Ecology 71(3). 1990. pp. 1133-1143
O 1990 by the Ecolog~calSoc~etyof 4menca
SHIFTING DEMOGRAPHIC CONTROL OF A PERENNIAL BUNCHGRASS ALONG A NATURAL HABITAT GRADIENT1 KIRKA. MOLONEY
Department of Botany, Duke University, Durham, North Carolina 27706 USA'
Abstract. The relative importance of competition vs. other environmental factors in
determining the local distribution of a perennial bunchgrass species, Danthonia sericea,
was investigated in two experiments, one in 1984 and another in 1985. Seeds were planted
in a two-way split-plot experimental design under field conditions on the campus of Duke
University, Durham, North Carolina, USA. One treatment, four planting regions in distinct
demographic zones along a soil/vegetation gradient, was used to investigate the effect of
gradient position on germination rates, early establishment success, growth rates, and early
reproduction of Danthonia sericea. The second treatment, vegetation left intact or vegetation removed, was used to assess the relative importance of competition from neighboring
herbaceous plants in regulating the same demographic processes. The demographic success
of Danthonia sericea was enhanced at all positions along the gradient by vegetation removal.
However, the magnitude of the response varied with year of experiment and with gradient
position, indicating that the intensity of competition is dependent upon a variety of factors
that shift over time and space. It is suggested that a complete understanding of the importance of any one factor, such as competition, in regulating species distributions can
only be obtained by conducting experiments over a range of habitats and years.
Key words: competition; Danthonia sericea; demographic control; experimental demography; ecotone: gradient; year-to-year variability.
Any attempt to explain the distribution of a plant
species along a complex habitat gradient must account
for at least four tightly linked elements: (i) competitive
interactions with other plant species; (ii) interactions
with herbivores, pollinators, etc.; (iii) changes in physical environment; and (iv) the differential response to
the first three elements at different life history stages
(cf. Austin and Austin 1980, Lubchenco 1980, Louda
1982, Hay 1984, Austin et al. 1985, Goldberg 1985).
Changes in demography that affect populations might
be confined to one life history stage or be spread over
several stages (Caswell and Werner 1978, Sebens 1982,
Neilson and Wullstein 1983, Hamada et al. 1985). In
plant populations, the inability to germinate or to establish seedlings can produce a barrier to dispersal beyond current boundaries (Lazenby 1955, Harris 1967,
Tripathi and Harper 1973, Turkington et al. 1979,
Fowler 1984). In other cases, where germination and
initial establishment are possible, mortality may occur
before age of reproduction (Quinn 1975) or reproduction may be limited in individuals reaching reproductive age, prohibiting the development of a self-perpetuating population (Cavers and Harper 1967, Watkinson
1985). By determining the life history stages that are
most important in limiting the distributions of species,
' Manuscript received 23 February 1989; revised 26 September 1989; accepted 2 October 1989.
* Present address: Center for Environmental Research, Corson Hall, Cornell University, Ithaca, New York 14853-2701
USA.
a more thorough understanding of the processes regulating species' distributions can be obtained.
A large number of field experiments investigating
interspecific competition has been reported in the literature over the last several years (see the reviews by
Connell 1983 and Schoener 1983). Many provide evidence that competition is an important process in natural populations. However, most d o not explicitly consider the impact of other components of the
environment on competition in their analyses (although see Austin and Austin 1980, Keddy 198 l , Tilman 1984, Austin et al. 1985). One question that remains is the relative importance of competition in
determining the distribution of plant species among
different habitats of a natural resource gradient.
The research I present in this paper investigates the
relative importance of competition in determining the
distribution of the bunchgrass Danthonia sericea Nutt.
along a known soiVvegetation gradient (Moloney 1989).
The study employed a simple, two-way split-plot experimental design. One treatment, location along the
soil/vegetation gradient, investigated the effects of a
concurrent change in the abiotic and biotic environment on demographic processes in Danthonia sericea.
The second treatment, permitting competition in some
quadrats and not in others, investigated the relative
importance of competition. The work examined several life history stages, including germination, early
establishment, postestablishment growth, and reproduction. (Postestablishment growth and reproduction,
although touched upon briefly here, were the primary
topics of another experiment involving tiller trans-
KIRK A. MOLONEY
1134
plants and will be discussed in greater detail in a later
Species
Danthonia sericea, downy oatgrass, is a long-lived,
perennial bunchgrass species, which grows primarily
on well-drained, sandy soils at the edges of pine-oak
forests on the coastal plain and piedmont of eastern
North America. (Danthonia sericea will hereafter be
referred to as Danthonia.) Danthonia grows primarily
during the spring and fall. Individual genets form relatively discrete clusters of tillers. Most germination
occurs in early spring after overwintering of dormant
seed in the soil. At the study site discussed in this paper,
culm formation began in early spring, pollination occurred in mid-May, and seed was dropped in early
June. The species is discussed in greater detail in Moloney (1988).
