e l l y s i l t
loams ( 4 ) . According t o K i t t r e d g e , a d r y f o r e s t f l o o r under
f i e l d conditions r a r e l y contains l e s s than 1 0 t o 15 percent
m o i s t u r e (12)
.
INFILTRATION
&
PERCOLATION
The f o r e s t f l o o r f a c i l i t a t e s t h e e n t r a n c e of w a t e r
i n t o t h e s o i l body i n s e v e r a l ways.
Being h i g h l y porous,
it o f f e r s l i t t l e r e s i s t a n c e t o t h e downward movement o f
r a t e r toward t h e m i n e r a l s o i l , y e t it p r o t e c t s t h e m i n e r a l
s o i l a g a i n s t r a i n f a l l impact.
T h i s p r o t e c t i v e p r o p e r t y ext e n d s t o t h e s h e l t e r and food it p r o v i d e s t h e s o i l fauna
whose burrowing s e r v e s t o i n c r e a s e s o i l p o r o s i t y and i n f i l t r a t i o n . Aggregation, t h r o u g h i n t e r a c t i o n o f s o i l p a r t i c l e s
with organic matter, a l s o increases porosity.
Finally, t h e
f o r e s t f l o o r forms a n o b s t r u c t i o n t o s u r f a c e r u n o f f , i n c r e a s i n g t h e f r i c t i o n a l r e s i s t a n c e t o o v e r l a n d flow, t h e r e b y
i n c r e a s i n g t h e d e p t h o f s u r f a c e d e t e n t i o n s t o r a g e and perm i t t i n g i n f i l t r a t i o n t o t a k e place f o r a longer period.
These q u a l i t a t i v e r e l a t i o n s h i p s a r e w e l l recognized.
Q u a n t i t a t i v e l y , t h e i n f l u e n c e o f a f o r e s t f l o o r on i n f i l t r a t i o n has been g i v e n some s t u d y by measuring i n f i l t r a t i o n o r
s u r f a c e runoff on p l o t s w i t h and w i t h o u t l i t t e r and/or
humus. The c l a s s i c example i s LowdermilkDse a r l y s t u d y
in
C a l i f o r n i a (Q), where he found t h a t s u r f a c e r u n o f f from
p l o t s on which t h e f o r e s t f l o o r had been burned o f f was 3,
9 , and 16.5 times g r e a t e r t h a n runoff from unburned p l o t s
f o r f i n e sandy loam, sandy c l a y loam, and c l a y loam s o i l s
r e s ~ e c t i v e l y : t h e f o r e s t f l o o r w a s most e f f e c t i v e on t h ?
finest-textured s o i l .
I n Connecticut, Lunt found t h a t about twice a s much
r a i n f a l l percolated through &-inch deep lysimeters with
l i t t e r - c o v e r e d s o i l a s through lysimeters with bare s o i l
Two i n f i l t r o m e t e r s t u d i e s provide a d d i t i o n a l evidence. I n t h e upland oak type of t h e Ozarks, Arend found a n
average i n f i l t r a t i c r , -rate of 2.12 inches p e r hour f o r unburned p l o t s compared t o 1.32 inches p e r hour f o r p l o t s t h a t
had been burned over annually f o r 5 o r 6 years. T h i s was an
average reduction of 38 percent; reductions f o r t h e various
Comparative r a t e s
s o i l types ranged from 20 t o 62 percent.
f o r undisturbed and raked p l o t s were 2.36 and 1.94 inches
r e s p e c t i v e l y , a reduction of 18 percent (1). Johnson, i n
Colorado, found t h a t removing t h e f o r e s t f l o o r reduced inf i l t r a t i o n capacity from 1.52 t o 0.92 inches, a reduction of
about 40 percent (11).
I n a 3-year record of s u r f a c e runoff from lysimeters
i n s t a l l e d under Ponderosa pine i n C a l i f o r n i a , Rowe found a n
average surface runoff of 0.33 inch (from 36.80 inches of
annual r a i n f a l l ) a s compared t o 13.30 inches from a l y s i meter kept bare
(a).
A l l of t h e s e s t u d i e s i n d i c a t e t h a t high i n f i l t r a t i o n
r a t e s a r e a s s o c i a t e d with a normal f o r e s t f l o o r . Once it i s
removed by burning o r raking, i n f i l t r a t i o n r a t e s a r e sharply
This
reduced and surface runoff i s increased several-fold.
e f f e c t i s g r e a t e r f o r fine-textured s o i l s t h a n f o r coarse,
and g r e a t e r on burned p l o t s t h a n on raked p l o t s .
These res u l t s a r e a p p l i c a b l e t o a r e a s where i n t e n s e ground f i r e s
destroy t h e f o r e s t f l o o r , and t o t h e occasional i n s t a n c e s
where l i t t e r i s removed f o r mulch o r barn straw.
Since t h e b e n e f i c i a l e f f e c t s of organic m a t t e r i n
i n c r e a s i n g i n f i l t r a t i o n r a t e s and i n reducing s u r f a c e runoff
have been proved, t h e question then can be narrowed down
from Does t h e f o r e s t f l o o r a f f e c t i n f i l t r a t i o n ? t o What
d i f f e r e n c e s i n i n f i l t r a t i o n r e s u l t from d i f f e r e n c e s i n humus
type and/or depth?
As t o t h e e f f e c t of humus type: t h e r e have been no
large-scale s t u d i e s designed s p e c i f i c a l l y t o
determine
whether mors o r mulls o r t h e i r v a r i a n t s , with comparable
depths, have d i f f e r e n t i n f i l t r a t i o n r a t e s . Occasionally one
w i l l come upon a reference t h a t a s h i n g l e e f f e c t of r e c e n t l y
f a l l e n hardwood leaves tends t o i n c r e a s e runoff.
Again,
q u a l i t a t i v e d a t a a r e lacking.
There i s , however, some evidence t h a t f o r e s t - f l o o r
depth a f f e c t s i n f i l t r a t i o n .
I n C a l i f o r n i a , Rome found s u r f a c e runoff of 5.9, 2.2, 0.5, 0 . 3 , and 0.5 i n c h e s f o r b a r e
s o i l , and 1/4-,
2
, 3 4 , and l & - i n c h depths of f o r e s t
floor, respectively.
I n t h i s i n s t a n c e , i n c r e a s i n g depths
beyond 1/2 i n c h had l i t t l e o r no e f f e c t (3).
Table 2 . - - I n f i l t r a t i o n
Average
humus
depth1
Forest
condition
Inches
i n d i c e s f o r 2 inches of storm r a i n f a l l
Deep
well-drained
sqils2
Shallow
well-drained
soils3
Inches
Imperfectly
drained
soilsL
-1
Poorly
draine'?
soils
Inches
per hr.
per hr.
per hr.
Inches
per hr.
0.49
0.28
Sawtimber and
paletimber,
poorly stocked;
seedling and
sapling
1.0
1.04
0.41
Sawtimber and
poletimber,
f a i r stocked
1.7
1-43
.56
.67
.38
Poletimber,
w e l l stocked
2.3
1.71
.67
.79
.46
Sawtimber,
me11 stocked
3.3
1.98
.77
.92
.53
'Excludes
'?lore
litter.
than
24 inches deep.
' ~ e s s than 24 inches deep.
i*Restriction l a y e r between 18 and 24 inches.
5 ~ e s t r i c t i o nl a y e r between 8 and 1 8 inches.
T r i p p and hThelan have a l s o given d a t a t h a t show t h e
i n f l u e n c e o f humus depth on i n f i l t r a t i o n .
Infiltration
indexes a r e given i n t a b l e 2 f o r four s o i l storage-drainage
c o n d i t i o n s of f o r e s t l a n d i n t h e Kennebec R i v e r b a s i n i n
Maine.
The i n f i l t r a t i o n i n d e x i s d e f i n e d a s t h a t average
r a t e t h a t , when a p p l i e d t o a n a c t u a l storm r a i n f a l l p a t t e r n ,
w i l l y i e l d m a t h e m a t i c a l l y a volume of surface-runoff t h a t i s
e q u i v a l e n t t o t h a t observed.
Within each of t h e f o u r s o i l c o n d i t i o n s , such f a c t o r s
a s a n t e c e d e n t m o i s t u r e and s o i l compaction from l o g g i n g w i l l
a l s o a f f e c t t h e i n f i l t r a t i o n index.
It i s noteworthy t h a t
t h e i n d e x f o r c o n d i t i o n s of h i g h e s t i n f i l t r a t i o n i s about
t w i c e t h a t of t h e p o o r e s t c o n d i t i o n i n each s o i l c a t e g o r y .
