Reforestation and surface cooling in temperate zones: Mechanisms and implications

Land‐use/cover change (LUCC) is an important driver of environmental change, occurring at the same time as, and often interacting with, global climate change. Reforestation and deforestation have been critical aspects of LUCC over the past two centuries and are widely studied for their potential to perturb the global carbon cycle. More recently, there has been keen interest in understanding the extent to which reforestation affects terrestrial energy cycling and thus surface temperature directly by altering surface physical properties (e.g., albedo and emissivity) and land–atmosphere energy exchange. The impacts of reforestation on land surface temperature and their mechanisms are relatively well understood in tropical and boreal climates, but the effects of reforestation on warming and/or cooling in temperate zones are less certain. This study is designed to elucidate the biophysical mechanisms that link land cover and surface temperature in temperate ecosystems. To achieve this goal, we used data from six paired eddy‐covariance towers over co‐located forests and grasslands in the temperate eastern United States, where radiation components, latent and sensible heat fluxes, and meteorological conditions were measured. The results show that, at the annual time scale, the surface of the forests is 1–2°C cooler than grasslands, indicating a substantial cooling effect of reforestation. The enhanced latent and sensible heat fluxes of forests have an average cooling effect of −2.5°C, which offsets the net warming effect (+1.5°C) of albedo warming (+2.3°C) and emissivity cooling effect (−0.8°C) associated with surface properties. Additional daytime cooling over forests is driven by local feedbacks to incoming radiation. We further show that the forest cooling effect is most pronounced when land surface temperature is higher, often exceeding −5°C. Our results contribute important observational evidence that reforestation in the temperate zone offers opportunities for local climate mitigation and adaptation.

土地利用/覆盖变化（LUCC）是环境变化的一个重要驱动因素，它与全球气候变化同时发生，并且常常相互作用。在过去两个世纪里，造林和毁林一直是土地利用/覆盖变化的关键方面，并且因其对全球碳循环产生扰动的可能性而被广泛研究。最近，人们迫切希望了解造林在多大程度上通过改变地表物理特性（例如反照率和发射率）以及地 - 气能量交换直接影响陆地能量循环，进而影响地表温度。造林对地表温度的影响及其机制在热带和寒带气候中相对得到了较好的理解，但在温带地区造林对升温或降温的影响则不太确定。本研究旨在阐明温带生态系统中连接土地覆盖和地表温度的生物物理机制。为了实现这一目标，我们使用了美国东部温带地区六对位于同地的森林和草地的涡度相关塔的数据，这些塔测量了辐射分量、潜热和感热通量以及气象条件。结果表明，在年时间尺度上，森林地表比草地低1 - 2°C，这表明造林具有显著的降温效应。森林增强的潜热和感热通量平均具有 - 2.5°C的降温效应，这抵消了与地表特性相关的反照率升温（+2.3°C）和发射率降温效应（ - 0.8°C）的净升温效应（+1.5°C）。森林白天额外的降温是由对入射辐射的局部反馈驱动的。我们进一步表明，当地表温度较高时，森林的降温效应最为显著，通常超过 - 5°C。我们的研究结果提供了重要的观测证据，表明温带地区的造林为当地气候缓解和适应提供了机会。