THE SIGNIFICANCE OF ROOT ROT AND FROST DAMAGE
IN SOME DOUGLAS FIR PLANTATIONS1
R. E. FOSTER AND A. L. S. JOHNSONa
ABSTRACT
Evaluations are made o f the significance o f root rot and frost damage evident
in five 16- to 18-year-old Douglas fir plantations. Assuming that all affected
trees die, two effects o f mortality are considered, one beneficial in that it
eliminates trees restricting the growth o f neighboring trees, and the other
detrimental in that it creates stand openings large enough to support a crop
tree. Most of the mortality was o f the former class, and there was no evidence
that the stand openings created would effect any appreciable reduction in
future yield from the area examined.
Most of the Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) plantations
in the coastal region of British Columbia have been established with trees
spaced at the approximate interval of 6 x 6 ft, yielding initial populations
of approximately 1,200 trees per acre. Although these populations may be
augmented by the establishment of natural regeneration, it is inevitable, with
time, that mortality will effect a net decrease in the number of trees. This
mortality is a normal phase of stand development and is necessary if detrimental root and crown competition is to be avoided. Normal yield tables
show, for example, that fewer than 450 trees per ac should be expected at
age 70, even o n moderately low sites (McArdle, Meyer, and Bruce 1949). On
the basis of normal yield theory, growth would be restricted if densities
higher than this prevailed and the full potential of the area in terms of trees of
acceptable diameter would not be realized. From this and other considerations,
mortality could be classed as detrimental only if it created undesirable stand
openings or eliminated a tree suitable for utilization during an intermediate
harvest. The object of the present paper is to examine the significance of two
diseases in the latter terms.
Studies were carried out in five 16- to 18-year-old Douglas fir plantations
on Vancouver Island, as described in previous publications (Foster and Johnson
1963a, 196313). Reference is made here to root rot and frost lesions, two of
the several disorders encountered. Evaluations of the significance of the
two disorders were made in terms of the size and configuration of stand
openings which would be created if all affected trees died, and o n an estimation of the maximum size and age that the tree closest to the centre of each
Contribution No. 936 of the Forest Entomology and Pathology Branch, Department of Forestry,
Ottawa, Canada.
Respectively, Head, Forest Pathology Investigations, and Forest Research Technician, Forest
~ n t ~ m o l o g and
; Pathology ~aborator;, victor;, B.C. (Biographies in For. Chron. 29: 3 5 9 and
36: 30, respectively.)
SEPTEMBER, 1963
267
opening might have attained had it survived. The area occupied by tree
roots was estimated through application of a formula derived from data
from McMinn (1962); namely, that the radial extension of the roots of a
16- to 20-year-old Douglas fir is approximately equal to 1/6 of its height.
This formula was applied to Douglas fir of different height class in the area
peripheral to the opening, thus establishing, on a scale plan of the study
area, the rooting zones and the sizes of the openings resulting from mortality.
An estimate of the additional space required by older trees was made on
the basis of data presented by Barnes (1956). Barnes examined yield tables
based on intensively managed Douglas fir plantations in Great Britain which
had been thinned at 3-year intervals over a 45-year period. His data showed,
for example, that stands on site index 140 were thinned from 740 trees per
ac at age 16 to 187 trees per ac at age 40. Converting these values to the
spatial requirements of individual trees,Vt was determined that d 40-year-old
tree required an area approximately twice as large as one 16 years old.
This correction was applied to McMinn's formula to estimate the circular
area required by trees at age 40, the initial age at which intermediate
harvests might be contemplated. Other values presented by Barnes were
used to estimate the spatial requirements of trees 22 to 40 years old. Openings
were tabulated only if it was possible to extend the roots of the tree closest
to the centre of the opening to a diameter of at least 9.9 ft without contacting the roots of a peripheral tree (Table 1). Both size and shape of
opening were thus taken into consideration.
Root-rot infected trees were classed as isolated or were grouped with one
or more adjacent infected trees on the basis of the distance separating them
and on the presence or absence of intervening healthy trees. Several groups,
consisting of more than 15 trees, were partitioned into smaller units on the
assumption that adjacent groups had coalesced.
TABLE 1
Size class
(ft)
Equivalent
age
(~rs)
<9.9
9.9-10.9
11.0-12.0
12.1-13.1
13.2-14.0
14.1-15.2
<22
Frequency of Openings
Tsnble River
Robertson
Campbell River
Block I1 Block I
Block I1
River
Block I
22-24
25-27
28-3 1
32-35
36-39
> 15.2
>39
1
Area examined (ac)
9.00
Percentage of trees affected 1.44
Number of trees per ac
505
Diameter in ft.
