Simulating the response of theQuercus mongolicaforest ecosystem carbon budget to asymmetric warming

It has been found that nighttime minimum temperature has increased more rapidly than daytime maximum temperature at locations around the Northern Hemisphere in recent years. It is therefore important to understand the differential responses of terrestrial ecosystems to asymmetrical vs. symmetrical diurnal warming, and their consequent influences on ecosystem carbon cycling. We used a well-established, process-based ecosystem model (BIOME-BGC) combined with climate change scenarios, to examine effects of asymmetric temperature change on a Quercus mongolica forest ecosystem carbon budget in North China. Two mean temperatures increases (3℃ and 4℃) are applied to baseline daily climate data (1961–1990). The effects of three different scenarios of temperature change are analyzed (minimum and maximum temperatures raised equally; minima increased two or three times as much as maxima), under both current and elevated CO2 concentrations. Carbon sequestration was consistently negative under temperature change and current CO2 concentration, and negative effects of temperature declined when minima were increased more than maxima. Sequestration changed to positive under temperature change and elevated CO2 concentration, and positive effects of temperature increased when minima were increased more than maxima. Our results indicate that the potential effects of global change on Quercus mongolica forest ecosytem productivity (GPP and NPP) may be less severe with equal day-night warming than with asymmetric day-night warming. In this view, we suggest that estimates based only on average temperature change may to some extent overestimate the positive feedback of warm temperate deciduous broad-leaf forest ecosystems to climate warming. However, the model simulation results have not been confirmed by corresponding observation and experiment. Only by giving full consideration to minimum, maximum and average temperatures and their respective ecological effects via coordinated approaches, can we more realistically assess forest ecosystem carbon source/sink functions in the real world.