Acclimation of Leaf Respiration to Temperature Change and Implications for Coupled Climate-Carbon Cycle Feedbacks

Leaf maintenance respiration is known to increase in response to an increase in temperature. This knowledge has led to a variety of predictions that leaf and plant respiration will increase in response to the warmer temperatures of a future world of higher atmospheric CO2 concentrations. This expectation has implications for climate change itself. Higher leaf respiration with warmer temperatures could lead to the release of more CO2 to the atmosphere from terrestrial ecosystems, correspondingly higher atmospheric CO2 concentrations, and consequently more warming leading to still higher respiration and further warming in a positive feedback. However, it is also known that over time leaf maintenance respiration often acclimates to warmer temperatures. Leaves of plants experimentally grown at higher temperatures often show a reduced sensitivity to further increases in temperature compared to plants grown at cooler temperatures. Temperature acclimation of leaf maintenance respiration could reduce the positive feedback between climate and the carbon cycle in a warming world. Yet, while most coupled climate-carbon models include an increase in leaf and plant respiration in response to elevated temperature, few if any include an explicit time-dependent acclimation of plant respiration to increasing temperatures. To address this issue we are investigating the influence of temperature acclimation of leaf maintenance respiration (Rleaf_m) on simulated carbon dynamics and climate-carbon feedbacks at both the local ecosystem scale and globally. We have modeled a temperature-dependent sensitivity to temperature change for Rleaf_m and derived and parameterized an empirical formulation of the acclimation of Rleaf_m to temperature change using results from plant warming experiments. These functional forms are implemented in a site-scale ecosystem model (LoTEC) and a global terrestrial biogeochemistry model (GTEC 2.0). We compare simulations using these modified representations of leaf sensitivity to temperature with those using a more conventional fixed sensitivity equivalent to a constant Q10 value. The models are driven by scenarios of warming and climate change from the Parallel Climate Model. In the global simulations, acclimation to warming reduces global annual Rleaf_m in year 2100 by 14.8%, but the global annual total plant respiration is reduced by only 1.5%. With acclimation of leaf respiration, a large fraction of carbon not respired by leaves is allocated to other plant parts and released by the growth and non-acclimated maintenance respiration of these other plant parts. Similarly, global net primary production increases by only 1.1% with leaf acclimation to temperature, with a correspondingly small increase in litter and soil carbon. However, small annual increases accumulate over time, and over the simulation from 1930 to 2100, acclimation of R_m(leaf) increases the cumulative global terrestrial biospheric sink by 9.1%. We conclude that while the influence of acclimation of leaf maintenance respiration to temperature is not likely to dominate over other uncertainties, it is of sufficient magnitude to warrant inclusion in coupled climate-carbon simulations. Our analyses indicate a need for improved understanding of temperature acclimation by other plant parts and of the acclimation of respiration at lower temperatures characteristic of high latitudes. With this improved understanding, increasingly appropriate formulations of acclimation can be developed for inclusion in models of the global carbon cycle.