Fores;~;ology
Management
ELSEVIER
Forest Ecology and Management 92 (1997) 19-28
The role of hair-cap mosses (Polytrichu~n spp.) in natural
regeneration of white spruce (Picea gZauca (Moench) Voss)
William C. Parker *, Stephen R. Watson, Daryll W. Cairns
Ontario
Forest Research
Institute.
Box 969, 1235 Queen St. East, Sault Ste. Marie.
Ont. P6A 5N5, Canada
Accepted 22 October 1996
Abstract
Natural regeneration of many North American conifer species frequently occurs in association with the hair-cap mosses
spp.), common and widely distributed pioneer mosses of boreal and temperate coniferous forests. The role of
mosses in the natural regeneration of white spruce (Picea glauca (Moench) Voss) was investigated through a
comparative study of seedbed effects on seedling emergence, survival, morphology and the presence of competing
vegetation. Intact monoliths of Polytrichum
moss, mineral soil and undisturbed litter were collected from the forest floor of
a mixed conifer stand in Central Ontario. Half of the litter seedbeds were later burned with a propane torch. Replicates of the
four seedbeds (n = 24) were placed in a greenhouse and hand sown with white spruce seed. Percent effective seedling
emergence was significantly higher on mineral soil than on the other three seedbeds but did not differ among moss, litter and
burned litter. Seedling survival under a watering regime representative of the seedbed collection site did not differ among
seedbeds. Survival of a short drought stress treatment, however, was significantly improved in mineral soil and moss as
compared with the litter and burned litter seedbeds. Differential survival was related in part to the biomass of herbaceous and
woody competition that developed from the seed bank and rhizomes present in the four seedbeds.
Above- and below-ground
(Polytrichum
Polpichum
biomassof competitionon the litter andburnedlitter seedbeds
were significantlyhigherandten timesthat of mineralsoil
and moss seedbeds. This difference in biomass of competition was associated with lower average daily soil water potentials
in the two litter seedbeds. Seedlings growing in mineral soil, and to a lesser extent burned litter, generally exhibited larger
shoot, root and total dry massandproducedmorerobustshootsof highermechanicalstrength(i.e. shootdry mass/shoot
length). Seedlings grown on moss were taller with smaller epicotyls and stem diameters, indicative of an etiolation response
associated
with steeplight gradientsin this seedbed.
Seedlingsgrownin mineralsoil andmosshadhigherroot/shoot ratios
andshowedtrendstowardgreatertotal root lengthandroot branching.The ecologicalrelationshipbetweenwhite spruceand
Polytrichum
mossesand the comparativevalue of Poiytrichum
seedbedin silvicultural systemsto promote natural
regenerationof white sprucearediscussed.
Keywords:
emergence; Competition; Seedbed; Morphology; Seedling establishment; Forest regeneration
Seedling
1. Introduction
Increased interest in low cost forest renewal options has led to renewed emphasis on the use of
* Corresponding
author. Tel.: 705-946-2981;
0378-I 127/97/$17.00
P/I SO378-
fax: 705-946-2030.
natural regeneration systems. Silvicultural practices
that promote the natural regeneration of white spruce
(Picea gluuca (Moench) Voss) typically include partial cutting or creation of small openings (e.g. patch
or strip clearcuts) in combination with site preparation to create mineral soil seedbedrequired for initial
Copyright 0 1997 Elsevier Science B.V. All rights reserved.
1 127(96)03956-4
20
W.C. Parker
et al. /Forest
Ecolog.~ and Managrment
establishment of this species (Zasada and Gregory,
1969; Dobbs, 1972). The residual or adjacent overstory provides a source of seed and a sheltered
microenvironment
for germination (Place, 1955;
Hughes, 1967; Lees, 19701. Site preparation by mechanical scarification or prescribed burning removes
the thick organic layer of rotten wood, litter,
bryophytes, etc. that accumulates on the forest floor.
exposing the underlying mineral soil (Place, 1955:
Zasada and Gregory, 1969; Dobbs, 1972).
