| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Department of Ecology and Evolutionary Biology, U-42, University of Connecticut, Storrs, Connecticut 06269-3042 USA
Received for publication March 14, 2000. Accepted for publication July 18, 2000.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Key Words: Connecticut • forest regeneration • forest understory dynamics • Great Mountain Forest • hayscented fern • New York fern • successional dynamics
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
), water availability (Beals and Cope, 1964
; Lipscomb and Nilsen, 1990b
), the availability of nitrogen and other soil nutrients (Rice and Pancholy, 1973
; Finzi, 1996
), and microtopographical variation (Bratton, 1976a, b
; Beatty, 1984
). Some of the variation in the forest understory environment can be linked to species-specific influences of canopy trees in the quality and quantity of light transmittance (Canham, 1988
; Canham et al., 1994
), differences in stemflow quality and quantity (Gersper and Holowaychuk, 1971
; Crozier and Boerner, 1984
), and leaf litter chemistry (Lodhi, 1977
). Although less studied, biotic interactions such as competition among understory species within the herb layer may be important in determining the distribution and abundance patterns of understory plants (Maguire and Forman, 1983
; Collins and Good, 1987
). However, few studies have explored these biotic interactions and even fewer have attempted to examine the feedback between forest understory and canopy dynamics (Hill, 1996
).
Although there are many studies of the distribution and dynamics of temperate forest herbs and shrubs (e.g., Brewer, 1980
; Moore and VanKat, 1986
; Plocher and Carvell, 1987
; Collins and Pickett, 1988
), few have examined the role of these understory species in the successional dynamics of the forest (Hill, 1996
). Herbs and shrubs can play significant roles in determining the composition and abundance of tree seedlings in forest stands (Horsley, 1977a, b
; Maguire and Forman, 1983
; Phillips and Murdy, 1985
; Kolb, Bowersox, and McCormick, 1990
). These understory species can also have an important impact on forest dynamics. For example, the combined, negative effects of understory species (Dennstaedtia punctilobula, Thelypteris noveboracensis, Brachyelytrum erectum, and Lycopodium obscurum) often results in the failure of the forests to regenerate (arrested succession) in the Allegheny Plateau of Pennsylvania (Horsley, 1977a, b, 1985
) and New York (Drew, 1988, 1990
).
Hayscented fern (Dennstaedtia punctilobula) and New York fern (Thelypteris noveboracensis) are two prominent species that can interfere with forest regeneration (Horsley, 1977a, b
; Drew, 1988, 1990
; Hill, 1996
), yet our understanding of the ecology of these species is limited. The abundance of tree seedlings under these fern canopies can be dramatically (60–85%) reduced (Horsley, 1977a
; Hill, 1996
). Though the detrimental effects of hayscented fern on tree seedling growth and survivorship have been shown (Horsley and Marquis, 1983
; Drew, 1988
; Horsley, 1993
; Hill, 1996
), less work has been conducted on New York fern (Horsley, 1977b
). Since these understory species have the potential to influence forest regeneration patterns, understanding their distribution and dynamics is critical to understanding the successional dynamics of the forest.
Hayscented fern (Dennstaedtia punctilobula) and New York fern (Thelypteris noveboracensis) are abundant throughout the understory of secondary growth, oak–transition hardwoods–hemlock stands at Great Mountain Forest (GMF), Connecticut. The two species commonly co-occur and thus appear to have similar distributional patterns in the forest understory. However, they differ in their rhizome morphology (R. McCalley and J. D. Hill, unpublished data, Yale University) with hayscented fern growing 
9 cm/yr and New York growing 1–2 cm/yr. Also, hayscented fern has a continuous population of fronds and New York fern has more discrete patches of fronds. In addition, there may be subtle differences in the moisture requirements of the two species (J. D. Hill, personal observation). Both are native species but, based on current distribution patterns, hayscented fern appears to be more invasive, spreading aggressively in comparison to New York fern.
