Photosynthate Allocation Patterns Along a Fire-Induced Age Sequence in Two Shrub Species From the California Chaparral

Abstract

In the prolonged absence of fire, shrubs of the California chaparral have sometimes been reported to demonstrate an increase in the proportion of standing dead biomass and a decrease in productivity (''senescence''). To investigate the possible physiological basis of this phenomenon, seasonal patterns of the allocation of 14C-labeled photosynthate were studied in leaves of two chaparral species, Adenostoma fasciculatum H. & A. and Ceanothus greggii var. perplexans (Trel.) Jeps., along a fire-induced age sequence. Harvested leaves labeled with 14C were analyzed for percent of 14C in storage, structural, metabolic, and defense compounds. No age-specific trends in photosynthate allocation were found except in the spring. During spring, when demands on photosynthate for growth were high, shrubs in older stands allocated a much lower proportion ot their photosynthate to storage compounds compared to shrubs in younger stands. Older shrubs are apparently less able to meet the concurrent demands of storage and growth during spring than younger shrubs. This observation is consistent with the notion of physiological senescence in older shrubs, and has implications for questions about the optimum length of the fire-free interval for stands managed with prescribed burning.