In Central Europe, summer temperatures are expected to increase and precipitation to decrease in the coming decades. Heat and drought stress will have considerable impact on tree physiology, growth, and productivity. In the Swiss Rhone Valley, high drought-induced mortality in Scots pine (Pinus sylvestris) forests occurred in the past decades. In some stands, as in our long-term monitoring plot nearby Visp, up to 59% of all pines died between 1996 and 2004 as consequence of the severe drought years 1998 and 2003. The physiological mechanisms underlying the tree mortality are heavily debated, in particular with respect to hydraulic failure and carbon limitation determining growth decline and mortality.
We aim at investigating how winter reserve NSC (non-structural carbon) levels influence early spring regrowth in stressed, compared to non-stressed trees. Furthermore, we look at a potential mitigating effect of slight fertilization on the negative effects of stressors (drought, defoliation). For this, we carry out a controlled experiment in the 16 open-top chamber lysimeters of the Model Ecosystem Facility (MODOEK) at WSL. Within the chambers, we apply 3 different treatments: i) the experimentally controlled lysimeters are subjected to 4 different soil water availability levels. ii) half of the trees within a chamber grows under ambient nutrient levels, while the other half is treated with additional nutrients. iii) within the nutrient treatment, there will be 6 levels of debudding/defoliation.
During growing season, we measure soil respiration and monitor root growth with a root scanning system. Furthermore, we measure photosynthetic rates, stomatal conductance, and mesophyll conductance to study the effects of the treatments on physiological fluxes. During three points of time throughout the study period, we will harvest 1/3 of the trees and analyze the different tissues (roots, wood, needles) on their yearly increment, biomass, and non-structural carbon content.
In addition to this controlled experiment, carbon dynamics in trees subjected to natural drought are sampled within the Irrigation Experiment Pfynwald, allowing us to compare the physiological differences between mature trees, grown under ambient and irrigated conditions. We compare the winter, summer, and autumn levels of non-structural carbohydrates over a height gradient from roots (belowground), xylem (1-9 m) and needles (canopy). This will contribute to the question how spring regrowth is influenced by the level of non-structural carbon within the tree.
With this study, we are contributing to the controversial debate whether drought may or may not cause acute growth decline and mortality via carbon starvation.