The presence of conifer regeneration in association with the hair-cap mosses of the genus PO&
trichum (sub-class Polytrichidae) has been recognized for many years (Nichols, 1918; Gates, 1930;
Godwin, 1938). These ‘pioneer’ mosses are widely
distributed in temperate and boreal forests of Canada
and the United States (Crum and Anderson, 198 11
and commonly become established following timber
harvesting operations in coniferous and mixedwood
forests. These mosses form loose to fairly dense turfs
of erect, leafy vegetative shoots interconnected by a
network of underground rhizomes (Sarafis, 197 1I.
Vegetative reproduction through this perennial rhizome system, efficient internal water and carbohydrate conducting systems and adaptations to control leaf water status enable these mossesto inhabit
more exposed, disturbed microsites than most other
bryophytes (Sarafis, 1971; Bayfield, 1973; Callaghan
et al., 1978). Moss colonization of forest cutovers is
favored by forest practices and silvicultural treatments that open the forest canopy and expose mineral soil, factors that also enhancenatural regeneration of many conifers (Hughes, 1967: Zasada et al.,
1978; Geier-Hayes, 1987).
The co-occurrence of Polytrichum and natural
regeneration of white spruce and many other North
American conifer species is due in part to their
shared microhabitat requirements (Geier-Hayes,
1987; Fleming and Mossa, 1995). A positive ecological relationship between conifer regeneration and
Polytrichum has also been proposed, with thesepioneer mosses providing receptive seedbed and/or
affording seedlingssome protection from effects of
competition (Smith, 1951; Place, 1955; Ahlgren,
1976; Geier-Hayes, 1987). However, quantitative examinations of the apparent functional relationship
between Polytrichum and conifer regeneration are
few.
92 (19971 19-2X
A greenhouseexperiment was establishedto investigate the potential role of Polytrichum mossesin
the natural regeneration of white spruce. The objectives of this study were to examine Polytrichum and
other common seedbeds,with reference to: (11 comparative differences in seedling emergence.survival
and morphology and (2) the influence of competing
vegetation on emergence, growth and survival of
white spruce seedlings.
2. Materials
and methods
2.1. Seedbedcollection
Intact seedbed samples were collected from a
second-growth, mixed conifer forest located in Laurier township, near South River, Ontario (45”36’N.
79”34’W). This area was shelterwood harvested in
1985 to convert a Popu1u.sspp. dominated mixedwood to a white pine, (Pinus strobus L.) mixed
conifer stand. Basal area and canopy closure were
reduced to JO-14 m’ ha-’ and 50-60’%, respectively, with white pine, red pine (Pinus resinosrr
Ait.) and white spruce favored as residual overstory
trees. The site was scarified in fall, 1986 using a
Case 450 bulldozer with a straight blade to create a
mineral soil seedbedfor natural regeneration.
The forest floor of this site is characterized by a
patchwork of either litter or colonies of Polytrichum
moss. Separate, uniform blocks of undisturbed htter
(n = 48) and moss (n = 24) approximately 18.5 X
18.5 cm in area and 8- 10 cm deep were cut from the
forest floor using a serrated knife, gently lifted and
placed into aluminum foil baking trays. Care was
taken during excavation to maintain the integrity of
the surface layers and underlying mineral soil. The
surface organic horizon of litter seedbedsconsisted
of a 2-2.5 cm litter (L) layer. The L layer was
derived primarity from needles and small twigs of
the overstory conifers. Leaf litter of understorey
white birch (Bet&a papvrifera Marsh.) and pin
cherry ( Prunus pennsyluunica L.f.), and leaves and
stems of a variety of herbaceous species were of
secondary importance. The fermented (F1 and humus
(H) layers were a combined 1.O to 1.5 cm in thickness.The L layer of a sample(n = 24) of the litter
seedbedswas later burned using a propane torch to
W.C. Parker
et al./ Forest
Ecology
simulate a light surface fire. The moss seedbeds were
composed of a mixture of common hair-cap moss
(Polytrichum commune Hedw.) and juniper hair-cap
moss (P. juniperinum Willd.). The height of the
moss gametophyte measured at three random locations in each tray (n = 24) averaged 3.4 IfI 0.2 cm
(mean 5 standard error). The moss detritus layer,
that included moss rhizoids, rhizomes and L, F and
H layers, was 2-3 cm thick.
Mineral soil seedbeds (n = 24) were created by
manually removing the organic layers from undisturbed litter microsites and mixing the upper mineral
soil horizons to a depth of ca 10 cm to simulate
mechanical site preparation treatments. Large rocks
and cobble were removed and trays filled with this
mineral soil mixture.
