Modification of surface energy balance during springtime: The relative importance of biophysical and meteorological changes

In ecosystems characterized by strong seasonality in leaf area, the emergence of leaves during springtime modifies land surface energy balance by altering surface biophysical properties during a period when atmospheric conditions are also changing. However, the relative importance and interactions among surface biophysical and atmospheric variables in modifying the surface energy balance are not well understood. In this study, we use a physically-based attribution method to quantify the relative importance of covarying surface biophysical and atmospheric variables in modifying the surface energy balance during springtime. Results show that the widely observed decrease in the Bowen ratio that occurs with leaf emergence is not solely attributable to the sharp decrease in surface resistance caused by increasing leaf area. Rather, decreases in the Bowen ratio reflect the combined effects of changes in surface properties and atmospheric conditions. Specifically, decreasing surface resistance and increasing air temperature both act to reduce the Bowen ratio, while concurrent increases in specific humidity provide a negative feedback that constrains evaporative fluxes. In parallel, aerodynamic resistance tends to increase after leaf emergence largely because wind speed tends to decrease during springtime. These findings provide a refined characterization of surface energy balance dynamics during springtime when both surface and atmospheric conditions are changing rapidly and reveal previously understudied properties of the near-surface atmosphere that influence surface Bowen ratio and aerodynamic resistance.