ECR feature: Robert Weigel on growth synchrony and climate change-sensitivity in European forests

Robert Weigel is a postdoc at the University of Göttingen. He is a geoecologist with special interest in forest ecosystems. Here, Robert shares his recent work on climate sensitivity and within-stand synchrony of growth in a European beech–oak ecotone.

Robert Weigel (Photo Credits: Banzragch Bat-Enerel).

Personal links. Twitter.

Institute. University of Göttingen, Plant Ecology and Ecosystems Research.

Academic life stage. Postdoc.

Major research themes. Responses and adaptation mechanisms of trees and forest ecosystems to changing climate across seasons.

Current study system. Temperate forest trees, such as European beech, Scots pine, European oak species. Trees are cool study systems on their own, but studying these species is now a very hot topic because foresters urgently need advice on which tree species to choose for climate smart forestry.

Recent JBI paper. Kasper, J., Leuschner, C., Walentowski, H., & Weigel, R. (2022). Higher growth synchrony and climate change-sensitivity in European beech and silver linden than in temperate oaks. Journal of Biogeography.

Motivation behind this paper. European beech, the naturally dominant tree species in Central Europe, was long considered to be fairly climate-change resilient. However, severe defoliation events, e.g., the extreme hot drought in 2018, highlight the species’ potential drought vulnerability. Admixing more drought-resilient, thermophilic oak species might be a promising option to promote more drought resilient forest ecosystems closer to the near-natural state, compared to introducing exotic timber species. To directly compare the drought resilience of different species, we analysed growth dynamics in beech and oak populations in close proximity.

Key methodologies. We analysed tree-ring series in order to explore the climate–growth relationship of our study populations. As a quite novel approach, we rely on the strength of regional synchronization in tree growth dynamics as an indicator for climate change-induced abiotic stress, instead of only quantifying the drought signal in the tree-ring series by correlating annual increment rates with time series of various factors. The advantage of this synchrony analysis is that it summarizes across the isolated, non-stationary effects of the various climatic drivers of tree growth rates.

Samples of wood are the basis of our study. These samples were taken from European beech. They are glued to wooden strips and prepared with razor blade or sandpaper so that the annual rings are clearly visible on the smooth surfaces and can be measured and dated using a microscope (Photo Credits: Stella Gribbe).

Unexpected challenges. In the beginning, we did not anticipate how clearly the synchrony patterns would differ between the species. With the onset of the strong warming in the 1980s, growth synchrony clearly increased in European beech and silver linden, while it clearly decreased in the studied Central European to Pannonian oak species, although the synchrony patterns where quite similar before that time.

Major results. We think that, for increasing climate change resilience of forests, it may be wise to favour more stress-tolerant over high-yield timber species in vulnerable regions. The three studied oak species produce highly valued timber and would be a promising option for the transition to climate-smart forestry.

View into beech (left) and oak (right) forests in the study area in Western Romania (Photo Credits: Jan Kasper).

Next steps for this research. We are currently transferring the research to South America (Patagonia) in order to explore if we can find similar divergence in drought response and synchrony patterns when comparing among different southern beeches (Nothofagus). Here, less is known about how the drought resistance changes among species, especially when comparing to fast-growing introduced conifers.

If you could study any organism on Earth, what would it be? I have no preference for any study organism in particular. I would like to stay with studying temperate forest ecosystems, because we urgently need a sound and locally precise understanding of climate change-related stressors for tree growth, in order to provide precise suggestions for climate-smart forestry. Of course, the best advice for sustaining healthy forests is to reduce greenhouse gas emissions and slow down the climate change velocity as fast as possible!

Anything else to add? Climate change means not only hotter and drier summers. This being bad enough, temperate and boreal winters are also changing dramatically. Due to winter warming, the insulating snow cover may decline or vanish, which increases soil frost exposure to tree roots and the microbial community in the forest floor in some regions, while in other, somewhat warmer regions frost might become absent completely. These aspects have to be considered by studying climate change impacts across seasons instead of focussing on the growing season only!

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