In the global scramble to reduce carbon emissions, planting more trees is always near the top of the list of solutions. Pegged as a low-cost, natural, and scalable approach, projects like the Great Green Wall in North Africa, Pakistan’s 10 Billion Tree Tsunami, and New York City’s Million Tree Program raise the bar for this climate change mitigation strategy. While a new scientific study found there is untapped potential for carbon sequestration through planetary reforestation, other researchers are concerned about how growing new forests could reduce the focus on preserving existing old growth forests or negatively impact the water supply in developing countries.
The recent study published in Science, led by Thomas W. Crowther at ETH-Zürich, posits that an increase in 0.9 billion hectares (2.2 billion acres) of new forests, an amount that would cover about 14 percent of habitable land, could sequester 205 gigatons of carbon from the atmosphere. This means a forest roughly the size of the United States or China could sequester more than five times the annual carbon output of the planet.
Under current climate conditions, the Earth could support a maximum of 4.4 billion hectares (10.9 billion acres) of forests. Approximately 2.8 billion hectares (6.9 billion acres) are currently forested. This leaves 1.6 billion hectares (4 billion acres) were additional forest could be planted. The research team removed land used for crop-based agriculture or cities,”which are necessary for supporting an ever-growing human population,” leaving 0.9 billion hectares (2.2 billion acres) available for forest restoration.
Across the lifetime of these proposed new forests, the trees would sequester 205 gigatons of carbon from the atmosphere. For reference, we have released 1,510 gigatons of carbon to date (as of 2015), and some 55 percent of that has been sequestered by oceans and plants.
A sequestration strategy of this magnitude would make a sizable dent in the total carbon released into the atmosphere, but needs to be matched with reductions in fossil fuel use and other major forms of greenhouse gas emissions. The World Resources Institute (WRI) reports that 37.1 gigatons of carbon were released in 2018 alone. At this rate, more carbon will be released than can be captured by the new forests during the 50-100 years it will take for the trees to mature.
The research team is correct in asserting that global tree restoration is “our most effective climate change solution to date,” but some researchers fear that addressing one warning light may turn on others.
For example, focusing on planting new forests instead of preserving old growth trees can have negative impacts. Large, old trees, which support greater biodiversity and sequester more carbon than younger trees, are “declining in forests of all latitudes,” according to a 2012 study. Old growth forests are able to sequester more carbon than their younger counterparts because they are still rapidly growing and increasing their carbon storage capacity. Preserving older forests while implementing massive reforestation efforts would yield the greatest potential for carbon capture and forest ecosystem health.
Protecting large old trees is an important part of the climate mitigation effort, and something that landscape architects working at a variety of scales can support. Every reforestation effort, even in an urban park, should take into account existing trees and the role they play in ecosystems.
Trees need water to thrive. The renewed call for mass reforestation across the globe has some researchers worried about the effect this will have on local water supplies.
In a recent study published in Nature, Jaivime Evaristo and Jeffery J. McDonnell examine the impact of forest management practices, such as deforestation, conversion into agricultural land, regrowth, and afforestation (growing new forests), on the availability of water in watersheds. The study develops a vegetation-to-bedrock model, which considers the geology of a given region in relation to its capacity to store water.
The researchers found that deforestation and conversion of forests into agricultural land increases the volume of water present in almost all watersheds, while regrowth of forests and afforestation reduced the volume of water. “The vast majority of the water loss in afforested and reforested areas is from evapotranspiration, which is a combination of evaporation from soil and other surfaces and transpiration from plants.”
Afforestation and deforestation have the largest impacts on streamflow in watersheds. Deforestation can cause flash floods, but reforestation can lead to droughts.
The data also shows the percentage change in tree cover is correlated to the socio-economic status of a country. Developing and least developed countries lose the most tree cover while developed and emerging countries lose the least. The researchers think this correlation between tree-cover change and economic status “suggests that countries that have infrastructure in place for capturing and storing water may be least vulnerable to possible water supply shortages associated with planting schemes.”
Furthermore, the research team concludes the magnitude of a forest management technique is correlated with the water-yield response. Reforesting nearly 14 percent of the landmass is a massive change, one that would surely have consequences for local communities and ecosystems.
The researchers recognize their streamflow analysis could be used most prudently “for re-calibrating the cost-benefit matrix of climate change mitigation schemes (for example, planting and removal) in different geo-climate regions around the world.”