Modelling the climatic drivers determining photosynthesis and carbon allocation in evergreen Mediterranean forests using multiproxy long time series

[摘要] Climatic drivers limit several important physiological processes involved inecosystem carbon dynamics including gross primary productivity (GPP) andcarbon allocation in vegetation. Climatic variability limits these twoprocesses differently. We developed an existing mechanistic model to analysephotosynthesis and variability in carbon allocation in two evergreen speciesat two Mediterranean forests. The model was calibrated using a combinationof eddy covariance CO₂ flux data, dendrochronological time series ofsecondary growth and forest inventory data. The model was modified to beclimate explicit in the key processes addressing the acclimation ofphotosynthesis and the pattern of C allocation, particularly to waterstress. It succeeded in fitting both the high- and the low-frequency response ofstand GPP and carbon allocation to stem growth. This would support itscapability to address both C-source and C-sink limitations. Simulationssuggest a decrease in mean stomatal conductance in response to a recentenhancement in water stress and an increase in mean annual intrinsic wateruse efficiency (iWUE) in both species during the last 50 years. However,this was not translated into a parallel increase in ecosystem water useefficiency (WUE). The interannual variability in WUE closely followed that iniWUE at both sites. Nevertheless, long-term decadal variability in WUEfollowed the long-term decrease in annual GPP matching the local trend inannual precipitation observed since the late 1970s at one site. In contrast,at the site where long-term precipitation remained stable, GPP and WUE didnot show a negative trend and the trees buffered the climatic variability.In our simulations these temporal changes were related to acclimationprocesses at the canopy level, including modifications in LAI andstomatal conductance, but also partly related to increasing [CO₂]because the model includes biochemical equations where photosynthesis isdirectly linked to [CO₂]. Long-term trends in GPP did not match thosein growth, in agreement with the C-sink hypothesis. The model has great potential for use with abundant dendrochronological data andanalyse forest performance under climate change. This would help tounderstand how different interfering environmental factors produceinstability in the pattern of carbon allocation and, hence, the climatic signalexpressed in tree rings.

[学科分类] 地球化学与岩石