Plant respiration (R) is generally correlated with temperature and is expected to increase with rising temperatures in the absence of thermal acclimation. Increased drought is also predicted for future climate in many regions, which could reduce respiration due to reduced growth or if substrates become limiting. We examined the effects of temperature, drought, and their combination on foliar nighttime R and photosynthesis of mature piñon (Pinus edulis) and juniper (Juniperus monosperma) trees in a precipitation reduction and elevated temperature experiment.
CO2 efflux from maintenance and total respiration (Rm and Rt, respectively) showed signs of acclimation to temperature in piñon but not juniper, and drought reduced Rm in piñon. Over the growing season Rt in both species was often decoupled from temperature, but strongly correlated with pre-dawn leaf water potential, photosynthetic assimilation (A) rates, and in piñon, foliar carbohydrates. Heat in piñon and the combination of heat and drought in juniper caused far more days when the carbon balance (A-R) was negative than did drought. The consequences of drought and heat in piñon included higher Rt per unit growth, indicating that each abiotic stress forces a greater allocation of Rt to maintenance costs whereas juniper grew very little and no growth differences were observed across treatments . Drought and heat also reduced the number of days that foliar carbohydrates could sustain R in both species. The larger carbon balance (A-R) of juniper than piñon is consistent with their relative aniso/isohydric stomatal behavior; however, juniper’s lack of temperature acclimation suggests climate warming may have a greater relative impact on the juniper carbon balance than piñon.