Christopher R. Webster

MINIMUM OPENING SIZES FOR CANOPY RECRUITMENT OF MIDTOLERANT TREE SPECIES: A RETROSPECTIVE APPROACH

Minimum opening sizes and time required for successful canopy recruitment of tree saplings are not well known because the gap capture process is slow and difficult to monitor. In this study, we investigated canopy recruitment in harvest openings created by group selection in hemlock–hardwood forests to determine if yellow birch (Betula alleghaniensis Britt.), a species with intermediate shade tolerance, could successfully reach the canopy via small openings. We measured gap trees in openings ranging from 40 to 1000 m2 and up to 55 years old. Sample trees up to 25 m tall, spanning all stages of the gap capture process, were destructively sampled to determine height–growth rates, total age, and size at time of gap formation. Results indicate that yellow birch comprised >40% of the upper stratum of gap saplings in openings as small as 100–400 m2. The ultimately successful hardwood gap trees, on average, reached crown shoulder height of the mature gap border trees (73% of total canopy height) within 28–37 yea...

Ungulate Carcasses Perforate Ecological Filters and Create Biogeochemical Hotspots in Forest Herbaceous Layers Allowing Trees a Competitive Advantage

Ecological filters and nutrient heterogeneity are important in the function of ecosystems. Herbaceous layers alter forest ecosystems by filtering tree species during early stages of tree reproduction and influencing nutrient cycling. Important aspects about how tree species successfully establish below and extend above this ecological filter are unanswered in forest ecology. We experimentally tested the effects of large ungulate carcasses on the filtering function of herbaceous layers. Even well-utilized carcasses created unexpected disturbances that reduced herbaceous cover, which effectively perforated the herbaceous layer filter that can differentially influence tree reproduction. Carcasses also created lasting biogeochemical “hotspots” in forest soils that may help maintain plant biodiversity by creating resource heterogeneity and shifting competitive relationships. Because the spatial distribution of carcasses is influenced by predators, these data establish an unrecognized link between large carnivores, prey carcasses, and ecosystem processes. This link supports a novel understanding of disturbance by large herbivores in forest ecosystems by demonstrating an important interaction between predator–prey functional traits and tree seedling dynamics on either side of a major ecological filter.

Comparative growing space efficiency of four tree species in mixed conifer–hardwood forests

The influence of shade tolerance, canopy position, and tree size on growing space efficiency (GSE) in mixed stands of co-occurring conifer and hardwood species was investigated in hemlock–northern hardwood forests. Three alternative measures of two-dimensional growing space—total crown area (TCA), exposed crown area (ECA), and a projection of the total available growing space (AGS)—were investigated to clarify the comparative importance of shaded and illuminated crown regions and unoccupied space in the forest canopy. GSE was expressed as ratios of stem volume increment and biomass increment per unit of growing space. Late-successional, shade-tolerant species have often been portrayed as slow growing, inefficient users of their growing space; however, hemlock (Tsuga canadensis), which is one of the most shade-tolerant conifers in North America, was the most efficient canopy tree in our sample across all measures of GSE. Likewise, the mid-tolerant yellow birch (Betula alleghaniensis) tended to be less efficient than the more shade-tolerant maples (Acer rubrum and Acer saccharum). For all species, volume increment per unit of growing space increased with increasing tree height and canopy position, but within a given stratum decreased with increasing crown size. The relative efficiency of each species did not appear to be influenced by the measure of growing space employed. In most cases, volume and biomass increments per unit of ECA and AGS were significantly greater (p<0.05) for intermediate than dominant crown class trees. However, for a given level of ECA or AGS, efficiency did increase with increasing relative height, which suggests that efficiency is influenced by the relative vertical position of growing space in the forest canopy. In general, the shaded area of a crown (i.e., TCA−ECA) was not a significant predictor of volume increment once height and ECA were known, suggesting that once 100% canopy closure is reached, packing trees more tightly may not increase stand-level production. However, mean volume increment per unit of TCA scaled more accurately to the stand-level than mean volume increment per unit of ECA. Potential scaling problems associated with mixed-species stands are discussed.

Vegetative legacy of a protected deer herd in Cades Cove, Great Smoky Mountains National Park.

