Tree species diversity in forests significantly buffers temperature extremes by lowering heat peaks and elevating cold hours, according to an international study published in Ecology Letters. Researchers found that canopy density and structural variety drive this cooling effect, offering a nature-based strategy for climate adaptation and forest restoration.
How Tree Species Richness Buffers Extreme Forest Temperatures
Forests with high tree-species diversity are better at buffering heat peaks in summer and cold peaks in winter than a forest with fewer tree species, according to a study led by researchers from the German Centre for Integrative Biodiversity Research, Leipzig University, and the Martin Luther University Halle-Wittenberg. Temperatures are increasing worldwide due to rising greenhouse gases, altering temperature extremes. While winter cold peaks are becoming warmer, heat peaks are increasing.
Trees have long been known to reduce heat peaks beneath their canopies during hot weather. However, the exact role of tree species richness remained largely unknown until this new research, which was published in the journal Ecology Letters.
To investigate this phenomenon, scientists utilized the largest planted tree diversity experiment worldwide, located in subtropical China. The BEF-China project planted several hundred thousand trees into plots containing 1, 2, 4, 8, 16, or 24 different tree species. A joint Sino-German international research training group conducted forest temperature measurements across these plots over a six-year period from 2015 to 2020.
Measuring the Cooling Impact of Biodiverse Canopies
The findings demonstrated that plots rich in tree species lowered temperatures below the canopy during heat peaks more effectively than species-poor plots. This cooling effect peaked during midday summer heat.
Cooling reached up to 4.4°C stronger in experimental plots containing 24 species compared to plots with just a single species. At the same time, species-rich forests performed better at increasing temperatures during cold nighttime hours and throughout the winter. When researchers analyzed monthly averages, however, they observed no temperature differences between species-poor and species-rich forests.
Canopy Density and Structural Diversity as Driving Factors
The study identified a clear structural explanation for how species richness drives temperature buffering. Plots featuring many tree species developed higher canopy density—representing more leaf area per ground area—and higher structural diversity, such as a varied mix of smaller and larger trees.
These combined factors enhanced temperature buffering by reducing the mixing of air masses beneath the canopy. Former research has shown that the buffered temperatures below the tree canopy are important for forest biodiversity as they slow down the climate change-driven shift towards species that prefer warm temperatures
, noted co-first author Dr Florian Schnabel from the University of Freiburg.
A buffered microclimate creates more favorable conditions for ecosystems and protects the services they offer. Under a buffered climate, forests are likely to grow and regenerate more effectively, while soils function better, supporting greater biodiversity, improving nutrient cycles, and increasing carbon storage.
Protecting Forest Resilience Through Genetic and Species Diversity
In Canada, the National Tree Seed Centre in Fredericton operates an Indigenous seed collection program launched in 2022 to preserve native seeds threatened by climate change, invasive species, and other pressures.
Mary Knockwood, the Indigenous programs manager at the Atlantic Forestry Centre, explained that the collaborative program operates directly through communities.
One focus of these seed-saving efforts is black ash, a tree heavily impacted by the emerald ash borer. Anthony Taylor, a forestry professor at the University of New Brunswick, likened genetic diversity within tree populations to an insurance policy against pests and diseases.
Implications for Global Reforestation Initiatives
While typical tree monocultures planted worldwide provide essential timber, experts note they harbour less biodiversity and deliver fewer ecosystem services than diverse forests. The growing body of research highlights how species richness actively mitigates the negative impacts of global warming on whole forest ecosystems.
These findings provide scientific support for large-scale forest restoration initiatives and urban forestry projects aiming to reduce thermal stress as global temperatures rise. Whether through securing diverse seed collections or planting multi-species experimental plots, maintaining ecological variety remains a primary defense for forests under climate stress.
