STEM RESPIRATION OF PONDEROSA PINES GROWN IN CONTRASTING CLIMATES - IMPLICATIONS FOR GLOBAL CLIMATE-CHANGE

We examined the effects of climate and allocation patterns on stem res piration in ponderosa pine (Pinus ponderosa) growing on identical subs trate in the cool, moist Sierra Nevada mountains and the warm, dry, Gr eat Basin Desert. These environments are representative of current cli matic conditions and those predicted to accompany a doubling of atmosp heric CO2, respectively, throughout the range of many western north Am erican conifers. A previous study found that trees growing in the dese rt allocate proportionally more biomass to sapwood and less to leaf ar ea than montane trees. We tested the hypothesis that respiration rates of sapwood are lower in desert trees than in montane trees due to red uced stem maintenance respiration (physiological acclimation) or reduc ed construction cost of stem tissue (structural acclimation). Maintena nce respiration per unit sapwood Volume at 15 degrees C did not differ between populations (desert: 6.39 +/- 1.14 SE mu mol m(-3) s(-1), mon tane: 6.54 +/- 1.13 SE mu mol m(-3) s(-1), P = 0.71) and declined with increasing stem diameter (P = 0.001). The temperature coefficient of respiration (Q(10)) varied seasonally within both environments (P = 0. 05). Construction cost of stem sapwood was the same in both environmen ts (desert: 1.46 +/- 0.009 SE g glucose g(-1) sapwood, montane: 1.48 /- 0.009 SE glucose g(-1) sapwood, P = 0.14). Annual construction resp iration calculated from construction cost, percent carbon and relative growth rate was greater in montane populations due to higher growth r ates. These data provide no evidence of respiratory acclimation by des ert trees. Estimated yearly stem maintenance respiration was greater i n large desert trees than in large montane trees because of higher tem peratures in the desert and because of increased allocation of biomass to sapwood. By analogy, these data suggest that under predicted incre ases in temperature and aridity, potential increases in aboveground ca rbon gain due to enhanced photosynthetic rates may be partially offset by increases in maintenance respiration in large trees growing in CO2 -enriched atmospheres.