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For example, tree species of the northern hardwood forest are adapted to winters with consistent periods of snowpack. But for species specifically adapted to the northern hardwood forests of northeastern North America, a changing hydrological regime during the non-growing season may have a substantial influence on growth as well. Growing season moisture has been shown to be a driving factor of growth for numerous tree species (D'Orangeville et al. However, while precipitation rates have risen, this increase has not been distributed equally across all seasons (Kunkel et al. 2013), a pattern that is expected to continue or even increase as climate change progresses (Janowiak et al. The northeastern United States (US) and adjacent Canada have experienced an unprecedented amount of precipitation over the last several decades (e.g., Pederson et al. 2017) and growth (Martin-Benito and Pederson 2015 Bosela et al 2018 D'Orangeville et al. Recent large-scale studies in North America and Europe show that moisture and drought sensitivity are major drivers of change in tree species abundance (Mette et al. Of the four species studied, red maple had the fewest associations with environmental variables, which suggests that it may be less susceptible to growth reductions as the climate changes. Measures of pollution deposition were also correlated with growth for all species except American beech-a species with documented tolerance to pollutant inputs.
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However, our data suggest that winter snowpack may be more broadly relevant in sustaining tree growth in a region where snow has historically insulated the soil from freezing that can damage roots and lead to reduced aboveground growth. This last association was expected for sugar maple. For all species, summer moisture was positively correlated with growth, summer temperature was negatively associated with growth, and winter moisture or snow were positively correlated with growth. Throughout their chronologies (1945–2014), all four species exhibited increasing growth followed by plateaued growth indicative of a maturing forest. We also analyzed the link between growth and several factors for 690 trees in 45 plots throughout Vermont, USA: tree age and size, site elevation, and climate and acid deposition variables. To better understand the trajectory of the northern hardwood forest, we studied the growth of three tree species emblematic of it: sugar maple ( Acer saccharum Marsh), American beech ( Fagus grandifolia Ehrh.), and yellow birch ( Betula alleghaniensis Britton), plus a fourth species, red maple ( Acer rubrum L.), whose abundance has increased in the region. The future health and productivity of tree species in the northern hardwood forest of eastern North America are uncertain considering changes in climate and pollution loading there.