Don’t just plant trees, plant forests and restore biodiversity for the future

Around the world, people plan to plant more than 1 trillion trees this decade in an ambitious effort to slow climate change and reduce biodiversity loss. But if the past is prologue, many of the trees planted will not survive. And if they do, they could become biological deserts lacking the richness and resilience of healthy forests.

It doesn’t have to be this way.

The United Nations has declared 2021-2030 as the Decade of Ecosystem Restoration to encourage efforts to restore degraded ecosystems. Tree planting is at the heart of that effort, supported by initiatives such as the Bonn Challenge and the Trillion Tree Campaign.

However, many tree-planting efforts have serious flaws. We rely too much on monoculture plantations, large areas planted with just one species of tree.

Monoculture plantations are usually a one-way ticket to timber production. But these high-yielding plantations can be risky and surprisingly vulnerable. Droughts, pests, and forest fires can destroy entire monoculture plantations at once. In one example, nearly 90% of the 11 million seedlings planted in Turkey died within three months due to drought and lack of care.

Forests are more than just sawmills. They regulate water, store carbon, provide habitat for wildlife, cool the surrounding landscape, and even provide human health benefits.

Science now points to a prudent path to reaping both ecological and economic benefits while minimizing risk, rather than betting on a single species and hoping for the best. It’s about mixed-species plantations that reflect the biodiversity of natural forests, ultimately creating forests that grow faster and are more resilient in the face of constant threats.

We are community and landscape ecologists at the Smithsonian Environmental Research Center. Since 2013, we and our colleagues have rigorously tested this idea in a large ecosystem-scale experiment called BiodiversiTREE. The verdict is shocking. Trees in mixed forests not only survive, they outgrow monoculture trees and support dramatically more biodiversity.

Trees with diverse neighbors grow larger

Thirteen years ago, we worked with volunteers to plant approximately 18,000 seedlings on 60 acres of fallow fields on the campus of the Smithsonian Environmental Research Center near the Chesapeake Bay.

We didn’t just plant a single seed. We planted 16 native species from all types of trees. Some species are fast-growing timber species, and some are slow-growing species, which can take over 100 years to reach full size.

Some plots were planted with only a single species, and rows of the same species were planted over and over again. However, other trees were planted with random assignments of 4 and 12 species, reflecting the middle and upper ends of tree diversity in similar-sized areas of local forest.

We asked a simple question. What if we tried to mirror nature and plant a mixture of species rather than a monoculture?

The difference after more than 10 years is noticeable.

The surviving monoculture plots resemble the traditional plantation forestry that has historically dominated the rural southeastern and Pacific Northwest regions of the United States. There are rows of tall, narrow trees with sparse canopies and little life beneath them.

In contrast, mixed-species plots are layered, complex, and dynamic, with foliage filling the canopy and beneath which a diverse range of plants and animals thrive.

These visual contrasts reflect real ecological benefits. Trees grown in mixtures that include important wood species such as poplar and red oak can grow up to 80% larger than when grown alone. Mixed plots had reduced foliar pathogens, a higher presence of caterpillar communities that provided food for birds, and increased phytochemical diversity on foliage. We hypothesize that these leaf chemicals, some of which have the effect of deterring animals from eating the leaves, may have reduced feeding damage by hungry deer and ultimately promoted tree growth in mixed plots.

Plots planted with several tree species developed richer, denser leaf canopies, provided cooler, shady conditions, flourished understory plants, and hosted up to 50% more insects, spiders, and birds.

This pattern is not limited to this site. The BiodiversiTREE project is part of TreeDivNet, a global network of large-scale experiments spanning more than 1.2 million trees and hundreds of species. The results are consistent across continents and climates. Forests with a mix of species tend to grow larger, store more carbon, and withstand stress from drought, pests, and disease.

So why is monoculture still so common?

Despite decades of evidence, mixed species plantings remain relatively rare in practice. Most commercial forestry still relies on monocultures, and these plantations are the target of international tree-planting campaigns aimed at slowing climate change and reversing biodiversity loss.

The reasons are generally practical. Mixed plantings can be more complex to design, more expensive to establish, and more difficult to manage. Importantly, until recently, there was limited evidence that they could match or exceed the economic returns of traditional plantations.

A new experiment called the “Functional Forest” at the Smithsonian Environmental Research Center aims to bridge the gap between science and practice. We are developing intentionally designed tree combinations to test whether specific species mixtures can contribute ecological benefits. At the same time, we are testing whether they can also provide the timber and other services that humans need to support a thriving and sustainable economy.

Each of the 20 tree species in the Functional Forest Project is selected to provide one or more benefits, including timber, wildlife habitat, food for people, deer resistance, and climate resilience. However, no single species provides all these benefits.

Some of the approximately 200 plots contain a single species, while others contain combinations of five species carefully selected based on the functions they provide. Some sections are protected from deer access, while others remain exposed.

By comparing these approaches, we can test how different reforestation strategies perform across a variety of goals, from timber production to food production, biodiversity to climate resilience.

Landowners and communities have a variety of priorities, including producing timber, supporting wildlife, and creating forests that can withstand a changing climate. The idea behind functional forests is to design plantings that can provide multiple of these benefits at once, rather than optimizing for just one, essentially harnessing the positive effects of biodiversity to achieve real-world goals.

Plant 1 trillion trees wisely

The stakes are high. Restoration has become a major global investment, with hundreds of billions of dollars already being spent each year. Getting it wrong means wasting resources and missing opportunities to address the most pressing environmental challenges of our time.

If the world is going to plant 1 trillion trees, we believe it’s not enough to simply plant saplings in the ground. We need to rethink the nature of forests.

Growing trees is not the only purpose. It is about growing sustainable forests.