Climate limits on European forest structure across space and time

The structure of a forest dictates its function, vulnerability to mortality, and ecosystem services it provides. Many aspects of the environment and management determine forest structures, such as canopy height, stand density, carbon content, etc. Environmental factors, such as climate, limit the extent to which management can maximize structures of a forest. By understanding how climate limits forest structures over large landscapes we can better quantify the potential upper limit that a forest structure can achieve independent of management. Further, by quantifying how climate limits forest structures we can deepen our understanding of the impact climate change has had and will have on our forest resources and services. This type of information goes beyond quantifying how climate will impact the pools and fluxes of a forest, which is typically done for climate change studies over large landscapes. Estimating how climate change will impact structures will allow us to quantify how climate change will impact resources and services unquantifiable by pools and fluxes alone - such as biodiversity, habitat suitability, and market values. We quantified how maximum and minimum temperatures, and precipitation limit 3 forest structures, diameter at breast height, height, and basal area across the European continent. We found that climate zones exist that maximize each forest structure. Further, we estimated how climate change since the 1950's has influenced the potential structures of European forests by assessing eight individual forests throughout Europe and then Europe as a whole. All three forest structures are limited in different ways depending on their location in Europe. Though some individual forests have seen a benefit from climate change, European forests on average have lost 5.0%, 1.7% and 6.5% of potential forest diameter, height and basal area respectively. Further, the extremes of the climate values in our study, which may support endemic life, have already begun to vanish from the entire continent.