Afforestation in China cools local land surface temperature

Abstract

China has the largest afforested area in the world (∼62 million hectares in 2008), and these forests are carbon sinks. The climatic effect of these new forests depends on how radiant and turbulent energy fluxes over these plantations modify surface temperature. For instance, a lower albedo may cause warming, which negates the climatic benefits of carbon sequestration. Here, we used satellite measurements of land surface temperature (LST) from planted forests and adjacent grasslands or croplands in China to understand how afforestation affects LST. Afforestation is found to decrease daytime LST by about 1.1 ± 0.5°C (mean ± 1 SD) and to increase nighttime LST by about 0.2 ± 0.5°C, on average. The observed daytime cooling is a result of increased evapotranspiration. The nighttime warming is found to increase with latitude and decrease with average rainfall. Afforestation in dry regions therefore leads to net warming, as daytime cooling is offset by nighttime warming. Thus, it is necessary to carefully consider where to plant trees to realize potential climatic benefits in future afforestation projects. T he area of planted forest (PF) in China has increased by ∼1.7 million hectares per year (about 41% of the global afforestation rate) during the last 2 decades (1, 2). China had the largest PF area in the world in 2008, at ∼62 million hectares ( Fig. 1), or ∼23% of global plantation area (264 million hectares) (1, 2). The Chinese government launched several projects to convert croplands (CR) and marginal lands into forests, to reduce soil and water quality degradation, in the 1980s and 1990s (2). This afforestation contributed to increased carbon storage (3, 4) but also altered local energy budgets, which has the potential to offer feedback on local and regional climates (5-10).Forests generally have a lower albedo than grasslands (GR) and CR. Thus, afforestation increases the amount of absorbed solar radiation at the surface (9, 10). Surface cooling will result if this extra energy is dissipated as evapotranspiration (ET) (11) or heat convection (7); otherwise, afforestation will result in surface warming. The biophysical effects of afforestation on local climate can be much larger than the small global cooling effect resulting from uptake of CO 2 by growing forests (8,12,13). However, these biophysical effects are also complex and depend on "background" climate (14). Afforestation generally cools the surface in tropical areas but warms it in boreal lands (6,(8)(9)(10). The effects of afforestation in temperate regions are not clear. The large area under afforestation in China, the diversity of projects (over former CR, GR, or marginal lands), and the broad range of background climates (most plantations are in temperate regions with varying degrees of annual average rainfall) provide an interesting test bed to assess how afforestation affects local temperature.In this article, we investigate how plantations affect land surface temperature (LST) across China, using satellite-derived LST data sets from Earth Observin...

Global pattern for the effect of climate and land cover on water yield

“…In humid regions (P/PET41) where PET plays a dominant role in water yield or evapotranspiration, a decrease in PET will likely cause a significant increase in water yield. This is consistent with the findings that afforestation can increase water yield in large watersheds of humid regions 11,12 because afforestation cools local land surface temperature 39 and thus decreases PET.…”

Changes in Growing Season Vegetation and Their Associated Driving Forces in China during 2001–2012

“…The feedback between land cover change and regional climate has attracted much attention in recent studies on regional climate variability caused by afforestation [46], and conclusions vary at regional to global scales. Understanding the interactions between the land and atmosphere can facilitate sustainable development of the ecological environment.…”

Vegetation changes in recent large-scale ecological restoration projects and subsequent impact on water resources in China's Loess Plateau

“…Undoubtedly, changes in vegetation directly or indirectly modify biosphere-atmosphere interactions, including the hydrological cycle and energy budgets Chapin et al, 2005). A recent research has shown that vegetation growth cools local land surface temperature (Peng et al, 2014;Shen et al, 2015). However, the increased vegetation coverage result in changes in water resources, which is a dominant factor for fragile ecological regions, through inducing changes in land surface energy components, has not been well documented.…”