Turning the Tide: How Logged Tropical Forests Continue to Support Biodiversity

Understanding how various elements of tropical forests are influenced by logging and the transformation into oil palm plantations is crucial for identifying priority habitats for preservation and rehabilitation. It can also assist in land use decisions – for example, whether a logged forest should be conserved, revitalized, or converted into a plantation. Yet, to date, most investigations have concentrated on a narrow set of factors, complicating the overall evaluation of impacts on the entire ecosystem.

In this recent research, scientists examined over 80 parameters capturing multiple aspects of the structure, biodiversity, and functioning of the tropical forest ecosystem – including soil nutrients and carbon sequestration, photosynthesis rates, and the diversity of bird and bat species. These metrics were evaluated across study sites in three regions of Sabah, Malaysian Borneo, which included undisturbed old-growth forest, logged forest (either moderately or heavily logged), and previously logged areas transformed into oil palm plantations.

The research, unprecedented in exploring such a wide range of indicators for the health of tropical forest ecosystems within a single analysis, was enabled by the extensive network of study sites established and maintained by the South East Asia Rainforest Research Partnership. Overall, logging and conversion produced widespread effects, influencing most of the assessed properties – affecting 60 of the 82 ecosystem metrics. Nonetheless, distinct differences between the two were evident.

Generally, logging predominantly affected factors linked to forest structure and environment. Since logging in tropical regions is usually selective – targeting trees with specific commercial qualities – even minimal logging significantly alters the ecosystem. For example, the removal of older, larger trees creates gaps in the canopy, allowing for the emergence of rapidly growing species with markedly different traits, including less dense wood and thinner leaves that are more susceptible to herbivory.

In contrast, converting logged forests to oil palm plantations results in more significant impacts on biodiversity that transcend the effects of logging alone. Varieties of birds, bats, dung beetles, trees, vines, and soil microorganisms displayed greater declines in abundance and diversity on plantations compared to logged forests. This is likely attributable to substantial alterations in plant food resources and the transition to hotter and drier microclimates induced by the single layer of oil palm following conversion from logged forest.

Senior author Professor Andrew Hector (Department of Biology, University of Oxford), stated: ‘A crucial takeaway from the study is that selective logging and conversion differ in their impacts on the forest ecosystem, implying that the transition to plantations introduces new effects that compound those of logging alone.’

The research team asserts that this highlights the potential of logged forests to remain critical for sustaining biodiversity and should not be hastily ‘dismissed’ for conversion to oil palm plantations.

One unexpected finding for the research group was the variability in responses observed. Dr. Charlie Marsh (Department of Biology, University of Oxford during the study, currently at the National University of Singapore), the lead author, mentioned: ‘Our investigation indicates that concentrating on any single aspect of the ecosystem may lead to an incomplete understanding of its overall response. We were genuinely surprised by the considerable variability in how various components of the ecosystem reacted to deforestation. We observed increases, reductions, or occasionally no change at all. Certain aspects even exhibited growth in logged forests but saw declines in oil palm plantations. When making decisions regarding land management and conservation, we must take into account a broad spectrum of ecological properties.’

The study titled ‘Tropical forest clearance impacts biodiversity and function whereas logging changes structure’ has been published in Science.