2.2. Experimental design
Seedbed trays were placed in two adjacent greenhouse cubicles, each cubicle containing two benches.
Six trays of each of four seedbeds (undisturbed litter,
burned litter, moss and mineral soil) were randomly
located on each bench, yielding a total of 96 trays,
with 24 trays per seedbed. An additional ten moss
samples and four samples of the other three seedbeds
were also randomly positioned to determine the
amount of background viable white spruce seed present at the onset of the study.
2.3. Greenhouse environment and seedbed watering
regime
Natural lighting and photoperiod (ca 16 h) provided a photosynthetic photon flux density (PPFD)
of approximately 1300 pmol mm2 s-’ at the seedbed
surface at midday on clear, sunny days. Midday
PPFD was reduced to about 300 kmol me2 s-l
when skies were overcast. Midday air temperature in
the greenhouse averaged 27.9 f 0.4”C over the study
period and ranged from 21.4 f O.l”C to 37.2 + 1.8”C.
Vapor pressure deficit at midday averaged 1.3 1 +
0.01 kPa, with daily means ranging from 0.35 * 0.01
to 3.29 + 0.33 kPa.
Air temperature in the greenhouse was somewhat
higher (ca 3°C) than recommended for optimal
growth of white spruce container planting stock
(Nienstaedt and Zasada, 1990). However, the mi-
and Management
92 (1997)
19-28
21
croenvironment of the shelterwood understory where
seedbed samples were collected, monitored during
the same period, was quite similar to that in the
greenhouse
(W.C.
Parker,
unpublished
data).
Seedling growth responses exhibited are, therefore,
probably representative of white spruce germinants
in a natural, field environment.
A seedbed watering regime was developed based
on the 30-year (1961- 1990) average monthly precipitation (June-September) recorded at Burks Falls,
Ont., located approximately 45 km from the seedbed
collection site. The amount of water (ml) applied to
each tray in a given month was calculated as monthly
average rainfall (cm) X 342.25 cm2 per tray. This
monthly total (312 to 367 ml) was divided into ten
equal volumes and applied to each tray every 3 days.
A daily misting of deionized water (ca 3 ml per tray)
between scheduled waterings was required to maintain the health of the moss. So as not to alter water
relations among seedbed treatments, all trays of each
seedbed received this misting treatment.
Gypsum block type soil moisture sensors and a
Model K-SD1 Digital Soil Moisture Tester (Delmhorst Instrument Co., Towaco, NJ, USA) were
used to assess general seedbed effects on soil water
availability. Sensors were embedded in mineral soil
ca 1 cm below the H organic layer near the center of
three trays per seedbed. Soil water potential (at 1l:OO
a.m.1 was measured periodically throughout the
study.
A 6-day drought stress treatment was accidentally
imposed on Day 60, but provided the opportunity to
further examine comparative seedbed moisture relations. This event occurred about 3 weeks after cessation of seedling emergence. During this 6-day period, seedbeds were misted but received no water.
Drought was alleviated on Day 65 when all trays
were watered to field capacity and the normal irrigation schedule resumed.
2.4. Seed pretreatment
and sowing
White spruce seed used in this study was gathered
in bulked-stand collections from the provincial seed
zone of the seedbed collection site. Germination tests
indicated seed viability averaged 88%. Seed was
soaked in aerated water for 24 h, drained, sealed in a
plastic bag and stratified at 2°C for 30 days. After
22
W.C. Parker
et al. / Forest Ecology
stratification, 50 spruce seed were sown in each tray
(Day 0). Seeds were scattered uniformly over the
litter, burned litter and mineral soil seedbeds and
covered with a thin (ca 0.5 cm) layer of the surface
substrate. After seeding the moss seedbeds, the shoots
of the moss were lightly brushed by hand to facilitate
the downward movement of seeds to the surface of
the moss organic layer. All trays were watered to
field capacity on Day 0 after seeding.
and Mana~rmrnt
92 ( 1997) 19-2X
ground tissues and oven dried to determine total
shoot and root dry mass of competition. The aboveand below-ground biomass of the moss ecosystems
were harvested and separated to determine their dry
mass. However, due to difficulty in accurately separating moss rhizoids and rhizomes from roots of
competition, root dry mass of competition could not
be accurately measured for moss seedbeds.
