Central Appalachian Hardwood Forest Research Articles

Role of Tree Species, the Herb Layer and Watershed Characteristics in Nitrate Assimilation in a Central Appalachian Hardwood Forest

Forest plants that can assimilate nitrate may act as nitrate sink and, consequently, reduce nitrate losses from watershed ecosystems through leaching. This study, conducted at the Fernow Experimental Forest in West Virginia, quantified via nitrogen reductase activity (NRA) the nitrate assimilation of two tree species, red maple and sugar maple, and surrounding common herb-layer species at the tissue (foliage, roots) and plot level. NRA measurements were conducted in summer and spring. Furthermore, NRA was quantified under varying levels of soil nitrate availability due to fertilization, different stages in secondary forest succession, and watershed aspect. This study confirmed that NRA of mature maples does not respond to varying levels of soil nitrate availability. However, some herb-layer species’ NRA did increase with nitrogen fertilization, and it may be greater in spring than in summer. Combined with biomass, the herb layer’s NRA at the plot-level (NRAA) comprised 9 to 41% of the total (tree + herb-layer) foliar NRAA during the growing season. This demonstrates that the herb layer contributes to nitrate assimilation disproportionally to its small biomass in the forest and may provide a vernal dam to nitrate loss not only by its early presence but also by increased spring NRA relative to summer.

Nitrogen Fertilization, Stand Age, and Overstory Tree Species Impact the Herbaceous Layer in a Central Appalachian Hardwood Forest

Research Highlights: Herb-layer community composition, abundance, species richness, and Shannon–Wiener diversity index are shaped by nitrogen fertilization, disturbance history, and the overstory tree species in its immediate vicinity. Background and Objectives: While the herbaceous layer in deciduous forests is increasingly recognized for its importance in various aspects of forest ecosystem function, this study sought to describe the factors impacting the herbaceous layer. Specifically, this study’s objective was to quantify and compare herb-layer species composition, cover, and other community indices in watersheds with (a) different levels of N deposition, (b) different stand ages due to differing disturbance histories, and (c) different watershed aspects. This study also tested the hypothesis that herb-layer characteristics vary beneath tree species with contrasting nutrient dynamics (i.e., red and sugar maple). Materials and Methods: At the Fernow Experimental Forest in West Virginia (USA), the cover of all herb-layer species was recorded directly under nine red maple and nine sugar maple trees in each of four watersheds (WS): long-term fertilized WS3 and unfertilized WS7, both with a stand age of about 50 years, and two unmanaged watersheds with 110-year-old stands and opposite watershed aspects (south-facing WS10, north-facing WS13). Community composition and plot-level indices of diversity were evaluated with multivariate analysis and ANOVA for watershed-level differences, effects of the maple species, and other environmental factors. Results: In the fertilized watershed (WS3), herb-layer diversity indices were lower than in the unfertilized watershed of the same stand age (WS7). In the unfertilized watershed with the 50-year-old stand (WS7), herb-layer diversity indices were higher than in the watershed with the 110-year-old stand of the same watershed aspect (WS13). WS10 and WS13 had similar herb-layer characteristics despite opposite watershed aspects. The presence of sugar maple corresponded to higher cover and diversity indices of the herb-layer in some of the watersheds. Conclusions: Despite the limitations of a case study, these findings bear relevance to future forest management since the forest herb layer plays important roles in deciduous forests through its influence on nutrient cycling, productivity, and overstory regeneration.

Response of soil fertility to 25 years of experimental acidification in a temperate hardwood forest.

The effects of enhanced acid deposition from the atmosphere, and associated elevated inputs of N, are widely evident, especially for forests where excess N has led to a variety of deleterious effects. These include declines in biodiversity, a response that will likely require considerable time for recovery. The purpose of this study was to determine responses of plant nutrient availability in surface mineral soil to 25 yr of experimental acidification and N addition in a central Appalachian hardwood forest ecosystem. We hypothesized that chronic additions of (NH4 )2 SO4 will increase mineral N, decrease soil pH, P, and base cations, increase micronutrients (Mn2+ and Fe2+ ), and increase levels of Al3+ . Results supported these predictions, although Mn2+ did not vary significantly. Earlier work on these plots found no response of any of the extractable nutrients to 3 yr of treatment, yet after 25 yr, our results suggest that impacts are apparent in the top 5cm of the A horizon. We surmise that impacts in these soils may have lagged behind the onset of acidification treatments or that several years of treatment were required to overcome preexisting differences in soil ions. Generally, current findings confirm that (NH4 )2 SO4 treatments have lowered the pH, enhanced levels of exchangeable Al3+ , and increased stream-water exports of NO3 - and base cations-a process that further acidifies soil. The combination of these changes in surface soils, with their high proportion of fine roots, may contribute to the reduced growth and competitiveness of some hardwood species at the acidified site.

