Drought Acclimation and Recovery of Beech and Norway Spruce

The Kranzberg Roof Project (KROOF) has been initiated in 2013 and received funding from DFG through a joint proposal of three partners, i.e. Part A / Prof. Pretzsch (Forest Growth and Yield, TU München), Part B / Prof. Matyssek (Ecophysiology of Plants, TU München) and Part C / Prof. Munch/Dr. Pritsch (Soil Ecology, Helmholtz-Zentrum München). The three partners aim to jointly continue KROOF together with newly associated Part D / Prof. Lang (Soil Science, University Freiburg). The present proposal is a renewal of Part B by Rainer Matyssek and Thorsten Grams (TU München, Germany) and Barbara Beikircher (University of Innsbruck, Austria), forming a D-A-CH project (co- funded by FWF, Austria). It is one out four proposals (Parts A D) submitted in parallel to DFG. The focus of the present proposal is on ecophysiological aspects of the acclimation of European beech and Norway spruce after four years of prolonged summer drought and subsequent recovery upon re- watering. Experimentally induced intense summer drought in Kranzberg Forest using novel, rain- sensitive rainout-shelters is to be continued until early summer 2018. Subsequently, recovery will be initiated through controlled irrigation of individual plots, allowing for detailed assessments of recovery processes. In parallel, drought recovery is analyzed on five sites along a natural precipitation gradient in response to the extraordinary drought encountered in 2015. Research focus during repeated drought and subsequent recovery is detailed by the following five objectives: Objective 1: Tree hydraulic conductivities and vulnerability to drought-induced embolisms, comparing juvenile and adults trees Objective 2: Water uptake by aboveground organs and hydraulic redistribution studied via sap flow and deuterium labeling Objective 3: Increase of photosynthesis and stomatal limitation upon re-irrigation in response to growth/sink-limitation Objective 4: Concentration of non-structural carbohydrates after four years of summer drought and re- watering Objective 5: Depths of soil water uptake and recovery from drought analyzed along a natural precipitation gradient and experimentally induced drought Overall, the project aims at functional interrelations between water and carbon (C) pools and fluxes from tree rhizosphere to stand level of beech/spruce mixtures focusing on two hypotheses: (i) Upon restoring soil water availability, more anisohydric beech is less sensitive than more isohydric spruce in the aftermath of the multi-year summer drought. Sensitivity is assessed through indices of resistance, resilience and recovery time of various ecophysiological processes such as water potential, conductivity and photosynthesis. (ii) Dry site conditions reduce drought sensitivity of both beech and spruce, whereas mixture is expected to reduce drought sensitivity in beech only.

KRoof2 - Acclimation in beech and spruce after four years of prolonged summer drought and recovery upon re-watering. Forests fulfill a variety of functions. In addition to economic use, forests are important carbon and water reservoirs and of central importance for biodiversity. They serve local recreation and perform important protective functions. As in many parts of the world, Central European forests are increasingly affected by climate change. In recent years, there has been a significant increase in prolonged drought events, which have been further exacerbated by high temperatures. To make forests fit for the future, it is essential to know the resilience and adaptability of potential tree species. In recent years, there have been countless studies on this topic, but mostly only short-term effects of drought stress on juvenile plants were investigated. The 'Kranzberg Roof Project (KRoof)' allowed to study the effect of long-term drought stress on adult trees and to determine their ability to recover. For this purpose, 12 subplots were established in the Kranzberg forest near Freising (Germany), each of which had several 70- to 90-year-old spruce and beech trees growing on it. Automatic roofs were installed on six subplots to keep precipitation off the ground during the 2014-2019 growing seasons. In the summer of 2019, these roofs were removed, and all trees were irrigated. Before, during and after re-watering, drought tolerance and water status of trees was studied. Outcomes show different impacts of summer drought on growth, water balance, and drought tolerance, with spruce being significantly more affected than beech. While growth was severely restricted and water content in the trunk of drought-stressed trees significantly reduced, differences in water conductance and drought tolerance of the water transport system were minor. Re-irrigation quickly led to plant recovery and, with one exception, after one year no differences were detectable between drought-stressed and control trees. Analyses with the electrical resistivity tomography tough showed that internal water reservoirs in the trunk of spruce did not fully recover. Even after one year, significantly lower water contents in the trunk of drought-stressed spruce were observed, which may bear serious consequences in future drought events, as trees have less water reserves to compensate for water shortages. The findings provide important insights into the water balance of adult trees under summer drought, as well as information on the regeneration potential of the economically important species spruce and beech. They show that spruce is significantly more affected than beech, and that effects are still detectable after one year. Based on the results, it can be assumed that severe drought stress thus also affects spruce in the longer term and that there is an increased risk of drought-induced dieback with repeated occurrences of severe drought events.