Seasonal variability of forest cooling and warming effects and response to drought in mid-to-high latitudes of the Northern Hemisphere

Abstract

The cooling or warming effects of forests in mid-to-high latitudes of the Northern Hemisphere are still a highly controversial topic, while few studies link these effects to drought using large-scale in situ observation data. Here, the temperature difference between forests and non-forests (ΔT; ΔT > 0 represents the warming effect of forests, and vice versa) and its response to drought has been quantified by utilizing eddy covariance data across 31 FLUXNET paired sites, covering a large region in the mid-to-high latitudes (ranging from 40°N to 60°N) of the Northern Hemisphere. We found that high vapor pressure deficit (VPD) during summer (warm season) droughts decreased the cooling effect of forests from ΔT = -0.70 °C to −0.41 °C due to water deficit (precipitation < evapotranspiration). Conversely, winter (cold season) drought decreased the warming effect of forests, due to moderate VPD promoting forests evaporative cooling in water surpluses conditions (precipitation > evapotranspiration). The cooling effect of forests in the warm season is highly sensitive to water stress. Overall, the seasonal warming/cooling of forests responds differently to drought, which increases the difficulty of predicting climate. Our observations can help in the implementation of forestry policies for regulating climate change, especially when the frequency and intensity of drought increase in the future.