Low naturalness of Swiss broadleaf forests increases their susceptibility to disturbances

Abstract

The tree species composition of Swiss forests is influenced by both environmental conditions and centuries of forest management, resulting in varying degrees of naturalness. Here, we estimate the naturalness of Swiss forests by comparing the tree species composition (i.e., dominance and presence/absence of tree species) recorded by the national forest inventory with the idealised species composition of the potential natural forests. We aimed to (1) evaluate the naturalness of different forest types, (2) identify the main drivers of low naturalness and (3) investigate the influence of naturalness on the susceptibility to disturbances. Based on our analysis, 45 % of Swiss forests had a tree species composition classified as ‘not natural’ while 42 % were classified as ‘natural’ or ‘close to natural’. The vast majority (65 %) of the forests classified as ‘not natural’ were potential European beech (Fagus sylvatica L.) and other broadleaf forests which are currently dominated by conifers. In addition, at higher elevations, forests often showed a higher proportion of European larch (Larix decidua Mill.) than expected, based on the potential natural forest. Overall, forests classified as ‘natural’ had a significantly lower risk of being affected by stand-level disturbance events. Potential natural European beech and other broadleaf forests were significantly less affected by disturbances, namely insect outbreaks and wind throw, when comparing ‘natural’ to ‘not natural’ (i.e., afforested with conifers). The observed dieback and associated loss of ecosystem function of conifer afforestations (mainly Norway spruce; Picea abies (L.) H. Karst.) in the Swiss plateau is likely a combination of climatic stress and higher pathogen/parasite pressure. These Norway spruce afforestations already experience a significantly elevated temperature of about 2.5 °C compared to their climatic optimum (i.e., primary natural distribution within Switzerland), representing a large-scale, long-term transplantation experiment. We discuss the question of whether similar diebacks and problems of maladaptation are to be expected for many other species, including European beech, due to accelerated climate warming during the 21st century and the slow transformation of long-lived systems such as forests. Consequently, an adaptation of current management practices might be needed to allow a faster transition of forests minimizing the risk of large-scale diebacks.