Increased belowground tree carbon allocation in a mature mixed forest in a dry vs. a wet year

Climate change is expected to increase the frequency and severity of droughts in Mediterranean forests. Tree survival and storage of atmospheric CO₂ in these forests depend on how individual tree species allocate their carbon (C). Here, we measured a complete set of above- and belowground C pools and fluxes in five coniferous and broadleaf species co-existing in a mature evergreen forest. Our study period included a drought year, followed by an above-average wet year, and the seasonal long dry period characterizes Mediterranean climate. To quantify the exact timing and spatial distribution of belowground C allocation, we additionally applied ¹³CO₂ pulse labelling of one of the tree species (Quercus calliprinos). We found that during the dry versus wet year, photosynthetic C uptake decreased, C use in the C sinks remained unchanged and C allocation to belowground sinks increased. Among the five major C sinks, respiration was the main flux (~64%), while smaller fluxes like exudation (~9%) and reproduction (~1%) were those which increased the most in the dry year. To cope with seasonal drought, most trees relied on starch to maintain the C supply, but between years we found no significant differences in starch and sugars in aboveground tissues. Relative to the C storage dynamics, higher water use efficiency was found in conifers, while species-specific differences between dry and wet years were found among the broadleaves. Interestingly, in the wet season, after pulse labelling, C was allocated from the labeled leaves to the roots in two main time-lags: first after 3-5 days and second after 15-20 days. Labeled C reached fine roots at a distance of 0-6 m from the labeled tree. These detailed tree-level observations expose inter-annual and interspecific differences in C allocation among fluxes and tissues, specifically in response to varying water availability.