2.7. Seedling morphology
2.5. Seedling emergence and surrGcal
The amount and rate of seedling emergence were
monitored for 53 days after seeding. A seedling was
defined as having emerged when the hypocotyl had
broken the surface of the seedbed. The day of emergence was recorded and the location of germinants
marked with a toothpick. On successive measurement days, new germinants were tallied, their location physically marked and the number of new germinants added to the previous total.
The cumulative total number of germinants in
each tray and measuring date was used to calculate
percent seedling emergence as [(total seedlings
emerged)/(total
seeds planted X 0.88) X 1001, where
0.88 is average seed viability (St. Hilaire and
Leopold, 1995). A total of 11 spruce germinants
were counted by Day 53 in the ten unseeded moss
samples used to detect pre-sowing background seed
levels. No germinants were observed in unseeded
trays of the other three seedbeds. Therefore, only the
final value of percent seedling emergence of moss
seedbeds was adjusted for background seed by subtracting one germinant from the cumulative total on
Day 53. Percent survival during the 53-day germination period was calculated as [(number of germinants
present on Day 53)/(total
number germinated) X
IOO]. The percentage of seedlings that survived the
drought treatment was calculated as [(number of
seedlings present on Day 7 1)/(number of seedlings
present on Day 53) X 1001.
2.6. Competition
The shoot and root systems of competing vegetation were collected from each tray (i.e. seeded and
unseeded) on Day 106. This material was carefully
washed free of soil, divided into above- and below-
White spruce seedlings were destructively harvested on Day 106 for measurementof morphotogical traits. Seedbedswere lifted from the trays and
soil gently washedfrom the root systems.Hypocotyl
and epicotyl length, total height, root collar diameter
and total root system length (i.e. vertical extension)
were measured for each seedling (N = 734). The
number of first-order-lateral-roots (FOLR) and the
number of FOLR > 1 cm in length were measured
on 25 randomly selected seedlingsfor each of the
seedbeds.Roots and shootswere oven dried at 70°C
for 48 h to determine dry mass.The ‘robustness’or
mechanical strength of the shoots was estimated as
(shoot dry mass)/(shoot length) and expressedin mg
mm-’ (Peterson and Facelli. 1992).
2.8. Statistical analysis
Examination of diagnostic statistic5 of all measured variables indicated that percent emergenceand
survival data were not normally distributed and exhibited unequal error variances among seedbeds.
Variance stabilization transformations failed to correct the data. Therefore, a non-parametric statistical
procedure was used in analysis of emergence and
survival data. Seedbed treatment effects were assessedusing Friedman’s procedure modified for randomized complete block designswith more than one
observation per block (Conover, 1980). Multiple
mean comparisonswere performed using procedures
of Conover (1980).
Two-way analysis of variance was used to assess
seedbedtreatment effects on competition dry mass
and seedling morphology. A mixed general linear
model was used with seedbedas a fixed effect and
benches used in blocking identified as a random
effect. Data were analyzed using the General Linear
W.C. Parker
et al./Forest
Ecology
and Management
92 (1997)
19-28
23
Model procedure of SAS (SAS Institute, Inc., 1988).
For competition data, each tray provided a single
observation. In analysis of morphological features,
each individual seedling provided one observation.
Mean comparisons of competition and most morphological features among seedbed treatments were performed using a Tukey-Kramer
studentized range test
to control the maximum experimentwise error rate
for unequal sample sizes. Seedbed effects on numbers of FOLR and soil water potential were examined using means and standard errors.
SEEDLING
SURYIYAL
DROUCiH
SuR”L”AL
EMERGENCE
3. Results
3. I. Seedling emergence and survival
White spruce seedling emergence began 8-l 1
days after sowing (Fig. 1). The rate of emergence did
not differ among seedbeds. Total seedling emergence
pooled over all seedbeds was 50% and 75% complete in 21.1 + 0.9 and 25.9 f 1.0 days, respectively.
By Day 35, 97.2% of total seedling emergence for
all seedbeds had occurred. Seedbed had a significant
effect on percent cumulative emergence on Day 53
(P < 0.01) but not percent survival during this pe-
I
Fig. 2. Percent effective emergence, percent seedling survival and
percent of seedlings that survived
a drought for burned litter,
litter, moss and mineral soil seedbeds in the greenhouse.