The objectives of this study were to quantify the patterns of distribution, abundance, and within- and between-season dynamics of hayscented and New York fern in relation to canopy tree species and canopy tree gaps (1–2 tree gaps) in the forest and to assess whether these patterns are related to light and moisture availability.
|
MATERIALS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Study species
Hayscented fern (Dennstaedtia punctilobula (Michx.) Moore) is a homosporous, leptosporangiate fern native to the eastern United States and Canada (Cody, Hall, and Crompton, 1977
). In New England, hayscented fern is common in deciduous forests, along roadsides, and at the edges of old fields (Hammen, 1993
). Hayscented fern colonizes bare mineral soil via spores, but once established can spread aggressively through underground stems (rhizomes) (Conard, 1908
; Horsley, 1984
; Groninger and McCormick, 1991
). The basic architecture of the rhizome is linear with dichotomous branching at 8–24 cm intervals with fronds arising on alternate sides of the rhizome (Conard, 1908
). The rhizome of hayscented fern has the capacity to expand by either centrifugal (due to repeated branching) or linear growth (unbranched rhizome) of the perennial rhizome system (Hammen, 1993
; Hill, 1996
). The perennial rhizomatous habit allows the rapid formation of an early growing season canopy, often putting the species in direct competition with other herbs and tree seedlings (Horsley and Marquis, 1983
). Despite being a native species, hayscented fern is often considered a weed (Cody, Hall, and Crompton, 1977
; Hammen, 1993
), as it out-competes more desirable species.
New York fern (Thelypteris noveboracensis (L.) Nieuwl.) has long-creeping rhizomes that are slender and cord-like. Unlike hayscented fern, fronds of New York fern occur closely packed in tufts of three to four fronds or as solitary fronds. Although the rhizomes are linear in form like those of hayscented fern, rhizome growth rates are much slower in New York fern on the order of 1–2 cm annually (R. McCalley and J. D. Hill, unpublished data, Yale University). New York fern is prevalent in somewhat shady, moist to dry, rich woods and on swamp edges (Lellinger, 1985
). Both hayscented and New York fern occur in moderately acidic soils and appear to be similar in their niche requirements (J. D. Hill, personal observation). Additionally, New York fern appears to be a far less weedy and aggressive species, posing less of a problem with forest regeneration failure.
Quantifying fern distribution and abundance
Because hayscented and New York fern are clonal organisms, the strict definition of a genetic individual (genet) was difficult. To circumvent this problem, we used aboveground ramet density (fronds) to quantify fern abundance. Ramet density was assessed in 105, 1-m2 plots in July and August of 1991. Plots were placed out randomly after stratifying by canopy tree species types (see below). Plots were recensused in 1992, 1993, and 1994 to assess the dynamics of these fern populations (see Fern dynamics below).
To address whether species-specific canopy tree effects were important, census plots (N = 15 per canopy type) were located beneath specific canopy trees (Tsuga canadensis, Fagus grandifolia, Quercus rubra, Prunus serotina, Acer rubrum, or A. saccharum) or in small (1–2 canopy tree gaps) canopy gaps. Plots were chosen in a stratified random fashion, and although they span a range of understory light conditions (0.25–26.0% full sun), soil moisture conditions were in the mesic (6.5–30.0% moisture) range. The species of fern, the number of mature and immature ramets, and the average ramet height by plot were recorded on each census date.
The patterns of distribution and abundance of hayscented and New York fern with respect to canopy type, light availability, and soil moisture availability were determined using the 1991 data set. Assumptions of normality and homogeneity of variance were evaluated throughout using graphical diagnostics. If necessary, log or power transformations were utilized, or nonparametric approaches were employed. Distributions in terms of presence–absence were evaluated using Wilcoxon two-sample tests (SAS, 1987
). The analysis of abundance patterns with respect to resources was performed at the canopy type level using one-way ANOVA and Kruskal-Wallis tests (with Bonferroni t tests for comparisons of means) and on a continuous basis with regression.
Quantifying resource availability
Light availability was assessed using hemispherical photographs (see Canham 1988
for detailed methods) taken 1.5 m above the ground in each plot in July of 1991. An index of whole-growing-season light availability (GLI, following Canham, 1988
) was computed for each photograph. The GLI index integrates the seasonal and diurnal movements of the sun, the mix of diffuse and beam radiation, and the spatial distribution of canopy openness into a single index in units of percentage of full sun. This index is correlated with total photosynthetically active radiation (PAR) under closed and open canopies and in gaps (Canham, 1988
; Canham et al., 1994
). Soil moisture availability (percentage of volume) was quantified using time domain reflectometry (Trase Instrumentation, Soilmoisture Equipment Corp., Goleta, California, USA; see Gray and Spies [1995]
for detailed methods) in each of the 1-m2 plots. Soil moisture measurements were taken in both 1991 and 1992 at the mid-growing season census, as the average of three measurements (spatial replicates) in each of the 1-m2 plots.
