Download Ashton, P.M.S., and Larson, B.C. 1996. Germination and seedling

lores~~~ol0g-y
Management
Forest Ecologyand Management80 (1996) 81-94
Germination and seedling growth of Quercu,s ( section
ErythrobaEanus) across openings in a mixed-deciduous forest
of southern New England, USA
Mark S. Ashton *, Bruce C. Larson
School of Forestry
& Environmental
Studies, Yale University,
New Haven,
CT 0651 I, USA
Accepted10 July 1995
Abstract
In this study threespeciesof the genusQuercus sectionErythrobulanus (Quercus coccinea, Quercus rubra, Quercus
uelurina)wereinvestigated.All occurtogetherascanopytreesin forestsof southernNew England.Acornsof eachQuercus
specieswere plantedin plots locatedin five zonesthat representa rangeof forest gap/canopy conditionsthat can occur
within a southernNew Englandforest. Thesefive zonesweredemarcatedadjacentto and acrosslarge openingsof two
physiographicsites-valley andridgetop.Experimentsweredesigned
to monitorgerminationandinitial growthof seedlings
for the first threegrowingseasons.
During the startof the first growingseason
germinationwasmonitored.At theendof the
first growingseasonmeasurements
of height andnumberof flushesweretakenanddestructivesamples
of seedlings
made
for dry massof root, stemandleaves.At the endof the third growing season
height wasrecordedfor surviving seedlings.
Comparisons
were madeof germinationand growth of seedlings
locatedin the different gap/canopy conditions.
Resultsdemonstratedclear differencesin patternsof germinationand early growth among speciesand among
gap/canopy conditionsof the sites.All speciesshowedan increasedlag in germinationwith reducedamountsof light.
Highestgrowth and flushingrate were in the centerconditionsof the openingsfor all speciesduring the fust growing
season.Quercus rubra had the greatestheight growth the first growing seasonbut a lower numberof flushesthan Q.
uelutina and Q. coccineu. In comparisonwith the other species,Q. rubru had the greatesttotal dry massin most
gap/canopy conditionsafter the first growingseason.
However, Q. velutinu hadthe greatesttotal dry massin the centerof
the ridgetopopening.In almostall gap/canopy conditionsQ. velutina hadgreaterproportionsof dry massallocatedto roots
comparedwith the otherspecies.
After 3 years,greatestheightgrowthin any of the gap/ canopyconditionswasrecordedfor all threespeciesin the center
of the valley site.Underthis conditionQ. rubru hadsignificantlygreatergrowth than Q. uelutina and Q. coccineu. Quercus
rubru alsohad significantlygreaterheight growth and survival beneaththe canopyconditionsof the valley site than the
otherspecies.On the ridgetopsiteregenerationfailed to establishbeneathcanopyconditionsthat providedlow amountsof
light. Quercus uelutina showedgreatestheight growth after three yearsin the centerand edgeconditionsof the ridgetop
openingcomparedwith the other species.Environmentalinfluencesthat determinespeciesgerminationand growth
performanceare suggested.
Keywords: Germination; Light; Quercus
Seedling growth
l
coccinea
(Muenchh.); Quercus
Corresponding author.
0378-I 127/%/$15.00
6 19% Elsevier Science B.V. All rights reserved
SSDI0378-1127(95)03636-9
rubra
(L.); Quercus
velutina
(Lam.);
section Erythrobalanus;
82
MS. Ashton.
B.C. Larson/
Forest
Ecology
1. Introduction
Research in temperate deciduous forests of eastem North America has shown that the size of canopy
opening created by a disturbance promotes differences in regeneration survival and growth among
tree species (Marquis, 1975; Runkle, 1981; Hibbs,
1982; Runkle and Yetter, 1987; Canham 1988a,
1989; Lorimer, 1989). Studies have also shown that
the different microenvironments that exist across the
ground surface of an opening (e.g. soil moisture,
nutrient status, solar radiation quality and quantity;
Reifsynder et al., 1970; Canham, 1988b) influence
regeneration survival and growth of tree species
within the opening itself (Smith, 1951; Poulson and
Platt, 1988, 1989; Smith and Ashton, 1993). The
studies imply that both opening size and microenvironmental variation within an opening can lead to
differences in tree species composition within southem New England forests.