Abstract Overabundant deer populations pose significant challenges to the conservation of native plant communities. We examined the effects of a protected white-tailed deer (Odocoileus virginianus Zimm.) population on forest plant communities in Cades Cove, Great Smoky Mountains National Park. The Cove deer herd reached 43 deer km−2 at its peak in the late 1970s. While current densities are somewhat lower, the diversity and coverage of many understory species remains greatly reduced. To elucidate the influence of contemporary deer browsing, an exclosure study was established in 1997. Ten 10-m × 10- m sample plots were randomly located at three sites, half of which per site were exclosed. Understory vegetation on all 30 plots was intensively sampled in August 2004. While the diversity and richness of herbaceous species was similar in exclosures and controls following eight years of deer exclusion, exclosures contained significantly more diverse sapling layers (tree species ≤2 m in height). Correspondingly, exclosures contained significantly greater sapling densities than control plots (P < 0.001). Herbaceous and sapling layer composition varied between exclosures and controls. Forbs, vines, and low woody shrubs were consistently more abundant within exclosures than control plots. Graminoids (both native and exotic) dominated both exclosures and controls, but typically had greater relative abundance on control plots. Given the modest recovery of the herbaceous layer and the persistence of non-preferred browse species following eight years of deer exclusion, plant community recovery would be slow and uncertain even if the density of deer within the Cove was reduced.

Broadening the ecological context of ungulate–ecosystem interactions: the importance of space, seasonality, and nitrogen

Spatial heterogeneity of soil resources, particularly nitrogen availability, affects herbaceous-layer cover and diversity in temperate forest ecosystems. Current hypotheses predict that ungulate herbivores influence nitrogen availability at the stand scale, but how ungulates affect nitrogen availability at finer spatial scales that are relevant to the herb layer is less understood. We tested the hypothesis that ungulate exclusion reduces the spatial complexity of nitrogen availability at neighborhood scales (1-26 m) apart from mean stand scale effects. This outcome was expected due to a lack of ungulate nitrogenous waste deposition within exclosures and seasonally variable ungulate habitat use. To test this hypothesis we examined spatial patterning of ammonium and nitrate availability, herb-layer cover and diversity, and under-canopy solar radiation using geostatistical models. Our study sites included six stands of eastern hemlock (Tsuga canadensis) forest: three where white-tailed deer (Odocoileus virginianus) were excluded and three that were accessible to deer. Where deer were present, patch sizes of ammonium availability, cover, and diversity were smaller compared to deer exclosures, whereas mean site-level effects were not significant. Within deer exclosures cover and solar radiation were more similar in patch size than were cover and nitrogen availability. Our results suggest that browsing ungulates affect spatial patterns of herb-layer cover and diversity through the excretion of nitrogenous wastes in small, discrete patches. Ungulate-excreted nitrogen deposition and herbivory were concentrated in the dormant season, allowing herb-layer plants a greater opportunity to benefit from nitrogen additions. Therefore, the impact of ungulates on nitrogen cycling in forest ecosystems varies with spatial scale and the seasonal timing of ungulate impacts. In this way, ungulates may function as a seasonally dependent link between fine-scale and landscape-level ecological processes.

Herbaceous-layer impoverishment in a post-agricultural southern Appalachian landscape

Abstract Large ungulates are an important driver of plant community composition and structure. In eastern North America, white-tailed deer (Odocoileus virginianus) thrive in agricultural mosaics and fragmented forested landscapes, at times reaching unprecedented densities. Nevertheless, few long-term data sets are available that allow an assessment of the long-term consequences of chronic herbivory. We quantified herbaceous-layer change over a 26 y period in Cades Cove, Great Smoky Mountains National Park, USA. Cades Cove has a long and well-documented history of deer overabundance, with densities reaching 43 deer km−2 during the late 1970s. Over the 26 y sampling interval, mean coverage of herbaceous species declined significantly (P < 0.001) in the forests bordering Cades Cove. Although most plots only lost 1–2 species during the interval, 46 herbaceous species recorded on plots during the 1970s were wholly absent in 2004 (63% of which were forest species). Additionally, the herbaceous layer has become significantly more homogeneous over time. In contrast, species richness and cover on reference plots increased by 106 and 183%, respectively, over a similar time interval. Whereas some compositional changes were associated with forest succession, proximity to the Coves edge environments was the most informative environmental gradient, lending support to the hypothesis that deer foraging behavior results in a biotic edge effect in fragmented landscapes. Chronic herbivory may result in impoverishment and simplification of herbaceous layers in forests otherwise protected from habitat degradation and loss.

