Effects of spatial variations in soil evaporation caused by tree shading on water flux partitioning in a semi-arid pine forest

In dry environments, water availability is a major limitation to forest productivity while losses to soil evaporation (E) are a significant component in ecosystem hydrology. We report on a 3-year study (2004-2007) in a semi-arid pine forest in Southern Israel (40-year-old Pinus halepensis; leaf area index=1.5; mean precipitation 285mmyear⁻¹) that estimated soil E, assessed its spatial variability and identified the factors influencing it. We used a modified and specially calibrated soil respiration chamber to directly measure E on a weekly basis at 14 permanently installed soil collars across the range of soil surface conditions. Results showed that spatial variability in E was large, with SD of ±47% between measurement sites. E fluxes measured in sun-exposed areas were on average double those in shaded areas (0.11mmh⁻¹ vs. 0.06mmh⁻¹). The spatial variability in E correlated with radiation (measured in the photosynthetically active range), which was up to 92% higher in exposed compared to shaded sites, and with soil water content, which was higher in exposed areas during the wetting season but higher in shaded areas during the drying season. The fraction of shaded forest floor area was described as a function of canopy geometry (mean tree height, crown width and stand density) and the daily variation in solar altitude. Simple simulations based on the relationship between E and the shaded fraction indicated that E/P (precipitation) for the Yatir forest could decrease from 0.53 (undeveloped canopy of 10% cover) to 0.30 (full canopy closure). However, according to our analysis, increasing canopy cover will also increase intercepted precipitation and transpiration such that current precipitation inputs will not be able to support forest growth above a canopy cover of 65%. Combining direct measurements of environmental conditions and canopy characteristics with such simulations can provide a simple predictive and management tool to optimize tree water use in dry environments.