Nitrogen (N) Dynamics in the Mineral Soil of a Central Appalachian Hardwood Forest During a Quarter Century of Whole-Watershed N Additions

The structure and function of terrestrial ecosystems are maintained by processes that vary with temporal and spatial scale. This study examined temporal and spatial patterns of net nitrogen (N) mineralization and nitrification in mineral soil of three watersheds at the Fernow Experimental Forest, WV: 2 untreated watersheds and 1 watershed receiving aerial applications of N over a 25-year period. Soil was sampled to 5 cm from each of seven plots per watershed and placed in two polyethylene bags—one bag brought to the laboratory for extraction/analysis, and the other bag incubated in situ at a 5 cm depth monthly during growing seasons of 1993–1995, 2002, 2005, 2007–2014. Spatial patterns of net N mineralization and nitrification changed in all watersheds, but were especially evident in the treated watershed, with spatial variability changing non-monotonically, increasing then decreasing markedly. These results support a prediction of the N homogeneity hypothesis that increasing N loads will increase spatial homogeneity in N processing. Temporal patterns for net N mineralization and nitrification were similar for all watersheds, with rates increasing about 25–30% from 1993 to 1995, decreasing by more than 50% by 2005, and then increasing significantly to 2014. The best predictor of these synchronous temporal patterns across all watersheds was number of degree days below 19°C, a value similar to published temperature maxima for net rates of N mineralization and nitrification for these soils. The lack of persistent, detectable differences in net nitrification between watersheds is surprising because fertilization has maintained higher stream-water nitrate concentrations than in the reference watersheds. Lack of differences in net nitrification among watersheds suggests that N-enhanced stream-water nitrate following N fertilization may be the result of a reduced biotic demand for nitrate following fertilization with ammonium sulfate.

Response of foliar nutrients of Rubus allegheniensis to nutrient amendments in a central Appalachian hardwood forest

Abstract Foliar nutrient concentrations are often useful in inferring soil fertility and plant-soil interactions. This study examined pre- and post-senescent foliar macronutrients and Mn of Rubus allegheniensis to assess potential effects of two nutrient amendments on mineral soil nutrient availability in aggrading hardwood stands: additions of (NH4)2SO4 (simulating increases in acid deposition) and additions of (NH4)2SO4 with dolomitic limestone (simulating mitigation of effects of acidification). Foliar Mn analysis further addressed a prediction of the nutrient redistribution hypothesis, i.e., Rubus accumulates Mn in pre-senescent leaves and further concentrates Mn in post-senescent leaves to produce high-Mn litter. The study used a four plot × four block randomized design with three experimental treatments (Control, NS, NS + Lime). Rubus leaves were harvested randomly in plots of all treatment before and after senescence. Experimental treatments appeared to increase availability of N and P, with little effect on K. Simulated acidification decreased availability of Ca and Mg, an effect mitigated for Mg by addition of dolomitic limestone. By contrast, Mn in Rubus foliage was up to 100 times more concentrated than in most plant species, and was further concentrated by ∼60% during senescence. These latter observations support predictions of the nutrient redistribution hypothesis that Rubus redistributes Mn from the rooting zone to surface soil.

