Human disruption is driving ‘winner’ and ‘loser’ tree species shifts across Brazilian forests
Fast-growing and small-seeded tree species are dominating Brazilian forests in regions with high levels of deforestation and degradation, a new study shows.
Lancaster University
image:
Fragmented and degraded forests in the Santarem region
view moreCredit: Marizilda Cruppe and Rede Amazônia Sustentável.
Fast-growing and small-seeded tree species are dominating Brazilian forests in regions with high levels of deforestation and degradation, a new study shows.
This has potential implications for the ecosystem services these forests provide, including the ability of these ‘disturbed’ forests to absorb and store carbon. This is because these “winning” species grow fast but die young, as their stems and branches are far less dense than the slow growing tree species they replace.
Wildlife species adapted to consuming and dispersing the large seeds of tree species that are being lost in human-modified landscapes may also be affected by these shifts.
Authors of the study, published today in Nature Ecology and Evolution, say their findings highlight the urgent need to conserve and restore tropical forests, prevent degradation, and implement measures to protect and boost populations of the large-bodied birds like toucans and mammals such as spider monkeys that disperse the seeds of “losing” slow-growing large-seeded tree species.
An international team of researchers examined a unique dataset of more than 1,200 tropical tree species over more than 270 forest plots across six regions of Brazilian Amazon and Atlantic forests that have been altered by people through activities such as deforestation and local disturbances like logging, hunting and burning.
The researchers looked at the overall structure of the landscapes surrounding each forest plot and, using multiple statistical models, they were able to identify the causal effects of habitat loss, fragmentation and local degradation on the composition of forests, as well as identifying the attributes of so-called “winners” and “losers” species.
“We found that the tree species dominating landscapes with high forest cover tend to have dense wood and large seeds, which are primarily dispersed by medium to large-bodied animals typical of Brazil’s rainforests,” said Bruno X. Pinho, first author of the study who conducted most of the research while at the University of Montpellier (now at the University of Bern). “In contrast, in highly deforested landscapes, where remaining forests face additional human disturbances, these tree species are losing out to so-called ‘opportunistic’ species, which have softer wood and smaller seeds consumed by small, mobile, disturbance-adapted birds and bats. These species typically grow faster and have greater dispersal capacity”.
The researchers found this was happening despite differing geography, climate and land-use contexts.
This study highlights the urgent need to strengthening the conservation and restoration of tropical forests to preserve these vital ecosystems.
“The strong influence of forest degradation in some Amazonian regions demonstrates the importance of going beyond tacking deforestation and also combating forest disturbances, such as selective logging and fires,” said Senior Investigator Professor Jos Barlow, of Lancaster University.
Tropical forests constitute the most important reservoir of terrestrial biodiversity. They play a major role in absorbing greenhouse gasses and provide essential ecosystem services. Yet they are victims of rapid deforestation and fragmentation, with the loss of 3 to 6 million hectares per year over the last two decades. A large part of today's tropical forests are therefore found in landscapes modified by humans and exposed to local disturbances.
“These functional replacements have serious implications that urgently need to be quantified. They suggest possible deteriorations of essential processes of these ecosystems and their contributions to human populations, in particular through changes in carbon stocks – but also in fauna-flora interactions and forest regeneration,” explains Felipe Melo, second author of the study and researcher at the Federal University of Pernambuco in Brazil (now at Nottingham Trent University).
“There is broad consensus on the negative impact of habitat loss on biodiversity, but the independent effects of landscape fragmentation and local disturbance remain less well understood, in part because of the difficulty in disentangling cause-and-effect relationships on the one hand and non-causal associations on the other,” explains David Bauman, of the French National Research Institute for Sustainable Development (IRD) and co-author of the study.
The study also helps address these questions, and shows that policies should focus on preserving and enhancing forest cover and preventing degradation, and can worry less about how the remaining forests are distributed across the landscape.
The study, which received funding support from the UKRI National Environment Research Council, is outlined in the paper ‘Winner-loser plant trait replacements in human-modified tropical forests’ published in Nature Ecology and Evolution.
Dead trees and a regenerating understorey demonstrate species turnover in the Amazon (IMAGE)
Dead trees and a regenerating understorey demonstrate species turnover and winners and losers in the Amazon
Credit
Flavio Forner and Rede Amazônia Sustentável
A Tayra (Eira barbara) carrying a large fruit of a large-seeded Sapotaceae tree in the Balbina region.
Credit
Maíra Benchimol
Journal
Nature Ecology & Evolution
Method of Research
Data/statistical analysis
Subject of Research
Not applicable
Article Title
Winner-loser plant trait replacements in human-modified tropical forests
Article Publication Date
10-Dec-2024
Wrong trees in the wrong place can make cities hotter at night, study reveals
While trees can cool some cities significantly during the day, new research shows that tree canopies can also trap heat and raise temperatures at night. The study aims to help urban planners choose the best combinations of trees and planting locations to combat urban heat stress.
