Foliar carbon dynamics of piñon and juniper in response to experimental drought and heat

Abstract:

Plant respiration (R) is generally well-coupled with temperature and in the absence of thermal acclimation, respiration is expected to increase as climate change brings higher temperatures. Increased drought is also predicted for future climate, which could drive respiration higher if the carbon (C) cost to maintain tissues (R_m) or grow increases, or lower if substrate or other factors become limiting. We examined the effects of temperature and drought on R as well as photosynthesis, growth, and carbohydrate storage of mature individuals of two co-dominant tree species. Three mature, in-situ piñon (Pinus edulis) and juniper (Juniperus monosperma) trees were assigned to each of the following treatments: +4.8 °C; 45% reduced precipitation; a combination of both (heat + drought); along with ambient control and treatment controls. R_m measured prior to foliar and twig growth was far more sensitive to drought in piñon, and heat in juniper. Total respiration (R_t, R not partitioned) acclimated to temperature in piñon such that elevated temperature had minimal impacts on R_t; however, juniper exhibited higher R_t with elevated temperature, thus juniper did not display any thermal acclimation. R_t in both species was weakly associated with temperature, but strongly correlated with pre-dawn water potential, photosynthetic assimilation (A) rates, and in piñon, foliar carbohydrates. For both species, heat caused far more days where A-R was negative than did drought. The consequences of drought alone and heat alone in piñon included higher R_t per unit growth, indicating that each abiotic stress forces a greater allocation of R_t to maintenance costs, and both drought + heat in combination results in far fewer days that foliar carbohydrates could sustain R in both species. Notably, the much higher A and R of juniper than piñon is consistent with predicted superior carbon budget regulation of juniper than piñon during drought; however, juniper’s lack of temperature acclimation in contrast to piñon suggests climate warming may have a greater deleterious impact on juniper carbon balance than piñon.