-
Evaluation o f f l o o d - c o n t r o l e f f
4~ripp, Norman R., and Whelan, Donald E.
e c t s o f the a g r i c u l t u r a l program f o r the Kennebec River b a s i n . Northeast. Fore s t h p t . Sta.
45 pp., i l l u s . 1953.
Lovest i n d i c e s f o r f o r e s t l a n d were almost i d e n t i c a l t o
t h o s e determined f o r good p a s t u r e and hayland. The i n c r e a s e
of i n f i l t r a t i o n w i t h depth of humus i n t h i s stildy, c o n t r a r y
t o t h e C a l i f o r n i a f i n d i n g s , i s due t o t h e d i f f e r e n c e s i n
r e s e a r c h methods.
C a l i f o r n i a f i n d i n g s were based on p l o t l y s i m e t e r s t u d i e s i n which, w i t h s u c c e s s i v e i n c r e a s e s of
f o r e s t f l o o r d e p t h , t h e s i n g u l a r e f f e c t on i n f i l t r a t i o n soon
reached a p o i n t o f l i m i t i n g r e t u r n .
The s t u d y by T r i p p and
TJhelan was based on t h e o r e t i c a l r o u t i n g of w a t e r through t h e
s o i l p r o f i l e , u s i n g s o i l - c o r e p e r c o l a t i o n r a t e s and moisture-storage values f o r i n d i v i d u a l s o i l horizons i n t h e
d i f f e r e n t s o i l - c o v e r complexes.
So f a r , o n l y summer i n f i l t r a t i o n has been considered.
The r e l a t i o n s h i p o f a f o r e s t f l o o r t o i n f i l t r a t i o n under
w i n t e r f r e e z i n g c o n d i t i o n s i s i n f l u e n c e d by i t s e f f e c t on
f r o s t t y p e and depth. Though t h i s phase o f i n f i l t r a t i o n has
been d i s c u s s e d more t h a t it has been s t u d i e d , a few observat i o n s o f f r o s t c o n d i t i o n s i n open and f o r e s t e d l a n d have
shown t h a t f r o s t a s s o c i a t e d w i t h f o r e s t humus i s o f t e n more
permeable t h a n t h e
f r o s t t y p e found i n u n p r o t e c t e d open
areas.
It h a s a l s o been shown t h a t humus-protected s o i l
does n o t f r e e z e s o d e e p l y nor s o e x t e n s i v e l y a s open-land
Q u a n t i t a t i v e d a t a a s t o humus t y p e and d e p t h i n resoil,
l a t i o n t o f r o s t t y p e and depth and f r o s t - i n f i l t r a t i o n r e lationships a r e lacking.
I n a d d i t i o n t o i t s i n f l u e n c e on i n f i l t r a t i o n
(the
movement of w a t e r i n t o t h e soil.),
a Torest f l o o r
also
a f f e c t s p e r c o l a t i o n - - t h e movement o f w a t e r through t h e s o i l .
Trimble, Hale, and P o t t e r (32) compared p e r c o l a t i o n r a t e s
through humus l a y e r s and through t h e s u r f a c e s o i l o f cropl a n d and p a s t u r e .
Vorking w i t h s o i l c o r e s , t h e y found perc o l a t i o n r a t e s of 236 and 132 i n c h e s p e r hour f o r mors and
m u l l s i n t h e Allegheny watershed i n New York and Pennsylvan i a . R a t e s f o r row crops and pasture-hayland c o n d i t i o n s were
3 t o 7 and 8 t o 22 i n c h e s r e s p e c t i v e l y .
Firm m u l l under
grazed f o r e s t s t a n d s had a r a t e o f 30 i n c h e s p e r hour. Perc o l a t i o n r a t e s f o r f i n e , c o a r s e , and medium m u l l s d i d n o t
d i f f e r s i g n i f i c a n t l y from each o t h e r ; ungrazed f i r m m u l l had
a s i g n i f i c a n t l y lovier r a t e , about one-half t h a t of t h e o t h e r
mulls.
Subtypes o f m o r h u m u s g a v e p e r c o l a t i o n r a t e s tha.t
showed no s i g n i f i c a n t d i f f e r e n c e s
(a).
The e f f e c t of a f o r e s t f l o o r on p e r c o l a t i o n i n t h e
u n d e r l y i n g s o i l i s more d i f f i c u l t t o i s o l a t e . Lunt observed
t h a t a g g r e g a t i o n was g r e a t e r under r e d p i n e l i t t e r i n t h e
1- t o 3-inch d e p t h t h a n under b a r e s o i l (LJJ, which s u g g e s t s
t h a t p e r c o l a t i o n w a s f a c i l i t a t e d immediately belovr t h e litter.
From t h e above, it i s e v i d e n t t h a t p e r c o l a t i o n r a t e s
through humus a r e h i g h and--even f o r t h e most slowly d r a i n e d
t y p e - f i r m mull--are
not a l i m i t i n g f a c t o r i n soil-water
drainage.
The e f f e c t on p e r c o l a t i o n r a t e s immediately below t h e humus l a y e r i s n o t known, though i t i s r e a s o n a b l e t o
assume t h a t , because o r g a n i c m a t t e r f a c i l i t a t e s aggregat i o n and a g g r e g a t i o n f a c i l i t a t e s p e r c o l a t i o n , a n e f f e c t exists.
4
SUGGESTED
RESEARCH
Recognizing t h e h i g h summer i n f i l t r a t i o n r a t e s t h a t
a r e a s s o c i a t e d w i t h normal accumulations of d i f f e r e n t t y p e s
of humus under u n d i s t u r b e d f o r e s t s , we can conclude t h a t und e r t h e s e c o n d i t i o n s humus l a y e r s do n o t l i m i t t h e movement
of w a t e r i n t o t h e s o i l .
The same c o n c l u s i o n can be reached
f o r p e r c o l a t i o n r a t e s . Three a r e a s of r e s e a r c h remain:
1. Depth and type.--The r e l a t i o n s h i p of f o r e s t - f l o o r depth
and humus t y p e t o i n f i l t r a t i o n s h o u l d be i n v e s t i g a t e d .
It would seem p a r t i c u l a . r l y p e r t i n e n t t o determine f o r
each major humus t y p e t h e minimum d e p t h o f f o r e s t f l o o r
t h a t i s s u f f i c i e n t t o c o n t r o l s u r f a c e runoff a n d e r o s i o n .
2.
Winter i n f i l t r a t i o n . - - T h i s
represents
probably t h e
During w i n t e r f r e e z i n g ,
l a r g e s t gap i n o u r knowledge.
a n o t h e r f a c t o r t h a t a f f e c t s infiltration i s i n t r o d u c e d :
A humus depth t h a t i s j u s t s u f f i c i e n t t o
s o i l frost.
p r o t e c t t h e s o i l i n t h e summer a g a i n s t r a i n f a l l impact
may n o t b e s u f f i c i e n t t o p r e v e n t t h e f o r m a t i o n of hpermeable f r o s t i n t h e w i n t e r .
A s t u d y of m u l t i p l e r e l a t i o n s h i p s i s n e c e s s a r y t o determine t h e r e l a t i o n s h i p
o f humus t y p e and depth t o f r o s t t y p e and depth, a n d t o
determine t h e i n f l u e n c e of f r o s t t y p e and d e p t h on i n f i l t r a t i o n and p e r c o l a t i o n .
3.
Aggregation and percolation.--The i n f l u e n c e of humus on
a g g r e g a t i o n 3 f t h e u n d e r l y i n g s o i l and i t s e f f e c t on
p e r c o l a t i o n deserves study.
Particularly interesting
would be d e t e r m i n a t i o n s o f t h e t i m e r e q u i r e d f o r aggreg a t i o n t o o c c u r a f t e r humus accumulation, t h e depth of
aggregation, and t h e d i f f e r e n c e s i n aggregate formation
t h a t a r e a s s o c i a t e d w i t h m u l l and mor t y p e s .
I n such s t u d i e s , i n f i l t r a t i o n c o u l d be measured on
small p l o t s e i t h e r w i t h r i n g s o r i n f i l t r o m e t e r s o r by measu r i n g n a t u r a l r u n o f f . P e r c o l a t i o n c o u l d be measured on s o i l
c o r e s i n t h e l a b o r a t o r y , o r w i t h r i n g s i n t h e f i e l d by removing humus t o t h e d e s i r e d depth and s e t t i n g t h e r i n g diWith t h e s e methods, o n l y
rectly i n t o the material tested.
