The changing forest disturbance regimes of Europe

Abstract:

Canopy disturbances are important drivers of forest dynamics. Disturbance events such as wildfires, insect outbreaks and windthrows occur at very short time scales relative to forest development, yet influence ecosystem structure and composition for decades to centuries. Reports from individual studies suggest that disturbance regimes are changing in many ecosystems around the globe, yet quantitative information on forest disturbance regimes and their large-scale changes remain scarce. Here we quantified the forest disturbance regime for 210 Mill. ha of Europe’s forests at a spatial grain of 30 m. We manually analyzed 680,000 Landsat image chips for close to 20,000 locations throughout Europe, identifying canopy disturbances and testing for disturbance trends 1986 – 2016. We subsequently trained a machine learning algorithm to map forest disturbance regimes across Europe, quantifying disturbance frequency, size and severity. Between 1986 and 2016, 17% of Europe’s forest area was disturbed by anthropogenic or natural causes. We identified 25 Mill. individual disturbance patches with a mean patch size of 1.09 ha. On average, 0.52 disturbance events occurred per km² yr^-1, with a mean disturbance severity of 77% of the canopy removed. Based on our representative sample of locations we show that canopy disturbances in Europe have increased continuously from 1986 to 2016 at a rate of +1.5% per year. Changes in disturbance activity were primarily driven by an increase in disturbance frequency, while disturbance severity decreased over time. A further increase in disturbances could have profound impacts on the demographic structure of Europe’s forests, reversing the current trend towards older forest. We conclude that the forest disturbance regimes of Europe are undergoing profound changes. As these changes have important implications for forest carbon storage and biodiversity, they require increased attention from forest policy and management.