Evidence of thermophilizationin tropical tree communities, accelerated by logging disturbance, from 40 years of forest dynamics in M’Baïki(Central Africa)

Abstract:
More than 90% of woody species occurring in tropical forests across Africa could be at risk from the temperature rises that are predicted, of 2.4 to 4.3°C by 2085 depending on the scenarios. Such warming is expected to shift species distributions and lead to the thermophilization of forest communities, as already observed in montane tropical forests. Such trends have been barely explored in lowland forests due to the lack of long-term data, and the impact of human disturbances, particularly logging, is likely to exacerbate these processes.
Here, we analysed 40 years of forest plot monitoring at the Mbaïki experimental site, located on the northern edge of the Congo Basin, at the forest–savannah transition zone, an ideal biogeographical context for identifying early signals of climate-induced transition. Based on a tree census of 24,346 individuals belonging to 394 species, we estimated the thermal optima of species — derived from their distribution across the continent — and calculated the Community Temperature Index (CTI) in 29 permanent 1-ha plots subjected to a gradient of logging (controls, logged, logged with thinning). Linear mixed models were fitted to quantify temporal changes in CTI and to identify the demographic processes underlying these dynamics.
Our results show that thermophilization is widespread at M’Baïki, and the rate of thermophilization, ranging from 0.0009 to 0.0025 °C·yr⁻¹, was found to increase with the intensity of disturbance caused by logging and other silvicultural interventions in the plots. Changes in CTI were primarily driven by mortality and growth, consistently across treatments. These results indicate that forests on the northern edge of the Congo Basin are already responding to ongoing warming. More importantly, our results show for the first time that logging practices modulate adaptation trajectories, with direct implications for the sustainable management and conservation of tropical forests.