Increases in canopy mortality and their impact on the demographic structure of Europe’s forests

Pulses of tree mortality have been reported for many ecosystems across the globe recently. Yet, large-scale trends in tree mortality remain poorly quantified. Manually analyzing more than 680,000 satellite image chips at 19,896 plot locations, we here show that forest canopy mortality in Europe has continuously increased since 1985 (+1.5 ± 0.28 % yr-1), with the highest canopy mortality rate of the past 34 years observed in 2018 (1.14 ± 0.16 %). Using simulations, we demonstrate that a continued increase in canopy mortality will strongly alter forest demography, with the median forest age falling below 30 years in more than 50% of Europe’s countries by 2050. These demographic changes can have substantial cascading effects on forest regeneration, biodiversity, and carbon storage. The current trend of increasing canopy mortality is thus challenging the future of Europe’s forests, and should be a key priority of forest policy and management.

Role of forest regrowth in global carbon sink dynamics

Although the existence of a large carbon sink in terrestrial ecosystems is well-established, the drivers of this sink remain uncertain. It has been suggested that perturbations to forest demography caused by past land-use change, management, and natural disturbances may be causing a large component of current carbon uptake. Here we use a global compilation of forest age observations, combined with a terrestrial biosphere model with explicit modeling of forest regrowth, to partition the global forest carbon sink between old-growth and regrowth stands over the period 1981–2010. For 2001–2010 we find a carbon sink of 0.85 (0.66–0.96) Pg year−1located in intact old-growth forest, primarily in the moist tropics and boreal Siberia, and 1.30 (1.03–1.96) Pg year−1located in stands regrowing after past disturbance. Approaching half of the sink in regrowth stands would have occurred from demographic changes alone, in the absence of other environmental changes. These age-constrained results show consistency with those simulated using an ensemble of demographically-enabled terrestrial biosphere models following an independent reconstruction of historical land use and management. We estimate that forests will accumulate an additional 69 (44–131) Pg C in live biomass from changes in demography alone if natural disturbances, wood harvest, and reforestation continue at rates comparable to those during 1981–2010. Our results confirm that it is not possible to understand the current global terrestrial carbon sink without accounting for the sizeable sink due to forest demography. They also imply that a large portion of the current terrestrial carbon sink is strictly transient in nature.