12
.
r e l a t i v e r e s u l t s would be obtained.
For t h e time being,
t h a t i s a l l t h a t can be expected: with so l i t t l e knowledge
i n t h e s e f i e l d s of study, determination of even r e l a t i v e
e f f e c t s mould be a major c o n t r i b u t i o n .
WATER
STORAGE
The a d d i t i o n of organic m a t e r i a l s t o t h e s o i l and t h e
development of humus i n c r e a s e s t h e water-storage c a p a c i t y of
the soil.
I f t h i s i s not t h e most important f u n c t i o n of
humus i n watershed management, i t i s c e r t a i n l y t h e most complex.
The i n f i l t r a t i o n and p e r c o l a t i o n r e l a t i o n s h i p s of
f o r e s t humus and ( a s w i l l be discussed) i t s e f f e c t on evapor a t i o n a r e l a r g e l y s a t i s f i e d by having minimum humus depths
t o prevent r a i n f a l l splash and shade t h e s o i l surface.
The
storage f u n c t i o n i s ' m o r e complex i n t h a t it v a r i e s more
s p e c i f i c a l l y with type of humus and humus depth.
For forest-watershed management, t h e i n c r e a s e i n
water-storage c a p a c i t y by humus development has s e v e r a l
e f f e c t s . I t s most important p e r t a i n s t o f l o o d c o n t r o l : i n creased r e t e n t i o n storage provides g r e a t e r opportunity f o r
s t o r a g e of flood-producing r a i n f a l l s ; and increased detent i o n s t o r a g e slows t h e movement of t h e r a i n f a l l t o stream
channels.
The importance of t h e s e e f f e c t s i s i n proportion
t o t h e r e l a t i v e i n c r e a s e i n storage; t h i s , i n t u r n , i s i n
proportion t o t h e t o t a l s t o r a g e capacity of t h e s o i l , a
Shallom-soil
f u n c t i o n of i t s t e x t u r e and a v a i l a b l e depth.
areas a r e benefited p r o p o r t i o n a t e l y t h e most.
Increasing
storage c a p a c i t y of shallow s o i l s may a l s o serve t o provide
g r e a t e r moisture s u p p l i e s f o r growth, though t h e contrary
p o s s i b i l i t y has been suggested: t h a t accumulation of s u r f a c e
organic matter, by reducing t h e amount of p r e c i p i t a t i o n t h a t
reaches t h e urfderlying s o i l , may i n i t i a t e woodland degeneration
(a).
Factors t h a t a f f e c t humus type and depth d i r e c t l y
a f f e c t water s t o r a g e and w i l l be discussed from t h a t point
of vie-.
Before considering t h e s e f a c t o r s , a t t e n t i o n w i l l
be paid t o water-storage c a p a c i t i e s of humus.
STORAGE
CAPACITIES
Since organic m a t e r i a l added t o t h e s o i l i n c r e a s e s
i t s c a p a c i t y t o s t o r e moisture, t h e i n c r e a s e w i l l depend on
t h e t o t a l amount of organic m a t e r i a l added and i t s waterFor t h e o b j e c t i v e s of
r e t e n t i o n and -detention c a p a c i t i e s .
t h i s paper t h e s e have been estimated from d a t a on weight of
organic m a t e r i a l and humus depth.
On t h e weight b a s i s , t o t a l amounts of organic mater i a l added t o t h e s o i l have been shown by Lunt (16) t o vary
from about 11 t o 131 tons per acre i n New England ( t a b l e 3).
Estimated water storage capacities f o r t h e s e accumulations
ranged from 1.74 t o 0.14 inches, based on an estimated ret e n t i o n o r f i e l d capacity f o r t h e humus of 150 percent by
weight.
The average value f o r t h e four hardwood stands i n
Table 3.--1mht
of accmulated o r m n i c material and estimated water-retention c a ~ a c i t y
Estimated
F
Forest stand
capacity
P
P
a
p e r acre
Inches
Pounds
B
per acre
per a c r e
New Hampshire
White pine
100 years o l d
42,981
75,697
118,678
0.79
White pine
100 years o l d
21,107
46,531
67,638
.45
-
263,547
263,547
1.74
15,350
95,460
110,810
.73
Spruce--hardwoods--birch-maple
with spruce understory
Spruce-hardwood
Connecticut
Hardmood--oak,
beech, birch
Hardmood--oaks
Hardwood--oak, maple, ash
Hardwood--oak
Red pine
30 years o l d
Red pine
27 years old
White pine
27 years old
Table 4.--Humus
depth and moisture-storace
capacity of mature stands
i n t h e C o ~ e c t i c u tRiver ~ a t e r s h e d
Storage capacity
Humus
type
Forest type
Depth
-1
Available
retention
Detention
Inches
Inches
Medium-textured s o i l s (loams)
Temporary hardwoods (northern New ~ngland)'
Mull
3.5
0.91
0.98
Long-lived hardwoods (northern New ~ n g l a n d ) ~
Mull
& mor
7.3
2.19
2.19
Hemlock-spruce-fir
kr
7.6
2.51
2.43
Temporarg hardwoods (southern Nem ~ n g l a n d ) '
null
2.6
--
Long-lived hardwoods (southern New England)
Mull
& mor
3.5
--
---
Hemlock-spruce-fir
Mor
6.0
--
--
(northern Nem England)
(northern New ~ n g l a n d )
hemporary hardwoods:
2Long-lived hardwoods:
land incluaes t h e oaks.
Aspen, pin cherry, gray birch, and paper birch.
Northern hardwood type i n northern N e w England, but i n southern
New Eng-
Connecticut was 0.50 inch and f o r t h e t h r e e pine s t a n d s 0.20
inch, about one-half t o one-fourth t h e mean c a p a c i t y of t h e
f o u r New Hampshire s i t e s .
On t h e depth b a s i s , average maximum humus depths
under mature f o r e s t stands (estimated i n t h e course of
flood-control surveys
conducted by t h e Department
of
Agriculture i n t h e Connecticut River watershed) a r e given
i n t a b l e 4 f o r medium- and coarse-textured s o i l s .
By t h e s e
data, maximum depths a r e about 7.5 inches f o r mediumt e x t u r e d s o i l s f o r both mull and mor humus types under both
long-lived hardwood and spruce-fir-hemlock stands.
These
f i g u r e s a r e based on d a t a from New Hampshire, Vermont, and
t h e Berkshire s e c t i o n of Massachusetts.
They represent
conditions i n o l d undisturbed stands and a r e maximum values
from curves of humus depth accumulation over age, such a s
shown i n f i g u r e 1.
To estimate soil-moisture s t o r a g e c a p a c i t i e s f o r
t h e s e humus depths, a p p r o p r i a t e soil-moisture constants mere
'
0
10
20
30
40
50
60
70
80
90
0
10
20
30
40
SO
60
0
I0
20
30
40
50
60
70
80
90
100
AGE, IN YEARS
AGE. IN YEARS
FIGURE I.--Humus d e p t h
-
a g e r e l a t i o n s h i p s . The
numbered p o i n t s i n d i c a t e number o f p l o t s .
derived, based l a r g e l y on p h y s i c a l p r o p e r t i e s of humus a s
a l r e a d y noted.
Per-inch depth values f o r t h r e e humus types
on medium-textured s o i l s a r e given i n t a b l e 5; t h e values
f o r t h e mor and coarse mull were used t o d e r i v e t h e retent i o n and d e t e n t i o n c a p a c i t i e s given i n t a b l e 4. For each of
t h e t h r e e f o r e s t t y p e s l i s t e d i n t h i s t a b l e t h e maximum
amount of w a t e r h e l d i n t h e humus i s about e q u a l l y d i v i d e d
between a v a i l a b l e r e t e n t i o n and d e t e n t i o n s t o r a g e .
Storage
c a p a c i t y i s l e a s t f o r t h e temporary hardwoods (about 0.95
i n c h e s i n each t y p e o f s t o r a g e ) and g r e a t e s t f o r t h e
hemlock-spruce-fir t y p e s ( a b o u t 2.50 i n c h e s c a p a c i t y i n each
storage category).
Moisture-storage
c o n s t a n t s were n o t
a v a i l a b l e t o determine s t o r a g e c a p a c i t i e s f o r m u l l humus
l a g e r s of t h e c o a r s e - t e x t u r e d s o i l s .
