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

Abstract

Pulses of tree mortality have been reported for many ecosystems across the globe. Yet, 13 large-scale trends in tree mortality remain poorly quantified. Manually analyzing more than 14 680,000 satellite image chips at 19,896 plot locations, we here show that forest canopy mortality 15 in Europe has continuously increased since 1985 (+1.5 ± 0.28 % yr -1 ), with the highest canopy 16 mortality rate of the past 34 years observed in 2018 (1.14 ± 0.16 %). Using simulation modeling 17 we further demonstrate that a continued increase in canopy mortality will largely alter forest 18 demography, with the median forest age falling below 30 years in nearly half of Europe's 19 countries by 2050. These demographic changes will have substantial cascading effects on the 20 regeneration, biodiversity, and carbon storage. The current trend of increasing canopy mortality 21 reported herein thus challenges the future of Europe's forests, and should be a key priority of 22 forest policy and management. 23 24 25 26 7 29 countries, resulting in the analysis of 10,440 NLMs in total. From those NLMs, we derived the 126 median age of forests within each country for the year 2050 and for the three scenarios as 127 indicator of changes in forest demography. 128 Results and discussion 129 Trends in canopy mortality rates 130 Canopy mortality increased by 1.50 ± 0.28 % per year across Europe (Figure 1). The average 131 canopy mortality rate in the late 20 th century (1985 to 1999) was 0.79 ± 0.04 % yr -1 (i.e., a forest 132 area of 1.7 Mill. ha affected by canopy mortality each year) and increased to 0.99 ± 0.04 % yr -1 133 (i.e., 2.1 Mill. ha) in the early 21 st century (2000 to 2018). Mortality increased at an accelerating 134 pace throughout the observation period, with the highest canopy mortality rate of the past 34 135 years observed for 2018 (1.14 ± 0.16 % yr -1 ). 136 137 138

Mapping the forest disturbance regimes of Europe

“…We here show that the previously reported increase in disturbance activity 4,6,7 is primarily an effect of increasing disturbance frequency, while disturbance patch size distributions are becoming more variable and disturbance severities are decreasing. Variation in forest policy was a more important predictor of changing disturbance frequencies than the variation in the natural template, suggesting human resource use as a major driver of change 1,4,7,32 . In contrast, the widening of the patch size distribution likely results from the combined effects of management changes (towards smaller intervention sizes) and increased natural disturbance activity (resulting in large areas of canopy removal).…”

“…In essence, an interpreter inspects the temporal profile of a Landsat pixel and, with the help of Landsat image chips and very high-resolution imagery available in Google Earth, makes a well-informed call whether the trajectory represents a stable forest or a forest experiencing a mortality event 45 . We here used a previously established set of 19,996 interpreted Landsat pixels 4,7 . The initial sample was drawn at random within forests of Europe, with samples stratified by country.…”

“…Europe’s forests have expanded in recent decades 2 and accumulated substantial amounts of biomass due to intensive post-war reforestation programs, changes in management systems, and timber harvesting rates that remained below increment 3 . This success story of 20 th century forestry in Europe, however, also has side effects, as the resultant changes in forest structure have – in combination with climate change – led to an episode of increasing forest disturbances in recent decades 4–7 . Increasing forest disturbances have the potential to erode Europe’s carbon storage potential 8,9 and also impact other important services provided by Europe’s forests 10,11 .…”

Excess forest mortality is consistently linked to drought across Europe

“…We excluded all cells that consisted mostly of non-forest ecoregions according to Olsen et al 52 (e.g., tundra and grassland ecoregions in northern Scandinavia), which resulted in the exclusion of 200 grid cells (6%). Based on previous research we expected the average mortality rate to increase across Europe 53,54 . However, in some years there can be substantially higher mortality rates than what would be expected even under a long-term increasing trend, a fact that we here define as excess mortality.…”

Water Budgets of Managed Forests in Northeast Germany under Climate Change—Results from a Model Study on Forest Monitoring Sites

“…Forest planners must take account of the current trend toward drier climate and of uncertain future conditions. Moreover, the current trend of an increase in canopy mortality in Europe will cause demographic changes in European forests [99], thus altering the properties and water relation of the soil-plant-atmosphere continuum in forests.…”