Twenty‐five‐year response of the herbaceous layer of a temperate hardwood forest to elevated nitrogen deposition

AbstractIncreasing rates of atmospheric deposition of nitrogen (N) present a novel threat to the biodiversity of terrestrial ecosystems. Many forests are particularly susceptible to excess N given their proximity to sources of anthropogenic N emissions. This study summarizes results of a 25‐yr treatment of an entire central Appalachian hardwood forest watershed via aerial applications of N with a focus on effects of added N on the cover, species richness, and composition of the herbaceous layer. Research was carried out on two watersheds of the Fernow Experimental Forest (FEF), West Virginia. The long‐term reference watershed at FEF (WS4) was used as a reference; WS3 was experimentally treated, receiving three aerial applications of N per year as (NH4)2SO4 totaling 35 kg N ha−1 yr−1, beginning in 1989. Cover of the herbaceous layer (vascular plants ≤1 m in height) was estimated visually in five circular 1‐m2 subplots within each of seven circular 400‐m2 sample plots spanning all aspects and elevations of each watershed. Sampling was carried out in early July of each of the following years: 1991, 1992, 1994, 2003, and 2009—2014, yielding 10 yr of data collected over a 23‐yr period. It was anticipated that the N treatment on WS3 would decrease species richness and alter herb layer composition by enhancing cover of a few nitrophilic species at the expense of numerous N‐efficient species. Following a period of minimal response from 1991 to 1994, cover of the herb layer increased substantially on N‐treated WS3, and remained high thereafter. There was also a coincidental decrease in herb layer diversity during this period, along with a sharp divergence in community composition between WS4 and WS3. Most changes appear to have arisen from unprecedented, N‐mediated increases of Rubus spp., which are normally associated with the high‐light environment of openings, rather than beneath intact forest canopies. These findings support the prediction that N‐mediated changes in the herbaceous layer of impacted forests are driven primarily by increases in nitrophilic species.

Raccoon Spatial Requirements and Multi-Scale Habitat Selection within an Intensively Managed Central Appalachian Forest

Abstract We studied a raccoon (Procyon lotor) population within a managed central Appalachian hardwood forest in West Virginia to investigate the effects of intensive forest management on raccoon spatial requirements and habitat selection. Raccoon home-range (95% utilization distribution) and core-area (50% utilization distribution) size differed between sexes with males maintaining larger (2×) home ranges and core areas than females. Home-range and core-area size did not differ between seasons for either sex. We used compositional analysis to quantify raccoon selection of six different habitat types at multiple spatial scales. Raccoons selected riparian corridors (riparian management zones [RMZ]) and intact forests (> 70 y old) at the core-area spatial scale. RMZs likely were used by raccoons because they provided abundant denning resources (i.e., large-diameter trees) as well as access to water. Habitat composition associated with raccoon foraging locations indicated selection for intact forests, ripari...

Long-term effects of experimental forest harvesting on abundance and reproductive demography of terrestrial salamanders

Both observational and experimental studies have documented drastic reductions in salamanders after forest harvesting. Yet, the amount of time until salamander populations rebound and the factors limiting recovery after harvesting are unknown. We compared the effects of six oak regeneration practices to a control, representing a disturbance gradient from no treatment to silvicultural clearcut, on the relative abundance and reproductive demography of terrestrial salamanders through 13-years post-harvest. Following the experimental disturbance, relative abundance of terrestrial salamanders in treatments that opened the canopy were significantly and persistently lower than in either untreated control stands or midstory herbicide treatments. In general, this trend persisted through 9–13 years after treatment. Changes to demography of commonly captured salamanders varied by species, but Plethodon cinereus had a greater proportion of juveniles in unharvested treatments 7–13-years post-harvest, and Desmognathus ochrophaeus had a greater proportion of juveniles and a greater number of eggs/female in unharvested treatments 1–6-years post-harvest. Population modeling of P. cinereus indicated that adult survival had the greatest elasticity of the vital rates. Further, >60 years may be needed before P. cinereus reduced by the observed decline could reach pre-harvest levels of abundance. Of the treatments with canopy disturbance, the group selection harvest had the greatest abundances of salamanders 7–13-years post-harvest, but when coupled with future stand entries, the volume of wood fiber extracted, costs of harvesting, reduced sprouting of oaks, and soil disturbances, this method may not have the best balance of ecological and economic sustainability in central Appalachian hardwood forest.