Prior investigations have compared tree seedling
survival and growth for species that belong to very
different taxonomic groups (Marquis, 1975; Barden,
1979, 1981; Ehrenfield, 1980; Runkle, 1981, 1989;
Hibbs, 1982; Canham, 1985, 1989; Collins and Good,
1987; Platt, 1987; Poulson and Platt, 1988, 1989;
Connell, 1989; Lorimer, 1989; Collins, 1990). These
studies have been carried out in field conditions that
monitored recruitment and growth of advance regeneration in situ with no control over microenvironment location or seedling age and size. This makes it
difficult to measure differences in survival and
growth among species that have similar growth morphology and physiology. Many closely related tree
species have therefore been placed within the same
ecological or physiological grouping. However, understanding the processes of establishment
and
growth among related species of congeneric groups
is central to future sustainable forest management for
eastern North America. For example in southern
New England tree species richness and dominance is
represented by a few relatively large assemblages in
the genera ( Acer, Bet&a, Carya, Fraxinus, and
Quercus).
The objective of this study was to learn more
about differences in the fundamental niche of related
tree species that occur together within the same
forest landscape. To do this we examined the germi-
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80 (1996)
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nation and growth of Quercus section Erythrubalanus (Fagaceae) under certain controlled field conditions. Past studies have documented Quercus species
distribution in relation to site (see review in Burns
and Honkala, 1991), but these studies have not at-.
tempted controlled experiments to more clearly identify the fundamental niche of each species within a
site. Our study selected the greatest variety of microenvironments within a forest by choosing particular
physiographic sites at which a range of gap/canopy
conditions were created. Control of spacing and age
among Quercus species within selected forest microenvironments facilitated comparison and observation for differences in their germination and growth
morphology. The study tested the hypothesis that
closely related tree species have different rates of
germination and amounts of height growth and allocation to roots and shoots in different forest microenvironments. All three species chosen for the study
(Quercus coccinea (Muenchh.), Quercus rubru CL.),
Quercus uelutina (Lam.)) are considered to be intermediate to intolerant of shade (Bums and Honkala,
1991) and dominate the canopy of mid successional
stands of southern New England (Abrams, 1990;
Abrams and Downs, 1990; Abrams, 1992).
Only a few studies have selected specific physiographic sites and then manipulated the forest canopy
for control of forest microenvironment. This kind of
approach has been used to examine for differences in
survival and establishment of several closely related
tree groups (Acer, Sipe, 1990; Betula, Carlton and
Bazzaz, 1993; Be&a, Wayne and Bazzaz, 1993).
Other studies that controlled forest microenvironment have investigated tree species of different successional status (Latham, 1992). Sipe and Bazzaz
(1994) reported finding little difference among three
species of Acer, but investigations by Wayne and
Bazzaz (1993) and Carlton and Bazzaz (1993)
demonstrated differences among four species of Betula.
2. Materials
and methods
2.1. Study site
The study was done at the Yale-Myers Research
and Demonstration
Forest located in northeastern
MS. Ashton, B.C. Larson/Forest
Ecology
Connecticut (41”57’N, 72’07’W). The forest is classified as Central Hardwood-Hemlock-Pine
(Westveld, 1956). The pre-settlement forests of this region
were diverse in composition and structure (Day,
1953; Hem-y and Swann, 1974), with a natural disturbance regime that comprises hurricanes, tomadoes, ice storms, fire, insect and pathogen epidemics
(Siccama et al., 1976; Bormann and Likens, 1979;
Foster, 1988). This region was almost completely
cleared for agriculture after colonization by settlers
(1700- 1900) (Meyer and Plusnin, 1945; Raup, 1966;
Cronon, 1983). After 1850 much of the land was
abandoned and second growth forest has since developed. The experimental sites were in openings created by removal of the second generation of trees to
occupy the sites since agricultural abandonment.
The forest topography consists of ridges and valleys that range from 170 to 300 m above sea level.
The land surface is gently undulating with slopes
rarely exceeding 40%. The bedrock is metamorphic,
overlain by glacial till soils that are moderate to
well-drained stony loams. Local variation in parent
materials and slope can also produce poorly drained
and excessively drained sites. The regional climate is
cool temperate (summer mean 20°C; winter -4°C)
and humid, with precipitation (annual mean 110 cm)
distributed fairly evenly throughout the year.
