In Maryland, Smithsonian scientists planted 17,000 trees in plots with 1, 4, or 12 species; after years of monitoring, mixed forests grew faster and stored more carbon

A tree planting event is a classic feel-good event. A sapling, a shovel, and a quick photo for the group chat. Tree planting has become synonymous with "I’m making a difference to the climate," meaning many people will rarely ask any follow-up questions. What species of tree? How many species? What are they planted next to? Most won’t even think to ask these questions.

Scientists in Maryland have spent over a decade gathering data that answers exactly these questions. According to the Smithsonian Environmental Research Center, a new study showed that mixed-species forests grow faster than single-species forests and are more efficient at absorbing carbon due to their faster growth rate. Every year, 10 million hectares (around 25 million acres) of forest is lost due to deforestation globally. However, around half of this is offset through regeneration.

Inside a nearly 18,000-tree experiment


The experiment, known as BiodiversiTREE, was conducted on the Smithsonian Environmental Research Center’s campus in Edgewater, Maryland. Scientists and volunteers led by John Parker planted roughly 17,850 trees across 70 plots on 60 acres of land. These trees were divided into single-species, four-species, or twelve-species sections.

The experiment was designed to be long-term. As trees can live for such a long time, Parker said they couldn’t really determine anything in just two years. BiodiversiTREE is one of only a handful of large tree-diversity experiments like this in North America, and the project itself spans two sites: the original at Edgewater, and a second planted about a year later at the Smithsonian Conservation Biology Institute in Front Royal, Virginia, both run by the Smithsonian. It contained a different combination of native trees; however, the results were the same.

Why mixed forests are faster growing

Using LiDAR data from the National Ecological Observatory Network, the researchers discovered that having a mixture of species in the canopy made the canopy structure more complex. This meant different lengths and structures of branches in the canopy, creating a puzzle effect, where different tree shapes fit together more tightly for more efficient use of light, according to lead researcher Catherine Fahey.

When comparing single-species plots to twelve-species plots, Fahey and colleagues found increases in canopy complexity and productivity of up to 71% and 73%, respectively, in their 2025 paper “Canopy complexity drives positive effects of tree diversity on productivity in two tree diversity experiments,” published in the journal Ecology. This means canopy complexity may be a key driver in productivity increases caused by diversity in young forests.

Not just a two-forest story

Whilst two research locations are a good start, a 2016 study titled “Positive biodiversity-productivity relationship predominant in global forests,” by Liang et al., published in the journal Science, looked at 777,126 permanent forest plots across 44 countries. This showed that the relationship between biodiversity and productivity in forests was consistent across all areas. The study found that continued biodiversity loss is linked to declining global forest productivity, a relationship that, a decade on, is treated as established rather than speculative.

The catch: timber math

Despite mixed forests being so productive, many forestry and logging plots are single species. Per the Smithsonian Environmental Research Center, this is because it makes harvesting much easier. Those looking to regenerate for conservation efforts usually do plant a diverse range of trees; however, those looking to plant trees for timber production are harder to persuade. Evidence showing mixed forests can grow faster may encourage these industries to change their ways.

However, the Ecology paper frames its conclusions around young forests, so this is a story still being written as the trees mature. The next time a brand, campus, or friend announces they’re planting thousands of trees, it’s worth asking what species. The results from Maryland show that planting a diverse range of trees can increase growth. This means the same amount of land will absorb more carbon.