Abstract
Tree mortality is a key process underlying forest dynamics and community assembly. Understanding how tree mortality is
driven by simultaneous drivers is needed to evaluate potential effects of climate change on forest composition. Using
repeat-measure information from c. 400,000 trees from the Spanish Forest Inventory, we quantified the relative importance
of tree size, competition, climate and edaphic conditions on tree mortality of 11 species, and explored the combined effect
of climate and competition. Tree mortality was affected by all of these multiple drivers, especially tree size and asymmetric
competition, and strong interactions between climate and competition were found. All species showed L-shaped mortality
patterns (i.e. showed decreasing mortality with tree size), but pines were more sensitive to asymmetric competition than
broadleaved species. Among climatic variables, the negative effect of temperature on tree mortality was much larger than
the effect of precipitation. Moreover, the effect of climate (mean annual temperature and annual precipitation) on tree
mortality was aggravated at high competition levels for all species, but especially for broadleaved species. The significant
interaction between climate and competition on tree mortality indicated that global change in Mediterranean regions,
causing hotter and drier conditions and denser stands, could lead to profound effects on forest structure and composition.
Therefore, to evaluate the potential effects of climatic change on tree mortality, forest structure must be considered, since
two systems of similar composition but different structure could radically differ in their response to climatic conditions.
