Gershenson, Alexander (Univ. of California, Santa Cruz, UCSC Environmental Studies, 1156 High St, Santa Cruz, CA, 95064; Phone: 831-252-3514; Email: agersh@ucsc.edu)

Soil Moisture and Temperature Constraints on Fine Root Dynamics, Plant Growth, and Photosynthetic Activity in a Mediterranean Forest in the Sierra Nevada, CA

A. Gershenson *, L. Misson, J. Tang, A. Goldstein, W. Cheng

Root turnover is one of the major pathways for terrestrial carbon cycling. Although many investigations have focused on determining how various factors affect fine root dynamics, few have examined the effects of intra-seasonal variability associated with Mediterranean climates on fine root and related aboveground growth dynamics. Data collected during the 2003 growth season suggest that phenological development of Ponderosa pine-dominated ecosystems of Sierra Nevada may be highly dependent on prevailing climatic conditions. We monitored fine root development, trunk thickening, shoot and needle elongation, and shrub LAI changes throughout the season, as well as soil moisture, and soil and air temperature and canopy level photosynthesis. The site is characterized by cold, wet winters and extremely hot and dry summers. The onset of growth in this ecosystem appears to be controlled by the increase of the daily minimum temperatures above 5 degrees C in the middle of May, and the end of the growing season appears to be controlled by the decrease of available soil moisture to below 15% in the middle of July. We also monitored canopy photosynthetic activity, which dramatically increased in the beginning of Spring and decreased by the end of June. Current available climate modeling projections focused on this area of California predict significant snow accumulation decreases, and increases of springtime temperatures by as much as 4 degrees C within the next twenty years. These changes to the temperature and moisture regimes during the critical time period have the potential of radically altering the time available for ecosystem growth and carbon accumulation.