New paper out in @NatureComms! Using satellite-based disturbance maps and climate reanalysis data we show that excess forest mortality is consistently linked to drought in Europe. But what exactly is excess forest mortality? Let me explain…

Link: https://www.nature.com/articles/s41467-020-19924-1

Similar to the current COVID19 crisis, we calculated how forest mortality deviates from its long-term trend. If there is more forest mortality than would be expected, we call this excess mortality. See the figure below for a visualization of the concept.

We calculate excess forest mortality for regular grid of 0.5° cells and at annual temporal resolution. This leads to interesting maps of where and when more or less forests die than we expect.

We combined this with reanalysis data to test whether excess forest mortality was statistically linked to drought. We used the integrated climatic water balance (CWB) from March to June. We find that a low CWB leads to increased probability of excess forest mortality.

The relationship between CWB and excess forest mortality showed, however, threshold behavior. That is, up to 1.6 SD lower CWB did not really affect the probability of excess mortality, but passing this threshold, the probability increased rapidly.

Using this relationship, we could do a (conservative) guess of the total excess forest mortality related to drought. We estimate a total of 500,000 ha of excess forest mortality over the period 1986-2015. We note that this also includes drought-related fires.

We also map hotspots of drought-related excess forest mortality, which shows several well-known but also several unknown drought-related forest diebacks in Europe.

We conclude that drought is an important driver of European forest dynamics and might become more important in the future. That said, we always advise caution in interpreting our results because there is lot os uncertainty remaining. Have fun reading!

I finally would like to thank all my co-authors for their help: @christianzang Alan Buren ( @ForestCondition) @anjarammig and @RupertSeidl

Should be Allan Buras