People are planting lots of trees in the hopes it will help slow climate change. It turns out this will work better if they don’t plant the same tree all the time.
Any tree will suck carbon dioxide from the atmosphere and turn it into plant tissue. But it adding a little variety has all kinds of benefits when it comes to reforestation, including the amount of carbon absorbed, according to new research.
There have been clues that a forest with a mixture of species can store more carbon, but strong experimental evidence was in short supply. Now, work spanning nearly two decades offers a strong and detailed accounting of all the carbon moving through a replanted Panamanian jungle.
It shows that a forest with five different types of trees can store around 36% more total carbon in the trunks and branches than the kind of monoculture crop often deployed in tree-planting initiatives.
“Restoration schemes should prioritize mixed over monospecific planted forests,” the researchers conclude in a paper published Feb. 25 in the journal Global Change Biology.
The new insights come from one of the oldest experimental forests in a global network designed to better understand the role of tree diversity in forests. The Sardinilla experiment is set on a former jungle in Panama that was logged in the early 1950s, then spent the rest of the 20th century as pasture. Starting in 2001, scientists planted two dozen square plots, each roughly the size of half a U.S. football field, with between one and six different species of tropical trees. Scientists chose the species to cover a range of different traits. Some grew quickly. Others more slowly. Some needed lots of direct sun. Others thrived in the shade.
In the more than two decades since the scientists have been tracking the flow of carbon in and out of each patch of trees. Counting carbon in a forest is no easy task. You have to measuring the carbon dioxide being “breathed” in and out as it’s absorbed by plants and released by decomposing vegetation. Then there’s the carbon in the parts of the trees above the ground, the dead leaves and branches on the forest floor, the roots beneath the ground, and the organic carbon that breaks down into inorganic carbon in the soil.
As the researchers watched the forests grow between 2001 and 2017, they saw that the more species of trees there were, the more carbon was stored in the portion of the trees growing above ground. After 16 years, the trees contained nearly 36 metric tons of carbon per hectare in places with five species compared to a little less than 23 metric tons per hectare in a forest with a single tree species.
There were other benefits as well. The multi-species forests were better able to maintain steady and growing amounts of carbon sequestration in the face of a string of environmental stresses – unusually wet weather in 2010, followed by a severe drought and topped with the arrival of Hurricane Otto in 2016.
“This is important, because in the face of climate change, the long-term carbon balance of forests will depend largely on their stability to disturbances. Diverse forests exhibit greater ecological stability and the risk that the stored carbon is released back to the atmosphere is lower than in monocultures”, said Florian Schnabel, a forest scientist at Germany’s University of Freiburg who led the Sardinilla experiment.
As with so many natural systems, diversity came with benefits. The forests with more trees generally had more overall growth, especially in branches as the trees competed to fill in the canopy and get precious sunlight. The species mix also made them less vulnerable to fluctuations in growth or deaths that might hit one or two species particularly hard.
Despite these benefits, the researchers also offered a sobering reminder to not overstate the potential benefits from reforestation, even in a tropical jungle. They calculated that it would take a year’s worth of tree growth on 11 hectares of the planted forests to compensate for a single airplane flight from Frankfurt to Panama.
No matter how many species of you plant, it’s going to take a lot more than trees to tackle global warming.
Schnabel, et. al. “Tree Diversity Increases Carbon Stocks and Fluxes Above—But Not Belowground in a Tropical Forest Experiment.” Global Change Biology. Feb. 25, 2025.
Image by Axel Fassio/CIFOR