2.2. Experimental
design
Experiments were designed to identify: (i) differences in germination and growth for each Quercus
species across the ground surface microenvironments
of canopy openings; and (ii) differences in germination and early growth among species. Measurements
determined: (i) germination; (ii) seedling height and
number of flushes in the fiit growing season and
seedling height after the third growing season; (iii)
seedling dry mass and allocation to above- (stem,
leaves) and below-ground (roots) portions after the
first growing season.
The two sites chosen for the experiment were
each located in (i) a valley and (ii) a ridgetop.
Rectangular canopy openings of 100 m length by 30
m width were created to obtain the greatest range of
microenvironments, both across the openings and
beneath the adjacent canopies that was representative
of the disturbance regime for this forest region. All
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80 (1996)
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83
trees felled were’ removed from the opening. The
lengths of the canopy openings were aligned eastwest along the approximate axes of the valley and
ridgetop.
The height of the forest canopy at the valley site
was measured as 25 m and composed of Liriodendron tulipifera CL.1 and Q. rubra. The subcanopy of
the forest was dominated by Acer saccharum (Marsh)
with an understory of Cornus jlorida
CL.),
Hamamelis virginiana (L.), and Carpinus caroliniana (L). On the northern side, the site was adjacent
to the toe of a shallow slope (10%) that had a
seepage during most of the spring and early summer
months from snow melt. The soil was deep (over 2
m) and classified as a Paxton very stony fine sandy
loam (USDA Soil Conservation Service, 1981).
The forest canopy height of the ridgetop site was
22 m and composed of Carya glabra ([Mill.] Sweet),
Q. rubra and Q. velutina. The subcanopy comprised
Acer rubrum (L.) and Tsuga canadensis CL.1 Carr.
The soil of the ridgetop site was shallow (less than 1
m) with some bedrock extrusions. This soil was
classified as Hinkley very stony loam (USDA Soil
Conservation Service, 1981). Though there was no
Q. coccinea in the forest canopy at this site it was
found on sites similar to this one in other parts of the
forest.
The area in and around the canopy opening of
each site was subdivided into five gap/canopy conditions: (i) the understory adjacent to the southern
edge of the opening; (ii> the southern edge of the
opening, which receives only diffuse solar radiation
and exhibits temperature fluctuations that are moderated by the shelter of the canopy; (iii) the center of
the opening which receives direct radiation and has
the greatest fluctuations in temperature; (iv) the
northern edge of the opening which receives direct
solar radiation; (v) the understory adjacent to the
northern edge of the opening which can also receive
some direct solar radiation (Canham, 1988b).
To make valid comparisons of germination and
early growth within and among species for this
study, acorns were planted at regular spacing in
plots. One plot was located in each of the gap/
canopy conditions, from north to south across each
gap; understory 20 m to the south of the gap, southem gap edge, center, northern gap edge, and understory 20 m to the north of the gap. Gap edge and
84
MS.
Ashton,
B.C.
Larson/
Forest
Ecology
understory plots were increasingly shaded from direct radiation. It is important to recognize that these
gap/ canopy conditions have boundaries that are hard
to delimit. In reality the plots are situated along a
microenvironmental continuum. It must also be recognized that the nature of these manipulations precludes considering seedlings in this study as advance
regeneration, which is the established seedling or
sapling stage (0.5- 1.O m) that naturally experiences
gap conditions.
2.3. Description
merits
of the gap/canopy
microenuiron-
The soil at both sites was classified as an inceptisol (USDA Soil Conservation Service, 1975). Soil
moisture was measuredat each plot by taking weekly
samplesof soil at 5-10 cm depth from 1 April 1989
to 30 October 1989. Moisture percentages were determined from the difference in wet and dry weights.
During the experiment the valley site showed virtually no deficiencies in top-soil moisture (Table 1).
The ridgetop site exhibited more pronounced deficiencies with longer periods of dryness during the
months of July and August.
