Comparative growth trends of five northern hardwood and montane tree species reveal divergent trajectories and response to climate

In the northeastern US, tree declines associated with acid deposition-induced calcium depletion have been documented, notably for red spruce (Picea rubens Sarg.) and sugar maple (Acer saccharum Marsh.). There is conflicting evidence if co-occurring tree species have capitalized on these declines or suffered similar growth reductions; and, how growth has fluctuated relative to environmental variables. We examined five species along three elevational transects on Mt. Mansfield, Vermont: sugar maple, red spruce, red maple (Acer rubrum L.), yellow birch (Betula alleghaniensis, Britton), and balsam fir (Abies balsamea, [L.] Mill.). We found baseline differences in growth. Red maple and yellow birch had the highest growth, sugar maple and red spruce intermediate, and balsam fir the lowest. While some of year-to-year declines were associated with specific stress events, protracted patterns, such as recent increases in red spruce and red maple growth, were correlated with increased temperature and cooling degree days (heat index). For most species and elevations, there was a positive association between temperature and growth, but a negative association with growth the following year. Based on our comparisons, for some species growth at Mt. Mansfield aligns with regional trends and suggests that patterns assessed here may be indicative of the broader region.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Growth of canopy red oak near its northern range limit: current trends, potential drivers, and implications for the future

Red oak (Quercus rubra L.) is projected to expand into the northern hardwood forest over the coming century. We explored the connection between red oak basal area growth and a number of factors (tree age and size, stand dynamics, site elevation, and climate and acid deposition variables) for 213 trees in 11 plots throughout Vermont, USA. Red oak growth generally increased over the course of the chronology (1935–2014) and has been particularly high in recent decades. Growth differed among elevational groups but did not differ between age or size groups. Summer moisture metrics were consistently and positively associated with growth, whereas fall moisture was associated with reduced growth in recent decades. Higher summer temperatures were often negatively associated with growth, though there was evidence that low temperatures in the summer (higher elevations) and fall (lower elevations) constrain growth. Several pollution metrics were associated with reduced growth, a surprising result for a species not known to be sensitive to inputs of acid deposition that have predisposed other species in the region to decline. While red oak growth is currently robust, increases in summer temperatures, reductions in growing season precipitation, or increases in fall precipitation could reduce future growth potential.The accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities

Recent research using repeat photography, long-term ecological monitoring and dendrochronology has documented shrub expansion in arctic, high-latitude and alpine tundra ecosystems. Here, we (1) synthesize these findings, (2) present a conceptual framework that identifies mechanisms and constraints on shrub increase, (3) explore causes, feedbacks and implications of the increased shrub cover in tundra ecosystems, and (4) address potential lines of investigation for future research. Satellite observations from around the circumpolar Arctic, showing increased productivity, measured as changes in 'greenness', have coincided with a general rise in high-latitude air temperatures and have been partly attributed to increases in shrub cover. Studies indicate that warming temperatures, changes in snow cover, altered disturbance regimes as a result of permafrost thaw, tundra fires, and anthropogenic activities or changes in herbivory intensity are all contributing to observed changes in shrub abundance. A large-scale increase in shrub cover will change the structure of tundra ecosystems and alter energy fluxes, regional climate, soil–atmosphere exchange of water, carbon and nutrients, and ecological interactions between species. In order to project future rates of shrub expansion and understand the feedbacks to ecosystem and climate processes, future research should investigate the species or trait-specific responses of shrubs to climate change including: (1) the temperature sensitivity of shrub growth, (2) factors controlling the recruitment of new individuals, and (3) the relative influence of the positive and negative feedbacks involved in shrub expansion

A review of studies dealing with tree rings and rockfall activity: the role of dendrogeomorphology in natural hazard research

Over the last few years, rockfall research has increasingly focused on hazard assessment and risk analysis. Input data on past rockfall activity were gathered from historical archives and lichenometric studies or were obtained through frequency– volume statistics. However, historical records are generally scarce, and lichenometry may only yield data with relatively low resolutions. On forested slopes, in contrast, tree-ring analyses may help, generally providing annual data on past rockfall activity over long periods. It is the purpose of the present literature review to survey the current state of investigations dealing with tree-ring sequences and rockfall activity, with emphasis on the extent to which dendrogeomorphology may contribute to rockfall research. Firstly, a brief introduction describes how dendrogeomorphological methods can contribute to natural hazard research. Secondly, an account is provided of the output of dendrogeomorphological studies investigating frequencies, volumes or spatial distributions of past rockfall activity. The current and potential strengths of dendrogeomorphology are then presented before, finally, the weaknesses of tree rings as natural archives of past rockfall activity are discussed and promising directions for further studies outlined