Is planting trees as good for the Earth as everyone says?

The scientific evidence on reforestation and forest restoration

Use the drop down menus to select data from your country of choice or to view data for a particular type of evidence. Click on the name of an outcome (such as forest cover or jobs) to display data specific to it. Click on a square to see what evidence the outcome is based on. You can find this and other infographics in our Conservation Effectiveness series at ConservationEffectiveness.org. Data visualization by GreenInfo Network

 

The verdict for environmental outcomes: Mostly positive change

Overall, most of the studies in our database show that reforestation and forest restoration does indeed increase forest cover. They have other positive environmental outcomes, too, like sequestering carbon, improving water quality and preventing erosion. However, projects generally take quite a long time, at least several decades, to provide old-growth forest levels of benefits. In some cases, all of them plantations of exotic species, reforestation actually proved worse than leaving the land in its current state, which underscores the idea that tree planters need to carefully consider the detailed context of the area they plan to reforest.

With regard to biodiversity, the findings are pretty consistent: most of the time, reforestation does increase plant and animal diversity relative to the land before reforestation, and sometimes even relative to natural regeneration (two common controls in the more rigorous studies). At the same time, biodiversity is almost never as high as in old-growth forests, although there are a few exciting exceptions, and, anecdotally, it seems that areas near mature forests recover faster. Some studies present sobering projections of the time it would take to recover to old-growth levels of biodiversity, on the order of 100 to 150 years longer than their study period.

One important, albeit unsurprising, finding is that monoculture plantations tend to decrease biodiversity, even compared to croplands, as Hua concluded in a 2016 study of China’s Grain for Green program.

We reached similar conclusions about biomass and carbon sequestration: reforestation almost always has a positive effect, in that it speeds up biomass accumulation and hence carbon sequestration compared to land before reforestation and naturally regenerating forests. However, here, too, replanted forests rarely match old-growth forests. For example, a 2016 study estimates that it would take another 96 years to reach old-growth levels of biomass accumulation in a replanted forest. Soil carbon seems to be especially slow to recover. Climate change mitigation strategies need to count on this: it can take more than a century to reach the amount of carbon stored in the soils of old-growth forest.

 

The survival rate of planted trees, whether they’re planted as seeds or seedlings that were germinated in nurseries, varies widely, from 0% to almost 100% success. In the studies we examined, we found a nearly even split between projects that report a greater than 50% survival rate (which we rank as a positive outcome in our database), and those that saw a less than 50% survival rate (we rank those as neutral).

It’s important to note that most studies of seedling survival in our database are highly controlled, rigorously designed, small-scale experiments. This enables a careful statistical analysis of the cause and effect of seedling survival, but does not necessarily reflect real-life conditions. High levels of seedling survival under tightly controlled conditions might not translate to successful reforestation projects at larger scales out in nature, where many additional factors may decrease seedling survival.

One 2020 study in the Philippines found that involving local people and organizations in a pilot reforestation project resulted in higher seedling survival and growth success (78% after three years) than did government-managed restoration programs. A key takeaway is that community-led restoration can work well so long as project organizers provide sufficient support, incentives, and decision-making opportunities to local communities.

Many reforestation projects are designed to decrease erosion and improve hydrology, and most of the studies we examined document improvements in these areas. However, only one study compared erosion and runoff in a reforestation project to that in a natural regeneration site, and natural regeneration came out ahead. In this category, there are cautionary tales of tree species planted where they don’t belong, causing water tables to dry up, like South Africa’s acacias (although we don’t actually have those studies in the database, for now).

It’s also worth remembering that all forest restoration interventions have different objectives and varying site quality. Many projects aim for biomass gain, for instance, so their biodiversity outcomes should not be compared to those of a project focused on biodiversity. The scale of restoration interventions (plot vs. landscape, etc.) is also important when interpreting results.

