Timberline - Legacy effects after summer and winter drought

Plant water transport is crucial under increasing drought, as expected due to climate change. Drought and freeze-thaw events can impair the transport of water from roots to leaves and cause shoot or even tree dieback. Plants try to avoid this by reducing transpiration or by investing in resistant wood structures. Some species can also repair the water transport system based on the formation of new wood (xylem) or on refilling of dysfunctional conduits. Trees at the timberline were demonstrated to be exposed to extreme impairments of the water transport system during winter and to survive by repair mechanisms. However, knowledge of combined stress effects, counter strategies and legacy effects, spatial hydraulic and stress patterns within trees and of the effects of different stress intensities on tree water transport are still poor. The project will focus on timberline tree species and the following topics: A. Effects of summer and winter stress: We hypothesize that combined summer drought and winter stress cause increased vulnerability and damage of the water transport system, and, despite repair mechanisms, legacy effects in water transport and growth. B. Xylem hydraulic architecture and hydraulic patterns: We hypothesize that timberline trees show pronounced hydraulic patterns within trees with high hydraulic conductivity and high vulnerability of the water transport system in bole and roots. C. Effects of stress duration and intensity: We hypothesize that young trees are more sensitive against drought stress than adult trees and that repair is more efficient after long stress periods of low intensity than after short but intense events. This project will make use of the unique possibility of an already installed rain exclusion site at the timberline, where trees during stress and repair periods, and legacy effects will be monitored. We will also compare water transport parameters in different tree parts down to the year ring level and analyze water extraction from different soil layers by roots. Furthermore, summer drought and winter stress in different combinations will be simulated on young trees and effects on the transport system and repair strategies studied. Besides various hydraulic, microclimatic and eco-physiological measurements, new methods to analyze hydraulics of bole wood will be developed. The project will be conducted by Stefan Mayr, Department of Botany and Michael Bahn, Department of Ecology, University of Innsbruck, both well-known scientists in the field of alpine plant eco-physiology and ecology. Cooperation partners are Andrea Nardini (Trieste, Italy), Hervé Cochard (INRA, France), Georg von Arx (WSL, Switzerland) and several national institutions. Outcomes will improve our understanding of plant water relations in general and of trees growing at high elevation and thus be relevant for future management and development of mountain ecosystems.

The increasing drought caused by climate change poses a challenge to plant water relations. Drought can particularly affect the transport system and, as a result, the vitality and growth of trees. In mountain forests, which are also exposed to intense winter stress, this is particularly relevant due to their protective function. In this project, conducted by Stefan Mayr and Michael Bahn of the University of Innsbruck in cooperation with various partners, the long-term effects of combined drought and winter stress, the effects of different stress intensities, counterstrategies, and the hydraulic architecture of alpine trees were investigated: (i) In a rain exclusion experiment at the tree line, spruce and larch trees were exposed to artificial drought over several summers. Repeated drought resulted in increasing water deficits (unfavorable water potential), reduced sap flow, and reduced growth, especially in spruce. In addition, the summer drought caused damage to the water transport system in some trees during the winter, but this was repaired by water uptake via the branches. The effects on growth were still detectable several years after the end of the experimental drought. (ii) Investigations into hydraulic architecture revealed clear differences between roots and branches in all coniferous and deciduous tree species studied. The roots showed lower resistance to unfavorable water potentials but higher hydraulic conductivity. This was also reflected in their anatomy. This hydraulic architecture, with some adaptations to resistance during drought, was particularly pronounced in conifers. (iii) Experiments with young spruce trees exposed to summer drought and winter stress also revealed impairments in growth. Since the water transport system was hardly damaged due to only moderate water potentials, this proves the high sensitivity of growth processes to drought. (iv) Several new methods were also established as part of the project. The analysis for electrotomography of tree trunks was optimized and a device for measuring flow in tree trunk cores was developed. Furthermore, the measurement of water potentials was improved, a measuring chamber for conifer needles was constructed, and a pressure apparatus for experimentally interrupting water transport in tree trunks was developed. The results show that both the water balance and the growth of Alpine trees are impaired by repeated dry periods in summer and winter stress. Accordingly, far-reaching but species-specific (e.g., due to hydraulic architecture) effects of climate change on tree vitality are to be expected. The observed effects are relevant for trees at the forest limit, but have also been demonstrated for trees at lower altitudes and shrubs, and also play an important role after forest fires, for example. Based on this project, the long-term observations will be continued and the investigations extended to other species.