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1 Disturbance may cause community composition across sites to become more or less homogenous, depending on the importance of different processes involved in community assembly. In north-eastern North America and Europe local (alpha) diversity of forest plants is lower in forests growing on former agricultural fields (recent forests) than in older (ancient) forests, but little is known about the influence of land-use history on the degree of compositional differentiation among sites (beta diversity). 2 Here we analyse data from 1446 sites in ancient and recent forests across 11 different landscapes in north-eastern North America and Europe to demonstrate decreases in beta diversity and in the strength of species-environment relationships in recent vs. ancient forests. 3 The magnitude of environmental variability among sites did not differ between the two forest types. This suggests the difference in beta diversity between ancient and recent forests was not due to different degrees of environmental heterogeneity, but rather to dispersal filters that constrain the pool of species initially colonizing recent forests. 4 The observed effects of community homogenization and weakened relationships between species distributions and environmental gradients appear to persist for decades or longer. The legacy of human land-use history in spatial patterns of biodiversity may endure, both within individual sites and across sites, for decades if not centuries.

Rapid climate change imperils many small-ranged endemic species as the climate envelopes of their native ranges shift poleward. In addition to abiotic changes, biotic interactions are expected to play a critical role in plant species’ responses. Below-ground interactions are of particular interest given increasing evidence of microbial effects on plant performance and the prevalence of mycorrhizal mutualisms. We used greenhouse mesocosm experiments to investigate how natural northward migration/assisted colonization of Rhododendron catawbiense, a small-ranged endemic eastern U.S. shrub, might be influenced by novel below-ground biotic interactions from soils north of its native range, particularly with ericoid mycorrhizal fungi (ERM). We compared germination, leaf size, survival, and ERM colonization rates of endemic R. catawbiense and widespread R. maximum when sown on different soil inoculum treatments: a sterilized control; a non-ERM biotic control; ERM communities from northern R. maximum populations; and ERM communities collected from the native range of R. catawbiense. Germination rates for both species when inoculated with congeners' novel soils were significantly higher than when inoculated with conspecific soils, or non-mycorrhizal controls. Mortality rates were unaffected by treatment, suggesting that the unexpected reciprocal effect of each species’ increased establishment in association with heterospecific ERM could have lasting demographic effects. Our results suggest that seedling establishment of R. catawbiense in northern regions outside its native range could be facilitated by the presence of extant congeners like R. maximum and their associated soil microbiota. These findings have direct relevance to the potential for successful poleward migration or future assisted colonization efforts.

AIM: Biogeographers have long known that plant species do not fully encompass their fundamental niche. Nonetheless, in practice, species distribution modelling assumes that plant distributions represent a reasonable approximation of their environmental tolerance. For ecological forecasting, projections of habitat loss due to climate change assume that many species will be unable to tolerate climate conditions outside of those found within their current distributional ranges. We aim to test how well occurrences in the native range approximate the climatic conditions in which plant species can survive. LOCATION: Continental USA. METHODS: We compared the climatic conditions between occurrences in the US native versus US non‐native ranges using 144 non‐invasive plant species. We quantified differences in January minimum temperature, July maximum temperature and annual precipitation as indicators of climatic tolerance. We also compared modelled potential distributions throughout the US based on native and total ranges to test how expanded climatic tolerance translates into predicted geographical range. RESULTS: Most species (86%) had non‐native occurrences in climates outside those described by their native distributions. For the 80 species with lower minimum temperatures at non‐native occurrences, the median expansion of minimum temperature tolerance was −2.9 °C. Similarly, for the 90 species with lower precipitation at non‐native occurrences, the median expansion of minimum annual precipitation was −23 cm. Broader climatic conditions at non‐native occurrences expanded the modelled potential geographical range by a median of 35%, with smaller range species showing larger expansions of potential geographical range. MAIN CONCLUSIONS: Our results show that plants' native ranges strongly underestimate climatic tolerance, leading species distribution models to underpredict potential range. The climatic tolerance of species with narrow native ranges appears most prone to underestimation. These findings suggest that many plants will be able to persist in situ with climate change for far longer than projected by species distribution models.

