Mark W. Kershner

The distribution of below‐ground traits is explained by intrinsic species differences and intraspecific plasticity in response to root neighbours

Summary 1. Large variation in tree root architecture and morphology has been reported for temperate forest communities. However, it is not clear whether this variation represents adaptation of species to specific soil properties, alternative resource acquisition strategies among co-occurring species, or canalized traits without a strong impact on the success of individuals in different environments. Here, our goal was to test these alternative hypotheses and quantify how community-aggregated and intraspecific root trait variations are explained by biotic versus abiotic mechanisms in a temperate deciduous forest. 2. We conducted our study in an Acer-Fagus-dominated forest in north-east Ohio, USA. Using molecular barcoding techniques, we identified 738 root systems belonging to 14 tree species. We measured seven functional root traits related to root architecture and morphology at the species and community-aggregated levels. 3. Although we found significant relationships between soil resource gradients and root trait distributions, intrinsic differences between coexisting species were more important than soil factors in explaining the distribution of root traits in the community. Additionally, root trait variation at the species level was also influenced by the presence of other species within cores. 4. Community-aggregated variation was more influenced by the combination of species present than soil properties in each sample, suggesting that biotic interactions play an important role in controlling community root trait distribution. 5. Synthesis. We propose that root trait differentiation between coexisting species is the result of inherent differences between species and plasticity-mediated responses to neighbours. Hence, the large variation in root traits reported in temperate forest seems to reflect alternative evolutionary pathways that allow individuals to exploit distinct niches in relatively close proximity.

Aggregated and complementary: symmetric proliferation, overyielding, and mass effects explain fine-root biomass in soil patches in a diverse temperate deciduous forest landscape.

Few studies describe root distributions at the species level in diverse forests, although belowground species interactions and traits are often assumed to affect fine-root biomass (FRB). We used molecular barcoding to study how FRB of trees relates to soil characteristics, species identity, root diversity, and root traits, and how these relationships are affected by proximity to ecotones in a temperate forest landscape. We found that soil patch root biomass increased in response to soil resources across all species, and there was little belowground vertical or horizontal spatial segregation among species. Root traits and species relative abundance did not explain significant variation in FRB after correcting for soil fertility. A positive relationship between phylogenetic diversity and FRB indicated significant belowground overyielding attributable to local root diversity. Finally, variation in FRB explained by soil fertility and diversity was reduced near ecotones, but only because of a reduction in biomass in periodically anoxic areas. These results suggest that symmetric responses to soil properties are coupled with complementary species traits and interactions to explain variation in FRB among soil patches. In addition, landscape-level dispersal among habitats and across ecotones helps explain variation in the strength of these relationships in complex landscapes.

Comparison of the Species Composition, Catch Rate, and Length Distribution of the Catch from Trap Nets with Three Different Mesh and Throat Size Combinations

Abstract Trap nets of varying design are commonly used to assess fish populations, but the effect of the design on gear selectivity is not well known. In particular, it may be advantageous to use multiple net designs with different mesh and throat sizes to maximize the catch of specific length-classes and to minimize the risk of predation on small fish by larger fish. We compared the species composition, catch rate, and length distribution of fishes caught by three trap net designs with dimensions differing only in mesh size and throat size (0.6-cm delta mesh and 3.8-cm × 3.8-cm square throats, 1.3-cm square mesh and 7.6-cm × 7.6-cm square throats, or 2.5-cm square mesh and 12.7-cm × 12.7-cm square throats). A total of 3,473 fish of 18 species were captured from Sandy Lake, Portage County, Ohio, during 24 sample dates from June to August 1999. The large net design had a significantly higher average number of species captured (mean = 11) than the medium or small net design (means = 9 and 8, respectively). ...

Vision is Important for Plant Location by the Phytophagous Aquatic Specialist Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae)

The aquatic milfoil weevil Euhrychiopsis lecontei Dietz (Coleoptera: Curculionidae) is a specialist on Myriophyllum spp. and is used as a biological control agent for Eurasian watermilfoil (Myriophyllum spicatum L.), an invasive aquatic macrophyte. We show evidence that visual cues are important for plant detection by these weevils. Weevils had difficulty locating plants in dark conditions and were highly attracted to plant stems in the light, even when the plant sample was sealed in a vial. However, weevils were equally attracted to both M. spicatum and another aquatic macrophyte, coontail (Ceratophyllum demersum L.) in vials. Turbidity (0–100 NTU) did not significantly influence visual plant detection by the weevils. This work fills a void in the literature regarding visual plant location by aquatic specialists and may help lead to a better understanding of when and where these weevils will find, accept, and damage their target host-plants.

