Forests are said to be the lungs of the Earth—they also provide a moist breath that travels tens to thousands of miles. Dense stands of trees release foggy clouds of condensation. The water molecules from those clouds ride wind currents before falling back down as rain. Places that rely on precipitation from this forest moisture have more stable rain cycles, meaning less variation month to month, than where rain comes from nonforest sources—this, according to a recent study in Global Change Biology. “The main takeaway is that forests around the globe help to regulate rainfall,” says lead author John O’Connor, a PhD student at Utrecht University in the Netherlands.
Massive tropical jungles, such as the Amazon and Congo, are known to maintain stable cycles of rain locally because they pump evaporated water all year. But it was an open question as to whether forests generally stabilize precipitation cycles worldwide, for instance in temperate areas, says senior author Maria João Santos, an Earth system scientist at the University of Zürich in Switzerland.
Santos and coauthors used climate models to divide the world into gridded cells and track the movement of water particles in the atmosphere. The models relied on known wind and atmospheric pressure patterns to predict the sources of rain for each cell. Cells receiving 50% or more of their water from forests saw 60% more stable precipitation regimes over a 15-year period, compared to cells getting most of their rain from nonforest and ocean sources. “We didn’t know globally if this was a characteristic just of tropical forests or of all forests,” Santos says. “We show that generally all forests in the world have this effect.”
Looking across 14 different biomes, the authors show that even in nonforest habitats, from temperate grasslands to savannahs, rain originating mostly from forests, propelled by winds, stabilizes precipitation patterns. Conversely, if 50% of the rain comes from nonforested terrestrial habitats, precipitation patterns are, on average, 68% less stable. The likely reason? Tree roots.
In a 2019 study, O’Connor showed that the deep roots of Amazonian trees allow the trees to access—and subsequently evaporate—much more water than bare soil could year-round. In nonforested habitats such as grasslands, plants growing steadily in the wet season might contribute lots of evaporated water to the atmosphere while they have access to rain. But as soon as the dry season comes, the plants die back, and then there are no deep roots to tap into groundwater supplies and keep pumping water molecules into the atmosphere. Grasslands vary more in the volume of water they put into the atmosphere as a result, and therefore, locations downwind have more variable rainfall patterns.
“This is an idea that’s been kicked around for a long time, and other people have answered it on a smaller scale,” says sustainability scientist Pat Keys at Colorado State University in Fort Collins. The global data, including many more forest samples, “helps you answer the question a lot more confidently,” he says. In the future, other researchers could replicate this study using different climate models, Keys adds, noting that varying approximations of where wind and pressure patterns carry rain, and the precise sources of water for different biomes, could produce slightly different results.
The next question, and final portion of O’Connor’s PhD, is tracking the reach of forests to calculate exactly how far their moist breath travels. “It’s important for global governance,” Santos says, since winds don’t obey political borders. Forests in Northwestern Canada, for example, could hypothetically contribute water that helps stabilize rain cycles down the US East Coast. In light of the Intergovernmental Panel on Climate Change’s Sixth Assessment Report, released August 9, which warns that climate change is intensifying droughts, Santos says an “obvious question” arises: How hot does it have to get for forests to stop performing their rain-stabilizing function? “Is this input of water broken at some point?” she asks; because if it breaks, the cascade would lead to even worse droughts.
A global analysis on the utility of forests for rain is timely, Keys says. “More reasons to protect forests is not a bad thing,” he adds. “And this is an under-recognized reason.”
Other recent papers recommended by Journal Club panelists: