Getting to the root of allocation change: identifying allocation trade-offs under drought

Forest ecosystems in Europe are under a growing threat from recurring drought and heat extremes. Resource acquisition and allocation strategies determine the ability of species to cope with such stresses, so better understanding their variation with the environment is key to forecasting species and forest resilience. Resilience to weather and climate extremes also varies with the efficiency of a plant’s hydraulic transport system, yet limited information is available on how hydraulics influence investments into root, stem, branch, and leaf growth. This calls for an exploration of how plant hydraulic function interacts with carbon allocation over time. Here, we propose a new experimental design for the paired monitoring of above- and belowground carbon allocation and water fluxes in two European oak species (Quercus robur and Quercus cerris) with different drought tolerance levels. The growth and functional status of 52 4-year-old saplings of each species, transplanted in two different soils, are to be recorded between March 2025 and December 2025, including during a 4-month-drought treatment (i.e., stepwise decreased watering until significant canopy damage is achieved). Both root and canopy dynamics (e.g., growth, desiccation/wilting) will be recorded on sub-daily timescales using automated robotic minirhizotrons and measurements of canopy transmittance; dendrometers will monitor stem growth. Diurnal canopy gas exchange and photosynthetic response curves will be measured monthly. Above-ground hydraulic variables and traits (water potential, hydraulic conductance, P₅₀) and non-structural carbohydrates in different organs will be measured periodically, as will synchronous leaf and woody anatomical traits. Taken together, the data gathered in our experiment will form a comprehensive picture of inter-species differences in whole-tree carbon allocation patterns and their hydraulic control under drought.