A local volunteer plants a mangrove sapling at the Trapeang Sangke inlet. The best time for transplanting is July, the peak of the wet season when the estuary is least salty. With a full team of volunteers, 4,000 saplings get transplanted in the most productive months. Photo by Matt Blomberg for Mongabay.
A volunteer plants a mangrove sapling at Trapeang Sangke estuary in Kampot, Cambodia, in November, 2017. Image by Matt Blomberg for Mongabay.

The verdict for social and economic outcomes: Inconclusive

We didn’t find enough data to allow us to make general conclusions about how effective reforestation and forest restoration are at achieving socioeconomic outcomes, such as increasing community well-being and financial security, supporting local livelihoods, and clarifying land ownership.

The studies that address the social aspect mainly cover employment, and how well the benefits of reforestation projects are shared among stakeholders. These tended to find that while reforestation projects did bring jobs to local communities, they didn’t necessarily reduce the gap between the rich and poor. Amid the general dearth of research on these topics, there is a need for more studies investigating how governments and other institutions can best work with local communities, particularly Indigenous peoples.

The few studies that cover the economic aspects of restoration tend to focus on the cost-effectiveness of tree planting versus natural regeneration and to consider the income that communities get from tree planting. They generally found that tree planting mostly did bring economic benefits to the local community. The studies that reported economic benefits often examined reforestation projects that included a payments-for-ecosystem-services scheme, such as China’s Grain for Green program. Opportunity costs of reforestation are particularly understudied, so we can’t say if enrolling one’s land in a reforestation project tends to confer more economic benefits than continuing to farm the land, for instance. It’s clear we need more studies like that, especially ones that factor in current carbon prices.

A popular narrative seems to have emerged in recent years that holds tree planting as the answer to the global climate crisis and a host of other environmental ills. Some researchers and practitioners have bolstered this perception by pushing tree planting as the number one nature-based climate solution, and large-scale campaigns that aim to plant billions or even trillions of trees have, too. From the research we examined, however, it’s evident that, just like every other strategy we’ve covered in the Conservation Effectiveness series, tree planting is no silver bullet. Tree planters need to ensure plantings makes sense in the local context, and even then there are numerous trade-offs and competing objectives to negotiate. And they need to understand that it may take a century before forests reach their full carbon-storage potential — yet we have just a decade to drastically curtail greenhouse gas emissions if we are to avert the worst impacts of runaway global climate change, according to the Intergovernmental Panel on Climate Change.

Since 1990, the world has lost as much as 420 million hectares (1.03 billion acres) of forest, much of it felled to make room for agriculture. Though the pace of forest destruction has decreased somewhat since 2015, the U.N.’s Food and Agriculture Organization says that “Deforestation and forest degradation continue to take place at alarming rates.” It might not ultimately make sense to replant all of those forests, but it’s clear that, in tandem with halting deforestation, planting trees can be a valuable part of efforts to combat global warming and restore other vital ecological functions that all life on this planet relies on.

So it’s at least partly true that “Trees are the answer,” as the slogan goes. We just have to make sure to do it right.

A young activist in Ghana, in May, 2016. Image by Plant for the Planet via Flickr (CC BY-NC 2.0).

Banner image: A man carries trees during a reforestation project in Tanzania in 2018. Image by David Wilfred, Sky Studios, Arusha for One Tree Planted. Copyright Matt Hill.

 

Writer: Mike Gaworecki

Researchers: Zuzana Burivalova, Rodrigo Mendes, Sharif Mukul

Editors: Rebecca Kessler, Willie Shubert

Copyeditor: Hayat Indriyatno

Infographic: Zuzana Burivalova, GreenInfo Network

Additional contributions: students from the Forests of the World course, University of Wisconsin-Madison

Related listening from Mongabay’s podcast: “Reforestation vs deforestation: Forest losses and gains this past year,” listen here:

 

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Editor’s note: Due to a dispute between various parties, we switched the header image for this story on June 4, 2021.

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