Eastern U.S. forests are witnessing an ecologically disruptive decline in one of the region's distinctive foundation tree species, the eastern hemlock (Tsuga canadenis). The exotic insect pests hemlock woolly adelgid (Adelges tsugae, HWA) and elongate hemlock scale (Fiorinia externa) have greatly altered many forest ecosystems previously dominated by this important evergreen conifer. The consequences for ecosystem processes are far reaching because hemlock is often replaced by deciduous tree species, such as black birch (Betula lenta), which have strongly divergent effects on forest floor microenvironments and nutrient cycling. We took advantage of an accidental experiment initiated by patch‐level timber harvesting ~30 yr ago to investigate how the removal of hemlock, and its replacement by deciduous trees, has affected leaf litter characteristics, soil organic layer mass, C:N content, and soil respiration rates. We also contrasted these areas to nearby forest plots where deciduous B. lenta has been dominant for almost a century. The inclusion of healthy, intact hemlock stands in the design, and the close proximity of plots, allowed for a powerful space‐for‐time approach to detect ecosystem changes that are likely to occur across the broader landscape with HWA‐induced hemlock loss in coming years. Three years of data collection from a series of plots in hemlock, young birch, and mature birch stands revealed dramatic differences in soil carbon pools and cycling. Between forests dominated by hemlock vs. mature birch, we saw a significant decrease in soil organic layer mass and in the C:N of the remaining organic material. Although hemlock and young birch stands showed no significant differences in soil respiration rates, mature birch stands had significantly higher soil respiration rates throughout the entire growing season, regardless of wet or dry years. Our results suggest that the carbon pool in the forest floor is likely to mobilize through greater decomposition with a 6.8× decline in soil organic layer C storage as hemlocks are replaced by deciduous trees, leading to a potential net release of ~4.5 tons C per hectare. We conclude that the ramifications of this change for carbon storage could be extensive, but may take decades to manifest.

Aim and location The research investigated the long-term effects of human disturbance, namely nineteenth century agricultural land-use, on the modern species composition, structure and distribution of Rich Mesic Forests (RMF) in western Massachusetts, USA. RMF are a species-rich north-eastern variant of the Mixed Mesophytic Forest Type of eastern North America. Methods Land-use history patterns were reconstructed for two towns (c. 16,000 ha) from the onset of widespread European settlement and agricultural land-use in the late eighteenth century until present. Vegetation and a range of environmental variables were sampled in sixty-one 10 × 10 m plots in thirty-four forest stands with varying histories of human disturbance. Vegetation data were ordinated (DCA) to identify patterns of variation and related environmental and historical factors. The distribution patterns of individual taxa in relation to historical land-use and environmental factors were analysed using G-tests of independence and logistic regression. Associations between species secondary forest colonization ability and life history characteristics (e.g. diaspore dispersal mode, degree of vegetative spread) were assessed. Results Persistent compositional differences were documented between the vegetation of primary forests and post-agricultural, secondary forests indicating that distribution patterns for many plant species still reflect the open, agricultural environment of the nineteenth century, despite the current predominance of forest cover in the study area. A major factor driving modern vegetation patterns in RMF is the ability and rate of colonization by forest herbs. In particular, species with seeds lacking morphological adaptations for dispersal (barochores) and those which produce seeds with elaiosomes to encourage ant dispersal (myrmecochores) are less frequent in secondary forests. Environmental differences between primary and secondary forests, although present, appear to be less important in influencing species distribution patterns. Main conclusions Widespread agricultural land-use represents a novel disturbance in the naturally forested ecosystems of eastern North America with long-term impacts on plant community composition and structure. Many secondary forest sites that are environmentally suitable for RMF vegetation do not support the suite of plant species typical of this community type, apparently because of the dispersal limitations of certain forest herbs. These poorly dispersed herb taxa are well adapted for growth in stable forest ecosystems characterized by local, small-scale disturbance (e.g. gap-phase dynamics), yet are maladapted for rapid population recovery and recolonization following severe disturbance (e.g. agricultural land-use).

Tsuga canadensis (Eastern Hemlock) is a key forest foundation species that is currently declining across the eastern US due to attack by exotic insect species, primarily Adelges tsugae (Hemlock Woolly Adelgid). In the northeastern US, declining Eastern Hemlock stands are typically replaced by fast-growing deciduous Betula lenta (Black Birch) trees, altering ecosystem processes and ecological communities. In this study, we used an approach that substituted space for time to explore how the forest floor's soil organic horizon, macrofungal communities, and bacterial abundance might be altered following Eastern Hemlock replacement by Black Birch. We compared intact, mature Eastern Hemlock forest plots at 2 sites in western Massachusetts to adjacent areas of vigorous regeneration of young Black Birch triggered by logging activity at the sites ∼25–30 years ago. Forest-floor soil organic horizons were significantly thicker under Eastern Hemlock forests and exhibited a higher carbon:nitrogen (C:N) ratio, suggesting slower decomposition and greater accumulation of organic material under Eastern Hemlock compared to deciduous Black Birch canopies. Macrofungal communities emerging from the forest floor did not differ strongly in morphospecies richness between Eastern Hemlock and Black Birch plots. However, a greater number of rare fungi taxa, defined in this study as those represented by a single observation, were detected in Eastern Hemlock plots, and the composition of Black Birch plots was more homogenized and less variable. The abundance of bacterial colony forming units (CFUs) in the soil organic horizon appeared to follow a seasonal pattern of variation between mature Eastern Hemlock versus young Black Birch plots; CFUs were most abundant in Black Birch soils in the fall, potentially tracking input of new deciduous leaf litter, whereas Eastern Hemlock plots had higher CFU counts in the summer. The results of this study suggest that forest-floor characteristics will be substantially altered by the impending loss of Eastern Hemlock, associated macrofungal communities may become simplified and homogenized, and the timing of peak bacterial abundance in the forest floor might be shifted to the fall.