Changes in Soil Bulk Density Resulting from Construction and Conventional Cable Skidding Using Preplanned Skid Trails

Abstract A harvesting system consisting of chainsaw felling and cable skidder extraction was studied to determine soil bulk density changes in a central Appalachian hardwood forest site. Soil bulk density was measured using a nuclear gauge preharvest and postharvest systematically across the harvest site, on transects across skid trails, and for a subset of skid trail transects closest to log landing after each of the first ten loaded machine passes. Bulk density was also measured in skid trails after their construction but prior to skidding. Bulk density did not change significantly across the harvest site, because the extraction equipment stayed on the preplanned skid trails. Bulk density increased on the skid trails as a result of construction by crawler bulldozer and during skidding. Bulk density in the skid trail increased by 30% because of construction by a crawler bulldozer. Fifty-five percent of the increase in bulk density attributable to skidding occurred after one loaded pass, and 80% of the bulk density increase was experienced after two loaded passes. Bulk density increased by only 5% between passes five and ten.

Effects of atmospheric nitrogen deposition on the herbaceous layer of a central Appalachian hardwood forest

Gilliam, F. S., A. W. Hockenberry (Department of Biological Sciences, Marshall University, Huntington, WV, 25755-2510), and M. B. Adams (U.S.D.A. Forest Service, Timber and Watershed Laboratory, Parsons, WV, 26287-0404). Effects of atmospheric nitrogen deposition on the herbaceous layer of a central Appalachian hardwood forest. J. Torrey Bot. Soc. 133(2): 240–254. 2006.—Additions of nitrogen (N) have been shown to alter species diversity of plant communities, with most experimental studies having been carried out in communities dominated by herbaceous species. We examined seasonal and inter-annual patterns of change in the herbaceous layer of two watersheds of a central Appalachian hardwood forest that differed in experimental N treatment. This study was carried out at the Fernow Experimental Forest, West Virginia, using two adjacent watersheds: WS4 (mature, second-growth hardwood stand, untreated reference), and WS3 (∼25-yr old, treated with 35 kg N ha−1 yr−1 as (NH4)2SO4 yr−1). Seven circular 0.04-ha sample plots were established in each watershed to represent its full range of elevation and slope aspect. The herbaceous layer was sampled by identifying and visually estimating cover (%) of all vascular plants ≤ 1 m in height within 10 1-m2 circular sub-plots in each sample plot, for a total of 140 1-m2 sub-plots. Sampling was carried out in mid-July of 1991 and repeated at approximately the same time in 1992. In 1994, these same plots were sampled each month from May to October. Seasonal patterns of herb layer dynamics were assessed for the complete 1994 data set, whereas inter-annual variability was based on plot data from 1991, 1992, and the July sample of 1994. There were no significant differences between watersheds for any sample year for any of the other herb layer characteristics measured, including herb layer cover, species richness, evenness, and diversity. Cover on WS4 decreased significantly from 1991 to 1992, followed by no change to 1994. By contrast, herb layer cover did not vary significantly across years on WS3. Cover of the herbaceous layer of both watersheds increased from early in the growing season to the middle of the growing season, decreasing thereafter, with no significant differences between WS3 and WS4 for any of the monthly cover means in 1994. Similar seasonal patterns found for herb layer cover—and lack of significant differences between watersheds—were also evident for species diversity and richness. By contrast, there was little seasonal change in herb layer species evenness, which was nearly identical between watersheds for all months except October. Seasonal patterns for individual species/species groups were closely similar between watersheds, especially for Viola rotundifolia and Viola spp. Species richness and species diversity were linearly related to herb layer cover for both WS3 and WS4, suggesting that spatial and temporal increases in cover were more related to recruitment of herb layer species than to growth of existing species. Results of this study indicate that there have been negligible responses of the herb layer to 6 yr of N additions to WS3.

Effects of Silvicultural Practices on Soil Carbon and Nitrogen in a Nitrogen Saturated Central Appalachian (USA) Hardwood Forest Ecosystem