Daily photosynthetic photon flux (PPF) was monitored for each gap/canopy condition on sunny days
during the spring and summer growing season of
1989. Light sensors(Li Cor 190SA and 19OSZ>were
set up to take readings of PPF every 10 s. Ten
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80 (1996)
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minute means for each gap/canopy condition were
simultaneously recorded over a 14 h period using a
data logger (Li Cor 1000). Sensorswere positianed
horizontally 30 cm above the ground within the
center of each plot. The plots in the understory at all
sites had a daily PPF that did not exceed 10% of that
in the open on sunny days (Table 1). In these
understory microenvironments direct radiation in the
form of sunflecks contributed between 13 and 5 1%
of the daily PPF received. The low amount of daily
PPF recorded for the southern understory condition
could be attributed to a subcanopy of Tsug~
canadensis, a deep-crowned conifer. The other understory condition on the ridgetop had only a few A.
rubrum trees in the subcanopy. The other conditions
that represented edges and centers of the canopy
openings had daily PPF that reflected a range of
intensities and durations. The center condition of the
canopy opening was exposed to the longest daily
duration of direct sun for both sites. The daily PPF
received in these conditions were equivalent to that
received in the full open. The northern edge gap/
canopy condition received approximately between 60
and 80% of the daily PPF received at the center of
the opening for each site, of which 90-93% compriseddirect sunlight. The southernedge gap/ canopy
condition received approximately between 10 and
20% of the daily PPF received at the center of the
opening for each site, of which 14-28% was direct
sunlight (Table I >.
Table I
A summary of PPF measurements
and number of droughty days for each of the gap/canopy
conditions
located at the ridgetop and valley
sites. Droughty
days were thos days from April to October where soil moisture at S-10 cm depth was < 15% of dry to wet mass of soil
(wet - dry/dry
x 100%). Values for PPF are means of total amounts received from 4 sunny days in June and July. Percentages represent
the proportion
of total mean PPF that occurs as direct sunlight at each gap/canopy
condition
No. of droughty
Ridge
Valley
PPF (mol m-’ day- ‘)
Ridgetop
(percent direct sunlight)
NE
40
00
72
10
3.68
Valley
NU, northern
NU
C
SE
su
21
28
67
00
00
07
days
understory;
NE, northern
27.44
35.47
(20 %)
1.82
(90 %)
24.7 1
(95 %)
38.21
(13%)
(93 %a)
(97%)
edge; C, center of the opening;
SE, southern
5.45
0.85
(51%)
(28
8)
4.70
(14%)
edge; SU, southern
2.01
(41%)
understory.
M.S. Ashron,
2.4. Experimental
B.C. Larson/Forest
Ecology
measurements and data analysis
Acorns of each species were collected from four
separate parent trees located in different areas of the
Yale-Myers Forest and its surroundings. Parent trees
on different sites and localities were selected to
insure a genetic variability representative of the
species populations for this region. After collection,
acorns were stratified in moist sand for 6 months,
beginning in November 1987. After stratification
viable acorns were sorted from those that were weeviled or rotten by floatation in water. An equal
number of acorns from each parent source were then
mixed together for each species and then planted at
3-4 cm depth into the plots of the two sites on 1
May 1988. Each plot was 2 m X 2 m and manually
weeded regularly during growing seasons for the
duration of the experiment to avoid competition with
other vegetation.
For each plot 48 acorns of each species were
arranged in four subplots. For each subplot three
groups of 12 acorns were planted. Group positions
within each subplot were allocated at random to each
of the three species. This would be classified as a
split-plot design with species nested within subplot.
Acorns within each subplot were initially spaced at
10 cm X 10 cm; plots were protected from rodent
predation by wire gauze with edges buried approximately 10 cm below the ground surface. All plots
were also protected from herbivory by ‘L-m-high
chicken wire fencing. Mean acorn mass of those that
were planted for each species were: Q. rubra (3.95
g), Q. coccinea (1.87 g) and Q. uelutina (2.05 g).
Because other suitable canopy openings were not
available site locations were not replicated, therefore
ANOVAs were done separately for each site. However, a one-way ANOVA was done to test if the two
sites (valley, ridgetop) were different from each other
overall. It should also be noted that no one plot for a
gap/canopy condition, therefore, had the same radiation regime as somewhere else within the opening.
The limitations imposed by this design were reduced
by the large number of acorns used for each plot and
the extended period of the experiment (3 years).
After the start of the experiment germination was
recorded for the first 50 days. Measurements of
height and number of flushes were made, and destructive samples were taken on germinated seedlings
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80 (1996)
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85
at the end of the first growing season (October 1988)
to determine dry mass. Flushes were recorded by the
number of times a seedling set bud and then reflushed during the first growing season. Measurements of height were again taken on surviving
seedlings at the end of the third growing season
(October 1990). An ANOVA procedure of the Statistical Analysis Systems (SAS) Institute Inc. (Ray,
1982) (with gap/canopy condition, and species as
main effects) was carried out on log transformed
data for first growing season measurements of height
and number of flushes. Measurements of height after
three growing seasons were done in the same way.