Study site
The research was conducted in a field dominated by
perennial grasses on the campus of Duke University,
Durham, North Carolina (36"01N,78'54'W). The field
was planted as gardens until abandoned during the
early 1940s and has been mown each summer since
(see Fowler 1978, Fowler and Antonovics 198 1, and
Moloney 1989 for a detailed description of the study
site). The experiments and demographic studies discussed in this paper were all situated in a 16 x 40 m
area of the field. The study site was characterized by
a slight elevational gradient paralleling the long axis,
with an overall elevational displacement of 3 m and
an aspect of 120". (A topographic map of the study site
can be found in Moloney 1989.) The gradient extended
from a stand of pine trees at the low point in the study
area to the top of a knoll at the high point, with soil
concentrations of several plant nutrients increasing
monotonically in the same direction (Moloney 1989).
The change in soil nutrients was also paralleled by a
change in plant species composition, producing a dominant soil/vegetation gradient that characterizes the field
(Moloney 1989).
The study site was located near an area originally
investigated by Fowler (1978). She found a correlation
between depth-to-clay and the local composition of
plant assemblages. She conjectured that Danthonla
characterized nutrient-poor sites. A more detailed
analysis by Moloney (1989) revealed that the distribution of Danthonla was related to intermediate levels
of nutrient and water availability and to the distribution of other plant species in the community. These
relationships involved both the primary soil/vegetation gradient and secondary vegetation gradients.
The distribution of Danthonia, as found by Moloney
(1989), varied widely within the confines of the study
area and can be divided, roughly, into five regions by
Ecology, Vol. 7 1, No. 3
cover values (Fig. 1). Cover estimates were obtained
from four 0.25-m2 quadrats located 4-m apart at each
of 10 transect locations in the field. The transects were
spaced at 4-m intervals along the primary soil/vegetation gradient. (Greater detail regarding the sampling
methodology is given in Moloney 1989.) In Region I,
near the pine trees at the lower end of the study site
(0-4 m in Fig. I), cover by Danthonia was at intermediate levels. Above this lay Region I1 (8-16 m in
Fig. 1) where cover by Danthonia was relatively low,
although individual plants were generally large and
robust. Region I11 (16-24 m in Fig. 1) contained the
densest population of Danthonia in the study site, with
a sharp ecotonal break located nearby in Region IV
(between 24-28 m in Fig. 1). A dense stand of Danthonia lay on one side of the ecotone and on the other
side virtually no Danthon~aplants were found. Curiously, there was no correspondingly sharp break in soil
structure at the ecotone (Moloney 1989). The ecotone
is apparently a stable characteristic of the local distribution of the Danthonia population as it has been present for at least 14 yr in approximately the same location
(K. A. Moloney, personal observation and J . Antonovics, personal colnlnunication). Region V (28-36 m in
Fig. 1) lay above the ecotone at the top of the knoll.
No Danthonia plants were present, except at the edges
of the field where some root intrusion from surrounding pines and hardwoods allowed Danthonia to become
established.
Results from the experiments reported in this study
will be compared to demographic patterns observed in
the natural Danthonia field population as reported in
Moloney (1988, 1989). The locations reported in the
earlier papers are associated with the regions in this
paper as follows: location A, Region I; locations B and
C, Region 11; location D, Region 111; location E, Region
IV; and location F, Region V (cf. Fig. 1 with Fig. 1 in
Moloney 1989).
Experimental des~gn
Common protocol .for 1984 and 1985 gerrninatlon
experiments. -Two Danthonia germination and establishment experiments were conducted; one was begun
in 1984 and another in 1985. Details of the protocol
differed between the two experiments, but the general
design was the same. Each experiment utilized a splitplot, two-way factorial design with two levels o f a vegetation treatment (intact and removal) and four levels
of a gradient treatment (four regions along the gradient). Experimental quadrats were isolated from the surrounding plant community by burying commercial lawn
0
edging around the perimeter to a depth of ~ 2 cm.
