Summer drought exposure, stand structure, and soil properties jointly control the growth of European beech along a steep precipitation gradient in northern Germany

Abstract

Increasing exposure to climate warming-related drought and heat threatens forest vitality in many regions on earth, with the trees' vulnerability likely depending on local climatic aridity, recent climate trends, edaphic conditions, and the drought acclimatization and adaptation of populations. Studies exploring tree species' vulnerability to climate change often have a local focus or model the species' entire distribution range, which hampers the separation of climatic and edaphic drivers of drought and heat vulnerability. We compared recent radial growth trends and the sensitivity of growth to drought and heat in central populations of a widespread and naturally dominant tree species in Europe, European beech (Fagus sylvatica), at 30 forest sites across a steep precipitation gradient (500-850 mm year −1 ) of short length to assess the species' adaptive potential. Size-standardized basal area increment remained more constant during the period of accelerated warming since the early 1980s in populations with >360 mm growing season precipitation (April-September), while growth trends were negative at sites with <360 mm. Climatic drought in June appeared as the most influential climatic factor affecting radial growth, with a stronger effect at drier sites.A decadal decrease in the climatic water balance of the summer was identified as the most important factor leading to growth decline, which is amplified by higher stem densities. Inter-annual growth variability has increased since the early 1980s, and variability is generally higher at drier and sandier sites. Similarly, within-population growth synchrony is higher at sandier sites and has increased with a decrease in the June climatic water balance. Our results caution against predicting the drought vulnerability of trees solely from climate projections, as soil properties emerged as an important modulating factor. We conclude that beech is facing recent growth decline at drier sites in the centre of its distribution range, driven by climate change-related climate aridification.

Comparing northern German Scots pine plantations with natural beech forests: Soil carbon pools and growth dynamics

“…Regions with largest declines in June precipitation as southern Lower Saxony, Saxony-Anhalt and the south of Brandenburg with reductions by up to 10 mm and significant negative growth trends contrast with coastal regions where June precipitation has increased, and beech growth trends were mostly positive. However, negative growth trends can also be detected in regions with June precipitation increase due to the influence of other, non-climate drivers (Weigel et al 2022). While the negative correlation between June precipitation trend and beech growth trend was highly significant (r = 0.86, p <0.001), our results do not allow defining a MAP threshold, which separates positive and negative growth trends in beech.…”

“…For example, did not find any relationship between radial increment and MAP across a short precipitation gradient in northern Germany. In contrast, a more comprehensive study with 30 beech stands in northern Germany detected a tipping point at about 360 mm growing-season precipitation (MGSP, April -September), which separated positive and negative growth trends (Weigel et al 2022). This matches the dendrochronological results of where June precipitation has recently increased (see Fig.…”

“…previously at some sites in Europe (Hacket-Pain and Friend 2017;Muffler et al 2020;Weigel et al 2022).…”

“…With increasing warming and climate aridification especially since the 1980s, the BAI of beech has ceased to increase or even declined since then in part of the study region, resulting in smaller tree rings which weakens the climate signal. Particularly influential is a decrease in June precipitation due to its large influence on radial growth in beech (Weigel et al 2022). In fact, regions with a decadal decrease in June precipitation show the greatest decrease in beech radial growth and also in growth sensitivity to climate since the 1960-1988 observation period.…”

Towards a causal understanding of the relationship between structural complexity, productivity and adaptability of forests based on principles of thermodynamics

“…Wang et al 2008). In fact, Weigel et al (2022) stated that it is likely that direct and negative effects of heat on leaf physiology are often underestimated.…”