Silvicultural treatments represent disturbances to forest ecosystems often resulting in transient increases in net nitrification and leaching of nitrate and base cations from the soil. Response of soil carbon (C) is more complex, decreasing from enhanced soil respiration and increasing from enhanced postharvest inputs of detritus. Because nitrogen (N) saturation can have similar effects on cation mobility, timber harvesting in N-saturated forests may contribute to a decline in both soil C and base cation fertility, decreasing tree growth. Although studies have addressed effects of either forest harvesting or N saturation separately, few data exist on their combined effects. Our study examined the responses of soil C and N to several commercially used silvicultural treatments within the Fernow Experimental Forest, West Virginia, USA, a site with N-saturated soils. Soil analyses included soil organic matter (SOM), C, N, C/N ratios, pH, and net nitrification. We hypothesized the following gradient of disturbance intensity among silvicultural practices (from most to least intense): even-age with intensive harvesting (EA-I), even-age with extensive harvesting, even-age with commercial harvesting, diameter limit, and single-tree harvesting (ST). We anticipated that effects on soil C and N would be greatest for EA-I and least with ST. Tree species exhibited a response to the gradient of disturbance intensity, with early successional species more predominant in high-intensity treatments and late successional species more predominant in low-intensity treatments. Results for soil variables, however, generally did not support our predictions, with few significant differences among treatments and between treatments and their paired controls for any of the measured soil variables. Multiple regression indicated that the best predictors for net nitrification among samples were SOM (positive relationship) and pH (negative relationship). This finding confirms the challenge of sustainable management of N-saturated forests.

Effects of harvesting on herbaceous layer diversity of a central Appalachian hardwood forest in West Virginia, USA

Clearcutting is a common harvesting practice in many eastern hardwood forests. Among the vegetation strata of these forests, the herbaceous layer is potentially the most sensitive in its response to harvest-mediated disturbances and has the highest species diversity. Thus, it is important to understand the response of herbaceous layer diversity to forest harvesting. Previous work on clearcut and mature stands at the Fernow Experimental Forest (FEF), West Virginia, has shown that, although, harvesting did not alter appreciably herbaceous layer cover, it influenced the relationship of cover to biotic and abiotic factors, such as tree density and soil nutrients, respectively. The purpose of this study was to examine the response of species diversity of the herbaceous layer to harvesting at FEF. Fifteen circular, 0.04 ha sample plots were established in each of four watersheds (60 plots in total) representing two stand age categories: two watersheds with 20 years even-age stands following clearcutting and two watersheds with mature second growth stands. All woody stems ≥2.5 cm diameter at breast height were identified, tallied, and measured for diameter. The herbaceous layer was sampled by identifying all vascular plants ≤1 m in height and estimating cover for each species in each of 10 (1 m 2) circular sub-plots per sample plot (600 sub-plots total). Species diversity for each plot was calculated from herbaceous layer data using the ln-based Shannon Index ( H′) equation. Ten stand and soil variables also were measured on each plot. Mean herbaceous layer cover for clearcut versus mature stands was 27.2±14.3% versus 20.2±8.1% ( P>0.05), respectively and mean H′ was 1.67±0.42 versus 1.55±0.48 ( P>0.05), respectively. Herbaceous layer diversity was negatively correlated with cation exchange capacity and extractable Ca and Mg in the mineral soil in clearcut stands. In contrast, herbaceous layer diversity was positively correlated with soil organic matter and clay content. Although, 20 years of recovery after clearcutting did not have significant effects on the species diversity of the herbaceous layer when examining stand age means alone, harvesting did appear to influence the spatial relationships between herbaceous layer diversity and biotic factors (e.g. tree density) and abiotic factors (e.g. soil nutrients).

Temporal and spatial variation of nitrogen transformations in nitrogen-saturated soils of a central Appalachian hardwood forest

We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreated); and WS3 (24-year-old, treated with (NH4)2SO4 since 1989 at the rate of 35 kg N·ha–1·year–1). Net nitrification was 141, 114, and 115 kg N·ha–1·year–1, for WS3, WS4, and WS7, respectively, essentially 100% of net N mineralization for all watersheds. Temporal (seasonal) patterns of nitrification were significantly related to soil moisture and ambient temperature in untreated watersheds only. Spatial patterns of soil water NO3– of WS4 suggest that microenvironmental variability limits rates of N processing in some areas of this N-saturated watershed, in part by ericaceous species in the herbaceous layer. Spatial patterns of soil water NO3– in treated WS3 suggest that later stages of N saturation may result in higher concentrations with less spatial variability. Spatial variability in soil N variables was lower in treated WS3 versus untreated watersheds. Nitrogen additions have altered the response of N-processing microbes to environmental factors, becoming less sensitive to seasonal changes in soil moisture and temperature. Biotic processes responsible for regulating N dynamics may be compromised in N-saturated forest ecosystems.

Temporal and spatial variation of nitrogen transformations in nitrogen-saturated soils of a central Appalachian hardwood forest

We studied temporal and spatial patterns of soil nitrogen (N) dynamics from 1993 to 1995 in three watersheds of Fernow Experimental Forest, W.V.: WS7 (24-year-old, untreated); WS4 (mature, untreate...