The edging extended well below the bulk of the rooting
zone, prohibiting root competition from outside the
treatment (cf. Fowler and Antonovics 198 1). In each
quadrat, vegetation was either left undisturbed (intact
treatment) or all aboveground vegetation, exposed
rootstock, and rhizomes were removed, disturbing the
June 1990
SHIFTING DEMOGRAPHIC CONTROL
Region
Location (m)
FIG. 1. Cover estimates of Danthonia sericea along the primary soil/vegetation gradient at the Durham study site. Mean
cover values for 10 transect locations are connected by the solid line (-);
x ' s indicate individual cover values for the four
quadrats sampled at each transect location. Distances along the abscissa are measured relative to a reference transect located
at metre zero. (See Moloney 1989 and Methods: Study slte for greater detail regarding sampling methodology.)
soil as little as possible (removal treatment). Replicates
of the vegetation treatments were located in Regions
11-V to represent four distinct demographic zones in
the distribution of Danthonia along the primary soil/
vegetation gradient. (Region I was excluded for practical considerations of time and effort.)
Danthonia seeds were collected from plants located
in all regions of the field during the spring preceding
initiation of each experiment. The seeds were then
combined to form a composite sample of genotypes
from the field. Individual seeds were planted at the
base of toothpicks in each quadrat by inserting them
into the soil to the base of the awn and were left enclosed by the lemma and palea as under natural conditions. Hairs on the lemma of Danthonia seeds are
oriented towards the awn and help prevent them from
becoming dislodged once buried.
Protocol for 1984.-Danthonia seeds were collected
from the field in June 1983, air-dried, and stored at
room temperature in plastic bags until January 1984,
at which time a large number of seeds were cold stratified in sand following the protocol outlined in Lindauer and Quinn (1972). Three replicate blocks with
two 0.5 x 0.5 m quadrats in each, one for each level
of the vegetation treatment, were located in each of the
four regions of the experiment. Danthonia seeds (n =
100) spaced 3.5-cm apart were planted in each replicate
in a 10 x 10 array on 25 and 26 April 1984. Censuses
were begun immediately after planting and continued
through the flowering period of June 1986. Censuses
were conducted frequently in 1984 (4 May, 9 May, 13
May, 22 May, 27 May, 1 June, 11 June, 21 June, 10
July, and 14 August) and on 12 May 1985 and 18 June
1986.
Protocol for 1985.-Danthonia seeds were collected
from the field in June 1984, air-dried, and stored at
room temperature in plastic bags. Five blocks, each
consisting of four 0.25 x 0.25 m quadrats, were located
in each of the four regions. There were two replicates
of the removal and of the intact vegetation manipulations in each block. Seeds (n = 49) were planted into
each quadrat in a 7 x 7 array with a 2.5-cm spacing
during November 1984. Censuses were begun with the
first observation of emergence during February 1985
and continued through the flowering period of June
1986. Censuses were conducted frequently in 1985 (7
February, 16 February, 4 March, 19 March, 27 March,
20 April, 30 May, and 30 July) and on 10 June 1986.
Data collection and anab,sis
Seeds were scored as successfully germinating upon
first observation of emergence from the soil surface,
but only if the seed leaves emerged next to a toothpick
with the same orientation as the original planting position. This protocol insured that the large majority of
seeds scored as germinating were ones actually planted
into the experiment, particularly as the density of naturally germinating seeds in the field was generally quite
low (Moloney 1988). Data concerning survivorship,
growth, and reproduction of seedlings were obtained
at each census following germination. Growth was
characterized by determining the number of leaves per
individual at each census and categorizing individuals
into six size classes: (I) size class 1, 1-2 leaves; (2) size
class 2, 3-6 leaves; (3) size class 3, 7-1 3 leaves; (4) size
class 4, 14-27 leaves; (5) size class 5, 28-56 leaves;
and (6) size class 6, > 56 leaves (see Moloney 1986 and
KIRK A. MOLONEY
1136
1988 for a description of the technique used to determine appropriate size classes).
Germination success was defined to be the proportion of seeds emerging over the course of the experiment and was analyzed using analysis of variance for
a split-plot design. The block effect (B) was treated as
a random variable with both the vegetation (V) and
region (R) treatments being considered fixed. Data for
proportion of seeds germinating within treatments were
normalized by applying an arcsine-square-root transform, which is appropriate for an underlying binomial
distribution (Snedecor and Cochran 1980).
