Altitudinal Variation of Chlorophyll Concentration and Reflectance of the Bark of Populus tremuloides

Abstract

Although quaking aspen (Populus tremuloides Michx.) bark appears greener at higher elevations in the Sangre de Cristo mountain range of New Mexico, chlorophyll concentrations are inversely correlated (p = .001) with altitude. The higher concentration of chlorophyll at lower elevations is effectively masked by a white bloom of dead periderm cells which is also characteristic of exposed site trees. At higher elevations the bloom is yellow—brown and more translucent, thus allowing the chlorophyll layers beneath to show through. Therefore, trees at higher elevations appear greener even though they have lower cortical chlorophyll concentrations. The altitudinal gradient in bark reflectance characteristics may be of adaptive value. If cortical photosynthesis is more important at higher elevations, the translucent bloom would be advantageous in making more light available for photosynthesis. Increased light absorption might also be important in raising bark temperatures for physiological activity during the winter months. At lower elevations where air temperatures are higher, the highly reflective bloom may protected the trees from high bark temperatures and sunscald.