Fern dynamics
In addition to the 1991 data set, census data were also collected in June, July, and August of 1992, and July of 1993 and 1994. As with the 1991 census data, species of fern, the number of mature and immature ramets, and the average ramet height by plot were recorded on each census date.
Abundance patterns within the growing season (seasonal) were extracted from the 1991 and 1992 data sets. Paired comparisons t tests were used to assess differences in the mean abundance (number of fronds per square meter) across census dates. In addition, correlation and regression analysis were used to evaluate changes in frond density with respect to initial density, light availability, soil moisture availability, and the density of the other fern species.
Changes in abundance between growing seasons were compared using the entire data set (1991–1994). Patterns of fern dynamics were evaluated using mean midseason (July) abundance for all years in paired comparison t tests. In addition, correlation and regression analyses were used to evaluate the importance of initial frond density, light availability, and soil moisture availability to changes in frond density over three growing seasons. All data analyses were performed using the SAS statistical program (SAS, 1987
) unless otherwise indicated.
To further assess the importance of light in determining the distribution and abundance patterns of hayscented fern, two small-scale experiments were performed. The shading experiment explored the dynamics of fern following reductions in light. The invasion experiment involved measuring the dynamics of fern following increases in light. In the shade experiment, aboveground ramet density and light incident on the fern canopy were measured in 40 1-m2 plots. A shading treatment was then applied to 20 of these plots using a shade box 1.5 x 1.5 m in size, 1.5–2.0 m tall, and consisting of 92% shade cloth. In other words, 8% of the light incident on the shade box transmitted through to the fern. Frond density was measured in these plots 1 and 2 yr after initiation of the shading treatment. Light beneath the shade boxes was calculated as a percentage of light incident on the shade box. Pretreatment light levels ranged from 1.4 to 10.1% of full sun; after shading, light levels above the fern ranged from 0.1 to 0.8% of full sun. Shading treatment did not affect soil moisture as no difference between shaded and control plots (t test, P > 0.05) was observed. Posttreatment (future) fern abundance was expressed as a function of pretreatment (past) fern abundance and light availability using maximum likelihood regression analysis.
The invasion experiment involved increasing the light over the fern census plots. Light was increased by either cutting experimental gaps in the forest canopy or by locating areas in the forest that had been harvested for timber within the last 1–2 mo. Initial fern abundance was quantified in 40 1-m2 plots immediately following canopy removal. Light was quantified following the removal of the forest canopy using hemispherical photographs and ranged from 2 to 37% of full sun. Fern abundance was then quantified 1, 2, and 3 yr following the initiation of the canopy removal treatment. Posttreatment (future) fern abundance was modeled as a function of pretreatment (past) fern abundance and light availability using maximum likelihood regression analysis.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
|
= -0.2555, P < 0.01) and 1992 ( = -0.2296, P < 0.05), the abundance of hayscented and New York fern were negatively correlated.
Differential distribution of hayscented fern with respect to canopy trees might result from species-specific differences in environmental conditions beneath trees (Canham et al., 1994
). As a preliminary test, we looked at the patterns of light and soil moisture availability beneath the different canopy types. There were clearly differences in light availability beneath the canopies of the various tree species (Fig. 2A). Percentage of full sun available was lowest under eastern hemlock and highest where there was no canopy tree (NONE [1–2 tree gaps]) directly above the sampling point (one-way ANOVA; F6,51 = 20.06; P < 0.0001; r2 = 0.70; Fig. 2A). Furthermore, a subset of areas in the forest without hayscented fern had lower mean light levels (N = 13, mean = 3.13% full sun, SE = 0.94) at 1.5 m above the ground than a subset of areas with hayscented fern (N = 92, mean = 6.13% full sun, SE = 0.57) (Wilcoxon two-sample test; 
2 = 5.798; df = 1; P < 0.01). The abundance of hayscented fern, although quite variable, increased with increasing light availability up to a point, reaching an asymptote at 15% full sun (r2 = 0.341, P < 0.001, Fig. 2B). In contrast, light availability had little impact on the distribution or abundance (data not shown) of New York fern. In fact, mean light levels for areas without (N = 71, mean = 5.47% full sun, SE = 0.61) and with New York fern (N = 34, mean = 6.38% full sun, SE = 0.57) did not differ (Wilcoxon two-sample test; P > 0.05).
