Growth Trends and Environmental Drivers of Major Tree Species of the Northern Hardwood Forest of Eastern North America

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 load-ing there.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),Ameri-can 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.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.Throughout their chronologies(1945-2014),all four species exhibited increasing growth followed by plateaued growth indicative of a maturing forest.For all species,summer mois-ture was positively correlated with growth,summer temperature was negatively associated with growth,and winter moisture or snow were positively correlated with growth.This last associa-tion was expected for sugar maple.However,our data suggest that winter snowpack may be more broadly relevant in sustain-ing tree growth in a region where snow has historically insu-lated the soil from freezing that can damage roots and lead to reduced aboveground growth.Measures of pollution deposition were also correlated with growth for all species except American beech—a species with documented tolerance to pollutant inputs.Of the four species studied,red maple had the fewest associa-tions with environmental variables,which suggests that it may be less susceptible to growth reductions as the climate changes.