Structure, environmental patterns and impact of expected climate change in natural beech-dominated forests in the Cantabrian Range (NW Spain)

Abstract The European beech (Fagus sylvatica L.) occurs in the Cantabrian Range (NW Spain), at the southwestern limit of the wide distribution area of the species in Europe, forming relatively unmanaged forests of high biodiversity value. In this study, we measured three-dimensional positions, diameter at breast height and height of all the trees present in 112 inventory plots established in beech-dominated forests in the north-western Cantabrian Range, in which hemispherical photographs were taken and a detailed floristic inventory was carried out. In addition, we measured 56 spatially continuous environmental variables in each plot to enable examination of environmental patterns in structural features and prediction of the effects of climate change. Forest structure was analyzed by using indices that evaluated spatial tree distribution, plant richness and tree species diversity, diversity of tree dimensions and vertical structure, stand density and average tree size, standing deadwood, canopy geometry and light regime. The stands exhibited a moderate clustered spatial arrangement at young stages, becoming more regular as they matured. The stands are generally monospecific, with low plant richness, never monostratified, with very close canopies, greater variation in diameter than in height and are usually overstocked. Only 25% of the stands included some standing dead trees. Random Forest models were used to describe structural features as a function of environmental variables. Although some of the models were complex and included many predictor variables, they revealed some interesting patterns. Thus, we found that spatial tree distribution was only related to lithostratigraphy, and tree species richness and vertical structure were related to isothermality. Shrub and herbaceous richness were related to soil pH and several thermal variables, while intermingling of tree species was mainly explained by soil-related variables. Climatic variables explained differences in tree diameter, whereas edaphic variables were more important for predicting differences in tree height. Stocking degree was mainly related to soil variables, while dominant height was related to thermal variables and standing dead wood to climatic variables. Projections under the moderate RCP 4.5 and pessimistic RCP 8.5 climate change scenarios predict a shift in beech forests towards increased shrub and plant richness and species diversity, but also increased stocking degree and standing deadwood basal area. These findings appear to confirm a drastic reduction in the suitable habitat for beech in the region (deterioration of future growth conditions), which could anticipate a loss of competitive advantage over other species and indicate a shift in this beech-dominated forest to more resilient mixed stands.