Climate modifies tree interactions in terms of basal area growth and mortality in monospecific and mixed Fagus sylvatica and Pinus sylvestris forests

Net interactions between trees vary depending on environmental conditions and other factors such as stand density, age, or between-species complementarity and/or facilitation. According to the stress gradient hypothesis, positive or facilitative interactions are more frequent in high-stress environments whereas negative or competitive interactions occur in benign environments, although recent studies highlight the influence of species composition, type of stress, ontogeny, etc. on the interaction-stress gradient pattern. The aim of this paper is to analyze whether site climatic variables are a key factor in tree interactions in mixed stands of beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.). To test how site climatic conditions modify the effect of inter-specific competition on tree basal area growth and tree mortality, growth and mortality models were fitted using monospecific and mixed sample plots located in matching site conditions selected from the Spanish National Forest Inventory in the Navarra Province. Tree competition status was broken down into four terms according to size-symmetry (size-symmetric vs. size-asymmetric) and species identity (intra-specific vs. inter-specific). The results showed that the size-symmetric inter-specific component was non-significant on beech basal area growth and had a negative effect on pine growth. The effect of size-asymmetric inter-specific competition was always non-significant, resulting in a higher basal area growth when the admixed species are of a larger size. The interaction between annual precipitation and this inter-specific competition effect was much more pronounced on beech than on pine. Inter-specific competition had a negative effect on pine growth under better climatic conditions and a positive effect at dryer sites, while beech always benefited from the presence of pine, although the benefit was greater where climatic site conditions were better. In tree mortality models, pine mortality increased with the proportion of beech, while beech mortality was lower as the proportion of pine increased. Precipitation modified the inter-specific competition effect on tree mortality although the site influence was less relevant than on tree growth. For pine mortality, the negative effect of beech admixture was stronger at lower mean annual precipitation, while in the case of beech the positive effect of pine increased at higher levels of precipitation. The influence of climate on the effect of competition, the variation in their strength depending on the mode of competition (size-symmetric or size-asymmetric), along with the inter-specific competition component, highlight the importance of considering the effect of site conditions on between-species interactions when modeling tree growth and mortality. The species-specific patterns of variation in tree interactions along climatic gradients and the differences in tree growth and mortality corroborate the need to consider the nature of stress-limiting factors and species composition and the importance of analyzing both dynamic processes simultaneously.