Survivorship, for the purposes of analysis, was defined to be the proportion of seedlings that survived
from germination until 14 August 1984 for the 1984
experiment and until 30 July 1985 for the 1985 experiment. These dates were chosen to characterize survivorship through the critical months of seedling establishment. Data transformation and the design of the
analysis for survivorship were similar to those used to
study germination success. However, since survivorship was calculated as the number of surviving seedlings divided by the number of germinating seedlings,
a weighted analysis of variance was used to account
for differences in variance expected for observations
based on different initial sample sizes (see Snedecor
and Cochran 1980).
Differences in the growth rates of Danthonia seedlings among regions were analyzed using contingency
table analysis. (In this case, growth was represented by
the distribution of Danthonia seedlings among size
classes 1 yr after planting.) Only data from the cleared
vegetation treatment were used as there were too few
survivors in the intact vegetation for a comparison
between vegetation treatments. Surviving individuals
were categorized by size class for each replicate and
were then pooled over replicates to produce one observation of size class distributions for each region.
Region V was omitted in the 1985 analysis as there
were no survivors in either vegetation treatment. Further pooling among the smaller size classes was also
necessary. Data for size classes 1 through 3 were pooled
for the 1984 experiment and data for size classes 1 and
2 were pooled for the 1985 experiment.
Germination
Germination by Danthonia occurred over a short
period of time in the 1984 experiment. Over 99% of
the germinating seedlings emerged over a 10-d period
(4 May-13 May 1984). In the 1985 experiment, seedling emergence began earlier (February as opposed to
May) and took longer to reach 99% total germination
(over 72 d). The 1985 results were more indicative of
natural germination patterns, due primarily to differences in experimental protocol (cf. Lindauer and Quinn
1972): seeds in the 1985 experiment overwintered in
Ecology. Vol. 7 1. No. 3
the field, whereas seeds in the 1984 experiment were
stratified in a cold room and planted in the spring. The
proportion of Danthonia seeds germinating in the 1985
experiment was greater than in the 1984 experiment
(Fig. 2). However, with the exception of Region 111,
the between-year differences in the intact vegetation
treatment were less dramatic than in the removal treatment.
The effect of vegetation removal on germination success was highly significant in both germination experiments (Table l), with a greater proportion of seeds
germinating in the removal quadrats (Fig. 2). The effects of region and region-by-vegetation interaction on
germination were also significant for the 1985 experiment, but not the 1984 experiment. With the exception of Region 11, the proportion of seeds germinating
across regions had the same rank order for both experiments and both vegetation treatments (Fig. 2). Germination was greatest in Region 111, intermediate in
Region IV, and least in Region V, paralleling the natural distribution of plants in the field (cf. Fig. 1 and
Fig. 2). Results for Region I1 were inconsistent between
experiments. Region I1 had the highest germination
rate for removal quadrats in the 1985 experiment and
the lowest rate in the 1984 experiment. Intermediate
levels of germination for intact vegetation quadrats
were seen in Region I1 for both experiments. T o a large
extent, the significant region-by-vegetation interaction
in the 1985 experiment can be attributed to differences
in germination success between vegetation treatments
in Region 11.
Postgerlnination survival
In the 1984 experiment, survival was significantly
greater in the removal quadrats in all four regions, with
very few individuals surviving in the intact vegetation
(Table 2, Fig. 3). Only Region I11 had an appreciable
number of individuals surviving in the intact vegetation through the August census of the first growing
season, and in this case most of the survivors died over
winter (Fig. 3). In contrast, there was little mortality
after the June 1984 census in the removal quadrats
regardless of region.
Survival rates in the 1985 experiment were generally
low from time ofgermination through the midsummer
census on 30 July 1985 (Fig. 3). Survivorship from
July 1985 through June 1986 was relatively high in
most treatment combinations (> 70%) with the exception of the intact quadrats of Region 111 (average survivorship of 28% over the period) and the intact and
removal quadrats of Region V, where there was complete mortality by June 1986. Overall, survivorship in
the 1985 experiment was lower than in the 1984 experiment for both removal and intact vegetation treatments (Fig. 3). The one major exception occurred in
the removal quadrats of Region 11, where individuals
had an average survival rate of 0.52 through the 30
July 1985 census. The high survivorship with vege-
June 1990
SHIFTING DEMOGRAPHIC CONTROL
1984 Experiment
1137
T.ZBLE
1. Analysis of variance for total germination in the
1984 and 1985 split-plot germination experiments.
SignifSum of
icance
F
level
Year
df squares
3
0.147
0.64*
NS
1984
8 0.612
1 0.165
13.631. ,006
NS
3 0.106
2.901.