=
43,560
n
0
4.80
3.78
778
0
2.40
3.36
1,118
3
2.88
21.46
758
5
1.44
32.29
927
-
FORESTRY CHRONICLE
268
Most of the openings created by the elimination of trees affected by root
rot were smaller than 9.9 ft in diameter, or of insufficient size to support a
tree more than 22 years old. Of 574 infection centres examined only 47
or 8.2% were greater than this size and only 9 openings or 1.6% exceeded
15.2 ft in diameter and were large enough to support a tree approximately 40
years old (Table 1). Most of the large openings were in the severely infected
Tsable River populations (Figs. 1 and 2), but even here there were only
8 openings, or 1.9 per acre, large enough to support a crop tree. The infrequent
occurrence of large openings created by root rot may be attributed in part
to the high proportion of focal centres consisting of only one or two infected trees (Table 2).
TABLE 2
POPULATION
VALUESASSOCIATED
WITH ROOTROT
Population value
Robertson
River
Area (ac)
9.00
Number of focal centres 50
Study area
Campbell River
Block I1
Block I
2.40
55
Number of individuals
per focal centre
1
1
4.80
81
Tsable River
Block I
Block I1
2.88
227
1.44
161
(Frequency)
1
42
38
58
122
75
2
3
4
5
6
7
8
9
3
4
1
8
5
1
3
7
8
2
4
1
0
1
47
24
20
5
6
1
1
1
31
22
11
6
4
6
3
3
1.70
1.60
Mean number of
individuals per
focal centre
1.28
1.99
2.45
Very few stand openings created by the elimination of frost-lesion-affected
trees were of substantial size (Table 3), even in areas containing a high proportion of damaged trees (Figs. 3 and 4). The greatest frequency of large
openings was created in the Robertson River study area; 24 openings or
2.7 per ac were determined to be of sufficient size to support a crop tree.
In two other areas having the same or higher incidence of damage, however,
the frequency of large openings was 0.0 and 0.7 per ac, respectively.
SEPTEMBER, 1963
269
TABLE 3
Frequenw of Openings
Size class
(ft)
Equivalent
age
(ma)
~~b~~~~~~
River
Area examined (ac)
9.00
20.40
Percentage of trees affected
Number of trees per ac
505
Campbell River
Block I Block I1
4.80
31.85
778
2.40
19.92
1,118
Tsable River
Block I
Block 11
2.88
6.46
758
1.44
2.37
927
The effective size of a stand opening has been estimated in terms of the
space likely to be unoccupied by tree roots and on the assumption that roots
extend laterally in direct relation to tree height. Although there is evidence
of the latter relationship (McMinn 1962), it is doubtful that rooting areas
can be estimated with a high degree of precision on this ,basis alone. Crown
area, expressing the photosynthetic capacity of a tree, offers another approach
to the evaluation of optimum density and spatial requirement. Crown width
may be measured directly, and is known to be highly correlated with tree
diameter and height (Briegleb 1952) and to provide an index to optimum
growth (Warrack 1959, Smith and Ker 1960). It may be rewarding in future
studies of the present nature to consider crown width as a direct measure
of the extent to which trees of different size utilize available growing space.
Even with the assumption that all affected trees will die, an assumption
perhaps justified in the case of root rot but not with frost-damaged trees,
the analysis did not provide any evidence that the present levels of occurrence
of the two disorders would be high enough to cause a significant reduction
in future yield. A number of openings were large enough to support a crop
tree, but the frequency of openings of this size did not exceed 3.5 per ac
in the case of root rot or 2.7 per ac in the case of frost-damaged trees.
The analyses of root rot and frost damage were not undertaken concurrently and larger openings would have been indicated in some cases had
all affected trees been eliminated simultaneously.
Although there is no evidence that the elimination of all frost-damaged
trees would materially reduce future yield, this reduction might arise, paradoxically, if frost-damaged trees survived. A number of damagd trees have
FORESTRY CHRONICLE
Captions on Page 271
SEPTEMBER, 1963
been found (Foster and Johnson 1963a) to provide suitable infection courts
for wood-inhabiting fungi and a considerable development of incipient decay
has been noted in some cases. Losses of the latter nature may be of greater
significance than mortality; affected trees which survive will continue to occupy
growing space, yet may contribute little to net volume production if any
substantial amount of internal defect develops.
No provision in the analysis has been made for the subsequent development of root rot in new infection centres or through the extension of
existing foci. Utilizing the Tsable River data, it has been found that the
probability that an infection centre creates an undesirable opening increases
with the number of trees per focus. With increase in size of existing foci,
therefore, it would be necessary to attach greater significance to root rot
than indicated by the present data.
There was no evidence of a simple correlation between the number or
percentage of affected trees and the number of large openings created by
their elimination. Several other criteria require consideration; for example,
pattern, segregation, and density.
