Planting forests with diverse tree species increases likelihood of success

A decade-long investigation suggests that the diversity of tree species planted in forests can significantly contribute to their survival, according to the latest findings from the Smithsonian Environmental Research Center (SERC) and The Nature Conservancy. 

These organizations embarked on a monumental study, BiodiversiTREE, which was conceived as a century-long exploration of the impacts of various tree planting strategies on sapling survival and the broader ecosystem.

Following nature’s design

Nature’s own design features forests brimming with diverse species. This biological assortment confers several advantages, including a built-in resistance to pests and diseases, the resilience to adapt to changing climates, and the creation of an expansive habitat for wildlife. 

But this truth is often overlooked, as many forest plantations and restoration projects resort to monocultures – planting a single species on a given tract of land.

Such an approach leaves forests, managed or otherwise, susceptible to shifting conditions, placing their ecological and economic stability at risk. Monocultures also harbor a heightened risk of planting failures, leading to early sapling death.

Biodiversity is a well-established catalyst for healthier ecosystems, as evidenced by hundreds of scientific studies. This fact hints at the potential for greater tree survival rates when diversity is included in the planting process. 

How the study was conducted

Until BiodiversiTREE, few tests of this theory involving trees were conducted. Scientists constructed this project to examine the influence of tree diversity on the ecosystem function, setting a new record as the most extensive study of its kind in North America.

A decade ago, a team of scientists and volunteers planted 20,000 saplings on former farmland near Chesapeake Bay to establish BiodiversiTREE. They opted for varying levels of species diversity, ranging from single-species sections to plots with four or twelve different species. 

Throughout the years, around 8,000 trees in the project have been monitored, first annually for three years, then every two to three years following that.

What the researchers learned

After ten years of study, the findings revealed that reforestation initiatives involving diverse species have a higher likelihood of planting thriving species. This diversity reduces the risk of planting failures, enhancing the overall success of the projects.

Rachel King is a forest ecologist at SERC and the lead author of the study, which is publishedi in the journal Restoration Ecology. “Even planting four species instead of just one significantly reduced variation in survival in our plots,” said King, emphasizing that small increases in diversity could have substantial impacts on the success of tree planting efforts.

King’s observations echo those of prior tree diversity experiments, which demonstrated that monocultures present a risky gamble. With single-species plots in BiodiversiTREE, results varied from a “boom or bust” scenario. 

For instance, sycamore species enjoyed a 99% survival rate, whereas hickory plots languished at around 21%. In contrast, diverse plots exhibited much greater stability, showing a variability of survival approximately half that of the single-species plots.

Impact of forests on climate change mitigation

Forests envelop roughly one-third of the Earth’s land, acting as a refuge for most terrestrial species and providing sustenance for millions of people worldwide. Crucially, forests are key combatants in the fight against climate change. 

It’s estimated that the restoration of forest cover in the United States alone, covering up to 148 million acres, could absorb 535 million metric tons of carbon dioxide annually. This is akin to eliminating 116 million cars from our roads.

Susan Cook-Patton, a senior forest restoration scientist at The Nature Conservancy and co-author of the paper, acknowledges the importance of forest restoration in mitigating climate change. 

She cautions, however, that “tree planting alone will not solve climate change, but done right, has significant potential as a natural climate solution.” Cook-Patton advocates for diversity in tree planting initiatives, emphasizing that “it is important to make sure those new forests are set up for success by employing a diverse set of native species.”

BiodiversiTREE’s location, the Chesapeake Bay area, holds its own environmental significance. This region houses the largest estuary in the United States and ranks third largest globally, encompassing six states and Washington, D.C. 

Overfertilization and other human activities have unfortunately compromised the productivity of this once thriving estuary. Planting trees in this critical watershed will aid in the rehabilitation of both the land and its waters.

The role of forests in successful ecosystems

Jamie Pullen, co-author and head technician in the Terrestrial Ecology Lab at SERC, highlights the integral role forests play in sustaining ecosystems: “Forests filter out nutrients, and reforesting a whole watershed – like we did with BiodiversiTREE – can provide valuable insights into improving the health of the Chesapeake Bay.”

As the urgency to combat climate change amplifies, reforestation and tree planting are emerging as prominent strategies in natural climate solutions. However, according to John Parker, senior scientist at SERC and director of BiodiversiTREE, many such projects overlook the critical step of monitoring their survival outcomes or impacts on other ecosystem functions. 

BiodiversiTREE, together with a network of similar projects under the TreeDivNet global consortium, seeks to bridge these knowledge gaps and offer robust scientific data for more effective ecosystem management.

Ultimately, the BiodiversiTREE project offers an encouraging path forward for reforestation efforts. By promoting diversity in tree planting, we can enhance the survival rates of young trees, boost ecosystem health, and contribute to global efforts to combat climate change. A focus on biodiversity in our forests not only promises a healthier environment but also a more resilient future for all inhabitants of Earth.

More about forests, trees, and the environment

Climate change significantly impacts forests and trees, and consequently, the environment and humanity in several ways.

Altered Growth Patterns and Forest Health

Changes in climate, such as increased temperatures, altered precipitation patterns, and elevated levels of carbon dioxide, can impact the growth and health of trees and forests. Some trees may grow faster in response to higher levels of carbon dioxide, while others might suffer from heat and water stress due to increased temperatures and altered rainfall. 

These changes can disrupt the balance of different tree species in forests and potentially lead to shifts in the geographical range of certain species.

Pest and Disease Outbreaks

Warmer conditions may lead to increased outbreaks of pests and diseases, impacting forest health and productivity. Pests that were once limited by cold temperatures could spread to new areas, causing extensive damage.

Increased Wildfire Risk

Climate change contributes to drier and hotter conditions in many regions, escalating the risk of wildfires. These fires not only destroy vast tracts of forest but also release large amounts of carbon dioxide into the atmosphere, further exacerbating climate change.

Reduced Biodiversity

Forests are biodiversity hotspots, but climate change threatens this rich diversity. As the habitats of various species shift due to changing temperatures and precipitation, some species may not be able to adapt quickly enough, leading to localized extinctions.

On the other hand, forests and trees play an essential role in mitigating climate change and supporting human life:

Carbon Sequestration

Forests absorb about a third of the carbon dioxide released by burning fossil fuels, effectively acting as carbon sinks. This ability to sequester carbon is crucial in offsetting greenhouse gas emissions and mitigating climate change.

Supporting Biodiversity

Forests are home to more than half of the world’s terrestrial species. This biodiversity is critical for ecosystem health, supporting functions like pollination, nutrient cycling, and pest control that are necessary for human agriculture and economies.

Livelihoods and Economy

Forests support the livelihoods of hundreds of millions of people worldwide, directly and indirectly. They provide timber, non-timber forest products, and ecosystem services such as water filtration and soil erosion control that are vital for agriculture.

Climate Regulation

Beyond carbon sequestration, forests influence local and global climates in other ways. They help regulate rainfall patterns and provide cooling through shade and evapotranspiration.

Given these impacts, the protection and sustainable management of forests are pivotal for climate change mitigation and adaptation. 

Embracing strategies such as reforestation and afforestation (planting trees where none were before), sustainable forest management, and biodiversity conservation can help enhance the resilience of forests in the face of climate change. 

At the same time, these strategies contribute to biodiversity conservation, sustainable development, and the overall wellbeing of the planet and humanity.

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