Abstract：
Vegetation regulates microclimate stability through biophysical mechanisms such as evaporation, transpiration and shading. Therefore, thermal conditions in tree-dominated habitats will frequently differ significantly from standardized free-air temperature measurements. The ability of forests to buffer temperatures nominates them as potential sanctuaries for tree species intolerant to the increasingly challenging thermal conditions established by climate change. Although many factors influencing thermal conditions beneath the vegetation cover have been ascertained, the role of three-dimensional vegetation structure in regulating the understory microclimate remains understudied. Recent advances in remote sensing technologies, such as terrestrial laser scanning, have allowed scientists to capture the three-dimensional structural heterogeneity of vegetation with a high level of accuracy. Here, we examined the relationships between vegetation structure parametrized from voxelized laser scanning point clouds, air and soil temperature ranges, as well as offsets between field-measured temperatures and gridded free-air temperature estimates in 17 sites in a tropical mountain ecosystem in Southeast Kenya. Structural diversity generally exerted a cooling effect on understory temperatures, but vertical diversity and stratification explained more variation in the understory air and soil temperature ranges (30%-40%) than canopy cover (27%), plant area index (24%) and average stand height (23%). We also observed that the combined effects of stratification, canopy cover and elevation explained more than half of the variation (53%) in understory air temperature ranges. Stratification\'s attenuating effect was consistent across different levels of elevation. Temperature offsets between field measurements and free-air estimates were predominantly controlled by elevation, but stratification and structural diversity were influential predictors of maximum and median temperature offsets. Moreover, stable understory temperatures were strongly associated with a large offset in daytime maximum temperatures, suggesting that structural diversity primarily contributes to thermal stability by cooling daytime maximum temperatures. Our findings shed light on the thermal influence of vertical vegetation structure and, in the context of tropical land-use change, suggest that decision-makers aiming to mitigate the thermal impacts of land conversion should prioritize management practices that preserve structural diversity by retaining uneven-aged trees and mixing plant species of varying sizes, e.g., silvopastoral, or agroforestry systems.
摘要：
植被通过蒸发等生物物理机制调节小气候稳定性，蒸腾作用和阴影。因此,以树木为主的栖息地的热条件通常与标准化的自由空气温度测量有很大不同。森林缓冲温度的能力使它们成为无法忍受气候变化所建立的日益具有挑战性的热条件的树种的潜在庇护所。尽管已经确定了许多影响植被覆盖下的热条件的因素，三维植被结构在调节林下小气候中的作用仍未得到研究。遥感技术的最新进展，例如地面激光扫描，使科学家能够高精度地捕获植被的三维结构异质性。这里,我们研究了从体素激光扫描点云参数化的植被结构之间的关系，空气和土壤温度范围，以及肯尼亚东南部热带山区生态系统中17个地点的现场测量温度和网格自由空气温度估计值之间的偏移量。结构多样性通常会对林下温度产生冷却作用，但垂直多样性和分层解释了更多的变化在林下空气和土壤温度范围（30％-40％）比树冠覆盖（27％），植物面积指数(24%)和平均林高(23%)。我们还观察到分层的综合效应，树冠覆盖和海拔解释了林下空气温度范围变化的一半以上（53％）。分层的衰减效应在不同的海拔水平上是一致的。现场测量和自由空气估计之间的温度偏移主要由海拔控制，但是分层和结构多样性是最大和中值温度偏移的影响因素。此外，稳定的林下温度与白天最高温度的大幅偏移密切相关，这表明结构多样性主要通过冷却白天的最高温度来促进热稳定性。我们的发现揭示了垂直植被结构的热影响，在热带土地利用变化的背景下，建议旨在减轻土地转换的热影响的决策者应优先考虑通过保留不均匀年龄的树木和混合不同大小的植物物种来保持结构多样性的管理实践，例如，silvopastoral,或农林系统。