Mean dry mass of 12 seedlings was calculated for
each species by gap/canopy condition at the end of
the first growing season. For a species, individuals
were selected from each subplot equally and at random for each gap/canopy
condition. The same
ANOVA procedure of the SAS Institute Inc. was
used on the log of seedling dry mass for the whole
seedling and on the ratio of above-ground mass
(stem, leaves) to below-ground mass (roots).
3. Results
3.1. Acorn germination
For all three species, germination at understory
and southern edge positions lagged behind that in the
centers and northern parts of the openings (Fig. 1).
However, in all gap/canopy conditions most germination had occurred by the 30th day after planting.
The northern understory condition of the valley site
had a noticeably lower amount of germination for all
Quercus species. The southern understory condition
of the ridgetop site also inhibited acorn germination
(Fig. 1). Over the duration of the experiment no
acorn predation from rodents or herbivory from deer
was recorded.
3.2. Height, flushing,
growing season
and dry mass gain after one
Analysis of variance for height growth and number of flushes showed most F values were significant (P < 0.001) among gap/canopy
conditions
(Table 2). No significant subplot effect was shown
86
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Ecology
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81-94
QUERCUS COCCINEA - FWGETOP
QUERCUS COCCINEA - VALLEY
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80 (1996)
N.EiXE
0
2
m
5
ii
10
0
0
10
20
TIME
30
(DAYS)
40
0
10
20
TIME
30
40
i
50
(DAYS)
Fig. 1. Number of acorns germinating successfully for Quercus species across gap/canopy conditions for each site over a 50 day period
afttr planting. Successful gtrmination was determined
by emergence and survival of the radiclt. S. UNDER, southern tdemhzy plot; S.
EDGE, southern edgt plot; CENTER, plot at the center of the.opening; N. EDGE, northern edge plot; N. UNDER, nor&em understory pkk
M.S. Ashron, B.C. Larson/Forest
Ecology
within gap/canopy condition and is not treated further. Differences between species within gap/ canopy
condition were analyzed and evaluated at the 5%
level of significance using Tukey’s Studentized
Range (Table 3).
For each species best height growth occurred in
the center of both sites (Fig. 2). In almost all gap/
canopy conditions greatest height was exhibited by
Q. rubru. On the ridgetop, Q. velurina had mostly
greater height than Q. coccinea. This was not the
case for the gap/canopy conditions in the valley.
For all species the highest number of flushes were
associated with gap/ canopy conditions that received
the largest amounts of daily PPF (Fig. 2). The lowest
mean number of flushes was observed for seedlings
growing in the southern understory condition of the
ridgetop site. In almost all gap/canopy conditions
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80 (1996)
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87
Q. velufina had a higher number of flushes than Q.
coccinea, which in turn had a greater number of
flushes tlpn Q. rubru in most cases.
Analysis of variance for total mass and shoot:root
ratio had F values which were significant (P >
0.001) among gap/canopy conditions (Table 2). No
significant subplot effects were demonstrated within
each gap/ canopy condition. Differences between
species within each gap/canopy condition were
evaluated in the same manner as height growth and
number of flushes (Table 3).
Seedlings in the centers of canopy openings gained
greater mass than those of corresponding edge and
understory conditions (Fig. 2). Comparison among
species demonstrated Q. rubru had significantly
greater mass than Q. velutina and Q. coccineu in
most conditions. However, an exception was in the
Table 2
F values and significance
(’ P < 0.05; * * P < 0.01;
* P < 0.001; * * * * P < 0.0001) for analyses of variance of height growth,
number of flushes, total dry mass and shoot : root ratio after the first growing season; and height growth atkr the third growing season. Sites
have been analyzed separately. For certain understory
conditions,
measures of height for the. third growing season could not be analyzed
because of uoor survival
l
First growing
season
Heiaht
Valley
Main treatments
Gap/canopy
condition
Subplot effects
Gap/canopy
X subplot
Sub-treatments-species
S. Under
S. Edge
Center
N.
