Published today (10 January) in Science, the study was led by the University of Oxford and involved Newcastle University. The results demonstrate that logging and conversion have significantly different and cumulative environmental impacts.

Understanding how different aspects of tropical forests are affected by logging and conversion to oil palm plantations is important for identifying priority habitats for conservation and restoration. It can also help aid decisions on land use – for instance, whether a logged forest should be protected, restored, or allowed to be converted into a plantation. Until now, most studies have focused on a limited number of factors, making the overall impact on the whole ecosystem difficult to assess.

Ecosystem impact

Study co-author, Professor Yit Arn Teh, Professor of Soil Science at Newcastle University’s School of Natural and Environmental Sciences, said: “Tropical forests in Southeast Asia play a central role in regulating global climate and are among the most biologically diverse ecosystems in the world. These important and unique ecosystems are under tremendous pressure from society to provide natural resources (e.g. timber, palm oil) and support the livelihoods of rural communities. Understanding how these ecosystems respond to different forms of human exploitation (e.g. selective logging, conversion to oil palm agriculture) are critical for designing more sustainable land management strategies that balance socio-economic needs against other key policy priorities, such as biodiversity conservation or climate change mitigation.”

In this new study, the researchers looked at over 80 metrics describing multiple aspects of the structure, biodiversity, and functioning of the tropical forest ecosystem – from soil nutrients and carbon storage, to photosynthesis rates and numbers of bird and bat species. These were measured in study sites in three areas of Sabah, Malaysian Borneo that were either in undisturbed old growth forest, logged forest (moderately or heavily logged), or in previous logged forests that had been converted to oil palm plantation.

The research, unprecedented in investigating such a broad spectrum of indicators for the health of tropical forest ecosystems in a single analysis, was made possible due to the wide range of study sites established and maintained by the South East Asia Rainforest Research Partnership. In total, logging and conversion had widespread impacts, affecting most of the measured properties - 60 of the 82 ecosystem metrics. However, there were clear differences between the two.

In general, logging mostly impacted factors associated with forest structure and environment. Since logging in the tropics is generally selective - focusing on trees with particular commercial qualities - even low levels of logging alter the system. For example, when older, larger trees are removed, this creates gaps in the canopy, enabling rapid-growing species to emerge that have very different characteristics, including less dense wood and thinner leaves that are more vulnerable to herbivores.

Biodiversity implications

Converting these logged forests to oil palm plantations, however, has greater impacts on biodiversity that go beyond those of logging alone. Species of birds, bats, dung beetles, trees, vines, and soil microorganisms all showed greater reductions in abundance and diversity on plantations compared with logged forests. This is likely due in part to the major changes in plant food resources and the shift to hotter and drier microclimates under the single layer of oil palm that follows conversion from logged forest.

Senior author Professor Andrew Hector (Department of Biology, University of Oxford), said: "One of the key messages of the study is that selective logging and conversion differ in how they impact the forest ecosystem meaning that conversion to plantations brings new impacts that add to those of logging alone."

According to the study team, this demonstrates that logged forest can still be valuable for maintaining biodiversity and should not be immediately ‘written off’ for conversion to oil palm plantations.

Professor Ed Turner (University of Cambridge), who co-led the study, said: "A key message of this work is that old growth, intact forests are unique, but secondary logged forests are also valuable and important in terms of their biodiversity and ecosystem functioning relative to the much-reduced levels seen in oil palm plantations."

One surprise for the research team was how variable the responses were. Dr Charlie Marsh (Department of Biology, University of Oxford at the time of the study, now National University of Singapore), lead author of the study, said: "Our study demonstrates that focussing on any single component of the ecosystem may lead to incomplete understanding of how the ecosystem responds as a whole. We were really surprised by the huge variability in how different facets of the ecosystem responded to deforestation. We saw increases, decreases, or sometimes no change at all. There were even aspects that would increase in logged forest, only to decrease in oil palm plantations. When making decisions concerning land management and conservation, we must consider a broad suite of ecological properties."

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Adapted with thanks from the University Oxford.