Functional composition drives ecosystem function through multiple mechanisms in a broadleaved subtropical forest.

Understanding the role of biodiversity (B) in maintaining ecosystem function (EF) is a foundational scientific goal with applications for resource management and conservation. Two main hypotheses have emerged that address B-EF relationships: niche complementarity (NC) and the mass-ratio (MR) effect. We tested the relative importance of these hypotheses in a subtropical old-growth forest on the island nation of Taiwan for two EFs: aboveground biomass (ABG) and coarse woody productivity (CWP). Functional dispersion (FDis) of eight plant functional traits was used to evaluate complementarity of resource use. Under the NC hypothesis, EF will be positively correlated with FDis. Under the MR hypothesis, EF will be negatively correlated with FDis and will be significantly influenced by community-weighted mean (CWM) trait values. We used path analysis to assess how these two processes (NC and MR) directly influence EF and may contribute indirectly to EF via their influence on canopy packing (stem density). Our results indicate that decreasing functional diversity and a significant influence of CWM traits were linked to increasing AGB for all eight traits in this forest supporting the MR hypothesis. Interestingly, CWP was primarily influenced by NC and MR indirectly via their influence on canopy packing. Maximum height explained more of the variation in both AGB and CWP than any of the other plant functional traits. Together, our results suggest that multiple mechanisms operate simultaneously to influence EF, and understanding their relative importance will help to elucidate the role of biodiversity in maintaining ecosystem function.

Pyrus calleryana extracts reduce germination of native grassland species, suggesting the potential for allelopathic effects during ecological invasion.

Invasive plant species' success may be a result of allelopathy, or the release of secondary metabolites that are harmful for surrounding plant species. Allelopathy can be mediated through the abiotic environment by chemical sorption or transformation, so the substrate on which interactions occur can lead to differential outcomes in allelopathic potential. One aggressive invader, Pyrus calleryana, has become dominant in many ecosystems throughout Eastern US, and has reduced the abundance of native species where it invades. Thus, our goal was to identify if P. calleryana had allelopathic potential by testing the impact of leaf and flower leachate on gemination of six common grassland species (three grasses and three forbs) in either sterilized sand or field collected soils. Germination of five out of six tested species was reduced by P. calleryana leaf litter, with weaker impacts from flower leachate. This suggests that allelopathy is one mechanism driving the success of P. calleryana and that allelopathic effects may change with plant phenology. For instance, P. calleryana has late leaf senescence in the fall and copious blooming in the spring that may elongate the timeframe that allelopathic inhibition can occur. Further, germination was higher in sand than in soil, suggesting that the context of the abiotic environment can mediate this relationship. In our study, two grass species that could be overabundant in restored grasslands had higher germination rates in soil than sand and one was not altered by P. calleryana suggesting that this relationship could further promote the overabundance of grass species. Taken together, P. calleryana likely inhibits the germination of native species where it invades, but there is context dependency of this relationship with both soil chemistry and seasonality.

Thirty Years of Compositional Change in an Old-Growth Temperate Forest: The Role of Topographic Gradients in Oak-Maple Dynamics.

Ecological communities are structured in response to spatial and temporal variation of numerous factors, including edaphic conditions, biotic interactions, climatic patterns and disturbance regimes. Widespread anthropogenic factors such as timber harvesting can create long-lasting impacts, obscuring the relationship between community structure and environmental conditions. Minimally impacted systems such as old-growth forests can serve as a useful ecological baseline for predicting long-term compositional shifts. We utilized decadal tree species sampling data (1979-2010) divided into three strata (understory, midstory, overstory) to examine temporal changes in relative abundances and spatial distributions of dominant taxa, as well as overall shifts in community composition, in a relatively pristine Appalachian old-growth forest in eastern Kentucky, USA. Quercus and Carya species persisted mainly as mature canopy trees with decreasing juvenile recruitment, especially in mesic areas. In contrast, Acer, Fagus, and other mesophytic species were abundant and spatially widespread in subcanopy layers suggesting these species are more likely to recruit in gap-scale canopy openings. In the overstory, mesophytic species were spatially restricted to lower and mid-slope mesic habitats. Temporal changes in community composition were most evident in the understory and tended to be greater in mesic areas, a trend seemingly driven by recruitment failure among xerophytic species. In subcanopy vegetation we discovered a loss of distinction through time among the ecological community designations established following the 1979 survey (Chestnut oak, Mixed mesophytic, and Beech). The overstory was more stable through time, suggesting a storage effect where long-lived trees have maintained a particular community composition through time in areas where regeneration opportunities are minimal under current environmental conditions. Overall, sitewide canopy succession is occurring slowly in the absence of major disturbance, and topography-driven environmental variation appears to have an important local-scale filtering effect on communities.