8 0.097
1985
3
1.065
7.02* ,003
16 0.809
3.439 ,0008
V
1 1.907
154.99* ,0001
VxR
3 0.187
5.08* .01
V x B(R)
16 0.197
0.849
NS
Error
40 3.45
* F ratio calculated using the B(R) mean square as the error
term.
t Block(R) and B(R) denote Block nested within Region.
F ratlo calculated using the V x B(R) mean square as the
error term.
5 F ratio calculated using the model error mean square as
the error term.
Source of
variation
Region
Block(R)t
Vegetation
V x R
V x B(R)
R
B(R)
Region
*
1985 Experiment
2
n
0.0
1
I
II
I
III
I
IV
I
v
Region
Mean germination levels for Dunthonia sericea by
region of the field and vegetation manipulation (removal and
intact) in the 1984and 1985germination experiments. Means
were calculated using arcsine-square-root transformed data
and were backtransformed before plotting the figure. W germination rates in quadrats with vegetation left intact; germination rates in quadrats with vegetation removed.
FIG.2.
tation removal in Region I1 resulted in a highly significant region-by-vegetation interaction for the 1985
experiment (Table 2). Region was also highly significant as a main effect, due in part to the contrast between
very low survivorship in Regions 111, IV, and V a n d
high survivorship in the removal treatment of Region
11. Although significant, the vegetation treatment was
substantially less important than either region o r region-by-vegetation interaction in explaining survivorship.
Growth o f seedlings
Growth rates of established seedlings were. in general, much greater in the removal quadrats than in the
intact quadrats (Figs. 4 and 5). However, a few plants
in Region I1 showed appreciable growth even in the
presence of background vegetation (Fig. 5). Plants
growing with vegetation removed were able to attain
sizes comparable to the largest individuals of any age
in the natural Danthonia population (Moloney 1988).
A significant difference among regions for growth
rates in the removal quadrats was found through contingency table analysis in the 1984 experiment (G =
19.15, P < .05, df = 9), whereas there was n o detectable
difference in the 1985 experiment (G = 8.43, P < .38,
df = 8).
Early reproduction
Seed was produced by the first June census following
the year of germination in both experiments (Table 3).
Only one plant in the intact vegetation, which was
located in Region 11, produced seed in either experiment. In the 1984 experiment, culms were produced
by plants in the removal quadrats of all of the regions
and exhibited a threefold increase in output between
the 1985 and 1986 June censuses (Table 3). In contrast,
with the exception of two plants, reproduction in the
1985 experiment was confined t o the removal quadrats
of Region 11.
Vegetat~onefects on demographic processes
Vegetation removal greatly enhanced germination
rates and the establishment success of Danthonia. Soon
after emergence from the soil surface, individual Danthonla plants typically produced two small leaves on
a single tiller. Individuals emerging in intact vegetation
grew much more slowly, producing few, if any, new
leaves o r tillers over the course of the experiment. T h e
early leaves continued to elongate while working u p
through the surrounding vegetation, becoming spindly
and requiring support from surrounding plants. Even-
KIRK A. MOLONEY
Ecology, Vol. 7 1. No. 3
TABLE2. Weighted analysis of variance for survivorship in
1984 Experiment
the 1984 and 1985 split-plot germination experiments.
Year
1984
Source of
variation
Region
Block(R)t
Vegetation
VxR
V x B(R)
df
Sum of
squares
F
Significance
level
B(R)
v
II
Ill
IV
v
Region
*
1985 Experiment
II
Ill
IV
VxR
V x B(R)
Error
* F ratio calculated using the B(R) mean square as the error
term.
t Block(R)and B(R) denote Block nested within Region.
F ratio calculated using the V x B(R) mean square as the
error term.
5 F ratio calculated using the model error mean square as
the error term.
v
Region
Mean survivorshipfor Danthonia sericea by region
of field and vegetation manipulation (removal and intact) in
the 1984 and 1985 germination experiments. Means were
calculated using arcsine-square-roottransformed data and were
backtransformed before plotting the figure. W survivorship in
quadrats with the vegetation left intact;
survivorship in
quadrats with vegetation removed. Solid lines (-)
connect
means for a late summer census conducted during the first
growing season after emergence (14 August 1984 for the 1984
experiment and 30 July 1985 for the 1985 experiment). Dotted lines (. . . . .) connect means for the last census conducted
in each experiment (18 June 1986 for the 1984 experiment
and I0 June 1986 for the 1985 experiment). Note the change
in vertical scale for the 1985 census.