|
2 = 3.89; df = 1; P < 0.05; 1992: 2 = 6.21; df = 1; P < 0.01). Abundance patterns of New York fern, however, did not relate systematically to soil moisture availability (data not shown).
|
|
|
|
19% lower at the end of the growing season than at the beginning (1992 data set; Fig. 5B). This pattern is primarily due to the thinning of mature fronds, as no new fronds are initiated after the fern canopy closes (Hill, 1996
), and presumably relates to decreases in light over the course of the growing season. Hayscented fern short-term dynamics (i.e., seasonal) appear to be related to initial frond density ( = -0.6227, P < 0.0001, N = 94) and light availability ( = -0.4218, P < 0.0001, N = 94), but not soil moisture ( = 0.05760, P > 0.05, N = 94). Overall, the reduction in frond density over the course of the growing season was somewhat higher on denser plots (regression; F = 164.215; P < 0.0001; r2 = 0.6384; Y = -0.162465 x [Initial frond density]). The seasonal population dynamics of hayscented fern were not correlated with the abundance patterns of New York fern ( = 0.0746, P > 0.05, N = 94). New York fern dynamics had little relationship to canopy tree type (Table 1). In 1991, mean frond densities of New York fern decreased from July to August (paired comparisons t test, t = -2.14, P < 0.05). In 1992, mean frond densities did not differ significantly between June and July (t = 1.85, P > 0.05) but did differ between July and August (t = -2.26, P < 0.05). Unlike hayscented fern, however, density at the end of the growing season (August) did not differ from the beginning of the season (June) (paired comparison t test, P > 0.05, Fig. 6).
|
|
Year-to-year dynamics of hayscented fern appear to be correlated with initial frond density (Table 2). Overall, the reduction in frond density over the course of the three growing seasons was somewhat higher on denser plots (regression; F = 18.518; P < 0.0001; r2 = 0.1722; Y = -0.091848 x [Initial frond density]). There was little relationship between change in frond density and initial light conditions ( = -0.01894, P > 0.05) or change in frond density and initial soil moisture conditions ( = -0.14695, P > 0.05) (Table 2). The year-to-year population dynamics of hayscented fern were not correlated with the abundance of New York fern (Table 2).
|
Patterns of fern dynamics following light manipulation
In the shading experiment, aboveground ramet density measured 1 and 2 yr posttreatment was 73% of that prior treatment (Fig. 8A) indicating a very slow decline in abundance. On the other hand, when light availability was increased, as in the invasion experiment, a fairly rapid increase in abundance was observed (Fig. 8B), with frond density doubling within 3–5 yr. For plots with high initial frond density, this rapid increase was absent, suggesting a saturation effect.
|
|
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Both ferns decreased in abundance over the course of a growing season, with New York fern varying little in abundance from year to year. On the other hand, there was significant variation in hayscented fern abundance. For hayscented fern, those plots with the highest initial abundance had the greatest seasonal and between-year decrease in frond density. The seasonal and between-year dynamics of hayscented fern did not relate to the dynamics of New York fern and vice versa, suggesting independence in the dynamics of these two species over the limited number of plots (N = 35) where the two species co-occurred.
Distribution and abundance
Hayscented fern was least abundant under canopies dominated by eastern hemlock (Tsuga canadensis) and most abundant within canopy gaps. Many shade-tolerant species, such as Eastern hemlock and American beech (Fagus grandifolia), allow only very low quantities of light (<2% full sun) to be transmitted through their canopies (Canham et al., 1994
; this study), suggesting the importance of light availability in determining distributional patterns. Hammen (1993)
also found a positive correlation between light availability and frond density in hayscented fern populations in Rhode Island.
