• Dr. Mahesh K. Gaur, Dr. Victor R. Squires

Reviving Forests—A Call for Careful Planning


How Large Reforestation Projects Impact Water Cycles and Local Water Availability

Deforestation of the Madagascar highlands led to river flow instability.   ©Wikimedia Commons
Deforestation of the Madagascar highlands led to river flow instability. ©Wikimedia Commons

The need to reforest portions of Earth has been well documented. The known benefits of reforestation and afforestation are unquestioned and far-reaching, particularly when it comes to impacts on water. Reforestation greatly reduces annual rainwater runoff, for instance, leading to less soil erosion and unexpected flooding. Ample forest cover lowers surface temperatures and cools soils, lakes, rivers and landscapes.

Forests also enhance water quality and reduce carbon dioxide in the atmosphere. Planting trees can increase carbon sinks that absorb and store carbon.

In addition to climate-related benefits, reforestation can potentially protect endangered species. A restored forest can undo habitat loss and improve species health.

Still, with all these benefits, Earth’s forested lands continue to shrink.

Indonesia: Land cleared for a palm oil plantation.   ©aidenvironment/Flickr:Riau
Indonesia: Land cleared for a palm oil plantation. ©aidenvironment/Flickr:Riau

A Crippling Trend

According to the UN’s Food and Agriculture Organization (FAO), the total forested area of Earth is about four billion hectares—31% of the global land area—or about 50 x 100m per person. Of this area, only about one billion hectares are primary, native forests that are largely undisturbed.

The number of forested hectares is huge, but so is the number of hectares lost. According to the FAO’s 2020 edition of The State of the World’s Forests, the world lost about 420 million hectares (mha)— approximately 10% of its total forest area—to deforestation in the last thirty years.

Satellite image of deforestation in Haiti (left) vs Dominican Republic (right).   ©NASA
Satellite image of deforestation in Haiti (left) vs Dominican Republic (right). ©NASA

The FAO report estimates that between 2015 and 2020, the rate of deforestation was ten million hectares per year. That is down from sixteen million hectares in the 1990s. Even so, the area of primary forest worldwide has decreased by over eighty million hectares since 1990.


Tree Planting has Side Effects

The world is planting trees in response. The FAO says that 7% of the global forest area is currently planted. The area of naturally regenerating forests has decreased since 1990—at a declining rate of loss—but the area of planted forests has increased by 123 million ha.

It is not easy, however, to successfully create—or recreate—a forest. According to an April 8, 2021 article in Yale 360, scientists and environmentalists have concerns about large-scale tree planting programs, as they can reduce water supplies and negatively impact agriculture and associated livelihoods.

©M.W. Toews/Wikimedia Commons
©M.W. Toews/Wikimedia Commons

How might planting trees dry up water supplies? It has to do with a tree’s ability to absorb and evaporate water at relatively high rates. According to Filoso et al., the authors of a 2017 study inPLOS One, "forests have relatively high evapotranspiration (ET) rates in comparison to most other land use and cover types, which is why water yields usually decrease upon the conversion of different land uses into forests." While it is true that large scale plantings may generate more evapotranspiration and lead to higher rainfall, this is common. To be effective, it needs to cover an area about as big as Switzerland. The major concern is that many plantation species can tap the groundwater and make the conditions less favorable for local native forest species or can lead to creeks and ponds drying up.

Water Impacts Can Be Felt Far Away

It takes relatively few trees to intensify the water cycle. According to a UN University report, "more than two square kilometers of forest expansion can increase the possibility of rainfall."

What's more, when trees, through evaporation, move water vapor into the atmosphere, it can travel far distances through "wind-driven circulation," thus increasing "the possibility of precipitation in another location," states the study’s author. This, the author writes, "indicates that, at a global scale, afforestation can indeed bring benefits to the water cycle." However, this does not take into account losses en route, such as if wind driven rain-bearing clouds pass over parched areas.

Dijke et al. (2022) observed that the effects of "directly enhanced ET and indirectly enhanced precipitation" can cause shifting patterns of water availability. They found that "large-scale tree-cover expansion can increase water availability by up to 6% in some regions, while decreasing it by up to 38% in others." Actual decreases have been more commonly reported in places such as India, Ethiopia, and China instead.

