We have quantified the temperature difference between sites within forests and outside, for the first time across the globe. Measurements occurred at 98 locations spread across five continents, in the tropics, temperate zone and northern boreal forests. The data show that the maximum temperature in forests is, on average across the globe, 4 degrees lower than outside forests.
Forests as thermal insulation
With their foliage and branches in the canopy, trees create a thermal isolating layer above the forest, cooling the understory when ambient temperatures are hot and warming the understory when ambient temperatures are cold. Summer heat waves are thus strongly moderated below the tree canopy. Plants and animals inside forests will thus experience the current warming trend to a lower degree than species not living in forests. Since forests cover a quarter of the land surface of the globe and harbour two thirds of all biodiversity, this has important implications for predictions on the impact of climate change.
Buffer against climate warming
We also show that with increasing temperatures, the buffering capacity of maximum temperatures of global forests also increases. The warming of maximum air temperatures within forests is thus probably lower than previously anticipated. Forests can therefore truly be considered a buffer against climate warming. Accordingly, our findings stress the need to conserve the existing forests and enhance reforestation efforts.