Interactive effects of fallow age and Chromolaena odorata invasion on tree regeneration in Central African agricultural landscapes

Natural regeneration of trees outside forests (TOF) is a key process sustaining biodiversity and ecosystem functions in tropical agricultural landscapes, yet it is increasingly shaped by interactions between succession and biological invasion. This study quantified the combined effects of fallow age and Chromolaena odorata cover on tree regeneration in agricultural fallows of Mongala Province (Democratic Republic of the Congo). Seedling inventories were conducted around 300 seed trees across gradients of fallow age (0–3, 3–6, > 6 years) and invasion intensity (0–25% to > 75% cover). Seedling density, spatial patterns, and size structure were analysed using zero-inflated negative binomial models. Seedling density declined significantly with increasing C. odorata cover, particularly in early fallows, where predicted densities decreased from 1051 to 270 ind. ha⁻¹ (≈ 74% reduction). This effect weakened along the successional gradient and became negligible in older fallows, indicating a strong interaction between succession and invasion. Invasion effects were non-linear, with sharp declines occurring beyond intermediate levels of cover, suggesting threshold responses and increased probability of recruitment failure. Species responses varied markedly: Erythrophleum suaveolens and Pycnanthus angolensis showed strong declines, whereas Petersianthus macrocarpus increased under high invasion, indicating species-specific ecological filtering. Invasion also reduced spatial heterogeneity and limited progression to larger seedling size classes. These findings highlight that regeneration is governed by context-dependent interactions between succession and invasion intensity, with early fallows representing both a window of opportunity and a phase of high vulnerability. Accounting for non-linear invasion effects is critical for understanding forest recovery and designing targeted management strategies in tropical agricultural landscapes.