FIG.3.
tually the seedlings became quite etiolated and most
died, apparently from the inability t o photosynthesize
a t rates greater than the compensation point. This process could take > 1 yr. The developmental pattern exhibited by plants emerging o n bare ground depended
to a much greater extent o n region within the field and
showed greater variation between experiments. However, plants successfully establishing o n bare ground
produced new leaves and tillers fairly soon after emergence and were more robust in habit than plants growing in intact vegetation. In fact, the most robust individuals attained sizes comparable to the largest
individuals found in the natural population within a
single growing season.
Survivorship improved markedly with vegetation
removal in the 1984 experiment, whereas survivorship
was low in the removal quadrats of the 1985 experiment, showing little improvement over survivorship
in the vegetated quadrats. The latter result was apparently due to drought conditions during late spring a n d
early summer of 1985. The only exception to the general trend was provided by Region I1 where survivorship in the removal quadrats was relatively high during
1985.
The effect of vegetation removal o n postestablishment rates of growth and reproduction by Danthonia
was difficult to assess quantitatively as very few individuals survived in the intact vegetation. However,
surviving plants had substantially higher average growth
rates in the removal quadrats and, with one exception,
were the only ones to reproduce during the course of
the experiment.
T h e response by Danthonia to vegetation removal
is not surprising in and of itself. Several studies have
demonstrated that germination and establishment of
plants can be greatly enhanced by the removal of vegetation ( H a n i s 1967, Hagon 1977, Werner 1977), although the effect of vegetation removal may depend
o n patch size (Miles 1974, McConnaughay and Bazzaz
1987), stage of seedling development a t the time of
vegetation removal (Miles 1974, Cavers and Harper
1967), and other factors such as size of seed (Gross
1984). T h e physiological processes involved in inducing a differential germination response to vegetation
removal are complex and include a diversity of factors
that are often species specific (see Fenner 1985 for a
comprehensive review). For Danthonia, Quinn (1975)
found a high rate of germination under a variety of
1139
SHIFTING DEMOGRAPHIC CONTROL
June 1990
Region I1
Region Ill
Region IV
Region V
August 1984
.;;;
-
Q
May 1985
June 1986
n
Size Class
FIG.4. Proportional distribution of Danthonra sericea seedlings among six size classes at three census dates for the 1984
germination experiment. Data were pooled among replicates within each treatment combination. The proportional distribution
of individuals among size classes is indicated by hatched bars for the vegetation-removal quadrats and by solid bars for the
quadrats with vegetation left intact.
moisture conditions, with only seeds planted initially
into saturated soil exhibiting lower germination rates.
This result suggests that the change in moisture availability induced by vegetation removal would not be
enough to produce a differential germination response.
Lindauer and Quinn (1 972) found that the germination
response by Danthonia varied under different temperature regimes. Germination rates for Danthonia were
higher when the daily maximum temperature was relatively low (20'-25°C) and declined as the daily maxi m u m temperature increased above 30" (Lindauer and
Quinn 1972). Unfortunately, this suggests that germination rates should be greater under a closed canopy
where the daily maximum temperature would be lower
due to a deeper boundary layer, just the opposite of
the pattern observed in this study. Lindauer and Quinn
(1972) also found n o effect of light availability on germination rates. A significant germination response to
a shift in the red/far red ratio has been observed for a
large number of grassland species (Fenner 1980, Sil-
vertown 1980). However, this aspect of the germination ecology of Danthonia has not been investigated.
Interactions between vegetation and
other environmental factors
Although germination rates by Danthonia increased
dramatically with vegetation removal, there was also
a clear modifying effect of gradient position. A decline
in the germination response to vegetation removal from
Region 111 through Region V suggests that competitive
effects play a greater role in intermediate regions of the
field where Danthonia is most abundant. In Region I1
the effect of vegetation removal on germination rates
was extremely variable; there was a very strong germination response to vegetation removal in 1985 and
little o r n o response in 1984 (Fig. 2). However, since
the 1985 protocol was more indicative of natural germination processes (seeds overwintered in the soil rather than in a coldroom), it appears that vegetation has
the greatest negative impact on germination rates in
KIRK A. MOLONEY
Region II
Region Ill
Ecology, Vol. 71, No. 3
Region IV
Region V
July 1985
Size Class
FIG. 5. Proportional distribution of Danthonla sericea seedlings among six size classes at two census dates for the 1985
germination experiment. Data were pooled among replicates within each treatment combination. The proportional distribution
of individuals among size classes is indicated by hatched bars for the removal quadrats and by solid bars for the quadrats
with vegetation left intact.
