What we think we know about how to restore tropical forests is getting a second look. A new paper produced by scientists in Missouri Botanical Garden’s Center for Conservation and Sustainable Development (CCSD), the University of Hawaii’s Lyon Arboretum, and the University of Maryland Baltimore County points out an important bias in recent studies.
How should we restore forests in places where they have been lost? This is one of the main questions that we study in the CCSD, so we were surprised last year when a big synthesis paper that compiled data from many earlier studies said that, when it comes to restoration, doing nothing was the same as doing something.
That’s only a slight exaggeration. The paper, by Renato Crouzeilles and several other scientists, said that letting a forest regrow on its own (that is, natural regeneration) was usually more successful than planting trees (that is, active restoration). Their conclusion was based on comparing many studies done throughout the world’s tropical forest regions.
Apples to oranges
The problem with this paper (and several like it) was that the set of studies looking at natural regeneration were not really the same as the set of studies looking at tree planting. The natural regeneration studies focused on forests that already existed, while the tree planting studies focused on a wider range of sites, many of which started with no forest. In other words, the natural regeneration studies had already been filtered to exclude places with a weak ability to grow forests.
To understand the problem, it is helpful to look back at the history of tropical forest restoration research. For many years, scientists who wanted to know about how forests recover after a disturbance (like a hurricane or logging) would go out and find several forests that had been recovering for different amounts of time. If you take forests that are 5, 10, and 20 years old, you can try to compare them to each other in order to see how a forest might change over 20 years. In contrast, tree planting studies usually start with a piece of land that has no trees on it. Scientists who want to know how trees grow on this land will plant some and then observe their survival and growth over time. These trees may or may not create a forest there, as the land can vary in quality.
So where does that bring us with respect to this study? If you compare a forest that already exists with another potential forest where planted trees may or may not survive and grow well, it’s a safe bet that the pre-existing forest will have taller trees. It has a head start over the planted forests, and we argue in our paper that the comparison is not a fair one.
This means that letting forest regrow on its own is not always a better option than planting trees. In fact, there are many places – like overgrazed pastures, mine sites, and other heavily degraded lands – where forests have been cleared and most likely will not be able to grow back on their own.
While we were not convinced by studies that said that natural regeneration is better than tree planting, we also don’t want to take any options off the table. Natural regeneration and tree planting are not mutually exclusive – in fact, they are highly complementary. Our practical advice is that if you want to get forest back, the best option is to see if natural regeneration can do the trick before you invest in tree planting. Or better yet, set up a paired experiment comparing the two strategies at the same site.