Table 5.--Moisture-storale
capacities per inch of forest humua on medium-textured s o i l s
i n the Northeast
Humus
type
"lilting
percentage
Percent
Percent
40.0
7.4
0.33
45
79
40.0
13.8
.26
39
.28
66
40.0
10.9
.29
26
.l9
Specific
gravity1
0.22
1.52
85
.51
2.38
2.56
pore
space
Percent
Mor
Coarse mull
Firm mull
- 87
Detention
Retention
storage2
Bulk
density
(u
Available
storage
Inches
Detention
storage
corrected4
Percent
Inches
0.32
1
1
Mor was
Calculated using 1.5 a s a specific gravity of organic matter and 2.65 as s p e c i f i c gravity of mineral s o i l .
estimated t o contain 95 percent organic matter, coarse mull 15 percent, and firm mull 5 percent.
20n a volume basis, retention storage appears t o be i n the neighborhood of 40 percent (33) which i s about equivalent
t o p r w i o u s estimates of 150 percent by weight and 0.22 f o r bulk density.
Total pore
? ~ e t e n t i o npore space was determined by subtracting retention storage (40 percent) from t o t a l pore space.
space was calculated using bulk densities and s p e c i f i c gravities.
'since t o t a l detention storage i s never f u l l y u t i l i z e d i n moisture storage, these volumes were multiplied by a corr e c t i r n f a c t o r (0.72) a s determined by Trimble, Hale, and P o t t e r on core samples (2).
Any comparison of w a t e r - s t o r a g e c a p a c i t i e s i n m u l l
v e r s u s mor humus l a y e r s s h o u l d be made w i t h c o n s i d e r a t i o n o f
t h e n a t u r e o f t h o s e two t y p e s . An accumulation of mor humus
i s almost e n t i r e l y a n a . d d i t i o n t o t h e depth o f m i n e r a l s o i l
and t h u s a n a d d i t i o n t o t h e p r o f i l e s t o r a g e c a p a c i t y . But a
m u l l humus o f t h e same d e p t h r e p r e s e n t s a mixing w i t h t h e
m i n e r a l s o i l o f a much l e s s e r amount of o r g a n i c matter--and
t h e r e f o r e an a d d i t i o n of considerably l e s s p r o f i l e storage
capacity.
This r e l a t i o n s h i p has been r e p o r t e d by T r i a b l e
(32)
To summarize, water-holding c a p a c i t i e s v a r y w i t h
humus t y p e and, i n t h e c a s e o f m u l l s , w i t h t e x t u r e of t h e
m i n e r a l s o i l . About 40 p e r c e n t of t h e mor humus H l a y e r may
be c o n s i d e r e d r e t e n t i o n s t o r a g e , 1 5 p e r c e n t s o l i d s , and 34
percent detention storage.
Thus a 4-inch d e p t h o f mor c a n
r e t a i n about 1.60 i n c h e s of w a t e r ( i n c l u d i n g t h a t below t h e
w i l t i n g p e r c e n t a g e ) and d e t a i n a b o u t 1 . 2 8 i n c h e s .
A s shown
i n t a b l e 5, t o t a l s t o r a g e c a p a c i t i e s o f a n e q u a l d e p t h of
mull humus a r e a p t t o be somewhat l e s s ;
FACTORS
THAT
AFFECT
HUMUS
DEPTH
&
TYPE
.L
Other than t h e flood-control survey d a t a j u s t c i t e d ,
only a few d a t a on depth of humus accumulation a r e a v a i l a ble.
Lunt measured humus depths under a number of f o r e s t
stands i n Connecticut and New Hampshire.
He found t h a t t h e
F and H l a y e r s i n Connecticut accumulated t o a depth of
about 2 inches. Under ha.rdwoods, t h e F l a y e r averaged about
0.4 i n c h and t h e H l a y e r 0.8 inch.
Under hemlock, 3- t o 4inch l a y e r s were o c c a s i o n a l l y found. I n New Hampshire, Lunt
reported humus depths of 2 and 3-1/4 inches under white p i n e
and 3-1/4 and 5-3/4 inches under spruce-hardwoods (16). I n
oak stands i n t h e mountains of West V i r g i n i a , Trimble and
Weitvnan found humus depths of 1.8 t o 4.0 inches, varying
with s i t e index
It i s doubtful t h a t any of t h e s e
measurements
represent maximum accumulations under und i s t u r b e d conditions.
(s)
.
Storage functions of f o r e s t humus a r e r e l a t e d d i r e c t l y t o t h e type and amount of humus, which, i n t u r n , a r e
r e l a t e d t o a v a r i e t y of environmental and induced f a c t o r s .
Among t h e i n f l u e n c i n g environmental f a c t o r s a r e s o i l text u r e , s o i l drainage, climate, topography, . species composit i o n , and age; humus depth i s a l s o c o r r e l a t e d with s i t e
q u a l i t y through j o i n t r e l a t i o n s h i p s with s o i l and topographic f a c t o r s .
Induced f a c t o r s i n c l u d e disturbance from
c u t t i n g , grazing, and f i r e .
These f a c t o r s have been d i s cussed i n some d e t a i l by K i t t r e d g e (12) and by S a r t z and
a s they a f f e c t humus accumulation.
Here,
Huttinger (2)
emphasis w i l l be placed on water storage, n o t i n g i n passing
some d e f i c i e n c i e s i n information. The following discussions
a r e r e l a t e d t o humus l a y e r s on w e l l and moderately drained
s o i l s . On poorly drained s o i l s , humus conditions a r e governed l a r g e l y by t h e degree of waterlogging.
Climate
The c l i m a t i c e f f e c t on humus depth and s t o r a g e can be
v i s u a l i z e d from a statement i n Jenny's t e x t on s o i l format i o n (8):
*
I f we consider t h e c h a r a c t e r i s t i c ( v i r g i n ) f o r e s t
s o i l s from t h e s n b a r c t i c regions of e a s t e r n Canada t o
t h e Appalachian regions of Kentucky and Tennessee, it
S~ortheasternForest Soils Group. Report of the Committee on Classification
of Forest Humus on Poorly Drained Soils. 6 pp., unpublished.
17
n i i l be n o t i c e d t h a t t h e most conspicuous d i f f e r e n c e s
l i e i n t h e depth and condition of t h e sum t o t a l of
organic m a t e r i a l t h a t remains above t h e mineral s o i l
I n t h e northern extremes, t h e s e l a y e r s a r e excessively
deep and slov: t o decompose.
I n t h e southern example,
they a r e shallow and s u b j e c t t o r a p i d decomposition.
To i l l u s t r a t e t h i s , K i t t r e d g e has compiled a range of
f o r e s t - f l o o r weights i n c r e a s i n g with decreasing l a t i t u d e
from 1 . 7 metric tons p e r a c r e f o r an old-growth longleaf
pine stand i n Florida t o 119.2 tons f o r a birch, sugar
maple, and spruce stand i n New Hampshire (12). Recent measurements by Metz of t h e t o t a l weight of t h e f o r e s t f l o o r i n
South Carolina hardwood stands ( 2 l ) permit a comparison with
Lunt s measurements i n hardwood stands i n Connecticut (16).
Oven-dry weights of organic matter i n South Carolina under
yellow-poplar, hickory, and oak stands were 11,940, 8,000,
and 13,030 pounds p e r a c r e r e s p e c t i v e l y , averaging about
one-seventh of t h e average weight of '75,960 pounds f o r f o u r
measurements reported by Lunt ( t a b l e 3).
(9,
A s Jenny e t a 1 have shown
decomposition r a t e s of
organic m a t t e r a r e dependent on both temperature and r a i n f a l l . Temperature i s probably t h e most i n f l u e n t i a l c l i m a t i c
f a c t o r t h a t a f f e c t s decomposition i n t h e Northeast, varying
r e l a t i v e l y more with l a t i t u d e t h a n with r a i n f a l l .
Roughly,
t h e f r o s t season i n northern Maine i s 60 t o 80 days longer
t h a n t h a t i n southern Pennsylvania.
Also, average January
and J u l y temperatures a r e 10 and 1 5 degrees lower i n northe r n Maine.
These d i f f e r e n c e s a r e noteworthy i n l i g h t of
Spdulding and Hansbroughls statement t h a t slash-decay fungi
made l i t t l e o r no gromth a t temperatures below 40°F. and req u i r e temperatures of 7 0 O ~ . o r higher f o r most rapid decay
(31).
From d a t a a v a i l a b l e , such a s those i n t a b l e s 3 and 4,
g r e a t e r humus depths (and t h e r e f o r e
g r e a t e r s t o r a g e capacity) i n t h e White Mountains i n New
Hampshire may give humus a n important r o l e i n watershed
management i n t h a t region.
i t appears t h a t t h e
While d a t a show t h a t humus l a y e r s a r e deeper i n t h e
northern than i n t h e southern p a r t of t h e United S t a t e s ,
t h i s should not be construed a s an i n d i c a t i o n t h a t organic
matter i s unimportant f o r watershed purposes i n t h e South.