Edge
N. Under
Ridgetop
Main treatments
Gap/canopy
condition
Subplot effects
Gap/canopy
X subplot
Sub-treatments-species
S. Under
S. Edge
Center
N. Edge
N. Under
Flushes
20.63
0.75 NS
1.68 NS
l
performance
1.78
17.14
17.46
29.14
56.75
l
*
l
l
within
NS
* *
*
l
*
l
*
** * ’
17.31 * * * *
1.12 NS
0.94 NS
performance
within
35.85 * * *
3.50
3.51 *
10.45
*
14.69
*
l
l
l
Total mass
16.68
2.13 NS
1.22 NS
’
*
*
l
each gap/canopy
6.59
3.65
1.27 NS
*
NS
10.50
l
l
33.06
1.48 NS
0.95 NS
l
l
each gap/canopy
20.31
l
l
0.56 NS
2.74 NS
6.91
3.46
l
l
l
condition
18.94
5.63
2.06
7.14
9.86
l
l
0.50
33.09
1.69 NS
1.00 NS
*
’
l
Shoot : root ratio
*
9.88 * * ’ *
2.16 NS
1.84 NS
*
* *
*
*
*
*
l
l
l
52.02
2.78 NS
0.58 NS
’
5.20
3.49 NS
1.80 NS
9.79
4.12 *
l
*
*
l
Third growing
Height
121.66
1.45 NS
0.32 NS
l
l
0.91 NS
37.23 * * ’
10.84
57.52
l
l
l
4.68 * *
2.49 NS
0.69 NS
170.04””
0.42 NS
1.13 NS
0.64 NS
1.77 NS
4.99
0.59 NS
14.73
condition
3.99
l
2.01 NS
28.69
11.37
5.67
l
l
l
l
10.95
1.19
1.17
3.17
*
NS
NS
*
’
l
*
l
l
season
l
88
MS.
S. UNDER
S. EDGE
Ashton,
CENTER
B.C. Larson/
N.EDCE
Forest
Ecology
N. UNDER
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”
S. UNDER
80 (1996)
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81-94
CENTER
N. EDGE
-7
2.5
2.5 -
N. UNDER
RIDGETOP
VALLEY
0.5
2
0.0
/
/
0.5
0.0
S. UNDER
S. EDGE
CENTER
N. EDGE
N. UNDER
S. UNDER
S. EDGE
CENTER
N. EDGE
N. UNDER
(a)
Fig. 2. Measurements
of(i) height growth, (ii) number of flushes, (iii) total dry mass gain, and (iv) dry mass shoot:root ratio. Measurements
were taken the first growing season after acorn planting. Measurement
means along with their standard errors are depicted by species for
each gap/canopy
condition across the valley and ridgetop sites.
center gap/canopy condition of the ridgetop site
where Q. uelutina had greater mass than the other
Quercus species.
All species showed an increase in proportional
allocation to roots in gap/canopy conditions that
received low amounts of daily PPF, and in gap/
canopy conditions that were located on the ridgetop
site compared with the valley. The shoot:root ratio of
Q. uelutina showed greater proportional allocation to
roots than the other Quercus species. The shootxoot
ratio was noticeably greater for Q. velufiaa tlm the
other species at the edges of openings or under
closed canopy conditions.
A one-way ANOVA comparing sites (valley,
ridgetop) showed F values were significant for two
of the attributes investigated (number of fhrshes,
shoot:root ratio). The valley site had higher seedling
shoot:root ratio, and a lower number of flushes
MS. Ashion,
B.C. Larson/Forest
Ecology
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80 (1996)
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81-94
RIDGETOP
7
89
n
n
6
6
Qucrcus
Quercus
Quercus
rubra
velutina
coccinea
6
5
3
$4
93
2
1
0
S. UNDER
S. EDGE
S. UNDER
S. EDGE
CENTER
N. EDGE
N. UNDER
S. UNDER
S. EDGE
CENTER
S. UNDER
S. EDGE
CENTER
N. EDGE
N. UNDER
1.5
0
i: 1.0
2
s
2
io5
u-l .
0.0
0.0
CENTER
N. EDGE
N. UNDER
N. EDGE
N. UNDEI
(b)
compared with the ridgetop site. No significant difference was shown between sites for seedling height
and total dry mass.
