A forest floor evapotranspiration model incorporating forest structure for estimating under-canopy climate conditions in conifer plantations

This study focuses on forest floor evapotranspiration (Ef), a critical part of the water cycle involving the atmosphere, vegetation, and soil. It specifically involves transpiration from understory vegetation and evaporation from soil surface. We acknowledge that forest structure, such as stand density and tree height, influences Ef by altering under-canopy meteorological conditions (e.g., temperature, solar radiation, and wind speed). Despite its importance, few models incorporate changes in forest structure. We address this gap by developing a model based on the Penman equation, incorporating under-canopy meteorological conditions affected by forest structure. We introduce a relative yield index (Ry), calculated as the current timber volume to the maximum timber volume ratio for specific tree heights and stand densities, with a theoretical maximum value of less than one.  We tested our model in two Japanese cypress plantations with different structures (FM Karasawa and Kaisawa). Three and five micro-lysimeters were used to measure EF in a 12×13m plot in FM Karasawa and a 10×10m plot in Kaisawa, respectively. Measurements showed Ef variations from 0.0 to 2.1 mm/day in FM Karasawa and 0.1 to 2.5 mm/day in Kaisawa. The model estimated Ef in the range of 0.1 to 2.0 mm/day in FM Karasawa and 0.0 to 2.9 mm/day in Kaisawa. These results confirm the model's ability to estimate daily Ef, considering the impact of varying forest structures on micrometeorological conditions. Our findings highlight the model's potential for predicting Ef responses to different forest management strategies, offering valuable insights for sustainable ecosystem management.