Because of i t s l e s s e r depth, t h e f o r e s t f l o o r does not have
so g r e a t a s t o r a g e capacity a s i n t h e North; but i t s prot e c t i v e functions and i t s propensity t o f a c i l i t a t e i n f i l t r a t i o n remain.
Topography
Apparently t h e only information on t h e e f f e c t of
topography on humus depth was obtained
by S a r t z and
Huttinger ( 3 0 ) . They shorn t h a t t h e depth of mull humus i n
t h e Allegheny River watershed v a r i e d with aspect.
Signific a n t l y g r e a t e r accumulations were measured on northeast
slopes vihere t h e average depth f o r a l l stands x a s 3.33
inches; on l e v e l ground, t h e depth was 2.35 inches. With 40
percent r e t e n t i o n , t h i s i s a d i f f e r e n c e of 0.39 inch,
a
substa.ntia1 p o r t i o n of t h e f i e l d c a p a c i t y involved.
Q u a n t i t a t i v e information on t h e influence of s l o p e
percent and p o s i t i o n on slope i s lacking; b u t observations
i n d i c a t e t h a t humus i s deeper i n hollows t h a n on r i d g e s and
deeper on f l a t a r e a s and g e n t l e slopes t h a n on s t e e p slopes.
S t a n d C o m p o s i t i o n , A g e , 6 S i t e Q u a 1 ity
The e f f e c t of s t a n d composition on humus depth and
s t o r a g e may be e a s i l y confounded with climate and topographic e f f e c t s .
Generally, according t o Kittredge (12)
,
t h e r e a r e no marked d i f f e r e n c e s i n annual d e p o s i t i o n of
l i t t e r by deciduous and c o n i f e r species.
However, d a t a i n
t a b l e 3 i n d i c a t e t h a t i n t h e Northeast t h e r e may be s i g n i f i c a n t d i f f e r e n c e s i n accumulation by f o r e s t types. I n t h i s
connection, d a t a i n t a b l e 4 i n d i c a t e t h a t i n t h e Connecticut
River watershed t h e hemlock-spruce-fir s t a n d s develop deeper
humus l a y e r s t h a n t h e long-lived hardv~oodson both loams and
sandy loams. The c o n i f e r humus types a r e predominantly mors
and t h e hardwood a r e predominantly mulls.
This comparison,
hovtever, i s based on meager d a t a ; information from designed
s t u d i e s i s needed.
The r e l a t i o n s h i p of stand age t o humus depth has been
s t u d i e d i n t h e Northeast by measuring and p l o t t i n g t h e s e
v a r i a b l e s so a s t o show humus reduction a f t e r c l e a r - c u t t i n g
and t h e subsequent build-up of humus depth (22).
Several
examples a r e given i n f i g u r e 1. Rates of humus accumulation
a r e given i n t a b l e 6 f o r t h e periods when annual accumulat i o n exceeded decomposition.
As noted, it required from 45
t o 80 years t o reach maximum humus depths s t a r t i n g with
i n i t i a l depths ranging from 1.6 t o 3.3 inches.
Rates of
accumulation a r e v a r i a b l e , and t h e r e i s a tendency t o higher
r a t e s i n t h e more n o r t h e r l y l o c a t i o n s . S a r t z and H u t t i n g e r q s
%ese i n i t i a l depths are taken a t the low point o f the humus-depth over age
curves; they appear t o be reached between the 25th and 35th years o f regrowth
after clear-cutting.
-
data showed a reduction i n humus depth a f t e r maximum values
However, t h i s reverse t r e n d was based on
a r e reached (30).
very few data.
Humus depths a l s o vary with s i t e q u a l i t y , a s can be
noted i n t h e comparison of medium with coarse s o i l s i n t a b l e
4. Kittredge (12) has reviewed t h e l i t e r a t u r e on this rel a t i o n s h i p and concluded t h a t t h e b e t t e r t h e s i t e , t h e
deeper t h e humus.
For t h e Northeast, supporting evidence
has r e c e n t l y been reported by Trimble and Weitzman (2).
They found average humus depths of 1.8 t o 4.0 inches i n oak
stands i n Mest Virginia associated with s i t e indexes of 40
The humus types were mostly medium and duff
t o 90 f e e t .
mulls. I n terms of r e t e n t i o n water storage t h e s e depths a r e
equivalent t o about 0.7 t o 1.6 inches.
II
Also i n West Virginia, t h e senior author has observed
t h a t humus can be strongly influenced by s o i l o r i g i n . Under
comparable f o r e s t stands on s i m i l a r topography, humus on a
f e r t i l e limestone s o i l was about 4 t o 6 inches deep but 3 t o
4 inches deep on a nearby s o i l derived from sandstone and
shale.
Humus types a l s o d i f f e r e d : t h e humus on limestone
was predominately coarse and f i n e mull; t h e humus a s s o c i a t e d
with t h e sandstone and shale was medium mull.
and
has
Though a p o s i t i v e r e l a t i o n s h i p between humus depth
s i t e q u a l i t y appears t o be established f o r mulls, one
not been established s o conclusively f o r mors.
For
Table 6.--Rates
of humus accumulation
Humus depth1
Forest type
Time
interval
Location
Beginning
Maximm
Rate of
accumulation
i n 1 0 years
L
~
~
~
~
"
,
I
Long-lived
hardwoods (loams)
Co~ecticut
River
watershed
Inches
-
Inches
-
Years
Inches
3.3
7.3
45
0.89
[<I
Hemlock-sprucef i r (10)
t(
o
2.7
7.6
70
.70
(*I
Temporary hardwoods
(sand, sandy loams)
I,
n
1.6
2.6
35
.29
(*I
Long-lived hardm o d s (sand,
s a d y loams)
II
14
2.0
3.5
80
-19
(*)
I!
(1
2.0
6.0
60
.67
(")
Hemlock-spruce-fir
(sand, sandy loam*)
&
Northern hardaoods
(loams)
White pine-hemlo~k
Allegheny
River
watershed
11
11
2.3
1.6
3.6
2.7
M
.22
(m
80
.14
(30)
Morey, H. F. The r e l a t i o n s h i p of h m u s t o t h e vegetation-soil complex and i t s a p p l i c a t i o n t o t h e flood
c o n t r o l problem. Allegheny Forest Fxpt. Sta. (now Northeast. Forest. Eupt. S t a . ) . 35 pp., unpublished. 1941.
The F l a y e r i s generally deeper i n mors than
l ~ h humus
e
depths a r e composed of FH i n mars and FA1 l n mulls.
I n mulls,sometimes being a s deep as 1.5 inches i n t h e former while rarely measuring more than 0.5 inch i n mulls.
20
rnors, t h e s c a n t y d a t a a v a i l a b l e i n d i c a t e a n e g a t i v e c o r r e l a tion.
For example, Young found t h a t i n c r e a s i n g d e p t h s o f
mor humus were c o r r e l a t e d w i t h d e c r e a s i n g s i t e i n d e x o f
w h i t e p i n e i n Maine (36).
Extremely t h i c k accumulations o f
mor humus under s p r u c e s t a n d s on poor s i t e s have a l s o been
observed by t h e s e n i o r a u t h o r .
While t h e p r o d u c t i o n
of
o r g a n i c m a t t e r i s no doubt p o s i t i v e l y c o r r e l a t e d w i t h s i t e
q u a l i t y , t h e f a c t o r s t h a t a f f e c t t h e decomposition o f mor
humus have n o t been so c o r r e l a t e d t o d a t e .
Induced
Factors
The i n f l u e n c e of such induced f a c t o r s a s f o r e s t c u t t i n g , f i r e , and g r a z i n g on humus accumulation w i l l v a r y
o b v i o u s l y w i t h t h e i n t e n s i t y of t h e d i s t u r b i n g i n f l u e n c e and
t h e s i t u a t i o n and c o n d i t i o n of t h e humus, i n c l u d i n g i t s
l o c a t i o n and age.
When t h e f o r e s t canopy i s reduced by
c u t t i n g , humus accumulation and d e p t h a r e a f f e c t e d .
S a r tz
and H u t t i n g e r , f o r i n s t a n c e , show i n t h e s t u d y p r e v i o u s l y
c i t e d ( 3 0 ) t h a t humus under good s t o c k i n g (70 t o 100 p e r c e n t
canopy c o v e r ) was a b o u t 0.5 i n c h deeper t h a n i n f a i r - s t o c k e d
s t a n d s (40 t o 70 p e r c e n t ) .