3.3. Height afrer three growing
seasons
Analysis of variance for height growth showed
most F values were significant (P < 0.001) among
gap/canopy conditions (Table 2), and no significant
subplot effect was shown within gap/canopy condition. Differences between species within gap/ canopy
condition were therefore analyzed at the 5% level of
significance using Tukey’s Studentized Range (Table
3).
Greatest height growth in any of the gap/canopy
conditions was recorded for all three species in the
center of the valley site (Fig. 3). In the valley center,
Q. rubra had significantly greater growth than Q.
velutinu and Q. coccinea. Quercus rubra also had
significantly greater height growth and survival at
the edges and the southern understory of the valley
site than the other species. On both sites seedlings
failed to survive beneath canopy conditions that provided low amounts of daily PPF. For the southern
90
,
:
MS. Ashton,
B.C. Larson/Forest
Ecology
Y
S. UNDER
S. EDGE
CENTER
N. EDGE
N. UNDER
N. EDGE
N. UNDER
RIDGETOP
i
50
0
s. UNDER
S. EDGE
CENTER
Measurementsof height growth taken after the third
season. Measurementmeans along with their standard
errors are depictedby species for each gap/canopy condition
across the valley and ridgetopsites.
Fig. 3.
growing
understory of the ridgetop site this could be attributed to the dense coniferous canopy of Tsuga
canadensis. Quercus velutina showed greatest height
growth after 3 years in the center and northern edge
conditions of the ridgetop opening compared with
the other species.
4. Discussion
Though many studies (Bourdeau, 1954; Briscoe,
1961; Nowacki et al., 1990; Nowacki and Abrams,
and Management
80 (1996)
81-94
91
1992) have documented demographic patterns in
seedling survivorship and growth for Quercus (section Eryrhrobalanus)
our study is the first to clearly
identify comparative morphological differences in
allocation of seedlings among the species in relation
to forest microenvironment for southern New England.
All three species showed a greater lag in germination with lower levels of daily PPF. This also appeared to affect eventual germination success. Some
understory gap/ canopy conditions had reduced germination success that might be related to other environmental phenomena. Studies have shown that waterlogged soils inhibit oak germination (Hosner,
1957; Briscoe, 19611, and that hemlock (Tsuga
canadensis L.) needle litter has allelopathic effects
on acorn germination of Q. rubra (Ward and McCormick, 1982). Two locations might have been
affected by these factors. The southern understory of
the ridgetop site had a Tsuga canadensis overstory,
and the northern understory of the valley site was
noticeably water logged from a nearby seepage during the spring (May and June).
Highest growth and flushing rates for all species
were in the center conditions of the openings. These
locations received the greatest amount of PPF and
had top soil that, compared with adjacent edge and
understory gap/ canopy conditions, was relatively
moist during the spring and summer of the experimental period. This suggested that root competition
from overstory trees for soil moisture was reduced at
the center of these large openings (3000 m*). This
pattern in soil moisture variation across the gap-understory continuum has been well described elsewhere (Geiger, 1957). Other studies have demonstrated surface soil moisture conditions (at a depth of
5 cm and less) to be very different (Castilleja, 1991;
Ashton, 1992). These studies showed that the centers
of openings exposed to long periods of direct radiation can have very dry surfaces that provide an
inhospitable environment for germination. However,
in these studies the opening sizes were small (less
than 400 m*). Species that produce small seeds with
little protective covering or that lack food reserves to
withstand periods of environmental stress might be
particularly susceptible to drying (Kramer and Kozlowski, 1979). Still other studies have demonstrated
the relation to survival within inhospitable environ-
92
M.S. Ashton,
B.C. Larson/
Forest
Ecology
ments to seed size (Grime and Jeffrey, 1965; Spurr
and Barnes, 1992). However, Quercus species hold
substantial food reserves within their acorns that,
compared with other smaller seeded tree species,
allows them to germinate and grow within a wide
range of environments during the first growing season. This is one explanation of why the germination
of all Quercus species were relatively site insensitive
in most of our experimental plots.
Quercus rubru had the greatest height growth but
had fewer flushes than Q. velutina and Q. coccinea.
The larger acorn size (Schopmeyer, 19741, and therefore by implication the greater food reserves, of Q.
rubru might have promoted greater extension growth
of the first flush. Quercus
coccinea,
and most noticeably Q. velutina, had a first year growth pattern of
shorter but more frequent flushing.
