Mixed-species forests are emerging as a better path for large-scale restoration than single-species plantations, as nations and organizations pursue pledges to plant more than 1 trillion trees this decade to counter climate change and biodiversity loss. Past drives show many seedlings fail to survive, and monoculture stands that do persist can function as biological deserts with limited resilience.
The United Nations declared 2021-2030 the Decade on Ecosystem Restoration to spur recovery of degraded landscapes. Tree planting sits at the heart of that push, through efforts such as the Bonn Challenge and the Trillion Trees Campaign. Yet many commitments lean heavily on monoculture plantations, vast areas seeded with one species.
These plantations are geared for wood production, but they carry high risks. Drought, insects or wildfire can wipe out entire blocks at once. In one example cited by researchers, nearly 90% of 11 million saplings planted in Turkey perished within three months due to drought and inadequate maintenance.
Forests provide far more than timber. They regulate water, lock away carbon, shelter wildlife, cool surrounding landscapes and support human well-being.
Evidence increasingly points to a stronger option that captures ecological and economic value while spreading risk. Mixed-species forests that reflect natural diversity tend to grow faster and withstand stress better.
Mixed-species forests boost growth
Community and landscape ecologists at the Smithsonian Environmental Research Center near the Chesapeake Bay have tested this approach since 2013 through a large experiment known as BiodiversiTREE. With volunteers, the team established nearly 18,000 seedlings across 60 acres of former fields, using 16 native species that span fast-growing timber trees, mid-story species and slow-growing trees that may take a century to mature.
Some plots were planted as monocultures, while others featured randomized mixtures of four or 12 species, reflecting diversity levels found in nearby forests. The core question was straightforward. What happens when plantings emulate nature rather than uniform stands?
A decade on, the contrasts are clear. Surviving monoculture plots resemble conventional plantations, with narrow crowns and little life beneath the canopy. By comparison, mixed plots are layered and dynamic, with dense foliage and an active understory.
The ecological gains track with what researchers measured. Trees grown in mixtures, including poplar and red oak, are up to 80% larger than the same species planted alone. Mixed plots also harbored fewer leaf pathogens, more abundant caterpillar communities that feed birds, and greater chemical diversity in leaves. Researchers suggest that certain leaf compounds deter browsing by deer, which may help explain the higher growth in mixed plots.
Canopies in multi-species stands are fuller and denser, creating cooler, shadier microclimates that support understory plants and up to 50% more insects, spiders and birds.
The pattern extends beyond Maryland. BiodiversiTREE is part of TreeDivNet, a global network encompassing more than 1.2 million trees and hundreds of species. Across climates and continents, results are consistent. Diverse forests tend to accumulate more biomass, store more carbon and better tolerate drought, pests and disease.
So why are monocultures still common?
Despite decades of evidence, most commercial operations still rely on single-species stands, and these areas are often counted toward international reforestation goals. The reasons are mainly practical. Designing and establishing mixed plantings can be more complex and costly, and until recently there was limited proof that they could meet or exceed the financial returns of monocultures.
To bridge science and practice, researchers at the Smithsonian Environmental Research Center have launched a new experiment called Functional Forests. The project assembles intentional mixes of species to test whether specific combinations can deliver ecological benefits alongside timber and other services that support a sustainable economy.
The team selected 20 tree species, each chosen for one or more functions such as timber production, wildlife habitat, edible fruit, resistance to deer and climate resilience. Some of the nearly 200 plots are monocultures, while others combine five species tailored to produce multiple functions. Certain plots are protected from deer browsing and others left open.
By comparing these designs, the study will evaluate how different strategies perform across goals ranging from wood and food production to biodiversity and climate readiness. The aim is to help landowners and communities match plantings to their priorities while capturing the positive effects of biodiversity.
Planting 1 trillion trees wisely
Global restoration now attracts hundreds of billions of dollars annually. Missteps mean wasted resources and missed chances to address climate and nature crises. If the world sets out to plant a trillion trees, researchers argue that success requires rethinking what a forest should be. The objective is not just to raise trees. It is to regenerate forests that endure, and mixed-species forests offer a proven way to do it.
That regeneration will also shape the landscapes that migratory species depend on, from forest birds to long-distance songbirds whose routes are only now coming into focus through tools like barometric geolocation mapping of warbler migrations.