Microbial Biofilm Community Variation in Flowing Habitats: Potential Utility as Bioindicators of Postmortem Submersion Intervals.

Biofilms are a ubiquitous formation of microbial communities found on surfaces in aqueous environments. These structures have been investigated as biomonitoring indicators for stream heath, and here were used for the potential use in forensic sciences. Biofilm successional development has been proposed as a method to determine the postmortem submersion interval (PMSI) of remains because there are no standard methods for estimating the PMSI and biofilms are ubiquitous in aquatic habitats. We sought to compare the development of epinecrotic (biofilms on Sus scrofa domesticus carcasses) and epilithic (biofilms on unglazed ceramic tiles) communities in two small streams using bacterial automated ribosomal intergenic spacer analysis. Epinecrotic communities were significantly different from epilithic communities even though environmental factors associated with each stream location also had a significant influence on biofilm structure. All communities at both locations exhibited significant succession suggesting that changing communities throughout time is a general characteristic of stream biofilm communities. The implications resulting from this work are that epinecrotic communities have distinctive shifts at the first and second weeks, and therefore the potential to be used in forensic applications by associating successional changes with submersion time to estimate a PMSI. The influence of environmental factors, however, indicates the lack of a successional pattern with the same organisms and a focus on functional diversity may be more applicable in a forensic context.

Abiotic autumnal organic matter deposition and grazing disturbance effects on epilithic biofilm succession.

Stream epilithic biofilm community assembly is influenced in part by environmental factors. Autumn leaf deposition is an annual resource subsidy to streams, but the physical effects of leaves settling on epilithic biofilms has not been investigated.We hypothesized that bacterial and microeukaryotic community assembly would follow a successional sequence that was mediated by abiotic effects that were simulating leaf deposition (reduced light and flow) and by biotic (snail grazing)disturbance. This hypothesis was tested using an in situ experimental manipulation. Ambient biofilms had greater algal biomass and distinct ARISA community profiles compared to biofilms developed under manipulated conditions. There were no significant differences in biofilm characteristics associated with grazing, suggesting that results were driven by reduced light/flow rather than invertebrate disturbance; however, grazing appeared to increase bacterial taxon richness.Interestingly at day 38, all treatments grouped together in ordination space and had similar algal/total biomass ratios. We suggest that algal priming promoted a shift in ambient biofilms but that this effect is dependent upon successional timing of algal establishment. These data demonstrate that abiotic effects were more influential than local grazing disturbance and imply that leaf litter deposition may have bottom-up effects on the stream ecosystem through altered epilithic biofilms.

Evidence for facilitation of Culex pipiens (Diptera: Culicidae) life history traits by the nonnative invasive shrub Amur honeysuckle (Lonicera maackii).

Mosquitoes are one of the most globally important insect pests and vectors of human pathogens, and their populations may be facilitated or inhibited by anthropogenic environmental change. Invasive plant species are an important management concern and environmental modifier in many ecosystems; these plant invasions have the potential to exacerbate or diminish mosquito populations. The purpose of this study was to assess potential effects of a highly invasive plant, Lonicera maackii, on a common mosquito species Culex pipiens L., which is an important pathogen vector in the United States. Three microcosm assays were conducted to determine the responses of C. pipiens life history attributes of larval survivorship, growth, and pupation when subjected to leachate from two native plant leaves (Platanus occidentalis and Acer saccharum) and both the leaves and flowers of L. maackii. Only C. pipiens larvae exposed to L. maackii leachate pupated and emerged as adults. However, in all three assays there were statistically significant differences in survivorship and body size change among treatments, and in each assay the highest survivorship and maximum larval size was found in the L. maackii leachate treatments, suggesting positive effects on certain life history traits. This study is one of the first to demonstrate the potential facilitative effect of this invasive plant species on an insect vector and suggests that plant invasion could have positive feedbacks into mosquito population dynamics and, ultimately, human disease.