Modelling central European oak growth

Terrestrial ecosystem models are key tools for understanding and predicting climate and CO₂ impacts on vegetation. Despite decades of research, there remains considerable uncertainty as to how to simulate canopy photosynthesis and transpiration, respiration, growth, and turnover. This uncertainty includes the approach to use, its complexity, and its parameterisation, and covers all processes from light interception to soil water dynamics to demography. While simulated carbon fluxes are often compared with eddy-covariance measurements, simulated growth is more difficult to evaluate. Tree rings can provide a useful measure of interannual growth rates, and are therefore an important potential constraint on model representations. Here, we use a new central European oak ring width chronology, 1901-2015 CE, to evaluate a range of model assumptions regarding key processes. Capturing more than a limited amount of the interannual variability in the chronology is difficult (i.e. achieving Pearson’s r > 0.5, including the long-term trend). It is clear that soil hydrological dynamics are a key driver and need to be represented well, as do leaf phenology, respiration, and responses to temperature and CO₂. Spatial variability in climate and other factors such as soil type and depth also plays an important role. Less clear is how to explain some years with very strong or very weak growth. Explanations are sort in extreme temperatures (e.g. frost damage), extreme drought stress (e.g. causing xylem cavitation), recovery from storm damage, carry-over between years, and separation of controls on growth and photosynthesis. Recommendations are made regarding requirements for process representations and for future work to better understand drivers of tree-ring variability in temperate forest ecosystems.