Maximum stand density strongly depends on species-specific wood stability, shade and drought tolerance

The abundance of stems in crowded populations and the subsequent self-thinning is a key issue in forest stand dynamics. However, the mechanisms that control self-thinning are challenging to model. Although some attempts to include climate and structural traits like specific gravity (SG) are promising, they remain confined to North American species. In this study we aimed to disentangle how SG along with two major abiotic stress tolerances, i.e. shade and drought tolerance, contribute to the maximum density of a forest stand across a climatic gradient in Europe, and thus test the validity of the species-specific trait control over stand density. We propose a modelling approach that incorporates the tolerance to drought and shade in the determination of maximum relative stocking. Here, relative stocking refers to the degree of tree crowding in forest ecosystems. A relative stocking base model where specific wood density is inversely related to stand density is modified, adding normalized indices of drought and shade stress tolerance. We used available species tolerance rankings modulated by stress intensity to analyse the effects of abiotic stress polytolerance or trade-offs in the study area which represent an environmental gradient from Alpine to Mediterranean climate in northern Spain. Results indicated that the role of drought tolerance in controlling maximum stand density is stronger in warmer sites. The simultaneous tolerance to shade and drought results in less carrying capacity of sites. In those sites where there is no water limitation but minimum temperature is very low the tolerance to bending stress (i.e. specific gravity) explains better the maximum tree occupancy.