In contrast, in our study abundance patterns of New York fern were not related to light availability but rather tended to increase with soil moisture availability. Nitrogen availability, which was not assessed in this study, may also be an important factor in determining New York fern distribution. Red maple (Acer rubrum) and sugar maple (A. saccharum) trees, under which New York fern tended to be most prevalent, have been shown to be associated with higher soil nitrogen mineralization rates than for other tree species at Great Mountain Forest (Finzi, 1996
). At present neither our data nor published data are sufficient to adequately assess the distribution and abundance patterns of New York fern.
Distribution patterns might also be caused by differences in other abiotic or biotic conditions beneath different canopy tree species as different canopy tree species have been shown to form different understory microhabitats (Crozier and Boerner, 1984
). Several studies have noted lower species richness in the understory of hemlock compared to non-hemlock areas (Hicks, 1980
; Beatty, 1984
). This lower species diversity has been attributed to lower soil pH, poorer light quality and quantity, lower soil moisture availability, and thicker organic layers found beneath hemlock (Hicks, 1980
; Beatty, 1984
).
Despite the general pattern of increased abundance at increased light levels, the observed density of hayscented fern at any given light level was quite variable. This variability might be related to several factors. First, there is potential for physiological integration among ramets (as suggested by Hammen, 1993
). Clonal integration among shaded and unshaded ramets might result in sharing of assimilates, allowing frond density to be higher than can be maintained by the local light environment. Rhizomes of hayscented fern can often persist for long periods of time, in excess of 10 yr (Cody, Hall, and Crompton, 1977
; Hammen, 1993
; J. D. Hill, personal observation), and suggest the potential for long-term physiological integration among fronds in a heterogeneous environment. Although there are no published accounts of physiological integration in hayscented fern, data from our shading experiment provide circumstantial evidence. In a shading experiment, fern census plots (1 m2) were subjected to extreme shade (92% shade, such that all plots received <1% of incident radiation), but ramet connections were left intact (no severing). The rate of decline in frond density we observed was far slower than expected based on known fern abundance–light availability relationships (see Fig. 2B). Clonal integration among shaded and unshaded ramets is one explanation for the slow rate of decline in these experimental shade plots.
Secondly, the ability of ferns to use sunflecks (Hollinger, 1987
; Gildner and Larson, 1992
; Brach, McNaughton, and Raynal, 1993
) may be important. Even if ramets are not physiologically linked, there is strong evidence that ferns can use sunflecks to maintain positive net photosynthetic rates. Hollinger (1987)
showed that 68% of daily photosynthesis in bracken fern (Pteridium aquilinum) occurred during sunflecks (>100 µmol·m-2·s-1). Ferns may in fact respond to sunflecks extremely rapidly with little or no induction period as in Polypodium virginianum (Gildner and Larson, 1992
). It has been demonstrated experimentally that shade-grown hayscented fern can have significantly higher net photosynthetic rates (per dry mass) than sun-grown conspecifics (Brach et al., 1993
). One explanation of this pattern is the ability of shade leaves to use periodic sunflecks (Chazdon and Pearcy, 1991
; Gildner and Larson, 1992
).
Lastly, the ability to respond to sunflecks effectively, with or without physiological integration among ramets, may allow hayscented fern to persist in the forest understory under low light conditions. This persistence coupled with the ability of many temperate forest herbs to reproduce vegetatively rather than sexually (Sobey and Barkhouse, 1977
; Bierzychudek, 1982
) may allow colonization of favorable and retreat from unfavorable environments. In hayscented fern, vegetative spread via the rhizome system (Cody, Hall, and Crompton, 1977
; Hammen, 1993
; Hill, 1996
) might result in rapid colonization of canopy openings as they become available. For example, in our study, we found that local frond density increased dramatically from one season to the next when light was increased (invasion experiment) suggesting the potential for rapid expansion of existing clones. Likewise, in large canopy openings at Hubbard Brook Experimental Forest, Hughes and Fahey (1991)
found that hayscented fern abundance was 5 times higher just 3 yr following canopy opening.
Seasonal and between-year dynamics
Both hayscented and New York fern decreased in abundance over the course of a growing season, with New York fern abundance varying little from year to year, and hayscented fern abundance showing a cyclic pattern. The lack of dramatic changes in frond density over the short-term might indicate that abundance is at or near light-specific equilibrium densities or that the dynamics are slow. However, the fact that plots with highest abundance had the greatest seasonal and between-year decrease in frond density challenges this conclusion. In fact, it has been shown that in areas of low fern density (such as recently cut forests) hayscented fern density and leaf size increase rapidly (this study; Collins and Pickett, 1988
; Hughes and Fahey, 1991
). Similarly, position within clone influences the dynamics as areas at the edge of clones might experience rapid increases in abundance (Hammen, 1993
; Hill, 1996
).
