This story originally appeared on Undark and is part of the Climate Desk collaboration.

When most people conjure a forest, they imagine a dense network of trees, their crowns arching high above, with spots of sunshine flashing between the leaves. Some might also think of birdsong and insects, or summon thoughts of thick foliage in the understory, the crunch of leaves or pine needles underfoot, or overgrown trails meandering into the thicket.

Whatever the particular imagery, it’s undoubtedly more picturesque than that conveyed by the United Nations Food and Agriculture Organization’s definition: An area greater than 1.25 acres, populated by trees 16 feet or taller, with more than 10 percent canopy cover. While this simple and straightforward list of attributes might make it easy to classify land, it gives little insight into what a forest can and should look like, which is important because recent research suggests that not all are created equal.

From a climate perspective, forests are vital because they’re filled with vegetation, fungi, and microorganisms that draw carbon dioxide from the air and store it. Although just how much CO 2 they can absorb may have been overestimated, there’s no doubt that ample, healthy forests can provide a relatively low-tech way to help offset greenhouse gas emissions and combat climate change.

Indeed, the United Nations recognized this value when it launched its REDD+ program, which gives developing countries money to protect forests rather than cut them down, and then enshrined the scheme in the Paris Agreement on climate change in 2015. This followed on the Bonn Challenge, launched by Germany and the International Union for Conservation of Nature in 2011, which aimed to restore more than half a million square miles of deforested and degraded land around the world by 2020 and more than double that by 2030.

The private sector has also gotten in on the act, often to make up for other environmentally destructive activities. In April, oil giant Shell pledged $300 million to offset the carbon emissions of its customers though forest restoration projects in countries such as the Netherlands and Spain.

The problem: Some experts are concerned that these initiatives rely on such an anemic definition of what constitutes a forest that they will ultimately generate far fewer benefits than advocates imagine. In a commentary published in Nature in April, Simon Lewis, a professor of global change science at University College London, Charlotte Wheeler, a forest researcher at the University of Edinburgh, and their co-authors noted that almost half the area pledged under the Bonn Challenge is actually planned plantations that nurture single tree types — usually for timber or food crops. While this may increase the global tally of “forested areas” around the world, the researchers suggest that such plantations will do little to meet the initiative’s environmental goals.

“Although these can support local economies, plantations are much poorer at storing carbon than are natural forests, which develop with little or no disturbance from humans,” they wrote. “The regular harvesting and clearing of plantations releases stored CO 2 back into the atmosphere every 10 to 20 years. By contrast, natural forests continue to sequester carbon for many decades.”

And it’s not just about carbon. Healthy, mature forests support a broad variety of lifeforms, giving and taking nutrients, habitat, and shade. They catch, store, and filter water. They improve air quality by removing pollutants. And their impacts extend outside their borders; a functioning forest prevents land from being degraded and keeps it productive, can reduce the risk of flooding on lower lying ground, and provides a source of timber, food, medicine, and jobs for people.