Letters
above the vertical bars indicate results of Friedman’s
test of
seedbed treatment
effects. Means with the same letter are not
significantly
different (P I 0.05).
riod (Fig. 2). Mineral soil exhibited a significantly
higher percent emergence, and was about twice that
of the litter and moss seedbeds (Fig. 2).
3.2. Competition
Herbaceous and woody competing vegetation began to overtop spruce germinants in some trays by
Day 35, when emergence in all seedbeds was essentially complete. Competition consisted largely of
herbaceous species such as bush honeysuckle (Dieruifla lonicera (Mill.)),
columbine ( Aquilegia
Table 1
Mean above- and below-ground
dry
selected seedbeds. a Mean comparisons
on results of a Tukey-Kramer
test
Seedbed
I
0
6
12
18
24
DAYS
I
30
FROM
36
42
48
54
60
Burned
Litter
Moss
Mineral
litter
soil
mass of competition
for
for competition
are based
Shoot dry mass
Root dry mass
n
63)
n
k)
28
28
34
23
5.62a
6.08a
0.37b
0.55b
25
24
24
5.20a
4.60a
0.63b
SOWING
Fig. 1. Variation
in mean percent total effective emergence
of
white spruce with time after sowing on burned litter, litter, moss
and mineral soil in the greenhouse. Each data point represents the
mean of 34 observations.
’ Dry mass of moss shoots averaged 9.99kO.75
g per tray. Moss
rhizoid and rhizome dry mass averaged 17.18 f 1.75 g per tray.
Means followed by the same letter are not significantly
different
(P I 0.05).
24
W.C. Parker
et al. /Forest
Ecology
Table 2
Pooled mean and standard error (in parentheses)
for daily
water potential of the four seedbeds during the emergence
establishment
periods of the greenhouse study a
Seedbed
Soil water potential
Emergence
Burned
Litter
Moss
Mineral
litter
-0.048
-0.052
-0.051
- 0.054
soil
period
(0.001)
(0.001)
(0.001)
(0.003)
’ Pooled means for the emergence
calculated from 8 measurement
days.
lishment period (Days 36 to 73) are
ment days. Daily soil water potential
trays per seedbed treatment measured
soil
and
(MPa)
Establishment
-
0.074
0.157
0.050
0.050
period
(0.001)
(0.039)
(0.001)
(0.001)
period (Days 1 to 35) are
Pooled means for the estabcalculated from 21 measurevalues are the mean of three
at 1l:OO a.m.
canadensis L.) and a variety of grasses, with wild
raspberry (Rubus
spp.) being the main woody
species.
Seedbedhad a significant effect on above- (P 5
0.001) and below-ground (P I 0.01) dry mass of
competition present (Table 1). Much larger shoot
biomass of competition developed on the litter and
burned litter than the mossand mineral soil seedbeds.
Root dry massof competition was also significantly
higher in the litter and burned litter than in the
mineral soil. Although not measured.root dry mass
of competition in moss seedbedswas likely very
small due to the low shoot dry massof competition
observed. The development of a larger biomass of
competition influenced seedbedsoil water status(Table 2). Mean soil water potential did not differ
among seedbedsduring Days O-35, the period of
peak seedling emergence. However, after this date
during the establishment period, burned litter and
Table 3
Morphological
characteristics
among seedbeds A
Seedbed
Burned
Litter
Moss
Mineral
a Means
Seedling
litter
soil
of white
morphological
Hypocotyl
length
(mm)
Epicotyl
length
I8.Ob
19.lab
23.0a
15.6b
followed
spruce germinants
grown
and Management
92 f I9971 19-28
litter seedbeds supported much higher dry mass of
competition and exhibited lower soil water potential
(Tables 1 and 2).
Percent drought survival differed among seedbeds
(P I 0.01) and this was related to the amount of
competition present on the four seedbeds(Fig. 2).
The drought treatment occurred during the period of
seedling establishment when emergence was complete and competition began to influence seedbed
moisture status (Table 2). Soil water potentials during this time were generally higher in mineral soil
and moss than the two litter seedbeds.Seedlings
growing in mineral soil exhibited significantly higher
percent survival than the litter and burned litter
seedbedsduring the drought treatment. Drought surviva1 in moss was intermediate and averaged 62.@Z
(Fig. 2).
