Elevated temperature, soil moisture and seasonality but not CO2 affect canopy assimilation and system respiration in seedling Douglas-fir ecosystems.

摘要

We investigated the effects of elevated atmospheric CO2 and air temperature on C cycling in trees and associated soil system, focusing on canopy CO2 assimilation (Asys) and system CO2 loss through respiration (Rsys). We hypothesized that both elevated CO2 and elevated temperature would stimulate Asys and Rsys. The study was conducted in sun-lit controlled-environment mesocosms using Douglas-fir (Pseudotsuga menziesii Mirb. Franco) seedlings grown in reconstructed plant-litter-soil systems. A completely randomized design with two atmospheric CO2 and two air temperature levels was used. A mass-balance approach was used to calculate daily mean Asys and Rsys rates for 19 months. A mixed model analysis was used to test the effects of CO2 and air temperature on daily Asys and Rsys adjusted for covariates of time, light, soil moisture and seasonality. Elevated temperature stimulated Asys and Rsys but elevated CO2 did not. Elevated CO2 and temperature both increased light sensitivity and the light saturation level of photosynthesis. Both Asys and Rsys were controlled by temperature, soil moisture and endogenous seasonal processes. Temperature sensitivity of Rsys varied seasonally but there was no acclimatization. Because of the close linkage between assimilation and respiration, elevated CO2 failed to stimulate Asys and Rsys. Although CO2 is a substrate, assimilate is also controlled by its concentration. Needle-level studies established that increasing CO2 down regulates assimilation through changes in Rubisco, especially if resources are limited. This study shows that increasing CO2 also regulates assimilation allometrically through changes in needle area. Stimulation of assimilation is offset by a reduction in needle area such that the Asys and Rsys are similar in ambient and elevated treatments..