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Projected changes in temperature and drought regime are likely to reduce carbon (C) storage in forests, thereby amplifying rates of climate change. While such reductions are often presumed to be greatest in semi-arid forests that experience widespread tree mortality, the consequences of drought may also be important in temperate mesic forests of Eastern North America (ENA) if tree growth is significantly curtailed by drought. Investigations of the environmental conditions that determine drought sensitivity are critically needed to accurately predict ecosystem feedbacks to climate change. We matched site factors with the growth responses to drought of 10,753 trees across mesic forests of ENA, representing 24 species and 346 stands, to determine the broad-scale drivers of drought sensitivity for the dominant trees in ENA. Here we show that two factors--the timing of drought, and the atmospheric demand for water (i.e., local potential evapotranspiration; PET)--are stronger drivers of drought sensitivity than soil and stand characteristics. Droughtinduced reductions in tree growth were greatest when the droughts occurred during early-season peaks in radial growth, especially for trees growing in the warmest, driest regions (i.e., highest PET). Further, mean species trait values (rooting depth and w50) were poor predictors of drought sensitivity, as intraspecific variation in sensitivity was equal to or greater than interspecific variation in 17 of 24 species. From a general circulation model ensemble, we find that future increases in earlyseason PET may exacerbate these effects, and potentially offset gains in C uptake and storage in ENA owing to other global change factors. [ABSTRACT FROM AUTHOR]

Ericaceous shrubs can influence soil properties in many ecosystems. In this study, we examined how soil and forest floor properties vary among sites with different ericaceous evergreen shrub basal area in the southern Appalachian mountains. We randomly located plots along transects that included open understories and understories with varying amounts of Rhododendron maximum (rosebay rhododendron) and Kalmia latifolia (mountain laurel) at three sites. The three sites were a mid-elevation ridge, a low-elevation cove, and a high-elevation southwest-facing slope. Basal area of R. maximum was more correlated with soil properties of the forest floor than was K. latifolia. Increasing R. maximum basal area was correlated with increasing mass of lower quality litter and humus as indicated by higher C:N ratios. Moreover, this correlation supports oar prediction that understory evergreen shrubs may have considerable effect on rarest floor resource heterogeneity in mature stands. [ABSTRACT FROM AUTHOR]

Managed ecosystems experience vulnerabilities when ecological resilience declines and key flows of ecosystem services become depleted or lost. Drivers of vulnerability often include local management actions in conjunction with other external, larger-scale factors. To translate these concepts to management applications, we developed a conceptual model of feedbacks linking the provision of ecosystem services, their use by society, and anthropogenic change. From this model we derived a method to integrate existing geodata at relevant scales and in locally meaningful ways to provide decision-support for adaptive management efforts. To demonstrate our approach, we conducted a case study assessment of southeast Alaska, where managers are concerned with sustaining fish and wildlife resources in areas where intensive logging disturbance has occurred. Individual datasets were measured as indicators of one of three criteria: ecological capacity to support fish/wildlife populations ( provision); human acquisition of fish/wildlife resources ( use); and intensity of logging and related land-use change ( disturbance). Relationships among these processes were analyzed using two methods—a watershed approach and a high-resolution raster—to identify where provision, use and disturbance were spatially coupled across the landscape. Our results identified very small focal areas of social-ecological coupling that, based on post-logging dynamics and other converging drivers of change, may indicate vulnerability resulting from depletion of ecosystem services. We envision our approach can be used to narrow down where adaptive management might be most beneficial, allowing practitioners with limited funds to prioritize efforts needed to address uncertainty and mitigate vulnerability in managed ecosystems. [ABSTRACT FROM AUTHOR]

Inhibition of canopy tree recruitment beneath thickets of the evergreen shrubs Rhododendron maximum L. and Kalmia latifolia L. has long been observed in Southern Appalachian forests, yet the mechanisms of this process remain unresolved. We present a first-year account of suppression of oak seedlings in relation to Rhododendron and Kalmia basal area, light and resource availability, seedling performance and the rates of seedling damage (i.e., herbivory). We found no evidence of first-year seedling suppression or significant resource deficiencies beneath thickets of K. latifolia in mature mixed hardwood stands. Suppression beneath R. maximum was apparent during the first growing season. We found that seedling biomass, light availability prior to canopy closure, and seedling tissue C:N ratios were negatively correlated with R. maximum basal area. Basal area of R. maximum was positively correlated with seedling mortality rates, soil [Al], and early-growing season leaf herbivory rates. Seedling growth was positively correlated with light and tissue C:N, while negatively correlated with soil [Al]. Overall, our results support the inhibition model of shade-mediated carbon limitation beneath dense understory shrubs and indicate the potential importance of herbivory and aluminum toxicity as components of a suppression mechanism beneath R. maximum thickets. We present a causal model of first year inhibition beneath R. maximum in the context of our findings and the results of prior studies. [ABSTRACT FROM AUTHOR]