Rainfall limitation experiment

In order to investigate and quantify the effects of climate changes and, in particular, of drought on seedling development, an in situ rainfall limitation experiment is conducted in regeneration patches of the Lauzelle wood (Louvain-la-Neuve). This forest presents patches with well-established oak and beech regeneration in which experimental zones (blocks) were set up in March 2021. Each experimental zone consists into (i) a ‘treatment unit’ subject to artificial drought and (ii) a ‘control unit’ receiving natural throughfall (i.e., no artificial interception). In the treatment unit, drought is induced through the installation of a partial roof (4 x 4 m horizontal area) consisting of 25 cm wide transparent plastic strips spaced 12.5 cm apart (i.e., 2/3 covered area) above the seedlings. Besides, a plastic sheet was inserted vertically in the ground to a depth of 40 cm at the periphery of the roof to avoid lateral transfers of water. The control unit (2 x 2 m area) is delimited in the proximity of the roof: not too close to avoid an influence of the roof on the throughfall reaching the control unit area neither too far to stay in the same environmental conditions, especially with regards to light, soil and water supply. Such experimental zones were replicated three times for each considered species (sessile oak and European beech), resulting in a total of six blocks.

Measurements are carried out on seedlings over the complete area of the ‘control’ units (2 x 2 m) and over the central 2 x 2 m area of the ‘treatment’ units, considering thereby a 1 m wide peripheral buffer zone in this latter case. A set of 32 seedlings was selected in each of the 12 experimental units, covering the encountered height range and evenly distributed over the 0-25 cm, 25-50 cm, 50-100 cm and 100-150 cm classes. Seedling height and collar diameter are measured on each of these selected individuals. These seedlings will be labelled with a unique identifier. Besides, complete counting of the alive seedlings is carried out in each experimental unit. These observations will be repeated every year all over the project duration and will allow to study the effect of water limitation on seedling growth and survival.

In addition, soil water content within the upper 30 cm soil layers and temperature at soil surface are continuously monitored in each experimental unit using, respectively, time domain reflectometry (TDR) and thermistor sensors.

During the summer of 2022, these measurements were completed by the acquisition of physiological data in order to monitor the hydrodynamics of the seedlings and to study its dependence on the soil water status. Leaf water potential and stomatal conductance of seedlings were regularly measured using a Scholander bomb and a porometer, respectively. Moreover, one of the beech experimental zones has been equipped with psychrometers and tensiometers for more intensive monitoring of the stem and soil water potentials. Photosynthetically active radiation (PAR) sensors have also been installed in this zone.