Biogeographical And Evolutionary Processes Influencing The Assembly Of Deciduous Forest Plant Communities

Ecological theories of community assembly and structuring are often predicated on the overriding importance of small-scale processes operating over short periods of time. However, it is becoming increasingly clear that longer-term, larger-scale processes, such as migration and the diversification of evolutionary lineages, are also important factors influencing the distributions of individual species and the diversity of communities. In this investigation, I first examined the geographic distribution of an ant-dispersed forest plant, Jeffersonia diphylla, to assess whether the population structure of its geographic range in eastern North America exhibited patterns consistent with a distribution in equilibrium with the environment, or whether the species’ limited dispersal ability on local scales might lead to a non-equilibrial distribution at large geographic scales. Population size and performance did not decline toward the northern range edge and seed sowing within and beyond the species’ northeastern range edge demonstrated potentially suitable habitat up to 300 km outside its range. As such, the range of J. diphylla may not be in equilibrium with the environment and its restricted distribution in the Northeast may trace to limited post-glacial migration. These findings highlight the potential for limited migratory responses of plant species to climate change, raising the possibility that human intervention or ‘assisted colonization’ may be necessary to aid some species in tracking modern climate change. Finally, I investigated a prominent plant species richness gradient in the forests of the northeastern U.S. to determine whether long- term evolutionary processes, including phylogenetic niche conservatism, may contribute to high species richness in communities on fertile, calcium-rich soils. Analyses of the phylogenetic ‘depth’ of communities along this gradient demonstrated that species-rich communities on calcium-rich soils included a disproportionate representation of Basal Angiosperm and Lower Eudicot angiosperm taxa. Parsimonybased reconstructions of the ancestral calcium niche of lineages present in the study also suggested a key role for fertile soils in the early diversification of angiosperms in Temperate Deciduous Forests. These patterns suggest that calcium-rich soils may be an ecological ‘zone of origin’ for angiosperms and highlight the potential for longterm evolutionary processes to influence species diversity in contemporary communities

Decline of a Foundation Tree Species Due to Invasive Insects Will Trigger Net Release of Soil Organic Carbon

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 6.89 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

Niche syndromes, species extinction risks, and management under climate change

Copyright © 2013 Elsevier. NOTICE: this is the author’s version of a work that was accepted for publication in Trends in Ecology and Evolution. Changes resulting from the publishing process, such as peer review, editing, corrections, structural formatting, and other quality control mechanisms may not be reflected in this document. Changes may have been made to this work since it was submitted for publication. A definitive version was subsequently published in Trends in Ecology and Evolution Vol. 28 (2013), DOI: 10.1016/j.tree.2013.05.010The current distributions of species are often assumed to correspond with the total set of environmental conditions under which species can persist. When this assumption is incorrect, extinction risk estimated from species distribution models can be misleading. The degree to which species can tolerate or even thrive under conditions found beyond their current distributions alters extinction risks, time lags in realizing those risks, and the usefulness of alternative management strategies. To inform these issues, we propose a conceptual framework within which empirical data could be used to generate hypotheses regarding the realized, fundamental, and ‘tolerance’ niche of species. Although these niche components have rarely been characterized over geographic scales, we suggest that this could be done for many plant species by comparing native, naturalized, and horticultural distributions.FCT (Fundação para a Ciência e a Tecnologia

Limited Range-Filling Among Endemic Forest Herbs of Eastern North America and Its Implications for Conservation With Climate Change

Biodiversity hotspots host a high diversity of narrowly distributed endemic species, which are increasingly threatened by climate change. In eastern North America, the highest concentration of plant diversity and endemism occurs in the Southern Appalachian Mountains (SAM). It has been hypothesized that this region served as a refugium during Pleistocene glacial cycles and that postglacial migration northward was dispersal limited. We tested this hypothesis using species distribution models for eight forest herb species. We also quantified the extent to which the geography of suitable habitat shifted away from the current range with climate change. We developed species distribution models for four forest herb species endemic to the SAM and four that co-occur in the same SAM habitats but have broader ranges. For widespread species, we built models using (1) all occurrences and (2) only those that overlap the SAM hotspot in order to evaluate the extent of Hutchinsonian shortfalls and the potential for models to predict suitable habitat beyond the SAM. We evaluated the extent to which predicted climatically suitable areas are projected to shift away from their current ranges under future climate change. We detected unoccupied but suitable habitat in regions up to 1,100 km north of the endemic species’ ranges. Endemic ranges are disjunct from suitable northern areas due to a ∼100–150 km gap of unsuitable habitat. Under future climate change, models predicted severe reductions in suitable habitat within current endemic ranges. For non-endemic species, we found similar overall patterns and gap of unsuitability in the same geographic location. Our results suggest a history of dispersal limitation following the last glacial maximum along with an environmental barrier to northward migration. Conservation of endemic species would likely require intervention and assisted migration to suitable habitat in northern New England and Canada

Of Mutualism and Migration: Will Interactions with Novel Ericoid Mycorrhizal Communities Help or Hinder Northward Rhododendron Range Shifts?

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\u27 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

Learning and Controlling Silicon Dopant Transitions in Graphene using Scanning Transmission Electron Microscopy

We introduce a machine learning approach to determine the transition dynamics of silicon atoms on a single layer of carbon atoms, when stimulated by the electron beam of a scanning transmission electron microscope (STEM). Our method is data-centric, leveraging data collected on a STEM. The data samples are processed and filtered to produce symbolic representations, which we use to train a neural network to predict transition probabilities. These learned transition dynamics are then leveraged to guide a single silicon atom throughout the lattice to pre-determined target destinations. We present empirical analyses that demonstrate the efficacy and generality of our approach

The Potential for Indirect Negative Effects of Exotic Insect Species on a Liverwort, \u3ci\u3eBazzania trilobata\u3c/i\u3e (Lepidoziaceae), Mediated by the Decline of a Foundation Tree Species, \u3ci\u3eTsuga canadensis\u3c/i\u3e (Pinaceae)

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

Climate Change and Forest Herbs of Temperate Deciduous Forests

This chapter reviews what is known about the long-term, large-scale range dynamics of forest herbs in response to past climate change. It presents a new biogeographic analysis investigating how contemporary distribution and diversity patterns among a subset of rare forest herbs may relate to these past climate dynamics. It also discusses how forest herb species may be affected by contemporary climate change and considers options for species conservation