In our study, the seasonal and between-year dynamics of hayscented fern and New York fern were unrelated, and there was no evidence of negative correlations between abundance and dynamics of the two species, suggesting no current competition between the two species. However, initial abundance of New York and hayscented fern are negatively correlated, and the two species co-occurred on a small number of plots (N = 34). These observed differences in distribution might be explained by past competition resulting in niche differentiation, species-specific differences in response to disturbance history, or the limited sample size of the current study. In any case, further detailed studies on the niche requirements of the two species and their response to disturbance will be required to resolve this issue.
Long-term dynamics
Contributing to the success of hayscented fern at the landscape level is the influence of browsing by white-tailed deer (Odocoileus virginianus) (Horsley and Marquis, 1983
). Deer do not browse the unpalatable hayscented fern (Tilghman, 1989
), but browse potential competitors. In the Allegheny Plateau, Hough (1965)
and Rooney and Dress (1997)
documented a shift in forest understory dominance by hobble bush (Viburnum alnifolium) to ferns and herbaceous plants. This shift was attributed to increased browsing on the more palatable competitors of the fern, namely shrubs and tree seedlings (see Horsley and Marquis, 1983
). Thus, over time, increased deer densities have effectively released the fern from competition. This release coupled with increased light in the understory due to forest harvesting and disturbance has resulted in the landscape level increase of hayscented fern. On the other hand, New York fern appears to be less responsive to conditions brought on by disturbance, and this leads to slower dynamics, because of slower rhizome growth rates (R. McCalley and J. D. Hill, unpublished data, Yale University) and thus less rapid spread through the forest understory.
In conclusion, the long-term dynamics of hayscented fern will be dependent on several factors. In the initial stages of succession in a secondary forest, hayscented fern might be quite prevalent (Whitney and Foster, 1988
), if there is a local source of spores or low-level populations from which vegetative spread can occur. However, as succession proceeds, if canopy closure and the dominance of the forest by more shade-tolerant canopy species occurs (Eastern hemlock and American beech), the abundance of hayscented fern in the understory would diminish (Hill, 1996
). This pattern of gradual elimination of hayscented fern from the forest is predicted from the fact that shade-tolerant tree canopies cast deep shade (<2% full sun) with little contribution from sunflecks (<10% of PAR) (see Canham et al., 1994
). However, any event that maintained or promoted openness in the forest canopy such as canopy tree death by disease or windthrow, forest harvesting, or the elimination of a shrub layer by browsing will promote conditions to allow the persistence of hayscented fern.
|
FOOTNOTES |
|---|
The authors thank C. D. Canham, R. L. Chazdon, S. W. Pacala, C. Craddock, and two anonymous reviewers for advice and comments on an earlier draft of the manuscript; the Childs family for their generous hospitality, and for the use of field sites at the Great Mountain Forest; and the Bridgeport Hydraulic Company for access to additional field sites. This research was funded in part by grants from the National Science Foundation (BSR-8918616), Department of Energy (DE-FG02-90ER60933), the National Aeronautics and Space Administration, and the Department of Ecology and Evolutionary Biology of the University of Connecticut. 
2 Author for correspondence, current address: Yale University, Department of Ecology and Evolutionary Biology, P.O. Box 208106, New Haven, Connecticut 06520-8106 USA (jim.hill{at}yale.edu
).