In many ecological communities, the effects of exotic species are likely to extend beyond their direct interactions with natives, due to indirect effects. This dynamic might be particularly consequential in cases where invasive insects or other exotic herbivores target foundation plant species in the communities they invade. In this study at a site in western Massachusetts, we used experimental transplants to gauge the potential effects of a decline in the evergreen conifer Tsuga canadensis due to ongoing spread of two exotic insect pests on a liverwort, Bazzania trilobata, whose distribution is closely linked to dense stands of this conifer in the northeastern USA. After 4 years, transplanted B. trilobata samples moved to forest areas with lower abundance of T. canadensis declined significantly, particularly on sites with higher solar radiation, as determined by local slope and aspect. In addition, samples manually cleared of deciduous leaf litter were ~ 17% larger than those exposed to natural accumulation of leaf litter, indicating a direct negative effect of deciduous trees on the liverwort that might increase as these tree species replace T. canadensis lost to invasive pests. A parallel experiment documented high mortality of B. trilobata (55%) when subjected to open canopy conditions similar to those resulting from selective \"salvage\" logging of hemlock in the region. These results indicate that the spread of exotic insect pests targeting T. canadensis is likely to produce strong indirect negative effects on the liverwort B. trilobata, via diminished commensal interactions with the conifer and increased amensalistic effects from the deciduous tree species that commonly replace it.

In many ecological communities, the effects of exotic species are likely to extend beyond their direct interactions with natives, due to indirect effects. This dynamic might be particularly consequential in cases where invasive insects or other exotic herbivores target foundation plant species in the communities they invade. In this study at a site in western Massachusetts, we used experimental transplants to gauge the potential effects of a decline in the evergreen conifer Tsuga canadensis due to ongoing spread of two exotic insect pests on a liverwort, Bazzania trilobata, whose distribution is closely linked to dense stands of this conifer in the northeastern USA. After 4 years, transplanted B. trilobata samples moved to forest areas with lower abundance of T. canadensis declined significantly, particularly on sites with higher solar radiation, as determined by local slope and aspect. In addition, samples manually cleared of deciduous leaf litter were ∼ 17% larger than those exposed to natural accumulation of leaf litter, indicating a direct negative effect of deciduous trees on the liverwort that might increase as these tree species replace T. canadensis lost to invasive pests. A parallel experiment documented high mortality of B. trilobata (55%) when subjected to open canopy conditions similar to those resulting from selective “salvage” logging of hemlock in the region. These results indicate that the spread of exotic insect pests targeting T. canadensis is likely to produce strong indirect negative effects on the liverwort B. trilobata, via diminished commensal interactions with the conifer and increased amensalistic effects from the deciduous tree species that commonly replace it.

The high rate of anthropogenic climate change projected for coming decades and evidence of low migration ability for many species have led researchers to warn of a looming extinction crisis. This threat is expected to be most acute for small-ranged endemic species, which could see novel climatic conditions develop rapidly across the entirety of their limited geographic ranges. To avoid extinctions, some conservationists have proposed that climateimperiled species might be candidates for \"assisted colonization\" or \"managed relocation\" to new regions, outside their historical ranges. One major concern related to managed relocation is the possibility that some relocated species could later become problematic invasives where they are introduced. In this review, we consider how these emerging conservation challenges might unfold for the flora of New England. A range of evidence suggests that most plant species native to New England might be resilient to immediate extinction risk from climate change, as these species typically have broad geographic ranges and have migrated long distances in response to past climate change. In contrast, regions to the south, particularly hotspots of plant endemism in the southeastern US, harbor numerous small-ranged species whose current climatic niches could rapidly shift beyond their native ranges, leaving them vulnerable to extinction unless they colonize new regions to the north. Consequently, debates surrounding managed relocation in New England are likely to be focused primarily on the ecological risks versus conservation benefits of accepting climate-threatened endemic plant species from the southeastern US, and to hinge on concerns about the invasive potential of these species. To provide an empirically-grounded estimate of invasion risk from the introduction of US native plant species to New England, we reviewed invasive species lists for New England and tallied those species that are native to other parts of the contiguous US (versus other regions and continents). Between four and ten \"invasive\" or \"potentially invasive\" plant species reported from New England are from other regions of the contiguous US, depending in part on how issues of native versus exotic genotypes within taxa are resolved. A review of current floristic data from New England shows that these 4-10 problematic species are drawn from a larger pool of ~374 US native plant species reported as exotic in the region, suggesting that only 1.1-2.7% of species appearing spontaneously as adventives in the region are viewed as invasive. In light of this analysis, we suggest that managed relocation is not likely to spawn large numbers of new invasives, and might therefore be judiciously evaluated alongside other conservation options for climate-threatened plant species. We propose a collaborative effort among field botanists, land managers, conservationists, and academics in New England, partnering with botanists in the southeastern US, to initiate fundamental research to experimentally test the viability and ecological effects of climate-threatened endemic plant species from the southeastern US in the New England region.