US Geological Survey Water Resources, Reston, VA 20192;
Department of Crop Production Ecology, Swedish University of Agricultural Sciences, 750 07 Uppsala, Sweden.
Proc Natl Acad Sci U S A. 2021 Feb 16;118(7). doi: 10.1073/pnas.2021646118.
Mountain ranges generate clouds, precipitation, and perennial streamflow for water supplies, but the role of forest cover in mountain hydrometeorology and cloud formation is not well understood. In the Luquillo Experimental Forest of Puerto Rico, mountains are immersed in clouds nightly, providing a steady precipitation source to support the tropical forest ecosystems and human uses. A severe drought in 2015 and the removal of forest canopy (defoliation) by Hurricane Maria in 2017 created natural experiments to examine interactions between the living forest and hydroclimatic processes. These unprecedented land-based observations over 4.5 y revealed that the orographic cloud system was highly responsive to local land-surface moisture and energy balances moderated by the forest. Cloud layer thickness and immersion frequency on the mountain slope correlated with antecedent rainfall, linking recycled terrestrial moisture to the formation of mountain clouds; and cloud-base altitude rose during drought stress and posthurricane defoliation. Changes in diurnal cycles of temperature and vapor-pressure deficit and an increase in sensible versus latent heat flux quantified local meteorological response to forest disturbances. Temperature and water vapor anomalies along the mountain slope persisted for at least 12 mo posthurricane, showing that understory recovery did not replace intact forest canopy function. In many similar settings around the world, prolonged drought, increasing temperatures, and deforestation could affect orographic cloud precipitation and the humans and ecosystems that depend on it.
山脉为水资源提供云、降水和常年溪流，但森林覆盖在山地水文学和云形成中的作用还不太清楚。在波多黎各的卢奎洛实验森林中，山脉每晚都被云层笼罩，为支持热带森林生态系统和人类利用提供稳定的降水来源。2015 年的严重干旱和 2017 年飓风玛丽亚（Maria）造成的森林树冠（落叶）移除，为研究森林与水文气候过程之间的相互作用创造了自然实验条件。这些史无前例的、基于陆地的 4.5 年的观测结果表明，地形云系统对当地陆面水分和能量平衡高度敏感，而森林则对其起到调节作用。山坡上的云层厚度和浸润频率与前期降雨相关，将陆地再循环水分与山地云的形成联系起来；在干旱胁迫和飓风后落叶期间，云底海拔升高。温度和水汽亏缺的日循环变化以及感热与潜热通量的增加，量化了森林干扰对当地气象的响应。沿山坡的温度和水汽异常至少持续了 12 个月，表明林下植被的恢复并没有取代完整的森林冠层功能。在世界上许多类似的环境中，长时间的干旱、气温升高和森林砍伐可能会影响地形云和依赖它的降水以及人类和生态系统。