|
LITERATURE CITED |
|---|
|
TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONLITERATURE CITED |
|---|
Beatty, S. W. 1984 Influence of microtopography and canopy species on spatial patterns of forest understory plants. Ecology 65: 1406–1419[CrossRef][ISI]
Bierzychudek, P. 1982 Life histories and demography of shade-tolerant temperate forest herbs: a review. New Phytologist 90: 757–776[CrossRef][ISI]
Brach, A. R., S. J. McNaughton, and D. J. Raynal. 1993 Photosynthetic adaptability of two fern species of a northern hardwood forest. American Fern Journal 83: 47–53[CrossRef]
Bratton, S. P. 1976a Resource division in an understory herb community: responses to temporal and microtopographic gradients. American Naturalist 110: 679–693[CrossRef][ISI]
———. 1976b The response of understory herbs to soil depth gradients in high and low diversity communities. Bulletin of the Torrey Botanical Club 103: 165–172[CrossRef][ISI]
Brewer, R. 1980 A half-century of changes in the herb layer of a climax deciduous forest in Michigan. Journal of Ecology 68: 823–832[CrossRef][ISI]
Canham, C. D. 1988 An index for understory light levels in and around canopy gaps. Ecology 69: 1634–1638[CrossRef][ISI]
———, A. C. Finzi, S. W. Pacala, and D. H. Burbank. 1994 Causes and consequences of resource heterogeneity in forests: interspecific variation in light transmission by canopy trees. Canadian Journal of Forest Research 24: 337–349
Chazdon, R. L., and R. W. Pearcy. 1991 The importance of sunflecks for forest understory plants. BioScience 41: 760–766[CrossRef][ISI]
Cody, W. J., I. V. Hall, and C. W. Crompton. 1977 The biology of Canadian weeds. 26. Dennstaedtia punctilobula (Michx.) Moore. Canadian Journal of Plant Science 57: 1159–1168[ISI]
Collins, B. S., and S. T. A. Pickett. 1988 Demographic responses of herb layer species to experimental canopy gaps in a northern hardwoods forest. Journal of Ecology 76: 437–450[CrossRef]
Collins, S. L., and R. E. Good. 1987 Canopy–ground layer relationships of oak–pine forests in the New Jersey pine barrens. American Midland Naturalist 117: 280–288[CrossRef][ISI]
Conard, H. S. 1908 The structure and life-history of the hay-scented fern. Carnegie Institute of Washington Publication 94: 1–56
Crozier, C. R., and R. E. J. Boerner. 1984 Correlations of understory herb distribution patterns with microhabitats under different tree species in a mixed mesophytic forest. Oecologia 62: 337–343[CrossRef][ISI]
Drew, A. P. 1988 Interference of black cherry by ground flora of the Allegheny uplands. Canadian Journal of Forest Research 18: 652–656[CrossRef]
———. 1990 Fern and aster effects on black cherry shelterwood regeneration. Canadian Journal of Forest Research 20: 1513–1514[CrossRef]
Finzi, A. C. 1996 Causes and consequences of soil resource heterogeneity in a transition oak–northern hardwood forest. Ph.D. dissertation, University of Connecticut, Storrs, Connecticut, USA
Gersper, P. L., and N. Holowaychuk. 1971 Some effects of stemflow from forest canopy trees on chemical properties of soils. Ecology 52: 691–702[CrossRef][ISI]
Gildner, B. S., and D. W. Larson. 1992 Photosynthetic response to sunflecks in the desiccation-tolerant fern Polypodium virginianum. Oecologia 89: 390–396
Gray, A. N., and T. A. Spies. 1995 Water content in forest soils and decayed wood using time domain reflectometry. Canadian Journal of Forest Research 25: 376–385
Groninger, J. W., and L. H. McCormick. 1991 Effects of sulfometuron on hay-scented fern spore emergence. Canadian Journal of Forest Research 21: 942–943[CrossRef]
Hammen, S. C. L. 1993 Density-dependent phenotypic variation in the hay-scented fern, Dennstaedtia punctilobula. Bulletin of the Torrey Botanical Club 120: 392–396
Hicks, D. J. 1980 Intrastand distribution patterns of southern Appalachian cove forest herbaceous species. American Midland Naturalist 104: 209–233[CrossRef][ISI]
Hill, J. D. 1996 Population dynamics of hayscented fern (Dennstaedtia punctilobula) and its impact on forest composition, structure and dynamics. Ph.D. dissertation, University of Connecticut, Storrs, USA
Hollinger, D. Y. 1987 Photosynthesis and stomatal conductance patterns of two fern species from different forest understoreys. Journal of Ecology 75: 925–935[CrossRef][ISI]