Temperatures in cities are rising across the globe and urban heat stress is already a major problem causing illness, death, a surge in energy use to cool buildings down, heat-related social inequality issues and problems with urban infrastructure.
Some cities have already started implementing mitigation strategies, with tree planting prominent among them. But a University of Cambridge-led study now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.
The study, published today in Communications Earth & Environment found that urban trees can lower pedestrian-level air temperature by up to 12°C. Its authors found that the introduction of trees reduced peak monthly temperatures to below 26°C in 83% of the cities studied, meeting the ‘thermal comfort threshold’. However, they also found that this cooling ability varies significantly around the world and is influenced by tree species traits, urban layout and climate conditions.
“Our study busts the myth that trees are the ultimate panacea for overheating cities across the globe,” said Dr Ronita Bardhan, Associate Professor of Sustainable Built Environment at the University of Cambridge.
“Trees have a crucial role to play in cooling cities down but we need to plant them much more strategically to maximise the benefits which they can provide.”
Previous research on the cooling effects of urban trees has focused on specific climates or regions, and considered case studies in a fragmented way, leaving major gaps in our knowledge about unique tree cooling mechanisms and how these interact with diverse urban features.
To overcome this, the authors of this study analysed the findings of 182 studies – concerning 17 climates in 110 global cities or regions – published between 2010 and 2023, offering the first comprehensive global assessment of urban tree cooling.
During the day, trees cool cities in three ways: by blocking solar radiation; through evaporation of water via pores in their leaves; and by foliage aerodynamically changing airflow. At night, however, tree canopies can trap longwave radiation from the ground surface, due to aerodynamic resistance and ‘stomatal closure’ – the closing of microscopic pores on the surface of leaves partly in response to heat and drought stress.
Variation by climate type
The study found that urban trees generally cool cities more in hot and dry climates, and less in hot humid climates.
In the ‘tropical wet and dry or savanna’ climate, trees can cool cities by as much as 12 °C, as recorded in Nigeria. However, it was in this same climate that trees also warmed cities most at night, by up to 0.8°C.
Trees performed well in arid climates, cooling cities by just over 9°C and warming them at night by 0.4 °C.
In tropical rainforest climates, where humidity is higher, the daytime cooling effect dropped to approximately 2°C while the nighttime heating effect was 0.8 °C.
In temperate climates, trees can cool cities by up to 6°C and warm them by 1.5°C.
Using trees more strategically
The study points out that cities which have more open urban layouts are more likely to feature a mix of evergreen and deciduous trees of varying sizes. This, the researchers found, tends to result in greater cooling in temperate, continental and tropical climates.
The combined use of trees in these climates generally results in 0.5 °C more cooling than in cities where only deciduous or evergreen trees feature. This is because mixed trees can balance seasonal shading and sunlight, providing three-dimensional cooling at various heights.
In arid climates, however, the researchers found that evergreen species dominate and cool more effectively in the specific context of compact urban layouts such as Cairo in Egypt, or Dubai in UAE.
In general, trees cooled more effectively in open and low-rise cities in dry climates. In open urban layouts, cooling can be improved by about 0.4 °C because their larger green spaces allow for more and larger tree canopies and a greater mix of tree species.
“Our study provides context-specific greening guidelines for urban planners to more effectively harness tree cooling in the face of global warming,” Dr Ronita Bardhan said.
“Our results emphasize that urban planners not only need to give cities more green spaces, they need to plant the right mix of trees in optimal positions to maximize cooling benefits.”
“Urban planners should plan for future warmer climates by choosing resilient species which will continue to thrive and maintain cooling benefits,” said Dr Bardhan, a Fellow of Selwyn College, Cambridge.
Matching trees to urban forms
The study goes further, arguing that species selection and placement needs to be compatible with urban forms. The orientation of the ‘street canyon’, local climate zones, aspect ratio, visible sky ratio and other urban features that influence the effects of trees all need to be carefully considered.
Although a higher degree of tree canopy cover in street canyons generally results in more cooling effects, excessively high cover may trap heat at the pedestrian level, especially in compact urban zones in high temperature climates. In such locations, narrow species and sparse planting strategies are recommended.
The researchers emphasise that we cannot rely entirely on trees to cool cities, and that solutions such as solar shading and reflective materials will continue to play an important role.
The researchers have developed an interactive database and map to enable users to estimate the cooling efficacy of strategies based on data from cities with similar climates and urban structures.
Reference
H. Li et al., ‘Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait’, Communications Earth & Environment (2024). DOI: 10.1038/s43247-024-01908-4
Journal
Communications Earth & Environment
Article Title
Cooling efficacy of trees across cities is determined by background climate, urban morphology, and tree trait
Article Publication Date
10-Dec-2024
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