Increased drought mortality in fast-growing silver fir trees in the Black Forest

Julia Schwarz,Georgios Skiadaresis,Anna-Lena Reinhart,Jürgen Bauhus

doi:10.1016/j.foreco.2024.122441

Julia Schwarz, Georgios Skiadaresis + Show 2 more

https://doi.org/10.1016/j.foreco.2024.122441

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Silver fir (Abies alba Mill.) has been considered a suitable tree species for drought-resilient mountain forests but recent observations of increased mortality rates in Central Europe challenge this notion, prompting the need for in-depth assessments of drought-induced mortality. Here we analyzed whether the death of individual silver fir trees during drought was indicated by early warning signals in radial stem growth patterns and declines in drought-tolerance of radial growth. For this purpose, we compared tree-ring data of silver fir trees with drought-related dieback and their nearest living neighbors in three forest reserves in the Black Forest region (500-1000m asl). Early-warning signals in radial growth (EWS) were successful in separating dead from living trees. Long-lasting (ca. 30 years) growth reductions were found to be the strongest mortality signal in fir trees, and these were accompanied by increases in first-order autocorrelation of radial growth series. Dying firs exhibited initially faster growth rates and were larger compared to surviving neighbors at the onset of growth decline, which coincided with a period of increased air pollution in the 1970-80s. Growth of dead trees was more sensitive to soil moisture fluctuations compared to that of living trees, and this was accompanied by a lower growth resilience to drought in the former. In addition, decreases in growth recovery and resilience to drought were synchronized with the appearance of EWS (reductions in growth) in trees that eventually died during drought. The results of this study raise the question whether the long-term growth reductions preceding tree death, closely linked with declining drought resilience, are contributing factors to tree mortality or merely symptoms of rapidly decreasing vigor in initially fast-growing, larger trees—potentially due to factors such as higher exposure to pre-1980s air pollution or mistletoe infestation.

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