TABLE
3. Mean is^) within-treatment values in the 1984 and 1985 germination experiments for (i) proportion of planted
seed surviving as seedlings, (ii) proportion of surviving seedlings reproducing, (iii) culm production per reproducing individual, and (iv) culm production per replicate.* Values were determined for May 1985 (1984 experiment only) and June
1986 censuses.
Treatment combinations
Removal
I1
111
Removal
I1
Proportion surviving
May 1985
June 1986
. I 5 i .21
. I 8 i .10
.15 i .21
.18 i .10
Proportion reproducing
May 1985
June 1986
1984 experiment
.65 i . 2 0 t
.87 i .18t
. I 5 i .12
.64 i .19
Culms per individual
May 1985
*
2.42 0.81t
1.13 i 0.22
June 1986 *
5.83
3.07t
3.04 i 1.23
1985 experiment
111
Intact
IV V
I1
111
IV
V
* All means were calculated using within-replicate values (e.g., the average number of culms per reproducing individual
was calculated within replicates before averaging over replicates). Three replicates (one per block) were used in calculating
mean values for the 1984 experiment and 10 replicates (two per block) for the 1985 experiment, except as noted.
t Based on two replicates, one replicate having no survivors.
Based on one replicate, two replicates having no reproductive individuals.
6 Based on seven replicates, three replicates having no survivors.
// Based on one reproductive individual.
7 Based on three replicates, seven replicates having no survivors.
*
June 1990
SHIFTING DEMOGRAPHIC CONTROL
Region 11, in keeping with the general trend of a decreasing effect of vegetation on germination from Region I11 through Region V.
Survivorship was strongly affected both by competition and by other environmental factors associated
with gradient position, yet the relative importance of
these factors varied markedly from region to region
and from year to year. In the 1984 experiment, there
was a general decline in survivorship in the absence of
competition along the gradient from Region I1 through
Region V, with the highest survival rates in Region I1
and the lowest rates in Region V. In contrast, more
plants germinating in the intact vegetation died over
the course of the experiment, irrespective of region.
This suggests that survival rates along the gradient are
most strongly affected by competitive interactions where
Danthonia is most abundant, and are most strongly
affected by factors other than competition where Danthonia is absent. However, the pattern observed in the
1985 experiment suggests a very different relationship.
Survivorship in the presence of vegetation in the 1985
experiment was low, but equaled, or nearly equaled,
survivorship in the absence of vegetation. (Again, Region I1 was the exception to the general trend.) Drought
conditions during the spring and summer of 1985 evidently overwhelmed any influence of vegetation on
survivorship. The worst conditions were observed in
Region V where plants growing in the absence of vege-
TABLE
3. Continued
May 1985
Culms per replicate June 1986 1984 experiment 16.3 i 18.9
4.33 5.77
12.3 k 11.0
1.33 i 2.31
0
0
0
0
1985 experiment
*
48.7
45.7
50.3
23.3
i 53.5
i 53.2
i 47.3
i 32.9
0
0
0
0
1141
tation were pushed by their elongating radicals out of
the soil, which had become very loose and friable. The
plants produced short, twisted leaves, most likely in
response to desiccation stress and damage. Although
survivorship in Regions I11 and IV was also low during
1985, patterns of growth and development were not as
extreme.
In Region 11, the survivorship response provided a
marked constrast to the response observed in other
regions of the field. Survivorship was greatly enhanced
by the removal of vegetation, even during the drought
of 1985. Not only was survivorship in 1985 relatively
high in Region 11, but growth by survivors was rapid
even where the vegetation had been left intact (Fig. 5).
This result may be due to the fact that Region I1 was
situated in a partial seep where water is available for
longer periods during dry conditions (K. A. Moloney,
personal observation), thus ameliorating stress conditions during the 1985 drought.
Comparisons to the natural.field population
The general decline in demographic performance by
Danthonia from Region I11 through Region V mirrors
a parallel decline in the natural Danthonia population
(Moloney 1988, 1989). In addition, the extreme variability of demographic rates in Region 11, both within
and between vegetation treatments, can be related to
the structure of the natural field population, which consisted of a few scattered, but extremely robust, individuals in Region 11. Opportunities for recruitment are
apparently limited in Region 11, but the conditions for
growth are extremely good, even in the presence of
intact vegetation.
