Forest dynamics and the growth decline of red spruce and sugar maple on Bolton Mountain, Vermont: a comparison of modeling methods

Montane forests in the northeastern United States have experienced symptoms of declining vigor, such as branch dieback and increased mortality, over the last half-century. These declines have been attributed to the cumulative impacts of acid deposition, but reconstructing these declines from tree-ring records has proved difficult because of confounding factors that affect low-frequency growth patterns, including climate and natural growth trajectories following disturbance. We obtained tree-ring records of red spruce ( Picea rubens Sarg.) and sugar maple ( Acer saccharum L.) from three elevations on Bolton Mountain, Vermont, and applied traditional dendroclimatological analyses that revealed a profound declining growth–climate correlation since ca. 1970 for sugar maple but much less so for red spruce. We then applied a new multifaceted statistical approach that conservatively detrends tree-ring records by minimizing the influences of tree size, age, and canopy disturbances on radial growth. In contrast with the traditional analysis, this approach yielded chronologies that were consistently correlated with climate but with important exceptions. Low-elevation sugar maple suffered distinct episodes of slow growth, likely because of insect defoliators, and also a progressive decline since ca. 1988. Red spruce experienced subdecadal episodes of decline that may be related to freeze–thaw events known to injure foliage but showed no evidence of a progressive decline. This analysis was supported by a forest plot resurvey that indicated major declines in these species.