Résumé
Question: Tree mortality can be modeled using two complementary covariates, tree size and tree growth. Tree growth is an integrative measure of tree vitality while tree diameter is a good index of sensitivity to disturbances and can be considered as a proxy for tree age which may indicate senescence. Few mortality models integrate both covariates because classical model calibration requires large permanent plot data-sets which are rare. How then can we calibrate a multivariate mortality model including size and growth when permanent plots data are not available?
Location: To answer this question, we studied Abies alba and Picea abies mortality in the French Swiss and Italian Alps.
Method: Our study proposes an alternative semi-parametric method which includes a random sample of living and dead trees with diameter and growth measurements.
Results: We were able to calibrate a mortality model combining both size-dependent and growth-dependent mortality. We demonstrated that A. alba had a lower annual mortality rate (10%) than P. abies (18%) for low growth (< 0.2 mm year(-1)). We also demonstrated that for higher diameters (DBH >= 70 cm), P. abies had a higher mortality rate (0.45%) than A. alba (0.32%).
Conclusion: Our results are consistent with the mechanisms of colonization-competition trade-off and of successional niche theory which may explain the coexistence of these two species in the Alps. The method we developed should be useful for forecasting tree mortality and can improve the efficiency of forest dynamics models.