3.3. Seedling
morphology
Seedbed had a significant effect on the growth
and morphology of white spruce germinants (Table
3). Best seedling growth was exhibited in mineral
soil. Seedlingsgrowing in mineral soil were shortest
but had strong, robust stems of largest diameter.
White spruce germinants growing in mineral soil
also had relatively high total dry mass (Table 3).
Greater root development in mineral soil was suggested by higher root dry mass (Table 3) coupled
with the higher numbers of FOLR (Table 4).
Seedlings growing in moss seedbedshad stemswith
comparatively long hypocotyls, short epicotyls. small
diameter and shootsof lower dry mass.As a result of
their tall, thin stems, these seedlingsalso exhibited
on four seedbeds and results of a Tukey-Kramer
test of mean differences
attributes
Root collar
diameter
(mm)
Shoot dry
mass (mg)
(mm)
Total
height
(mm)
Shoot
strength
tmg mm-
8.Oa
6.6a
3.3b
6.la
26.0a
25.7a
26.3a
21.7b
0.50b
0.47bc
0.40~
0.57a
20.6a
l-1.31,
n.oc
17.8ab
0.78a
0.54b
0.31c
0.81a
by the same letter are not significantly
different
(P I 0.05)
Root dry
mass (mg)
Total dry
mass (mg)
Root/shoot
(g g-- ’ )
Root system
length (mm)
5.6b
26.la
I8.5b
11.8c
26.5a
0.30b
0.28b
0.51a
0.51a
bl).Oab
44.6b
’1
4.2b
3.8b
8.7a
71.7a
71.la
W.C. Parker
et al. / Forest Ecologp
Table 4
Mean and standard error (in parentheses)
for numbers of firstorder-lateral-roots
(FOLR)
of two size classes for white spruce
seedlings grown in four seedbeds for 106 days
Seedbed
Burned
Litter
Moss
Mineral
litter
soil
n
Number
25
25
25
25
5.5
7.0
10.0
13.6
(0.8)
(1.1)
(2.0)
(1.1)
FOLR
Number
1.1
2.0
1.3
4.6
FOLR
2 1 cm
(0.3)
(0.5)
(0.4)
(0.6)
significantly lower shoot strength. The burning of
litter seedbeds resulted in increased shoot and total
dry mass and shoot strength relative to unburned
litter but other morphological features were unaffected by burning treatment. Seedlings grown in
litter and burned litter had lower root/shoot ratios,
and seedlings in the litter seedbed had lower root
system length than those grown in mineral soil and
moss.
4. Discussion
4.1. Seedling emergence and survival
Natural regeneration is dependent on the efficient
dispersal of viable seed to ‘safe’ microsites for germination, establishment and growth. Safe, or receptive, seedbeds supply the biological requirements for
seed germination, seedling emergence and establishment (Harper et al., 1961). These seedbeds also
provide a microhabitat protected from predators and
pathogens, free of competing vegetation and supply
water, nutrients and light adequate to support growth
and survival (Harper et al., 1961). Seedbeds of higher
receptivity require fewer viable seed to produce a
single seedling. The creation of adequate percent
coverage of safe, receptive seedbed is an important
operational objective of natural regeneration systems.
Mineral soil is the preferred seedbed of white
spruce and can be viewed as a control with which to
evaluate the comparative quality of other seedbeds.
Mineral soil seedbeds exhibited greater white spruce
seedling emergence than litter, burned litter and moss,
but no differences in rate of emergence or survival
were observed. These results are likely associated
with differences in seedbed moisture relations. Min-
and Management
92 (1997)
19-28
25
eral soil is a superior seedbed to litter, particularly in
more exposed habitats where these organic surface
layers can become very dry (Day, 1963; Zasada and
Gregory, 1969; Dobbs, 1972; Potts, 1985). The coarse
texture and low water holding capacity of litter
seedbeds can severely inhibit regeneration (Potts,
1985). As seeds were well hydrated prior to sowing,
reduced seedling emergence in litter and burned
seedbeds likely resulted from seeds drying out before
germination and/or from poor radicle-substrate contact and drought mortality of germinants prior to
emergence. The burning treatment used here did not
decrease the thickness of the F and H organic layers
sufficiently to improve spruce establishment as reported following wildfire and prescribed burning
(Place, 19.55; Rowe, 1955; Hughes, 1967; Lees,
1970).
