Ecophysiological and morphological responses of balsam fir and red spruce to elevation and the canopy light gradient in the mountains of the northeastern United States

This study focuses on the response to environment of two coniferous trees, Abies balsamea [L.] Mill (balsam fir) and Picea rubens Sarg. (red spruce), with an emphasis on leaf (needle) structure and function. Research was conducted in the Adirondack Mountains (New York), Green Mountains (Vermont), and White Mountains (New Hampshire). High-elevation spruce-fir forests in these ranges are dominated by these two species, although there is often a significant Betula papyrifera var. cordifolia (Regel) Fern. (mountain paper birch) component, too. Balsam fir is a short-lived tree but has a wide geographic and ecological range. In comparison, red spruce is a long-lived tree with a much more restricted geographic range and comparatively narrow ecological niche. Samples were collected from sun and shade needles to represent the canopy light gradient, and below, at, and above the treeline, to represent an elevational gradient.; In terms of needle anatomy and morphology, the two species exhibited similar responses to the elevational gradient. In response to the canopy light gradient, shoot morphology appeared to be more plastic than needle morphology or anatomy. Although the response to the canopy light gradient differed somewhat between species, there was little evidence that sun/shade plasticity in balsam fir was greater than that of red spruce, at least in these forests where the two species grow together. These results do not support the hypothesis that the capacity for plasticity is correlated with ecological breadth. Results did, however, support the hypothesis that plasticity is reduced in a harsh growth environment, as both species exhibited significantly less sun/shade plasticity at the highest elevation sites than at either the mid or low elevation sites.; The physiological response to elevation and crown position was comparable for the two species, which suggests that they share surprisingly similar ecophysiologies when grown together in an unfavorable environment. In contrast to the morphological and anatomical results, however, chlorophyll fluorescence and spectral reflectance suggested a physiological divergence of sun and shade needles with increasing elevation. Sun needles became progressively more stressed with increasing elevation, whereas shade needles did not.; Many other studies commonly focus on just the full-sun response to environment. These results demonstrate, however, that studies of foliage from only one extreme crown position cannot hope to capture the whole-plant response to environment, because foliage from the other extreme may reveal very different patterns.