Which tree species fix the most carbon?
INRAE - National Research Institute for Agriculture, Food and Environment
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Pine tree in Brazil
view moreCredit: INRAE - Hervé Cochard
Forests provide many ecosystem services, including microclimate regulation, biodiversity preservation, air and water purification, and soil protection. Together with the oceans, they are one of the two most important carbon sinks, due to their capacity to store carbon in the soil and in tree biomass.
As such, promoting fast-growing trees could strengthen efforts to mitigate climate change. This raises a key question for forest managers: which tree species have the greatest mitigation potential?
INRAE and Bordeaux Sciences Agro conducted a study to identify the tree characteristics (also known as functional traits) that favour growth and thus CO2 sequestration in biomass. The researchers coordinated an international consortium involving the French National Forest Office (ONF) and the French National Center for Private Forest Ownership (CNPF) to study the growth of 223 tree species planted in 160 experimental forests across the world (Western Europe, United States, Brazil, Ethiopia, Cameroon, South-East Asia, among others). The species were representative of all the major forest biomes.
Prevailing theory: acquisitive species grow quickly
Previous research had shown that under controlled conditions (often greenhouse experiments) species capable of efficiently acquiring resources (light, water, nutrients) generally grow quickly (e.g. maples, poplars, English oak, sessile oak). These acquisitive species have traits that help them maximize resource use (large specific leaf area, high specific root length) and improve their capacity to convert these resources into biomass (high maximum photosynthetic capacity, high nitrogen concentration in the leaves). Meanwhile, species that are more efficient at conserving their internal resources (nutrients, water, energy) than extracting external resources are known as conservative species (e.g. fir, downy oak, holm oak) and are assumed to grow more slowly.
New understanding: conservative species grow faster in forests
However, under real-world conditions in boreal and temperate forests, the researchers showed that conservative species generally grow faster than acquisitive species. This finding can be explained by the fact that these forests are generally located in areas with unfavourable growing conditions (low soil fertility, cold or dry climate), which gives conservative species an advantage since they are better able to resist stress and manage limited resources. In tropical rainforests, where the climate is potentially more favourable to plant growth, the two types of tree species show no differences on average.
The key role of local climate and soil for species choice
Beyond general trends at the major biome[1] scale, the researchers have shed light on the decisive role of local conditions. Growth conditions in some situations are sufficiently favourable for acquisitive species to grow faster than conservative ones. But the key is to ensure that species are adapted to their local environment. This means that in favourable climates and fertile soils, acquisitive species such as maples and poplars will grow faster and therefore fix more carbon than conservative species such as holm oaks, downy oaks and many types of pine trees. Conversely, in unfavourable climates and poor soils, conservative species will have the greatest potential to accumulate carbon in the biomass. This recent study gives forest managers yet one more tool to help mitigate climate change.
[1] The major terrestrial biomes represent vast geographical areas characterized by climate conditions and the species that grow there: tundras, deserts, savannahs, temperate forests, tropical forests, boreal forests, grasslands and the Mediterranean biome.
Journal
Nature
Article Title
Widespread slow growth of acquisitive tree species
Article Publication Date
19-Mar-2025
Could the layout of trees impact human health?
Beyond creating a serene and open atmosphere in urban areas, trees and parks also contribute to human well-being. There are various reasons for this: trees filter pollutants out of the air, provide shade, lower the ambient temperature in hot weather and encourage people to spend more time outdoors. Many governments have set ambitious tree-planting targets for the decades ahead, partially in response to climate change and rising temperatures. In densely developed cities, however, space for new green space is at a premium. In this context, the key question is how to plant trees in existing green spaces to optimal effect.
This is a question that occupies urban planning researchers and practitioners alike, because any answer must take account of specific, local spatial circumstances and climatic conditions. ETH researchers are tackling this issue – not only in Switzerland, but also in Asia. In the course of their work, researchers from Future Cities Lab operated in Singapore by ETH Zurich and the National University of Singapore (NUS) discovered interesting links between tree management and the health of urban residents.
Data on over six million people analysed
To begin with, the researchers examined high-resolution tree canopy data to determine the structure of tree-covered green spaces within a radius of 500 metres of a person’s place of residence. In addition to recording the total area covered by all tree clusters, they also identified the proximity and connectedness of tree clusters, their geometrical complexity and the fragmentation level.
They linked this information with the survival time of the resident in the respective neighbourhood for over six million adults, i.e. looking exclusively at natural-cause deaths due to illness and old age. This data, supplied by the Swiss Federal Statistical Office, covers over a ten-year period (2010–2019). In order to protect privacy, the Federal Statistical Office rounded the coordinates of citizens’ residences to the nearest 50 metres.
Tree quantity and positioning both matter
Data analysis shows that both the tree canopy cover in residential areas and their spatial arrangement correlate with mortality. The study identified a significantly lower mortality risk in people who live in neighbourhoods with large, contiguous and well networked areas of tree canopies than for people who live in areas with fewer, fragmented areas of tree canopies with complex geometries. This correlation is particularly evident in densely developed peri-urban and urban areas with poor air quality and high temperatures: if such areas feature well-structured forested green spaces, the residents may receive more health benefits than other areas.
Yet, while this study represents an important first step, it is still not possible to draw conclusions regarding the causes. The researchers are not yet able to state with precision the pathways through which tree canopy configuration influences human health. Nevertheless, the study’s findings at the individual level are generally consistent with the results of similar studies at the community level in Philadelphia, Tehran and Taipei.
Isolated forested green spaces should be joined up
Dengkai Chi, a postdoctoral researcher at the ETH Future Cities Lab and the first author of the study, says: “Although we can’t yet define a direct causal link, when we have addressed factors such as age, gender and socio-economic status, the data shows clear correlations. Our results provide plausible indications that human health may be influenced not only by the quantity of trees but also by their spatial distribution.”
The findings underline the importance of carefully considering the layout of forested green spaces and adopting a targeted approach to tree placement. “In order to fully exploit trees’ potential to support human health, cities should strive to not only increase the number of trees but also to connect isolated green spaces – including by creating tree-lined boulevards,” says Chi.
The study also suggests that compact, geometrically simple areas of tree canopy – including circular and rectangular forms – could have a greater positive effect on health than irregular, fragmented tree coverage. One possible explanation is that simply structured areas offer a larger core area, promote biodiversity and consequently attract residents to use these spaces.
Further research and clear indicators needed
“We’re still at the very outset of this research,” explains Chi. The study was unable to take account of many specific influencing factors, such as whether people have pre-existing illnesses, smoke or actually use green spaces. In addition, the results of this study pertain to the neighbourhood level and do not necessarily translate to an entire municipal area. Initial indications suggest that, at the level of an entire city, the health-promoting effects of green spaces correlate with their more even distribution throughout the city, so that as many residents as possible have access to them. The researchers hope to examine these issues in further studies to better understand these links.
Chi explains that, when it comes to developing recommendations for future action by political decision-makers and urban planners, the researchers will have to quantify their results more effectively and define specific thresholds.
Reference
Chi D, Manoli G, Lin B, Aerts R, Yang J, Hahs A, Richards D, Meili N, Zhu Y, Qiu Y, Wang J, Burlando P, Fatichi S, Tan PY. Residential tree canopy configuration and mortality in 6 million Swiss adults: a longitudinal study. Lancet Planet Health 2025: 9. Doi: https://www.thelancet.com/journals/lanplh/article/PIIS2542-5196(25)00022-1/fulltext
Journal
The Lancet Planetary Health
Article Publication Date
20-Mar-2025
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