Symptoms of nitrogen saturation in two central Appalachian hardwood forest ecosystems

By synthesizing more than twenty years of research at the Fernow Experimental Forest, we have documented 7 symptoms of nitrogen saturation in two adjacent watersheds. The symptoms include: 1) high relative rates of net nitrification, 2) long-term increases in stream-water concentrations of nitrate and base cations, 3) relatively high nitrate concentrations in solution losses, 4) little seasonal variability in stream-water nitrate concentrations, 5) a high discharge of nitrate from a young aggrading forest, 6) a rapid increase in nitrate loss following fertilization of a young aggrading forest, and 7) low retention of inorganic nitrogen when compared with other forested sites. These data support current conceptual models of nitrogen saturation and provide a strong, and perhaps the best, example of nitrogen saturation in the United States.

Herbaceous Layer and Soil Response to Experimental Acidification in a Central Appalachian Hardwood Forest

AbstractThe herbaceous layer (vascular plants ≤1 in in height) is an important component of forest ecosystems and a potentially sensitive vegetation stratum in response to acid deposition. This study tested several hypotheses concerning soil and herbaceous layer response to experimental acidification at the Fernow Experimental Forest in north‐central West Virginia. Fifteen circular sample plots (0.04 ha) were established in each of three watersheds: WS3 (an ≈ 20‐yr‐old watershed receiving acidification treatment with (NH4)2SO4), WS4 (>80‐yr‐old control), and WS7 (≈ 20‐yr‐old control). The herb layer was sampled intensively in 10 1‐m2 subplots within each sample plot, including determination of species composition, cover, and random biomass harvests. Harvested plant material was separated by species and analyzed for macronutrients, micronutrients, and Al. Soft was sampled from harvest subplots and analyzed for texture, pH, organic matter, and macro‐ and micronutrients. Few differences among watersheds for virtually all measured soil variables indicated minimal response of soil fertility to the acidification treatment. The herbaceous layer was also quite similar among watersheds with respect to cover‐biomass and species diversity; WS7, however, had ≈ 70% higher herb layer cover that both Ws3 and WS4, a result of the predominance of a few high‐cover fern species and attributable to the north‐facing aspect of WS7 vs. south‐facing aspects of WS3/WS4. There was a high degree of species similarity among watersheds, suggesting no shift in species composition in response to acidification. There was also minimal response of element concentrations to acidification, although Fe and Al exhibited evidence of increased uptake in WS3. We conclude that, contrary to our expectations, there has been little substantive response of the soil and herb layer to acidification, but hypothesize that herbaceous layer species may experience toxicity problems with increased mobility of Al and micronutrients in the future.

Herbaceous Layer Cover and Biomass in a Young Versus a Mature Stand of a Central Appalachian Hardwood Forest

GILLIAM, F. S. AND N. L. TURRILL (Department of Biological Sciences, Marshall University, Huntington, WV 25755). Herbaceous layer cover and biomass in a young versus a mature stand of a central Appalachian hardwood forest. Bull. Torrey Bot. Club 120: 445-450. 1993.-This study examined herb layer vegetation (all vascular plants c 1 m in height) of two montane watersheds of different stand ages in the Femow Experimental Forest, in north-central West Virginia (WS3, -20 yr; WS4, >80 yr). Mean herb layer cover was 19.3 and 26.4% for WS3 and WS4, respectively. Herb layer biomass was significantly (P < 0.00 1) correlated with herb cover for both watersheds. Mean herb layer biomass was 8.1 and 11.6 g/m2 for WS3 and WS4, respectively. Herb cover responded positively to elevation on both watersheds. Within WS3, herb cover was only weakly correlated with canopy characteristics, but was positively correlated with soil pH, organic matter, sand content, cation exchange capacity, Ca, K, Mg, and N03-N. Within WS4, herb cover was negatively correlated with understory basal area and density, but not significantly correlated with any soil variables except Ca. From these data we suggest a hypothesis that in early forest succession in these ecosystems, early herb layer development is influenced greatly by allogenic factors (such as soil fertility) but that autogenic factors, such as canopy closure, become more important as the stand matures and becomes more stratified.