Third year Q. vektina had greater total dry mass
than Q. rubra or Q. coccinea in the center of the
ridge site. In all other plots, Q. rubru had the
greatest mass. However, in all plots Q. velutina
generally had greater proportions of dry mass allocated to roots as compared to the other species.This
pattern of root allocation among the species was
more apparent for seedlings that germinated and
grew in gap/canopy conditions of the understory
than in the open, and in gap/canopy conditions of
the ridge site compared with those of the valley site.
The proportionately greater root massof Q. velutinu,
would make it more tolerant of soil drought or
nutrient deficiency. Many past ecological studies
have shown that Q. velutina is often restricted to
drier and nutrient poorer sites than other membersof
section Erythrobalanus (e.g. Bourdeau, 1954, Hannah, 1968). Some studies have demonstrated the
physiological and morphological advantagesof leaves
of Q. velutina that promotes their establishmenton
thesesites(Seidel, 1972; Bahari et al., 1985; Abrams,
1990; Ashton and Berlyn, 1994). These advantages
include higher water-use and nutrient-use efficiency
in environments that provide high amounts of PPF,
lower stomatal area per unit area of leaf, thicker
cuticle, and greater plasticity in leaf structure with
difference in amount of PPF received. Our study has
demonstrated that these advantages might also be
related to differences in morphological growth and
allocation to roots.
Though there are distinct growth and allocation
and Management
80 (1996181-94
differences between Q. uelutina and Q. rubru, Q.
coccinea sharesgrowth characteristicsof both. In our
study area of southern New England no clear difference in morphological attributes measuredbetween
Q. coccinea and the other speciesdescribed its geographic distribution. However, this might suggest
that Q. coccinea has an intermediate ecological status between Q. rubra and Q. r;elutinu. For example,
Q. coccinea
showed height growth and flushing rates
during the first growing seasonlike that of Q, w
Zutinu, but pattern in shoot:root ratio was similar to
Q. rubru. This is supportedby ecological studiesthat
have investigated the distribution patterns of Q. CWcinea
(Bums and Honkala, 19911, and by genetic
studies(Overlease, 1975; Kriebel et al.. 1976; Little,
1979) that have demonstrated that hybridization
maintains the anatomical, physiological and morphological linkages among all three species.
There are several factors that provide possible
explanations for Quercus
species distribution patterns across the topography of the Yale-Myers For
est. Based on findings from this study and others
(Bourdeau, 1954; Nowacki et al., 19901. soil moisture fluctuations might be one factor affecting
seedling distribution of Quercus species with the
more drought tolerant specieson the ridgetops and
the more drought sensitive speciesrestricted to the
valleys. Another factor affecting Quercus
species
distribution patterns appears to be the amount of
light received at the ground story. The amount can
be related to differences in germination and seedling
establishmentof the Quercus
speciesacrossthe zones
of the gap-understory light continuum. Quercus
rubru had greater survival and establishmentthan its
associatesin the northern understory adjacent to the
seepageof the valley sit.e. In circumstances where
understory conditions are low in light and droughty
little establishmentof any Quercus
speciesoccurred.
Both Q. velutina and Q. coccinea appeared to require greater amounts of light in understory conditions for survival.
Our study demonstrated that although there is
considerable morphological overlap among Quercus
that belong to a related group, Q. velutinu and Q.
rubra specieshad specific growth characteristics that
allowed each to establish and grow better than its
relative in particular forest microenvironments. Our
study therefore further elaboratesupon work that has
MS.
Ashton, B.C. Larson/
Forest
Ecology
documented these differences among species that
belong to very different taxonomic groups (Paulson
and Platt, 1988, 1989). Findings from our study
imply that silvicultural practices concerned with regenerating these QuercuS species in southern New
England should be specific to site and stand microenvironment.
However, there were no clear differences in morphology between Q. coccinea and the other two
species. Thus, morphological patterns in seed germination and regeneration establishment of these three
species provide only a partial explanation for their
coexistence.
Acknowledgments
We express sincere thanks for advice to colleagues Dr. D.M. Smith, Dr. G.P. Berlyn and Dr.
M.J. Kelty. We would also like to thank Dr. D. L.
Malcolm and Dr. T.L. Poulson for reviewing the
manuscript. Financial support for this study was
partially provided by the School Forests of Yale
University.
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