Spatio temporal variation of root water uptake in a mixed deciduous forest and a grassland

Vegetation relies on soil water to meet transpiration demands. Furthermore, the canopy intercepts precipitation and introduces spatial heterogeneity in water entering the soil not only in forests but also in grasslands. Some studies showed that in mixed forests, trees tap water from deeper layers in response to long dry spells to compensate transpiration demands. In contrast, recent observations indicate that water uptake from deeper layers is almost negligible, which may be the main reason for the drought susceptibility of many forests in Central Europe. Moreover, canopy induced variability in precipitation distribution at the ground could influence water uptake patterns, which is rarely considered in forests and not investigated in grasslands. Therefore, we examined root water uptake and soil water patterns considering the impact of canopy-driven heterogeneity in subsurface processes through field observations. The research site consists of a mixed deciduous forest (1 ha) and an adjacent grassland (0.045 ha) site in Hainich Critical Zone Observatory, Thuringia, Germany. The forest site is dominated by European beech and hosts other species such as sycamore maple, European ash, while the plant community in the grassland is characterized by different functional plant groups such as graminoids, legumes and herbs. Both sites were equipped with closely paired (within 1 m) throughfall and soil moisture measurements (n_forest = 34, n_grassland = 22). We sampled throughfall weekly at both sites in 2019 (March-August) and 2022 (May-September) along with gross precipitation and grass height measurements. At both sites, we derived root water uptake from diurnal fluctuations in soil water content at two depths during rain-free periods.

The growing season in 2022 was drier than in 2019 (P_{gross, cum,2019} < 200 mm), resulting in less than 100 mm of cumulative gross precipitation within the sampling period. In addition, dry spells were longer and more frequent in the 2022 growing season. At the forest site, the topsoil layer held more water than the deeper layer throughout the sampling period in 2019 and early in the season 2022. In the grassland plot, the topsoil layer stored precedingly less water in both years through the growing season, especially after summer mowing, which is probably due to preferential flow. In the forest, the average water uptake depth systematically shifted to deeper layers in the dry growing season of 2022, so that after mid-July roots mostly tapped water from deeper layers. In 2019, the relatively wetter growing season, changes in uptake depths were also related to tree size. The average daily transpiration reached 3 mm in 2019 while it decreased to less than 2.5 mm in 2022 despite the higher evapotranspiration demand, indicating a strong drought effect. In the grassland plot, in both years, the deeper soil layer facilitated higher water uptake over the growing season in line with grass development and remained so even after summer mowing. Our results suggest that severe droughts can alter water uptake strategies in mixed species forest and grassland sites.