Crayfish Effects on Fine Particulate Organic Matter Quality and Quantity

Coarse particulate organic matter (CPOM), especially leaves, plays a key role in food webs of many streams and can be converted to fine particulate organic matter (FPOM) by specific macroinvertebrate functional groups (i.e., shredders). In this study, we examined crayfish [Orconectes obscurus (Hagen)] effects on FPOM generation from two leaf types, red maple and white oak, that differ in recalcitrance and decomposition rates. Further, we examined potential quality differences between FPOM generated by crayfish via fragmentation and defecation. Crayfish were fed stream-conditioned maple or oak leaves in hanging 1-mm mesh-bottom baskets in aquaria. After 12 h, C:N ratios and dry/ash-free dry weights of remaining CPOM, FPOM fragments that fell through the mesh, and crayfish feces (collected using finger cots that encased the crayfish abdomens) were determined. Loss of CPOM attributable to crayfish feeding was higher for maple than oak; fragment FPOM and crayfish feces generation were also higher for maple. Maple CPOM percent organic matter was lower than oak CPOM but feces and fragment FPOM percent organic matter did not differ among leaf species suggesting that crayfish actions homogenize the properties of particulate organic matter. Both C:N ratios and bacterial abundance were also altered by crayfish processing and digestion underscoring potential crayfish effects on FPOM bioavailability. Overall, crayfish altered the ontogeny of the detritus, which may, in turn, affect stream FPOM dynamics.

Habitat-specific effects of particle size, current velocity, water depth, and predation risk on size-dependent crayfish distribution

This study assessed effects of abiotic (current velocity, water depth, particle size) and biotic (predation risk for crayfish, size distribution and densities of predatory fish) variables on habitat- and size-specific distribution patterns of lotic crayfish (Orconectes obscurus) using field surveys and tethering experiments. Additionally, particle size manipulations were used with predation assays to assess habitat-specific interactions since the average particle size increased from deep pools to shallow pools to riffles. Large crayfish had the highest densities in deep pools and were associated with increased water depth, whereas small and medium crayfish had the highest densities in shallow pools and were strongly associated with increased particle size and decreased water depth. Regardless of size, crayfish in deep pools had significantly lower survival than in shallow pools and riffles. However, only small crayfish showed consistent differences in predation risk by habitat type and were significantly more vulnerable to predation than larger crayfish. Additionally, large rocky refugia resulted in significantly higher survival of small crayfish in the combined particle manipulation/tethering experiment. Overall, predation appears to be a key mechanism structuring habitat-specific distribution patterns for only small O. obscurus. Large substrates may be particularly important in habitats where both small crayfish density and predation risk are high.

Placing the Effects of Leaf Litter Diversity on Saprotrophic Microorganisms in the Context of Leaf Type and Habitat

Because of conflicting results in previous studies, it is unclear whether litter diversity has a predictable impact on microbial communities or ecosystem processes. We examined whether effects of litter diversity depend on factors that could confound comparisons among previous studies, including leaf type, habitat type, identity of other leaves in the mixture, and spatial covariance at two scales within habitats. We also examined how litter diversity affects the saprotrophic microbial community using terminal restriction fragment length polymorphism to profile bacterial and fungal community composition, direct microscopy to quantify bacterial biomass, and ergosterol extraction to quantify fungal biomass. We found that leaf mixture diversity was rarely significant as a main effect (only for fungal biomass), but was often significant as an interaction with leaf type (for ash-free dry mass recovered, carbon-to-nitrogen ratio, fungal biomass, and bacterial community composition). Leaf type and habitat were significant as main effects for all response variables. The majority of variance in leaf ash-free dry mass and C/N ratio was explained after accounting for treatment effects and spatial covariation at the meter (block) and centimeter (litterbag) scales. However, a substantial amount of variability in microbial communities was left unexplained and must be driven by factors at other spatial scales or more complex spatiotemporal dynamics. We conclude that litter diversity effects are primarily dependent on leaf type, rather than habitat type or identity of surrounding leaves, which can guide the search for mechanisms underlying effects of litter diversity on ecosystem processes.