Large-scale tree-cover expansion can increase water availability by up to 6% in some regions, while decreasing it by up to 38% in others.

The effects of drying out local regions and increasing rainfall in other places can be extraordinarily far-reaching, write the authors. "Tree-cover change in the Amazon forest could impact precipitation in Canada, Northern Europe and all the way into Eastern Asia."

The study’s authors added that "several so-called hot spots for reforestation could lose water, including regions that are already facing water scarcity today." Such effects may not show up on trees themselves, but they could be impacting the water tables and small streams.

Local Winners and Losers

Dijke and colleagues predict local water-supply winners and losers (following reforestation), even though they see overall benefits for the planet. They write "that for half of Earth’s surface (47%), the indirect moisture recycling effects of large-scale tree restoration could offset the direct evaporation effects, thus resulting in slight increases in water availability rather than decreases."

Where do the authors think local water losses from reforestation will occur? The United Kingdom (UK) is one such place. They write that the UK "has a high tree-restoration potential and therefore a high increase in evaporation." This will result in "low evaporation recycling [rainfall] due to the dominant westerly moisture transport [aided by winds] from the country towards Eurasia."

New afforestation looking into Rand Wood, UK.   ©Alan Murray-Rust (CC BY-SA 2.0)
New afforestation looking into Rand Wood, UK. ©Alan Murray-Rust (CC BY-SA 2.0)

Possible winners? "Low latitudes" enjoy an increase in water availability because local evaporation recycling is high. This is due to "strong convection above the tropical forest [and short] travel distances of the atmospheric moisture," according to Dijke et al.

Possible Effects on Rivers

Dijke and colleagues predict varying effects on streamflow (following reforestation) by combining the direct effects of reforestation "via increased evaporation" and indirect effects "through increased precipitation" for twenty-one large river basins from the Yangtze to the Mississippi. For all of them, "enhanced evaporation reduces streamflow (up to 9%)," they found.

Afforestation along the Yangtze River, China.   ©Vmenkov/Wikimedia Commons
Afforestation along the Yangtze River, China. ©Vmenkov/Wikimedia Commons

Why the potentially different streamflow outcomes? Some river basins benefit "when evaporated water rains out within the same river basin," say the authors. Those same basins might also recycle rain from regions upwind. For river basins in the tropics that enjoy high local evaporation recycling, the gains could make up for losses via evaporation, they write.

What About Arid Regions?

River basins with limited water (arid regions} have a low tree-restoration potential because arid regions often lack enough groundwater to support tree growth. In such cases, state the authors, there is likely to be "a small absolute change in evaporation and precipitation" following reforestation.

The authors speculate that some arid regions might benefit from tree planting because trees can increase soil porosity and soil organic carbon, thus promoting the infiltration capacity and water storage capacity of local soil.

There is also a need to factor in seasonal impacts for arid areas that receive most of their precipitation on a few occasions per year.

Trees in manmade water catchment, Negev Desert.   ©David Shankbone/Wikimedia Commons
Trees in manmade water catchment, Negev Desert. ©David Shankbone/Wikimedia Commons

Despite these factors, the authors affirm their hypothesis that post-reforestation "streamflow will decrease for most of the world’s important river basins despite the indirect effect of evaporation recycling."

In the past, afforestation has failed unless it is done with an aggressive weedy tree such as the Acacia nilotica or Prosopis juliflora, which was officially grown in the Thar Desert of India in the 1930s to afforest the desert wasteland.

India: Learning from Possible Miscalculations

Just because an area is without tree cover does not mean it is time to start planting. According to a 2015 study, The World Resources Institute (WRI) and the International Union for Conservation of Nature (IUCN) once "misidentified nine million square kilometers of ancient grassy biomes as providing ‘opportunities’ for forest restoration." In reality, establishing forests in such grasslands, savannas, and open-canopy woodlands would "devastate biodiversity and ecosystem services," according to the study’s authors.

Fortunately, missteps based on miscalculations can be avoided. In the case of India,