When Susan Cook-Patton was doing a postdoc in forest restoration at the Smithsonian Environmental Research Center in Maryland seven years ago, she says, she helped plant 20,000 trees along Chesapeake Bay. It was a salutary lesson. “The ones that grew best were mostly ones we didn’t plant,” she remembers. “They just grew naturally on the ground we had set aside for planting. Lots popped up all around. It was a good reminder that nature knows what it is doing.”
What is true for Chesapeake Bay is probably true in many other places, says Cook-Patton, now at the Nature Conservancy. Sometimes, we just need to give nature room to grow back naturally. Her conclusion follows a new global study that finds the potential for natural forest regrowth to absorb atmospheric carbon and fight climate change has been seriously underestimated.
Tree planting is all the rage right now. This year’s World Economic Forum in Davos, Switzerland, called for the world to plant a trillion trees. In one of its few actions to address climate concerns, the US administration—with support from businesses and nonprofits such as American Forests—last month promised to contribute close to a billion of them—855 million, to be precise—across an estimated 2.8 million acres.
The European Union this year promised 3 billion more trees as part of a Green Deal; and existing worldwide pledges under the 2011 Bonn Challenge and the 2015 Paris Climate Accord set targets to restore more than 850 million acres of forests, mostly through planting. That is an area slightly larger than India, and it provides room for roughly a quarter-trillion trees.
Planting is widely seen as a vital “nature-based solution” to climate change—a way of moderating climate change in the next three decades as the world works to achieve a zero-carbon economy. But there is pushback.
Nobody condemns trees. But some critics argue that an aggressive drive to achieve planting targets will provide environmental cover for land grabs to blanket hundreds of millions of acres with monoculture plantations of a handful of fast-growing and often nonnative commercial species such as acacia, eucalyptus, and pine. Others ask: Why plant at all, when we can often simply leave the land for nearby forests to seed and recolonize? Nature knows what to grow and does it best.
Cook-Patton’s new study, published in Nature and coauthored by researchers from 17 academic and environmental organizations, says estimates of the rate of carbon accumulation by natural forest regrowth, endorsed last year by the UN’s Intergovernmental Panel on Climate Change, are on average 32 percent too low, a figure that rises to 53 percent for tropical forests.
The study is the most detailed attempt yet to map where forests could grow back naturally and to assess the potential of those forests to accumulate carbon. “We looked at almost 11,000 measurements of carbon uptake from regrowing forests, measured in around 250 studies around the world,” Cook-Patton told Yale Environment 360.
She found that current carbon accumulation rates vary by a factor of a hundred, depending on climate, soils, altitude, and terrain. This is much greater than previously assessed. “Even within countries there were huge differences.” But overall, besides being better for biodiversity, the study showed, natural regeneration can capture more carbon more quickly and more securely than plantations.
Cook-Patton agrees that as climate change gathers pace in the coming decades, rates of carbon accumulation will change. But while some forests will grow more slowly or even die, others will probably grow faster due to the fertilization effect of more carbon dioxide in the air, an existing phenomenon sometimes called global greening.