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.

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.

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