The e f f e c t of f i r e i s l i k e w i s e r a t h e r v a r i a b l e , b e i n g
a f u n c t i o n o f i t s i n t e n s i t y and humus t y p e and c o n d i t i o n
--which a l s o r e a c t on i n t e n s i t y .
Obviously b o t h f i r e and reduced s t o c k i n g m i l l have a
n e g a t i v e e f f e c t on humus accumulation, t h e magnitude o f
which i s p r o b a b l y p r e d i c t a b l e , g i v e n s u f f i c i e n t b a s i c i n f o r mation n o t now a v a i l a b l e .
For p r a c t i c a l a p p l i c a t i o n , t h e r e
may b e l i t t l e reward i n d e t e r m i n i n g f i r e intensity-humus
depth r e l a t i o n s h i p s because f i r e may b e c l a s s e d as a n unplanned e v e n t , t h e consequence o f which must b e a c c e p t e d r e gardless of i t s effects.
P r e d i c t i n g t h e e f f e c t s of c u t t i n g
may be more importa.nt i n a p r a c t i c a l sense i f marked reduct i o n s o f humus r e s u l t from p a r t i a l c u t t i n g s . One of t h e imp o r t a n t o b j e c t i v e s might be t o determine t h e s t o c k i n g i n
various-aged s t a n d s a t which a n n u a l d e p o s i t i o n o f l i t t e r
exceeds a n n u a l r a t e o f decay.
Like f i r e , g r a z i n g has a d e l e t e r i o u s e f f e c t on humus
accumulation. Trimble e t a 1 (2)n o t e t h a t t h e volume weight
of g r a z e d mulls was 0.92 as compared t o 0 . 5 1 f o r ungrazed
c o a r s e , medium, and f i n e m u l l s . This i s e q u i v a l e n t t o a b o u t
a 45-percent r e d u c t i o n i n t o t a l p o r e space.
Retention
s t o r a g e i n p e r c e n t by volume Q;as n o t a f f e c t e d , b u t d e t e n t i o n
I n a study i n
s t o r a g e dropped from 23.4 t o 12.6 p e r c e n t .
w e s t e r n North C s r o l i n a Johnson r e p o r t e d t h a t g r a z i n g reduced
t o t a l p o r o s i t y 49, 13, and 5 p e r c e n t i n t h e 0 t o .!+-inch s o i l
d e p t h s i n cove ha.rdwood, oak-hickory,
and pine-oak t y p e s
(10).
-
Differences r e f l e c t d i f f e r e n t i n t e n s i t i e s of grazing.
The e f f e c t of induced f a c t o r s on humus type has perhaps been l e s s c l e a r l y defined t h a n t h e e f f e c t of t h e s e
f a c t o r s on humus depths.
However, a s previously mentioned,
t h e type of cover--which i s a f a c t o r s u s c e p t i b l e t o modification--has been shown by i n v e s t i g a t o r s t o be r e l a t e d t o t h e
formation of mull and mor humus.
Probably t h e most obvious e f f e c t of a f o r e s t land-use
p r a c t i c e on humus type i s t h e e f f e c t of woodland grazing i n
This condition was widely observed
producing a firm mull.
during U. S. Department of Agriculture flood-control surveys
i n t h e Northeast.
To summarize : moisture-storage p r o p e r t i e s of f o r e s t
humus depend on t h e q u a n t i t y accumulated and i t s waterholding capacity.
Both vary, though t h e r e i s r e l a t i v e l y
more v a r i a t i o n i n t h e former than i n t h e l a t t e r .
Data
giving maximum accumulations a r e inconclusive; but a s a
rough estimate, hardwood stands i n New England develop humus
l a y e r s 2.5 t o 7.5 inches deep while humus under softwoods
may average s l i g h t l y deeper.
Among n a t u r a l environmental f a c t o r s t h a t a f f e c t humus
type and depth a r e climate, s o i l , topography, s t a n d comp o s i t i o n , and age.
Man's a c t i v i t i e s such a s c u t t i n g , f i r e ,
and grazing may d r a s t i c a l l y a l t e r t h e n o m l humus-formation
processes.
SUGGESTED
RESEARCH
The obvious and major gap i n our knowledge of t h e
water-storage r e l a t i o n s of f o r e s t hurrms p e r t a i n s t o humus
depth and type under various types and conditions of f o r e s t
cover. The d a t a c i t e d i n t h i s paper permit only an estimate
of t h e range of normal accumulation.
Considering t h a t f o r
a t l e a s t a half-century f o r e s t humus has g e n e r a l l y been bel i e v e d t o i n f l u e n c e t h e d i s p o s i t i o n of p r e c i p i t a t i o n and
streamflow, it i s somewhat s u r p r i s i n g t h a t b e t t e r q u a n t i t a t i v e d a t a a r e not a v a i l a b l e .
This paradox may be due i n
p a r t t o t h e f a c t t h a t h e r e t o f o r e q u a l i t a t i v e recognition of
t h e r e l a t i o n s h i p s u f f i c e d , and q u a n t i t a t i v e understanding
wasnot c a l l e d f o r .
I t m a y a l s o bedue i n p a r t t o t h e
f e e l i n g t h a t humus acc~umulationi s extremely v a r i a b l e , even
under undisturbed conditions, and t h e r e f o r e i s d i f f i c u l t t o
study.
Information a t hand i n d i c a t e s t h e research needed t o
define b e t t e r t h e e f f e c t of s t a n d and environmental f a c t o r s
on humus development and storage c a p a c i t i e s :
1.
Determine t h e mater-holding c a p a c i t i e s of l i t t e r
humus l a y e r s of r e p r e s e n t a t i v e f o r e s t types.
and
2.
Determine t h e ranges. of humus depths and humus types
under r e p r e s e n t a t i v e f o r e s t types i n undisturbed sapl i n g , pole, and sawtimber stands.
Factors of climate,
s o i l s , and topography should be s t r a t i f i e d o r held
constant so t h a t t h e i r influence on humus development
w i l l not confound t h e e f f e c t s of f o r e s t type and age.
-
3.
Study t h e i n f l u e n c e of c l i m a t i c , topographic, and s o i l
v a r i a t i o n s onhumus accumulation.
Aminimumgoal of
i
humus formation
such a study might be t o determine f
i s modified by such v a r i a t i o n s i n t h e White Mountain
a r e a and, i f SO, t h e d i r e c t i o n of such modifications.
4.
Determine t h e e f f e c t of d i f f e r e n t degrees of c u t t i n g on
humus accumulation and t h e physical p r o p e r t i e s of humus.
5.
Determine t h e time required t o transform a firm m u l l t o
a humus type of more favorable moisture r e l a t i o n s h i p s .
EVAPORATION
Evaporation of moisture from t h e f o r e s t f l o o r has two
f a c e t s : t h e reduction i n s o i l evaporation through t h e i n s u l a t i n g e f f e c t of organic l a y e r s , and t h e evaporation of
moisture from t h e f o r e s t f l o o r p e r s e during t h e course of
successive wettings and dryings.
A s f a r a s i s known, no
research i n t h e Northeast has been conducted on e i t h e r process.
According t o Kittredge, evaporation from a s o i l
covered by a f o r e s t f l o o r ranges from 10 t o 80 percent of
t h a t from bare s o i l (and evaporation from a bare s o i l i s
g e n e r a l l y l i m i t e d t o t h e upper f o o t ) .
The reduction v a r i e s
with t h e type of humus and i n c r e a s e s with f o r e s t - f l o o r depth
up t o 2 inches (12). ( ~ e ~ tofh mull humus cannot be compared with depths of mor humus o r l i t t e r when considered i n
More r e c e n t l y , Rowe has found t h a t
t h e sense of a mulch.)
i n pine f o r e s t s i n t h e S i e r r a Nevadas, a f o r e s t f l o o r 1/2
i n c h deep was a s adequate a s g r e a t e r depths f o r c o n t r o l l i n g
evaporation f rom- underlying s o i l
(a)
.
Rowe a l s o makes t h e i n t e r e s t i n g point t h a t although
annual evaporation from a f o r e s t f l o o r can reach important
amounts, it can be more than compensated f o r by saving i n
evaporation from t h e t o t a l s o i l .
The 3-year average of
evaporation from l y s i m e t e r s under a Ponderosa pine f o r e s t
f l o o r 2.5, i n c h e s deep was 7.91 i n c h e s a s ccmpared t o 13.61
i n c h e s from a l y s i m e t e r k e p t b a r e , a d i f f e r e n c e of 5.70
inches.