Hair-cap mosses have been reported to be more
receptive seedbed than conifer litter provided the
organic mat of the moss remains moist and is not too
thick (Smith, 1951; Place, 1955). Generally, Polytrichum seedbeds are no longer receptive within 5
years of scarification (Place, 1955; Hughes, 1967;
Fleming and Mossa, 1995). The moss seedbeds used
in our study averaged 3.4 cm in height, had an
organic mat of 2-3 cm and were probably 5-7 years
old when collected (Zasada et al., 1978; Chambers,
1991; Fleming and Mossa, 1995). As moss colonies
age, the moss detritus layer increases in depth and
becomes more characteristic of litter seedbeds, poor
media for germination. The age and above- and
below-ground
development of these moss ecosystems likely contributed to relatively low percent
emergence in our study.
4.2. Competition
Seedbed differences in drought survival were related to variation among seedbeds in competition and
substrate water status. Good root to soil contact, lack
of competition and higher soil water potentials probably contributed to improved drought survival in
mineral soil. The inability of white spruce germinants to extend a tap root through organic layers > 5
cm into moister substrates is a major source of
first-year mortality in the field (Place, 1955; Day,
1963; Dobbs, 1972). In our study, the organic layers
26
W. C. Parker
et al. /Forest
Ecology
of the litter and burned litter seedbeds probably acted
as a barrier to root access to moister mineral soil,
and seedlings of lower root length were predisposed
to drought mortality. Poor root penetration of the
moss litter layer also likely contributed to droughtinduced mortality in Polytrichurn seedbeds. However, lack of competition and higher soil water potential of underlying mineral soil in moss seedbeds
was associated with higher drought survival relative
to undisturbed and burned litter.
The elevated surfaces of rotten logs and decaying
stumps in mature spruce forests provide receptive
seedbed for white spruce regeneration that is largely
free of competition (Rowe, 1955; Zasada and Gregory, 1969). Our results provide evidence to support
earlier suggestions that isolation or protection from
competition is also a benefit provided to white spruce
regeneration by Polytrichum moss (Smith, 1951:
Place, 1955; Ahlgren, 1976). Comparatively
low
amounts of competing vegetation in Polytrichum
moss seedbed would reduce shading, increase soil
moisture availability and promote the early growth
and survival of white spruce regeneration (Shirley.
1945; Zasada and Gregory, 1969; Dobbs, 1972).
Because of the conservative water use of Polytrichum, competition for soil moisture between spruce
and this moss would be much less than competition
with herbaceous or woody species (Bayfield, 1973).
4.3. Seedling morphology
Variation among seedbeds in growth and morphology of white spruce seedlings was primarily
attributable to seedbed light environment. Subcanopy
shade is characterized by reduced PPFD and red/far
red light ratios that influence growth and carbon
allocation patterns of white spruce seedlings (Shirley, 1945; Logan, 1969; Hoddinott and Scott, 1996).
Seedlings grown in the unshaded, competition-free
mineral soil seedbeds were short with larger diameter stems, and higher total dry mass. Although Polytrichum seedbed was relatively free of competition,
shading by moss shoots resulted in development of
seedlings with tall, spindly shoots with small crowns
and reduced dry mass. Steep light gradients within
the canopy of older Polytrichum moss colonies (Skre
et al., 1983) may have stimulated an etiolation response and elongated shoots with relatively little
and Management
92 ( IYY7/ I Y-28
foliage were formed (Place, 1955). The very low
mechanical strength of these stems could predispose
them to a variety of physically damaging microdisturbances (McCarthy and Facelli, 1990). However,
mechanical injury of spruce germinants could be
tempered by additional physical support provided by
moss shoots.
