Excess forest mortality is consistently linked to drought across Europe - PubMed (original) (raw)

Excess forest mortality is consistently linked to drought across Europe

Cornelius Senf et al. Nat Commun. 2020.

Abstract

Pulses of tree mortality caused by drought have been reported recently in forests around the globe, but large-scale quantitative evidence is lacking for Europe. Analyzing high-resolution annual satellite-based canopy mortality maps from 1987 to 2016 we here show that excess forest mortality (i.e., canopy mortality exceeding the long-term mortality trend) is significantly related to drought across continental Europe. The relationship between water availability and mortality showed threshold behavior, with excess mortality increasing steeply when the integrated climatic water balance from March to July fell below -1.6 standard deviations of its long-term average. For -3.0 standard deviations the probability of excess canopy mortality was 91.6% (83.8-97.5%). Overall, drought caused approximately 500,000 ha of excess forest mortality between 1987 and 2016 in Europe. We here provide evidence that drought is an important driver of tree mortality at the continental scale, and suggest that a future increase in drought could trigger widespread tree mortality in Europe.

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Conflict of interest statement

The authors declare no competing interests.

Figures

Fig. 1

Fig. 1. The probability of excess forest canopy mortality in Europe’s forests related to the climatic water balance (CWB).

Values for different levels of mortality above the values expected from the long-term mortality trend are shown, with 100% excess mortality indicating a doubling of the annual area of canopy mortality. CWB is expressed as z-score, that is 0 indicates average CWB and ±1 indicate ±1 standard deviation below/above the average. Lines present the average estimate derived from n = 8,000 random draws of the full model, that is including parameter and model uncertainty. Ribbons represent the 95% credible interval derived by splitting all random draws into n = 100 bins and repeating the calculation for each bin.

Fig. 2

Fig. 2. Hotspots of excess forest canopy mortality in response to drought.

Hotspots were defined as regions and years where excess forest canopy mortality coincided with CWB values smaller than −1.6 standard deviations below the local average. See Supplementary Figs. 8 and 9 for hotspot maps using lower and upper bound thresholds. Note that we here report relative changes, whereas absolute estimates in terms of forest area are given in Fig. 3. Background maps are from

https://naturalearthdata.com

. The map was created by C. Senf.

Fig. 3

Fig. 3. Forest area affected by drought-related excess forest canopy mortality between 1987 and 2016 in Europe.

The red bars sum all excess forest canopy mortality across the n = 2,913 grid cells that co-occurred with CWB values smaller −1.6 standard deviation below the local average. The error-bars sum all excess mortality that co-occurred with CWB values smaller than −1.3 (upper bound) and −2.0 (lower bound) standard deviation below the local average, respectively. The error-bars thus show the potential range of drought-related excess mortality depending on different thresholds used for defining a drought that caused excess forest canopy mortality.

Fig. 4

Fig. 4. Percent of the total forest canopy mortality attributable to drought-related excess forest canopy mortality between 1987 and 2016 across Europe.

A value of 30 means that 30% of the total forest canopy mortality in this particular grid cell and over the whole period was drought-related excess forest canopy mortality given the definitions used in Fig. 2. See Supplementary Figs. 10 and 11 for maps using lower and upper bound thresholds. Background maps are from

https://naturalearthdata.com

. The map was created by C. Senf.

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