Abstract 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.

中文翻译：

---

春季表面能量平衡的改变：生物物理和气象变化的相对重要性

摘要 在叶面积季节性强的生态系统中，春季叶片的出现通过在大气条件也发生变化的时期改变地表生物物理特性来改变地表能量平衡。然而，表面生物物理和大气变量在改变表面能量平衡方面的相对重要性和相互作用尚不清楚。在这项研究中，我们使用基于物理的归因方法来量化共同变化的表面生物物理和大气变量在改变春季表面能量平衡方面的相对重要性。结果表明，广泛观察到的随着叶出现而出现的鲍温比下降不仅仅是由于叶面积增加引起的表面阻力急剧下降。相当，鲍温比的降低反映了地表特性和大气条件变化的综合影响。具体来说，降低表面阻力和增加空气温度都会降低鲍温比，而同时增加比湿度会提供限制蒸发通量的负反馈。同时，叶片出现后空气动力阻力趋于增加，这主要是因为春季风速趋于降低。这些发现提供了春季表面和大气条件都在迅速变化时的表面能量平衡动力学的精确表征，并揭示了以前未充分研究的影响表面鲍温比和空气动力阻力的近地表大气特性。