Seedlings grown under heavy competition in litter
and burned litter seedbeds exhibited morphological
features typical of shade grown white spruce (Shirley, 1945: Krasny et al., 1984). These seedlings had
taller, thinner stems, and lower root/shoot ratios and
seedling dry mass as compared with the unshaded
seedlings grown in mineral soil. Lower shoot strength
in litter than burned litter may be the result of shoot
etiolation in response to light levels in deep litter
seedbeds (Facelli and Pickett, 199 1). Although white
spruce seeds were sown at the same depth in all four
seedbeds in our study, settling of seeds to greater
depths in litter than burned litter seedbeds may have
occurred. The increased distance of upward penetration of shoots in undisturbed litter may have contributed to the development of relatively tall. thin
shoots of low dry mass.
The benefits of higher root length and deeper root
penetration to survival in first-year germinants of
white spruce and other small-seeded conifers are
well established (Day. 1963; Eis, 1965; Knapp and
Smith. 1982). Germinants on mineral soil, and to a
lesser extent on Polytrichum, exhibited trends toward higher root/shoot ratios, root length and more
highly branched root systems than those on litter and
burned litter seedbeds. Comparatively greater root
development and root/shoot
ratios in mineral soil
and moss seedbeds likely contributed to increased
survival of the drought treatment.
4.4. Polytrichum
regeneration
mosses and white spruce naturn/
Perhaps the greatest single disadvantage or deterrent to natural regeneration systems is difficulty in
timing site preparation activities to the year of a
large seed crop. Adequate coverage of mineral soil
seedbed is generally lost within 3-5 years of site
preparation due to ingress of competition and litter
deposition (Zasada et al., 1978; Fleming and &hsd,
1995). This reduction in availability of receptive
W.C. Parker
et al. /Forest
Ecology
seedbed increases the risk of regeneration failure if a
good seed year does not occur within l-2 years of
site preparation. Although results of greenhouse studies using excavated seedbeds are not strictly representative of responses in the field, our findings suggest silvicultural practices that encourage establishment of Polytrichum could broaden this regeneration
window (Smith, 1951). As young Polytrichum is
good seedbed for spruce (Place, 19551, aggressive
colonization of forest cutovers by hair-cap mosses
could serve to buffer the loss of mineral soil seedbed
and maintain site receptivity for comparatively longer
periods after site preparation.
The receptivity of Polytrichum seedbeds for natural regeneration of white spruce varies with age and
species of moss. The two most common species of
Polytrichum in Ontario, P. juniperinum and P. commune, differ in growth form and habitat distribution.
P. juniperinum has a shorter shoot growth habit and
occurs in drier, more open microsites than P. commune (Sarafis, 1971). As light intensity decreases
and soil moisture increases, taller shoots and looser
growth forms are favored in both species (Place,
1955; Sarafis, 1971). As the height of moss shoots
and depth of organic mat are the critical factors
affecting seedling emergence and morphology, Polytrichum juniperinum
may remain receptive to a
greater age as compared with P. commune (Sarafis,
1971).
Moss seedbeds may provide additional benefits to
natural regeneration not addressed in our study. The
vertically oriented vegetative shoots may facilitate
seed penetration to the mineral soil/organic surface
where germination occurs (Smith, 195 1). The surface
roughness of moss will reduce air movement near
the site of germination, allowing seeds to remain
stationary long enough to imbibe sufficient water for
germination (Johnson and Fryer, 1992). Wind may
move broadleaf litter off the elevated surfaces of
Polytrichum moss colonies and into microdepressions and reduce mortality by smothering, a significant source of white spruce mortality (Koroleff, 1954;
Gregory, 1966). Mechanical damage through crushing by grasses, bracken fern (Pteridium aquilinum
(L.) Kuhn), etc. may also be reduced due to the
comparative absence of competition in moss seedbeds
(Stoeckeler and Limstrom,
1942; Shirley, 1945).
Lastly, seedlings growing in Polytrichum moss may
and Management
92 (1997)
19-28
27
experience less mortality to frost heaving than on
mineral soil (Place, 1955).
Acknowledgements
The technical assistance of Nick Hoffman, Gary
Syroid, Angela Wilson, Chris Bean, Russ Finkbeiner
and the support of the Environmental Youth Corp
and the Summer Experience Program of Jobs Ontario are gratefully acknowledged. We also thank
Dave Deugo, Area Forester, for his cooperation,
Frank Schneckenburger for his help with the experimental design and statistical analysis, and John Paterson and Tom Noland for their thoughtful reviews
of an earlier version of this manuscript.
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