Vertical Soil Water Dynamics and Drought-Response are Affected by Stand density In a Mediterranean Dryland Forest

Abstract:
Mediterranean dryland forests are critically dependent on rainfall regimes and are increasingly exposed to prolonged droughts that disrupt soil–water–vegetation interactions and elevate the risk of tree mortality. Continuous monitoring of inter-depth soil moisture dynamics offers real-time insight into these processes, yet such data is very scarce. Based on long-term ecohydrological monitoring in HaKedoshim Pinus halepensis Forest, Israel, we quantified how drought alters soil moisture dynamics, water availability, and vegetation responses on shallow soil, hard rock (lime) terrains with contrasting canopy structures: dense forest plots (~550 trees hectare⁻¹) and thinned plots (~100 trees hectare⁻¹), where thinning was implemented 15 years earlier.

Volumetric water content (VWC) was continuously monitored using time-domain reflectometry (TDR) sensors at 0.5, 1.0, and 1.5 m depths during two consecutive hydrological years: HY23-24 (normal, ~561 mm year^-1 of rainfall) and HY24-25 (drought, ~251 mm year^-1). We derived seasonal soil-water subsidy metrics as the area under the excess VWC curve above the summer baseline (AUC_excess). We analyzed drought impacts using vertically aligned sensor sequences.

Despite greater understory development in the thinned plots over the years (55% cover vs. 35% in the dense forest), thinning significantly reduced tree mortality rate (six-fold). Annual evapotranspiration (ET) was also affected, with dense plots exhibiting 1.5 times greater ET compared to thinned ones.

In the thinned plots, higher volumetric soil moisture was measured in the upper soil layer (0.5 m) than in the dense plots, due to reduced rainfall interception and  water consumption by the forest vegetation. In contrast, in the deep soil (1.5 m), two interesting phenomena were observed: moisture in the dense plots was higher than in the thinned plots and, the deep layers in the dense forest responded earlier than the shallow layers to rainfall inputs.

Extreme drought (-55% rainfall) caused substantial reductions in soil water subsidy across depths. Dense plots experienced greater losses (-73% overall; -79% in topsoil) compared to thinned ones (-56% overall; -37% in topsoil). These moisture declines were associated with prolonged periods of limited water availability.

Our results show that canopy structure regulates not only interception and evapotranspiration but also the vertical coherence of soil moisture recharge, with dense forests promoting preferential deep flow and root-induced bypass flow via interception, stemflow and root flow while thinning enhances infiltration and shallow soil storage.

These findings demonstrate that 15 years after thinning, forest structure exerts a lasting control on water availability through depth-dependent hydrological pathways, providing mechanistic insight into drought-driven forest mortality under climate change.

Keywords: Drought, Soil moisture, Ecohydrology, Bypass flow, Thinning