Can the resistance and resilience of trees to drought be increased through thinning to adapt forests to climate change?
Zusammenfassung der Projektergebnisse
Recent and predicted increases in extremely dry and hot summers emphasize the need for silvicultural approaches to increase the drought tolerance of existing forests in the short-term, before adaptation through species changes may be possible. Thinning increases access of promoted trees to soil water, as long as this is available, but these trees may also be disadvantaged through a higher transpirational surface. The overarching aim of this project was to investigate, if the allocation of more growing space to individual crop trees through thinning has the potential to increase their drought tolerance. We hypothesized that reductions in stand density through thinning are unlikely to mitigate growth depressions, i.e., do not improve the growth resistance of crop trees during drought events (H1), whereas the recovery of stem growth of crop trees in the years following an extreme drought is expected to be accelerated by thinning (H2). In addition, we asked how overstorey density may influence growth resistance and resilience of understory trees, which comprise the future stand (H3). Finally, this study analyzed whether the tree response to thinning at a single stem height, i.e. breast-height, is representative of the drought response of the whole stem (H4). We collected tree discs and increment cores from promoted overstorey trees of thinning experiments of the three conifer tree species (Picea abies, Pinus sylvestris, P. ponderosa H1, H2 and H4) and from advance regeneration of Pseudotsuga menziesii and Fagus sylvatica that were planted beneath differently thinned canopies of P. menziesii and P. sylvestris, respectively (H3). Dendro-ecological analyses of radial stem growth and of carbon (C) and oxygen (O)- isotopic wood composition of growth rings were conducted to examine transpirational stress and growth reduction during previous droughts as well as their subsequent recovery. Results of this project confirmed that there was no - or even a negative - effect of thinning on the resistance of radial growth during extreme drought events in all tree species and at all stem heights (H1) but thinning was found to improve the growth recovery in the years following extreme droughts in all tree species and at all heights except the topmost (H2). The faster recovery resulted from a likely increase in water availability in recently thinned stands. In the medium term and after several thinning interventions, the more rapid post-drought recovery of stem growth may be attributed to structural advantages of large trees, i.e. high foliage area and fine-root biomass per tree, which allow them to take more quickly advantage of favourable conditions. The faster recovery of radial growth of trees from heavily thinned compared to unthinned stands was not accompanied by differences in recovery of isotopic composition. This may be an artefact of the dual isotope approach as stable O isotopes in wood (518Ow) can be influenced by several additional factors. To shed light on this, an additional study was conducted in a P. ponderosa thinning experiment in Arizona that included not only tree-ring data (radial growth and isotopes) but also in-situ measurements of gas-exchange. Results of a sensitivity analysis and modeling suggested that the more depleted 518Ow of faster-growing trees in heavily thinned stands was likely caused by a higher responsiveness to environmental variation when compared to slower-growing trees and to a lesser extent related to changes in gas exchange following thinning. Results from understorey trees indicate that there was no effect of overstorey density on either growth resistance during or growth recovery following droughts (H3). The last hypothesis, H4, was partly confirmed as basal area increments at breast height provided good estimates of the volume growth response to drought for the whole stem, but not for its recovery. The higher resilience of trees in heavily thinned stands against extreme drought is of high relevance, because these trees are most likely less susceptible to secondary pests and pathogens. This study showed that existing thinning experiments, which have experienced one or more drought events, are highly suitable to address the question whether increased growing space may increase the drought resistance or resilience of individual trees, and thus stands. Both the scope of and approaches employed by this project went far beyond those of existing studies: By including several tree species - comprising the two currently most important timber tree species of Germany (P. abies and P. sylvestris) - and multiple vegetation layers (over- versus understorey trees) as well as stem heights, we expect our results to be more relevant compared to those of previous studies. Finally, this is the first study to combine retrospective methods of dendro-ecology and stable isotopes in tree-rings in order to determine physiological drivers of differences in growth resistance and recovery of crop trees from differently thinned stands.
Projektbezogene Publikationen (Auswahl)
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(2012): Interactions of thinning and stem height on the drought response of radial stem growth and isotopic composition of Norway spruce (Picea abies). - Tree Physiology 32 /10, 1199-213
Sohn, J. A. , Kohler, M. , GeGler, A. , Bauhus, J.
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(2013). Mitigation of drought by thinning: Short-term and long-term effects on growth and physiological performance of Norway spruce (Picea abies), Forest Ecology and Management 308: 188-197
Sohn, J.A., Gebhardt, T., Ammer, C., Bauhus, J., Haberle, K.-H., Matyssek, R., Grams, T.E.E.
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(2014). Can thinning of conifer stands increase drought tolerance of crop trees?. Dissertation, Faculty of Environment and Natural Resources, Albert-Ludwigs-Universitat Freiburg, 195pp.
Sohn, J. A.
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(2014). Unthinned slow-growing ponderosa pine (Pinus ponderosa) trees contain muted isotopic signals in tree rings as compared to thinned trees. Trees 28(4), 1035-1051
Sohn, J. A., Brooks, J. R., Bauhus, J., Kohler, M., Kolb, T. E., & McDowell, N. G.
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Heavy and frequent thinning promotes drought adaptation in Pinus sylvestris forests. Ecological Applications Vol 26 Issue 7, October 2016, Pages 2190-2205
Sohn, J.A., Hartig, F., Kohler, M, Huss, J., Bauhus, J.