Whereas frost injuries occur more or less at random, it is known that
root rot tends to bc patchy in its occurrence. Thus, trees infected with root
rot are 'segregated' (Pielou, 1961) from uninfected ones; that is to say, the
two types of trees, diseased and healthy, are not randomly intermingled but,
instead, infected trees are more likely than healthy ones to have other infected
trees as their neighbors. As a result, openings of appreciable size may develop
where several infected trees occur close together. Previous studies (Foster
and Johnson 1963a) have shown that climatic disorders such as frost lesions
2nd sunscald are unsegregated; that is, healthy and diseased individuals are
frequently intermingled. These findings suggest that in areas in which root
rot and frost lesions occur at the same incidence, greater significance would
be attached to the former disorder.
Stand density is an additional factor which may influence the size of a
stacd opening; thus, one opening in excess of 15 ft occurred within the 500tree-per-ac population at Robertson River despite the very low incidence
of disease and the very few focal centres containing more than one infected
tree, but no openings in excess of 9.9 ft occurred within the 1,100-tree-per-ac
F I G U R E 1. Plan showing a heavy concentration of root r o t on a .03-ac sub-plot a t Tsable
River, with 9 of 2 5 Douglas fir infected ( 3 6 % ) . Hollow circles represent healthy trees, solid
circles represent root-rot-infected trees.
F I G U R E 2. Further details of sub-plot shown in Fig. 1. Root r o t trees have been eliminated,
rooting zones appropriate to 40-year-old trees have been indicated f o r surviving trees, and natural
regeneration has been added. Broken circle represents the size of the effective opening, 13.9
f t . in diameter.
F I G U R E 3. Plan showing a heavy concentration of frost-lesion-affected trees in Campbell River
Block I1 with 1 9 of 30 trees affected (63.3%) ). Hollow circles represent healthy trees, solid
circles represents frost-damaged trees.
F I G U R E 4. Further details of sub-plot shown in Fig. 3. Frost-damaged trees have been eliminated,
rooting zones appropriate to 40-year-old trees have been indicated f o r surviving trees, and natural
regeneration has been added. Broken circles represent effective openings, each less t h a n 7.0
f t. in diameter.
FORESTRY CHRONICLE
population in Campbell River Block I1 despite the higher level of root rot
and the occurrence of foci containing as many as eight trees.
Analyses were confined to Douglas tir 4.5 ft or more in height and no
consideration was given to small fir or to the natural regeneration of other
species. These species, mainly western hemlock (Tsuga heterophylla (Raf.)
Sarg.), western red cedar (Thuja plicata Donn), and western white pine
(Pinus monticola Dougl.) were present in every area and contributed as many
as 400 trees per ac at Tsable River (Foster and Johnson 1963b). There is
no assurance that all of the natural regeneration will survive, but even if
only a few trees survive they may have a substantial effect on future yield.
Surviving trees, many of which were in the 4-6 ft height class, will have an
additional period of approximately 50 years in which to contribute to a future
yield if the present stands are not harvested before 70 years. The apparent
freedom of natural regeneration from root infection may be related to its
relatively shallow rooting; as the roots become more extensive with time it
is probable that they will come in contact with viable inoculum in deeper
soil horizons and that a substantial number of those established in and near
infection centres will be eliminated from the stand.
These data suggest that appraisals establishing only the degree of infection
and mortality do not provide sufficient information on which to evaluate the
significance of a disease in terms of its effect on future yield. Some plantations
may be omrstocked and mortality may have a beneficial effect provided
large openings are not created.
Other plantations may be understocked for a given age and site and any
further mortality may be detrimental. The important criteria in the assessment of disease significance are the density and the spatial arrangement of
the residual stand after mortality has exacted its toll.
BARNES, G. H . 1956. Intermediate yields of Douglas-fir as interpreted from British yield tables.
J. Forestry, 54: 177-179.
BRIEGLEB, P. A. 1952. An approach to density measurement in Douglas fir. J. Forestry,
50: 529-536.
FOSTER, R. E. and A. L. S. JOHNSON. 1963a. Studies in forest pathology XXV. Assessments
of pattern, frequency distribution, and sampling of forest disease in Douglas fir plantations.
Can. Dept. Forestry, For. Ent. & Path. Br.
FOSTER, R. E. and A. L. S. JOHNSON, 196313. Amounts and distribution of natural regeneration
in three Douglas fir plantations on Vancouver Island. For. Chron. 39 (3): 260-5.
McARDLE, R. E. and W. H. MEYER. 1949. The yield of Douglas fir in the Pacific Northwnt.
U.S. Dept. Agr. Tech. Bull. 201.
McMINN, R. G. 1962. Personal communication.
PIELOU, E. C. 1961. Segregation and symmetry in two-species populations as studied by nearest
neighbour relations. J. Ecol. 49: 255-269.
SMITH, J. H. G. and J. W. KER. 1960. Growing Douglas fir and western hemlock at desired
rates. Univ. of B.C., Fac. of For., Res. Note 24.
WARRACK, G. C. 1959. Crown dimension, initial diameter and diameter growth in a juvenile
stand of Douglas fir. For. Chron. 35: 150-153.