Responses of plants and invertebrate assemblages to water-level fluctuation in headwater wetlands

Abstract Flood-pulsing is a key environmental factor that structures biotic communities in large-order river systems, but we focused our study on the effects of flood-pulsing in headwater systems. We used 10 mesocosm wetlands (10 m × 20 m) to test 2 treatments: a flood-pulse regimen in which natural flood events caused water levels to fluctuate and a static regimen in which water levels remained artificially stable. Abiotic characteristics, plants, and aquatic invertebrate communities were monitored from 2002 through 2005 in permanent pools, nonflooded banks in static wetlands, and intermittently flooded banks in flood-pulse wetlands. The flood-pulse treatment had minimal effects on environmental conditions of permanent pools, and submersed plant and aquatic invertebrate communities in permanent pools were similar in both treatments. This result suggested that resource subsidies from the floodplain to the pools were minimal. However, flood-pulsing caused observable changes to plant communities in the intermittently flooded zone (IFZ) above the permanently flooded pool. Overall plant diversity was higher in static wetlands, and % bare ground was higher in flood-pulse wetlands, results suggesting that the short, stochastic floods were a strong environmental stressor. In flood-pulse wetlands, the fluctuating water levels may have reduced the proportion of introduced, weedy, and upland plant taxa. Flood-pulse and static wetlands had distinctly different plant assemblage compositions, indicating that the abiotic stressors caused pronounced changes in the floodplain community. An indicator species analysis showed that taxa classified as obligate wetland plants were indicators in flood-pulse wetlands (e.g., Juncus canadensis, Ludwigia palustris, Dulichium arundinaceum, Eleocharis obtusa, Carex crinita, Carex lupulina, Carex vulpinoidea), but taxa classified as facultative wetland or upland plants were indicators of static wetlands (Cirsium arvense, Eupatoriadelphus maculatus, Plantago lanceolata, Bidens frondosus, Melilotus officinalis, Mentha arvensis, Daucus carota, Poa palustris). Many functional categories of plant species that were common in flood-pulse wetlands (e.g., obligate wetland plants, perennial, native and nonweedy species) are considered beneficial from a management perspective.

Algal-bacterial co-variation in streams: a cross-stream comparison

Algal-bacterial co-variation has been frequently observed in lentic and marine environments, but the existence of such relationships in lotic ecosystems is not well established. To examine possible co-variation, bacterial number and chlorophyll-a concentration in water and sediments of nine streams from different regions in the USA were examined. In the water, a strong relationship was found between chloro- phyll concentration and bacterial abundance. There was not a significant linear rela- tionship between the abundance of sediment bacteria and sediment or water chloro- phyll concentration. The linear regression results obtained between bacterial numbers and chlorophyll concentration in water were generally similar to those reported in other studies on lentic and marine systems suggesting that factors that cause this co-variation may be similar.

Decay of ecosystem differences and decoupling of tree community–soil environment relationships at ecotones

Ecotones are important landscape features where there is a transition between adjoining ecosystems. However, there are few generalized hypotheses about the response of communities to ecotones, except for a proposed increase in species richness that receives varying empirical support. Based on the assumption that transport of abiotic material and dispersal of organism propagules across ecotones are independent processes, we propose the new hypothesis that ecotones decouple community-environment relationships, increasing the importance of spatial structure that is independent of the environment. We tested this hypothesis by examining the effects of ecotones on relationships between trees and soil properties in a temperate deciduous forest. The study area included different landforms defined by topography, hydrology, and geomorphology, which we designated upland, bottomland, and riparian forests. The site also included a mowed herbaceous corridor. We found that soil properties and tree community composition significantly differed among landforms, and thus they could be treated as differing ecosystem types. However, inclusion of plots near ecotones significantly reduced the variance explained by landform due to introduction of increased noise, increased similarity of ecotone plots in different landforms, or both. To examine tree community-soil environment relationships, factorial kriging analysis was used to decompose variation in soil properties into structures associated with differing spatial scales, which were then used as predictors of tree composition using redundancy analysis. In agreement with the ecotone-decoupling hypothesis, we found that ecotones introduced significant unexplained variation into correlations between tree community composition and soil properties. In addition, spatial variation in tree community composition that was independent of soil properties was only detected when ecotones were included in the analysis, and little variation in tree community composition was attributed to small-scale soil property structures. Together, these results indicate that dispersal limitation and mass effects in the tree community take on increased importance near ecotones. We found no consistent changes in tree species richness associated with ecotones, and we suggest that the ecotone- decoupling hypothesis may correspond with a more general community-level pattern that warrants further testing. Decoupling of community-environment relationships near ecotones also has important implications for accuracy of models predicting community distributions from abiotic information.