Although a range of studies have suggested that competition plays a critical role in determining herbivore assemblages, there has been little work addressing the nature of interactions between competing invasive herbivores. We report the results of research on the hemlock woolly adelgid Adelges tsugae ('HWA') and elongate hemlock scale Fiorinia externa ('EHS'), invasive herbivores that both feed on eastern hemlock (Tsuga canadensis). HWA has been linked to hemlock mortality throughout the East Coast of the US; the loss of hemlock threatens to permanently alter surrounding ecosystems. We assessed the spread and impact of both species by resurveying 142 hemlock stands across a 7,500 km 2 latitudinal transect, running from coastal CT to northern MA, for HWA and EHS density as well as hemlock mortality. These stands had been previously surveyed in either 1997-1998 (CT) or 2002. While the number of HWAinfested stands has increased, per-stand HWA density has substantially decreased. In contrast, EHS distribution and density has increased dramatically since 1997-1998. Hemlock mortality was much more strongly related to HWA density than to EHS density, and many stands remain relatively healthy despite an overall increase in hemlock mortality. There was a positive correlation between HWA and EHS densities in stands with low mean HWA densities, suggesting the potential for host-plant-mediated facilitation of EHS by HWA. Our findings underline the importance of research explicitly addressing interactions between competing invasive species, and of determining the potential consequences of these interactions for the invaded ecosystem.

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Common buckthorn (Rhamnus cathartica L.) is one of the most abundant and ecologically harmful non-native plants in forests of the Upper Midwest United States. At the same time, European earthworms are invading previously glaciated areas in this region, with largely anecdotal evidence suggesting they compound the negative effects of buckthorn and influence the invasibility of these forests. Germination and seedling establishment are important control points for colonization by any species, and manipulation of the conditions influencing these life history stages may provide insight into why invasive species are successful in some environments and not others. Using a greenhouse microcosm experiment, we examined the effects of important biotic and abiotic factors on the germination and seedling establishment of common buckthorn. We manipulated light levels, leaf litter depth and earthworm presence to investigate the independent and interactive effects of these treatments on buckthorn establishment. We found that light and leaf litter depth were significant predictors of buckthorn germination but that the presence of earthworms was the most important factor; earthworms interacted with light and leaf litter to increase the number and biomass of buckthorn across all treatments. Path analysis suggested both direct and moisture-mediated indirect mechanisms controlled these processes. The results suggest that the action of earthworms may provide a pathway through which buckthorn invades forests of the Upper Midwest United States. Hence, researchers and managers should consider co-invasion of plants and earthworms when investigating invasibility and creating preemptive or post-invasion management plans.

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Improved understanding of the physiological mechanisms of tree mortality following fires is important with the predicted increase in wildfires under climate change, as well as continued use of prescribed fire for forest management. Disruption of water transport in the xylem from exposure to the heat plume of a fire has been hypothesized as a mechanism of delayed tree mortality. This heat plume rapidly increases vapor pressure deficit in the canopy, increasing transpiration and tension on the xylem causing cavitation, thus reducing water transport and leading to eventual tree death. We aimed to increase understanding of the mechanisms behind such unintended mortality by determining whether branches and roots of longleaf pine are more vulnerable to cavitation when exposed to temperatures expected to occur during prescribed or wild fires. Additionally, we modeled expected branch cavitation under fire conditions based on measured cavitation vulnerability. We heated branch and root segments in a water bath to 41 • C and 54 • C and simulated the negative xylem water potentials experienced during exposure to a heat plume using a double-ended pressure chamber. When branches and roots were pressurized under elevated temperatures, xylem in both organs was more vulnerable to cavitation. In branches, as temperature was increased from 23 • C-54 • C, the pressure at which 50% conductivity was lost (P 50 ) increased from −3.55 MPa to −2.79 MPa, while in roots, P 50 increased from −2.08 MPa to −1.36 MPa. When the P 50 values measured under elevated temperatures were included in plume and hydraulic models, branches were predicted to experience conditions leading to 50% loss of conductivity up to two meters higher into the canopy than under ambient temperatures. Overall, these results suggest that heating of branches and roots during fires can increase vulnerability to xylem cavitation, potentially leading to hydraulic disruption and delayed tree mortality.

Short‐term experiments cannot characterize how long‐lived, invasive shrubs influence ecological properties that can be slow to change, including native diversity and soil fertility. Observational studies are thus necessary, but often suffer from methodological issues. To highlight ways of improving the design and interpretation of observational studies that assess the impacts of invasive plants, we compare two studies of nutrient cycling and earthworms along two separate gradients of invasive shrub abundance. By considering the divergent sampling strategies and statistical analyses of these two studies, and interpreting their contradictory results in the context of other studies, we also aim to better describe the impacts of the focal invader, Rhamnus cathartica. In a new study of a single site in Minnesota, we observed positive correlations between buckthorn abundance and soil pH, soil nutrient pools, nutrient fluxes through leaf litterfall, earthworm abundance and root biomass. Multiple regression models showed these relationships persisted after accounting for variability in soil texture and tree species composition. For a separate, more expansive study in Illinois, other authors reported little to no correlation between buckthorn abundance and 10 soil properties, including earthworm abundance, pH and nutrient concentrations. However, like many other studies, their regression models only assessed predictors related to invader abundance. R2‐values for models of ecosystem properties ranged from 0 to .79 (adjusted‐R2) for our study in Minnesota and from <.05 to .16 (unadjusted) for the prior study in Illinois. Differences in sampling error and use of predictor variables between the two studies likely explain the contrasting results. Synthesis and applications. To reduce the uncertainty of conclusions from observational studies of invasive plants, future studies must ensure that heterogeneity of soils and vegetation is adequately accounted for in the sampling strategy and statistical analyses (e.g. analysis of covariance, multiple regression). Particular attention should be given to ecosystem properties with variability that likely pre‐dates the invader (e.g. geophysical features and tree community composition). In our study, effects of buckthorn on ecosystem properties were not only robust to the inclusion of potentially confounding predictors, but also consistent with expectations based on ecological stoichiometry and mass balance of element flow.

Theoretical predictions and empirical studies suggest that resident species diversity is an important driver of community invasibility. Through trait-based processes, plants in communities with high resident species diversity occupy a wider range of ecological niches and are more productive than low diversity communities, potentially reducing the opportunities for invasion through niche preemption. In terrestrial plant communities, biotic ecosystem engineers such as earthworms can also affect invasibility Keywords Biodiversity-ecosystem function Á Functional diversity Á Invasibility Á Lumbricus terrestris Á Rhamnus cathartica Electronic supplementary material The online version of this article (

The inverse relationship between size and abundance is a well‐documented pattern in forests, and the form of size–density relationships depends on the balance between growth and mortality rates in the community. Traditionally, studies of plant size distributions have focused on single‐species populations and forests, but here we examine diverse communities dominated by plants with varied life histories, including grasses, forbs, shrubs, and succulents. In particular, we test whether the parameters of the individual size distribution differ systematically across community types, whether they fit the contrasting predictions of metabolic or demographic theories, and whether they share a common cross‐community scaling relationship with forest communities and crop populations. All thirteen of our study sites better fit the predictions of demographic equilibrium theory, but interestingly, fits of both demographic and metabolic models showed little systematic variation across community types, despite large differences in environmental conditions and dominant life forms. Finally, analysis of the cross‐community scaling relationship demonstrates that natural and restored non‐forest communities conform to patterns of size and abundance observed among forest, plantation, and crop systems. Taken together, our results suggest that common ecological mechanisms govern plant community size structure across broad environmental gradients, regardless of the dominant plant life forms or limiting resources.