Diversity, productivity and landscape-level effects in North American grasslands managed for biomass production

Expanding markets for bioenergy will increasingly shape the composition and configuration of crop production. Growing interest in second-generation biofuels (e.g., lignocellulosic ethanol) is driving a discussion about the most productive and appropriate cropping systems. Interest in perennial grasslands as a bioenergy source has many people asking about the importance of plant community diversity in bolstering productivity, resistance to pest and pathogen pressure and wildlife habitat, among other ecosystem services. We review the current understanding of diversity–productivity relationships across multiple spatial scales, but also emphasize perspectives that have received less attention in the literature.

Wood δ13C, δ18O and radial growth responses of residual red pine to variable retention harvesting

Variable retention harvests are used to enhance the development of structural complexity in managed forests by retaining living trees and other structural legacies from the pre-harvest ecosystem. While harvesting should increase resource availability to residual trees, greater crown exposure may also increase environmental stress, which makes it difficult to predict growth in different structural environments. We used stable carbon isotope ratios (delta(13)C) of annual rings from red pine trees (Pinus resinosa Ait.) as an index of intrinsic water use efficiency (iWUE), the ratio of photosynthetic carbon assimilation (A) to stomatal conductance (g(s)), to better understand how differences in physiological performance relate to growth responses following harvests that left residuals dispersed, aggregated between small (0.1 ha) gaps or aggregated between large (0.3 ha) gaps. Stable oxygen isotope ratios (delta(18)O) were used as an index of g(s) to investigate the drivers behind changes in iWUE. Retention harvesting did not appear to affect delta(13)C or delta(18)O at the stand scale when compared to unharvested control stands, but there was a significant, negative correlation between residual tree delta(13)C and plot basal area in the second and third years after harvesting that suggests declining iWUE as overstory competition increases. Residual tree delta(18)O was similar across treatments and basal areas. Trees in variable retention harvests showed small but positive increases in radial growth from the pre-treatment to post-treatment measurement periods, while radial growth declined in unharvested control stands. There were no significant differences in radial growth among retention treatments. Our results suggest residual red pine in relatively open environments benefit from greater A but do not show evidence of changes in g(s) that would indicate altered water relations.

Ungulate winter habitat selection as a driver of herbaceous‐layer heterogeneity in northern temperate forests

Concentrated seasonal habitat use by ungulates may amplify their influence on ecosystem processes. For example, white-tailed deer (Odocoileus virginianus) depend on forests of coniferous trees, such as Tsuga canadensis (L.) Carr. (eastern hemlock), for winter cover and forage in the northern portion of their range. We hypothesized that winter habitat use influences summer herbaceous-layer plant communities through cascading indirect effects resulting from the deposition of nitrogenous wastes and browsing of woody plants. Deer use was monitored from 2006–2008 via fecal pellet group counts in 15 T. canadensis stands across the western Upper Peninsula of Michigan. Detailed vegetation and soil sampling was performed during the summer of 2008. Estimates derived from winter inputs of fecal pellets, suggest that overwintering white-tailed deer deposit 3.3 ± 0.8 kg N ha−1 yr−1. Results from nonmetric multidimensional scaling ordinations and permutational ANOVAs of distance to group centroids of ground-layer vegetation and soil attributes indicated that high levels of winter deer-use were significantly associated with greater heterogeneity in ground-layer community composition and relative soil nutrient variability within and between stands. Our results highlight potentially important feedbacks between habitat selection by ungulates, site productivity, and plant community structure in forest ecosystems.

Spatial dynamics of radial growth and growth efficiency in residual Pinus resinosa following aggregated retention harvesting

Variable retention harvest systems are encouraged to promote complexity in managed forests, and aggregated retention has been suggested as a means of reducing moisture stress in residual trees. We studied the impacts of within-aggregate position on growth and foliar physiology to better understand the spatial dynamics of residual-tree responses to aggregated retention harvests in even-aged Pinus resinosa Ait. stands. Distance from edge and edge aspect influenced radial growth, volume increment, and growth efficiency, but only edge aspect affected foliar nitrogen content. Spatial variables had no significant relationships with foliar carbon isotope ratios (δ13C). Increases in radial growth, volume increment, and growth efficiency following harvesting were greatest near edges and in the northeastern quadrants of aggregates that received mechanical understory release treatments, and lowest in the southeastern quadrant of aggregates and near aggregate centers. Foliar nitrogen content was highest in the southw...