I n c o n t r a s t , a n n u a l e v a p o r a t i o n from a 1-inch deep
p i n e f o r e s t f l o o r was 1.46 i n c h e s , and f o r one 3.6 i n c h e s
deep, 2.60 i n c h e s .
From s t u d i e s i n Nebraska o f t h e c o n t r o l of s o i l
e v a p o r a t i o n by wheat-straw mulch, R u s s e l h a s made s e v e r a l
p e r t i n e n t o b s e r v a t i o n s (28).
He found, f o r i n s t a n c e , t h a t
l i g h t a p p l i c a t i o n s o f straw t o a d e p t h o f 0.75 i n c h were
almost as e f f e c t i v e i n r e d u c i n g e v a p o r a t i o n a s d e p t h s up t o
About
6 inches--v!hich s u b s t a n t i a t e s Rove s f i n d i n g s
h a l f of t h e mulch's v a l u e was due t o i t s s h a d i n g e f f e c t , and
t h e remainder was due t o h e a t i n s u l a t i o n and o b s t r u c t i o n of
vapor escape.
A s soon as t h e s u r f a c e s o i l d r i e d , R u s s e l
noted, t h e s o i l i n e f f e c t provided i t s own mulch, r e d u c i n g
t h e i n f l u e n c e o f t h e o r g a n i c covering.
(a)
.
From a n upland hardwood f o r e s t f l o o r i n Tennessee,
t h e amount of w a t e r e v a p o r a t e d a f t e r i s o l a t e d storms w a s
e s t i m a t e d t o be 0.05 i n c h , amounting t o about 1 i n c h annually.
This i s roughly 4' p e r c e n t o f t h e a n n u a l evapot r a n s p i r a t i o n of about 26 i n c h e s .
This l o s s was c o n s i d e r e d
minor i n comparison w i t h t h e r e t a r d a t i o n e f f e c t of t h e
f o r e s t f l o o r on t o t a l s o i l e v a p o r a t i o n (2).
Rates o f e v a p o r a t i o n o f m o i s t u r e from humus a p p e a r t o
I n M i s s i s s i p p i , Broadfoot found t h a t a
vary with l a t i t u d e .
hardwood f l o o r l o s t 95 p e r c e n t of i t s f i e l d c a p a c i t y i n 5
days (2). I n t h e Tennessee s t u d y c i t e d above, 1 2 days were
r e q u i r e d t o r e a c h one-fourth f i e l d c a p a c i t y ( 2 ) .
G r e a t e r humus d e p t h c h a r a c t e r i s t i c s o f t h e N o r t h e a s t ,
compared t o more s o u t h e r n l a t i t u d e s , and a s s o c i a t e d g r e a t e r
w a t e r - s t o r a g e c a p a c i t i e s , would have t h e d u a l e f f e c t o f prov i d i n g maximum r e d u c t i o n o f s o i l e v a p o r a t i o n and g r e a t e r
Lower t e m p e r a t u r e s
e v a p o r a t i o n from t h e s o i l humus p e r s e .
o f t h e N o r t h e a s t , however, c o u l d reduce t h e d r y i n g r a t e between storms and t h u s t h e t o t a l amount o f m o i s t u r e l o s s .
U n t i l q u a n t i t a t i v e d a t a a r e obtained, t h e s e r e l a t i o n s h i p s
remain s p e c u l a t i v e .
SUGGESTED
RESEARCH
Two g e n e r a l p r o j e c t s a r e i n d i c a t e d :
1. Evaporation from t h e f o r e s t floor.--Determine amounts o f
m o i s t u r e e v a p o r a t e d from d i f f e r e n t t y p e s and amounts o f
litter.
2.
Evaporation from t h e soil.--Determine
t h e depths o f
d i f f e r e n t humus t y p e s a n d l i t t e r l a y e r s r e q u i r e d t o keep
e v a p o r a t i o n from s o i l a t a minimum.
GENERAL
SUMMARY
Watershed f u n c t i o n s of f o r e s t humus i n v o l v e i t s
e f f e c t on i n f i l t r a t i o n and p e r c o l a t i o n , w a t e r s t o r a g e , and
evaporation.
The d i r e c t i o n a n d m a g n i t u d e of t h e s e e f f e c t s
r e f l e c t t h e p h y s i c a l p r o p e r t i e s o f humus; namely, i t s l i g h t
weight, p o r o s i t y , and g r e a t water-holding c a p a c i t y .
These
Mor humus i s
p r o p e r t i e s o f h m s v a r y w i t h t h e humus type.
l i g h t e r i n weight and has g r e a t e r p o r o s i t y t h a n m u l l humus.
Within t h e m u l l t y p e s , f i r m m u l l i s t h e h e a v i e s t a n d t h e
l e a s t porous.
Most humus t y p e s have h i g h i n f i l t r a t i o n and p e r c o l a t i o n r a t e s ; s e v e r a l s t u d i e s have shown t h a t i n f i l t r a t i o n was
s h a r p l y reduced and s u r f a c e r u n o f f i n c r e a s e d many-fold a f t e r
humus was removed from t e s t p l o t s .
C o n s i d e r a b l e v a r i a t i o n e x i s t s i n humus d e p t h and
water-holding c a p a c i t y .
Generally, i n t h e N o r t h e a s t , depth
o f hardwood humus w i l l range between 2 and 5 i n c h e s v ~ i t h
w a t e r - s t o r a g e ( d e t e n t i o n ) c a p a c i t i e s of a b o u t 0.5 t o 2.0
i n c h e s . C o n i f e r humus, somewhat d e e p e r , w i l l v a r y between
3 and 6 i n c h e s w i t h s t o r a g e o f a b o u t 1 t o 2.5 i n c h e s .
Average maximum humus d e p t h s of b o t h c o n i f e r s and hardwoods
i n n o r t h e r n New England a r e i n t h e neighborhood o f 7 t o 8
inches.
I n s o u t h e r n New England t h e y a r e a b o u t one-half a s
much
.
Humus depth i s a f f e c t e d by a l a r g e number o f environmental and induced f a c t o r s , many of which a r e e x p r e s s e d
through s i t e q u a l i t y .
I n t h e Northeast, c o n i f e r humus
accumulates a t t h e r a t e of a b o u t 2 / 3 i n c h e v e r y 1 0 y e a r s .
Comparable . r a t e s f o r hardwoods a r e about 3/4 i n c h on mediumt e x t u r e d s o i l and about 1/4 i n c h f o r c o a r s e r s o i l s .
Humus t y p e a l s o i s a f f e c t e d by environmental a n d i n duced f a c t o r s .
C e r t a i n s o i l and s t a n d c o n d i t i o n s t e n d t o
f a v o r mor o v e r m u l l humus, b u t t h e r e l a t i o n s h i p s a r e n o t
c l e a r l y understood.
Humus reduces e v a p o r a t i o n from u n d e r l y i n g s o i l , t h e
r e d u c t i o n t e n d i n g t o more t h a n compensate f o r t h e m o i s t u r e
evaporated from t h e humus l a y e r s .
Few s t u d i e s have been made f o r t h e e x p r e s s purpose o f
determining q u a n t i t a t i v e l y t h e hydrologic r o l e of f o r e s t
humus inwatershedmanagement.
are:
Major gaps i n our knowledge
1. The water-holding c a p a c i t i e s of l i t t e r and underlying
hwnus l a y e r s of r e p r e s e n t a t i v e f o r e s t types.
2.
The r e l a t i o n s h i p of humus depth and type t o f o r e s t type,
age, and treatment a s influenced by t h e environmental
f a c t o r s of climate, s o i l , and topography.
3.
The i n t e r - r e l a t i o n s h i p of f o r e s t humus,
depth, and w i n t e r i n f i l t r a t i o n .
4.
The influence
tration.
5.
The influence of humus on aggregation and percolation.
6.
Evaporation of moisture from humus and underlying s o i l .
of humus type and depth
f r o s t type and
on summer i n f i l -
APPEND l X
KEY FOR
OF FOREST
CLASSIFICATION
HUMUS TYPES
(1)
The following humus c l a s s i f i c a t i o n system was developed by t h e C o r d i t t e e on Forest Humus C l a s s i f i c a t i o n , Forest
S o i l s Subdivision, S o i l Science Society of America (1).Thus
it replaces t h e e a r l i e r humus c l a s s i f i c a t i o n of Heiberg and
Chandler
on which it was b u i l t .
(a
A.
