Abstract
Since several studies have been reporting an increase in the decline of forests, a major issue in ecology is to better understand and predict tree mortality. The interactions between the different factors and the physiological processes giving rise to tree mortality, as well as the individual between-tree variability to mortality risk, still need to be identified and assessed.
This study is based on a survey of 4323 European beeches (Fagus sylvatica L.) since 2002 in a rear-edge population within a natural reserve. We combined two types of approaches: (1) statistical models were used to quantify the effects of competition, tree growth, size and decline on mortality and (2) an ecophysiological process-based model (PBM) was used to separate out the different mechanisms giving rise to temporal and inter-individual variations in mortality by simulating carbon reserves, hydraulic conductance and late frosts in response to climate.
The mortality rate at population level was associated to the combination of conductance loss, carbon reserve depletion and occurence of late frosts simulated with the PBM. In the statistical models, the individual probability of mortality decreased with increasing mean growth, and increased with increasing crown defoliation, earliness of budburst, fungi presence and increasing competition. The interaction between tree size and defoliation was significant, indicating a stronger increase in mortality associated to defoliation in smaller than larger trees. Finally, the PBM predicted a higher conductance loss together with a higher level of carbon reserves for trees with earlier budburst, while the ability to defoliate the crown was found to limit the impact of hydraulic stress at the expense of the accumulation of carbon reserves.
We discuss the convergences and divergences obtained between statistical and process-based approaches and we highlight the importance of combining them to identify the different processes underlying mortality, and the factors modulating individual vulnerability to mortality.
Footnotes
This is version 2 of the manuscript, revised after review by PCIEcology.