The trend for greater germination rates in the 1985
experiment was the opposite of the trend seen in the
natural population, where germination rates were very
high during 1984 and very low during all other years
of observation (for 1984, 1985, and 1986 see Moloney
1988; for 1983, 1987, 1988, and 1989, K. A. Moloney,
personalobservation). The higher germination rates observed in the natural population for 1984 might be
related to differences in the annual availability of viable
seed. However, the high levels of recruitment in 1984
followed a year of drought in 1983, which lowered
aboveground cover (Moloney 1986). This result, coupled with the experimental result that germination rates
increase after vegetation removal, suggests that there
is a feedback mechanism in Danthonia that increases
germination rates when aboveground competition is
minimized.
Rates of postemergence survivorship and growth
among recruits in the natural population were lower
than for recruits originally planted in the quadrats
cleared of vegetation, but were higher than rates for
recruits planted in intact vegetation (Moloney 1988).
Time to first reproduction was greater among natural
recruits than among recruits originally planted into
cleared quadrats. The earliest observed reproduction
1142
KIRK A. MOLONEY
among naturally germinating seedlings was 2 yr after
emergence, and then only in a few scattered individuals
located i n Region I1 (K. A. Moloney, personal ohserr a t ~ o n ) . In contrast. reproduction among seedlings
planted in the cleared quadrats was widespread only 1
yr after emergence, a t least in the 1984 experiment.
The intermediate level of demographic success for naturally germinating seedlings suggests that a majority
of the recruits in the field germinate in safe sites that
provide greater demographic potential than d o sites
chosen by planting a t random within the intact vegetation (cf. Harper 1977, Silvertown 198 1, Fenner 1985).
The natural safe sites, however, d o not provide conditions as favorable as sites from which the vegetation
has been removed, a n d a s such the natural safe sites
represent a relaxation, not elimination, of competition.
T h e complete absence of Danthonia from Region V
in the natural population appears t o be due t o a cumulative failure t o germinate a n d establish after germination, a n d not t o a n inability to grow once established; indeed, the few plants surviving in Region V
during the 1984 experiment exhibited fairly substantial
growth rates. What is difficult t o explain within the
context of the experiment is the abrupt decline in abundance of the natural Danthonia population a t the ecotone located in Region IV. There is n o correspondingly
abrupt change in the edaphic environment across the
ecotone; in fact, earlier work has shown that there is a
monotonic increase in soil fertility from Region I
through Region V (Moloney 1986, 1989). In addition,
there does not appear t o be a radical shift i n the demographic response across the ecotone for the characters investigated, although survivorship of emerging
seedlings may be substantially reduced above the ecotonal boundary.
An explanation for the presence of the ecotone may
lie in a population model introduced by Watkinson
(1 985). The model demonstrated that a n abrupt decline
in the abundance of a population along a complex environmental gradient can be produced by a n interaction between density-independent and density-dependent factors affecting the population if there is a slight
nonlinearity in the response across the gradient. Watkinson was considering intraspecific density-dependent interactions, but the argument can be extended
t o include interspecific interactions a s well. Under this
scenario, the ability of Danthonia t o become established collapses a t the ecotone d u e t o a nonlinear response t o competition against a gradually changing environmental background.
Conclusion
Although the removal of vegetation generally had a
positive effect o n the demographic success of D a n thonia, the magnitude of the effect varied greatly across
regions and between years. This suggests that a n interaction between competition a n d other environmental factors must be taken into account if we are t o
Ecology. Vol. 7 1. No. 3
explain the causal relationship between population
structure a n d environmental variation along natural
gradients in the field. In fact, a n experiment that is
designed t o test solely for competitive effects in one
location might find that competition is important in a
species like Danthonia, but might just as easily find
that competition is not a very important process if the
experiment is conducted a t a n adjacent site o r during
a different year. Only by designing field experiments
across a range of habitats a n d across a number of years
can we begin to determine the relative importance of
competition, o r any other factor, in shaping natural
distributions.
I would like to thank Janis Antonovics, my advisor, for his
help in seeing this project to completion. Joy Belsky, Don
Burdick, Deborah Clark, Nick Howell, Norma Fowler, Bill
Schlesinger, Don Stone, Henry Wilbur, and the reviewers
provided many useful suggestions. I would especially like to
thank Martha and Naomi Rappaport, and Frosty Levy for
their help in the field. Si Levin graciously provided a conducive environment for writing the final manuscript. Computing facilities were provided by DUCC and TUCC at Duke
University and CIT at Cornell University. The research was
supported in part by the Ecosystem Research Center at Cornell (for which this is publication ERC-215), a grant-in-aid
from Sigma XI, and by National Science Foundation grant
BSR-8806202.
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Shifting Demographic Control of a Perennial Bunchgrass along a Natural Habitat Gradient
Kirk A. Moloney
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