Reforestation and surface cooling in temperate zones: Mechanisms and implications

Abstract

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 landatmosphere 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 ZHANG et Al. | 3385

The Duality of Reforestation Impacts on Surface and Air Temperature

“…The surface temperature is substantially cooler in the forests compared to the grasslands. The difference in T surf is greatest during midday periods (Figure ), especially during the growing season, when it often exceeds 5 °C (see Juang, Katul, et al, , and Zhang et al, , for a thorough discussion of the mechanism driving T surf dynamics). In contrast, the differences in potential air temperature measured on the tower are much smaller (compare Figure a to Figure b), rarely exceeding 1 °C on average, and nearly negligible when comparing OF to PP over the entire diurnal cycle.…”

“…Multiple studies have investigated the question of where and why T surf varies between forests and grasslands in the study region (Burakowski et al, ; Juang, Katul, et al, ; Zhang et al, ). At the annual time scale, the T surf of forests is substantially cooler than the T surf of grasslands, by approximately 1–2 °C in general, and by approximately 1 °C in the study sites (described later).…”

“…At the annual time scale, the T surf of forests is substantially cooler than the T surf of grasslands, by approximately 1–2 °C in general, and by approximately 1 °C in the study sites (described later). The cooling effect of forests is enhanced in the summer when albedo differences between forests and grasslands are smaller but evapotranspiration (and the associated evaporative cooling) is enhanced over forests (Juang, Katul, et al, ; Zhang et al, ). Given this substantial body of preexisting work, this study will not investigate in detail the mechanisms driving differences in T surf in the study sites here.…”

“…The sensible heat flux, whose efficiency varies with the mean wind and turbulence conditions overlying the surface, plays a dual role and may contribute to warming or cooling (Huang et al, ). With rapid advancements and proliferation of remote sensing products, observational evidence has emerged to suggest that the combined influence of increased sensible and latent heat in temperate forest ecosystems has an overall surface cooling effect that outweighs albedo‐driving warming by a magnitude of 1–2 °C, annually averaged, across a wide range of temperate ecosystems (Bright et al, ; Burakowski et al, ; Juang, Katul, et al, ; Wickham et al, ; Zhang et al, ).…”

High Heterogeneity in Canopy Temperature Among Co‐occurring Tree Species in a Temperate Forest

“…There are also relevant studies comparing surface temperature between co‐located forests and grasslands in the eastern United States using radiometric measurements; for instance, Novick and Katul (2020) compared surface temperature ( T surf ) of hardwood forest, pine forest, and grasslands using a radiometric approach and showed that T surf of either type of the forests were cooler than the grassland (often exceeding 5°C) during the midday of a growing season. Zhang et al (2020) reported that T surf of grasslands was higher by 1–2°C than co‐located forests at annual scale, with growing season temperature difference largely attributable to greater water use by forests.…”