Seasonality of Potentail Stem Photosynthesis/Respiration in Four Hardwoods

Abstract

We observed an increasing rellectance or stems or four hardwoods in early spring. We hypothesized that it indicates an acclimation to spning conditions, likely associated with increased stem photosynthetic potential which is followed by re-acclimation in the fall to winter conditions. To test for changes in stem photosynthetic and respiratory capacity across seasons we contrasted, under laboratory conditions favorable to photosynthesize, CO2 evolution rates of hardwood stems acclimated to field conditions of winter, spring, summer and fall. The four species studied included two that brighten strongly in spring [red-osier dogwood (Cornus sericea), brittle willow (Salix Fragilis)] and two that brighten less [quaking aspen (Populus tremuloides), and black cottonwood (P. trichocarpa)]. Stem photosynthesis fixed, on average, 90 percent of carbon lost through respiration. We hypothesize that additional unmeasured photosynthesis is also occurring deeper within the stem. In all four species photosynthetic capacity increased in spring andsummer and Fell through fall to winter. In winter potential for photosynthesis fell markedly in both aspen and cottonwood and was eliminated in dogwood and willow. Responses or dogwood and willow provide the first known example of a complete down-regulation of stem photosynthesis in winter. Selective and selection-neutral hypotheses for this phenomenon are offered, i.e., that cessation or photosynthesis in winter has either been randomly fixed in shrub, (selection neutral) or created by selection of grazing animals against winter photosynthesis (palatability) in low shrubs greater than in trees with inaccessible twigs (natural selection). In contrast to photosynthesis, the potrntial for stem respiration appeared in all species and all seasons, and was usually lowest in winter and was highest in spring/summer.