Horsley, S. B. 1977a Allelopathic inhibition of black cherry by fern, grass, goldenrod, and aster. Canadian Journal of Forest Research 7: 205–216
———. 1977b Allelopathic inhibition of black cherry. II. Inhibition by woodland grass, ferns, and club moss. Canadian Journal of Forest Research 7: 515–519[CrossRef]
———. 1984 Hayscented fern rhizome development in uncut and thinned Allegheny hardwood stands. American Journal of Botany 71: 80–81 (Abstract)[CrossRef][ISI]
———. 1985 Reforestation of orchard stands and savannahs of Pennsylvania's Allegheny Plateau. Northern Journal of Applied Forestry 4: 22–26
———. 1993 Mechanisms of interference between hayscented fern and black cherry. Canadian Journal of Forest Research 23: 2059–2069[CrossRef]
———, and D. A. Marquis. 1983 Interference by weeds and deer with Allegheny hardwood reproduction. Canadian Journal of Forest Research 13: 61–69
Hough, A. F. 1965 A twenty-year record of understory vegetational change in a virgin Pennsylvania forest. Ecology 46: 370–373[CrossRef][ISI]
Hughes, J. W., and T. J. Fahey. 1991 Colonization dynamics of herbs and shrubs in a disturbed northern hardwood forest. Journal of Ecology 79: 605–616[CrossRef]
Kolb, T. E., T. W. Bowersox, and L. H. McCormick. 1990 Influences of light intensity on weed-induced stresses of tree seedlings. Canadian Journal of Forest Research 20: 503–507
Lellinger, D. B. 1985 A field manual to the ferns and fern-allies of the United States and Canada. Smithsonian Institution Press, Washington, D.C., USA
Lipscomb, M. V., and E. T. Nilsen. 1990a Environmental and physiological factors influencing the natural distribution of evergreen and deciduous ericaceous shrubs on northeast and southwest slopes of the southern Appalachian Mountains. I. Irradiance tolerance. American Journal of Botany 77: 108–115[CrossRef][ISI]
———, and ———. 1990b Environmental and physiological factors influencing the natural distribution of evergreen and deciduous ericaceous shrubs on northeast and southwest slopes of the southern Appalachian Mountains. II. Water relations. American Journal of Botany 77: 517–526[CrossRef][ISI]
Lodhi, M. A. K. 1977 The influence and comparison of individual forest trees on soil properties and possible inhibition of nitrification due to intact vegetation. American Journal of Botany 64: 260–264[CrossRef][ISI]
Maguire, D. A., and R. T. T. Forman. 1983 Herb cover effects on the tree seedling patterns in a mature hemlock–hardwood forest. Ecology 64: 1367–1380[CrossRef][ISI]
Moore, M. R., and J. L. Vankat. 1986 Responses of the herb layer to the gap dynamics of a mature beech–maple forest. American Midland Naturalist 115: 336–347[CrossRef][ISI]
Phillips, D. L., and W. H. Murdy. 1985 Effects of rhododendron (Rhododendron maximum L.) on regeneration of southern Appalachian hardwoods. Forest Science 31: 226–233
Plocher, A. E., and K. L. Carvell. 1987 Population dynamics of rosebay rhododendron thickets in the southern Appalachians. Bulletin of the Torrey Botanical Club 114: 121–126[CrossRef][ISI]
Rice, E. L., and S. K. Pancholy. 1973 Inhibition of nitrification by climax vegetation. II. Additional evidence and possible role of tannins. American Journal of Botany 60: 691–702[CrossRef][ISI]
Rooney, T. P., and W. J. Dress. 1997 Species loss over sixty-six years in the ground layer vegetation of heart's content, an old-growth forest in Pennsylvania, USA. Natural Areas Journal 17: 297–305[ISI]
SAS. 1987 SAS/STAT guide for personal computers, version 6 edition. SAS Institute, Cary, North Carolina, USA
Sobey, D. G., and P. Barkhouse. 1977 The structure and rate of growth of the rhizomes of some forest herbs and dwarf shrubs of the New Brunswick–Nova Scotia border region. Canadian Field Naturalist 91: 377–383[ISI]
Tilghman, N. G. 1989 Impacts of white-tailed deer on forest regeneration in northwestern Pennsylvania. Journal of Wildlife Management 53: 524–532[CrossRef][ISI]
Whitney, G. G., and D. R. Foster. 1988 Overstorey composition and age as determinants of the understorey flora of woods of central New England. Journal of Ecology 76: 867–876[CrossRef][ISI]
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
