Environmental and functional drivers of tree radial growth and water status in the Congo basin rainforest

High temperature, and atmospheric vapor pressure deficit and soil water stress increasingly threaten tree functions that regulate forest ecosystem productivity. Tree growth reflects the dynamic balance between carbon acquisition and allocation, and is highly sensitive to water availability, yet the mechanisms linking short-term water status to radial growth remain poorly understood, particularly in tropical forests experiencing intensifying dry seasons. Here, we explored the environmental and functional drivers of tree growth and water status in the Congo basin rainforest, the second largest tropical forest on Earth.

Specifically, in the Dja faunal reserve of eastern Cameroon, we quantified radial growth (RG) and tree water deficit (TWD) over an entire year and assessed their drivers, with a particular focus on understanding tree responses during dry periods. High-frequency (15-minute resolution) automatic dendrometer data were used to quantify dynamics of growth and stem shrinkage in 100 individuals of 16 tree species along a water availability gradient (wet vs dry conditions). Key functional traits related to resource use strategies and drought response syndromes were also measured on the same trees, including specific leaf area (SLA), wood density, water storage capacity, capacitance, turgor loss point, minimal conductance and variations in leaf water potential and relative water content over the dry season, enabling analyses of individual-level growth and water status, response to edaphoclimatic drivers, and functional trait syndromes.

In line with the view that species with rapid carbon acquisition capitalize on short favorable periods but remain highly sensitive to water limitations, we hypothesized that trees with acquisitive resource-use traits (e.g., high SLA, low wood density) and larger tree size would grow faster, but that trees with higher drought tolerance traits would show smaller growth reductions during dry periods and sustain functions under higher TWD. We found that prolonged dry seasons extended the duration of stem shrinkage, delayed post-drought growth recovery, and reduced the proportion of dry-season growth relative to annual growth, particularly in drought-sensitive species. By jointly analyzing individual growth, water status and functional traits, this study revealed how contrasting strategies of resource acquisition and drought tolerance regulate growth-water trade-offs and shape tropical forest resilience under increasing climatic stress, with implications for ecosystems functioning under future climate extremes.