No H-layer;
A1-horizon an i n t i m a t e mixture o r organic
matter and mineral s o i l , with gradual t r a n s i t i o n between
t h e A 1 and t h e horizon beneath.
F l a y e r may, o r may not
be p r e s e n t .
.MULL (2,3,L+) 7
...............................
1. A 1 e s s e n t i a l l y single-grain o r massive, without
aggregates.
Organic m a t t e r appears t o be more or
l e s s uniformly d i s t r i b u t e d throughout.
(a)
Massive and firm with generally
percent organic matter by weight..
l e s s than 5
...
.Firm Mull
(b)
Loose, with
low t o medium
organic-matter
content (usually l e s s than 10 p e r c e n t ) and
c o n s i s t i n g of a mixture of mineral s o i l and
organic m a t t e r a s s i n g l e grains.
Typically on
sandy s o i l s . . . . . . . . . . . . . . .
Sand Mull
............
2.
A horizon granular o r crumb s t r u c t u r e .
Concentralt l o n of organic mater and granular s t r u c t u r e most
pronounced i n t h e upper A 1 and decrease gradually
with depth.
(a)
Coarse granular o r crumb s t r u c t u r e ; many granu l e s 1/8 inch (2/3 mm) o r l a r g e r . Usually 5-20
percent organic matter..
Coarse Mull
............
(b)
Medium granular o r crumb s t r u c t u r e ; t h e l a r g e r
granules about 1/16 inch ( 2 mm) o r s l i g h t l y
smaller. Wide range of organic-matter content,
Medium Mull
u s u a l l y 5-30 percent
................
(c)
Fine granular s t r u c t u r e ; f r e q u e n t l y has t h e
appearance of f i n e black sawdust ; organic-mat-
'hkunbers r e f e r t o Explanatory Notes a t end o f key.
t e r high, u s u a l l g o v e r 30 p e r c e n t . .Fine Mull ( 6 ) .
3.
Complex mull t y p e s . D i s t i n c t s t r u c t u r a l d i f f e r e n c e s
between l a y e r s w i t h i n t h e zone o f o r g a n i c m a t t e r
incorporation.
(a)
B.
.
F i n e m u l l u n d e r l a i n by coarse- o r medium mull..
Twin Mull
......
H
and F l a y e r s p r e s e n t w i t h a n u n d e r l y i n g A 1 h o r i z o n
e s s e n t i a l l y similar t o t h a t o f a t r u e mull.
Gradual
t r a n s i t i o n from H t o A 1 and m i n e r a l s o i l beneath.
h his
t y p e p o s s e s s e s some o f t h e c h a r a c t e r i s t i c s of b o t h m u l l s
.DUFF MULL (4,5)
and mors )
. ..............................
1. Combined F and H l a y e r s more
.....................................Thick
than 1 inch thick.
Duff Mull
Combined F and H l a y e r s l e s s
......................................Thin
than 1 inch thick.
Duff Mull
2.
C.
H l a y e r p r e s e n t ( e x c e p t i n 3 below).
P r a c t i c a l l y no
mixing of o r g a n i c m a t t e r w i t h m i n e r a l s o i l .
Abrupt
t r a n s i t i o n from s u r f a c e o r g a n i c m a t t e r t o u n d e r l y i n g
horizon.
O
R (4)
......................................
1. The H l a y e r more t h a n 1/2 i n c h t h i c k .
H layer
has
s fine
......THICK
granular
MOR
(a)
The
(b)
H l a y e r s t r u c t u r e l . e s s , f e e l s g r e a s y when wet,
Greasy Mor
b u t hard and b r i t t l e when d r y
(c)
H l a y e r f e e l s and l o o k s f e l t y , due t o presence
of f u n g a l hyphae and/or p l a n t r e s i d u e s b u t n o t
Mor
l i v i n g roots ..........................Felty
...................................
structure.
Granular Mor
........
...........THIN MOR
2.
H l a y e r l e s s t h a n 1./2 i n c h t h i c k .
3.
H l a y e r lacking o r presnnt only a s a t h i n f i l m i n
d e p r e s s i o n s ............................IMPERFECT
MOR
Exfilanatory
Notes
(1) This key does n o t a p p l y on a r e a s where t h e upper A
h o r i z o n shoplls evidence o f prolonged w a t e r s a t u r a t i o n ,
such a s m o t t l i n g , p e a t l a y e r s , o r bog c o n d i t i o n s .
(2)
After disturbance of t h e
develop on a n o l d podsol.
f o r e s t c o v e r , a m u l l may
A s a r e s u l t , a remnant o f a
jeached l a y e r may be p r e s e n t i n t h e p r o f i l e even though
t h e l a y e r above i t resembles t h e A1 o f a mull. I n such
a c a s e , t h e humus t y p e i s t y p e d a s a mull on t h e b a s i s
of t h e c h a r a c t e r i s t i c s o f t h i s A 1 horizon.
A complete d e s c r i p t i o n o f a m u l l o r duff-mull t y p e
s h o u l d f u r n i s h t h e depth of organic-matter i n c o r p o r a t i o n i n inches.
For grouping d a t a and reconnaissance
u s e , t h e f o l l o w i n g depth c l a s s e s a r e suggested: v e r y
s h a l l o ~ v , l e s s t h a n 1 i n c h ; shallow, 1 t o 2 i n c h e s ;
deep, 2 t o 4 i n c h e s ; and v e r y deep, more t h a n 4 i n c h e s .
For example, a sand mull w i t h o r g a n i c m a t t e r incorporat e d t o a depth o f 1$i n c h e s would b e a ffShallow Sand
Mull. l1
( 4 ) When i t i s a p p a r e n t t h a t plowing o r g r a z i n g have modif i e d o r e l i m i n a t e d t h e n a t u r a l humus t y p e , t h i s s h o u l d
b e i n d i c a t e d by a d d i n g t h e l e t t e r U P w o r I1G" t o t h e
name of t h e humus t y p e .
For example, Firm Mull-P o r
Firm Mull-G;
o r Firm Mull-PG i f b o t h plowing and grazi n g have caused p r e s e n t c o n d i t i o n s .
On p r e v i o u s l y
c u l t i v a t e d l a n d , t h e r e i s f r e q u e n t l y a n o l d plow l a y e r
t h a t i s comparatively homogeneous throughout b u t may
u s u a l l y b e recognized by t h e s h a r p l i n e o f demarcation
a t t h e base of t h e plow l a y e r .
The humus t y p e s h o u l d
be based o n t h e c h a r a c t e r i s t i c s o f t h e H and/or A 1
h o r i z o n , and n o t on t h e p r o p e r t i e s o f t h e e n t i r e plowed
horizon.
Grazing c a u s e s compaction o f t h e o r g a n i c
h o r i z o n s and may reduce a m u l l w i t h g r a n u l a r s t r u c t u r e
t o f i r m m u l l , o r may m i x t h e H-layer o f a mor w i t h
mineral s o i l , c r e a t i n g a mull-like condition.
Again,
humus t y p e s h o u l d be based on t h e H and/or A 1 horizon,
a d d i n g t h e l e t t e r IfGI' t o i n d i c a t e t h a t g r a z i n g w a s
responsible.
A s s t a t e d i n e x p l a n a t o r y n o t e No. 3, t h e d e p t h o f
o r g a n i c m a t t e r i n c o r p o r a t i o n s h o u l d be g i v e n i n t h e
description.
The a d j e c t i v e s f o r t h e d e p t h c l a s s e s
should be used a s p r e f i x e s i n d e s c r i b i n g t h e A 1 p o r t i o n
o f t h e duff-mull.
For example, "Thick Duff Mull w i t h
shallow A l f l would b e used t o d e s c r i b e a duff-mull w i t h
F and H l a y e r s more t h a n 1 i n c h t h i c k and t h e A 1
h o r i z o n 1 t o 2 i n c h e s deep.
( 6 ) Because o f t h e h i g h organic-matter c o n t e n t i n t h e A
A
h o r i z o n of f i n e m u l l it may o c c a s i o n a l l y be confuse
w i t h t h e H l a y e r of g r a n u l a r mor. T h i s i s p a r t i c u l a r l y
t r u e when t h e h o r i z o n o r l a y e r i s shallow o r t h i n .
In
t h i s case, i f t r a n s i t i o n t o t h e mineral s o i l horizon
below i s r a t h e r a b r u p t and t h e o r g a n i c c o n t e n t s o high
t h a t it cannot be determined i n t h e f i e l d , whether i t
i s a c t u a l l y fine mull o r a g r a n u l a r rnor, t h e l a y e r
should be c l a s s e d a s H l a y e r and typed a s mor.
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