Effect of Tree-to-Shrub Type Conversion in Lower Montane Forests of the Sierra Nevada (USA) on Streamflow

Higher global temperatures and increased levels of disturbance are contributing to greater tree mortality in many forest ecosystems. These same drivers can also limit forest regeneration, leading to vegetation type conversion. For the Sierra Nevada of California, little is known about how type conversion may affect streamflow, a critical source of water supply for urban, agriculture and environmental purposes. In this paper, we examined the effects of tree-to-shrub type conversion, in combination with climate change, on streamflow in two lower montane forest watersheds in the Sierra Nevada. A spatially distributed ecohydrologic model was used to simulate changes in streamflow, evaporation, and transpiration following type conversion, with an explicit focus on the role of vegetation size and aspect. Model results indicated that streamflow may show negligible change or small decreases following type conversion when the difference between tree and shrub leaf areas is small, partly due to the higher stomatal conductivity and the deep rooting depth of shrubs. In contrast, streamflow may increase when post-conversion shrubs have a small leaf area relative to trees. Model estimates also suggested that vegetation change could have a greater impact on streamflow magnitude than the direct hydrologic impacts of increased temperatures. Temperature increases, however, may have a greater impact on streamflow timing. Tree-to-shrub type conversion increased streamflow only marginally during dry years (annual precipitation全球气温升高与干扰强度加剧，正导致诸多森林生态系统内树木死亡率显著上升。上述驱动因子同时还会限制森林更新，进而引发植被类型转换。针对加利福尼亚州内华达山脉区域，目前学界对该类植被类型转换如何影响径流（streamflow）——这一支撑城市供水、农业灌溉与生态环境的关键水源——的认知仍较为匮乏。 本文针对内华达山脉两处低山地森林流域，探究了树木向灌丛的植被类型转换与气候变化共同作用下的径流响应。研究采用空间分布式生态水文模型（spatially distributed ecohydrologic model），模拟了植被类型转换后径流、蒸发与蒸腾的变化过程，并重点关注植被冠层尺寸与坡向的调控作用。 模型结果显示，当树木与灌丛的叶面积差异较小时，植被类型转换后径流变化可忽略不计或仅小幅下降，这在一定程度上源于灌丛更高的气孔导度（stomatal conductivity）与更深的根系深度（deep rooting depth）。与之相反，若转换后的灌丛叶面积低于原树木，则径流可能出现上升。 模型估算还表明，植被变化对径流量的影响程度，可能超过气温升高直接带来的水文效应；但气温升高对径流时序的调控作用更为显著。在干旱年份（年降水量，原文未完成），树木向灌丛的植被类型转换仅会小幅提升径流。