Dynamics of tree stems and biomass in old-growth and secondary forests across gradients in liana dominance, elevation, and soil

Lianas, or woody vines, are key components of many tropical forests that can have substantial impacts on the dynamics and functioning of these important ecosystems. Their competition with trees for resources — in particular light — can hamper the recovery of forests from disturbances. Yet, it is unclear how forest disturbance interacts with liana–tree ratio (LTR), topography, and soil properties, to shape tree dynamics and the trajectories of forest succession. Here, we use temporal data from Tanzania’s Kilombero Valley and Udzungwa Mountains to demonstrate how tree stem and biomass dynamics vary across secondary and old-growth forests with changes in liana dominance and environmental gradients. Greater tree recruitment and mortality in secondary forests compared to old-growth forests suggested rapid regeneration processes and faster turnover. Yet, there was no significant difference in annual tree number or biomass changes between secondary and old-growth forests. Our findings also showed that higher LTRs were positively associated with stem mortality but also with biomass growth of trees, indicating a nuanced ecological role of lianas in forest ecosystems, which warrants further investigation to fully understand the causal factors at play. Net changes in tree stem numbers and biomass of recruits significantly declined with elevation, implying climatic constraints on forest regeneration at higher elevations. Soil cation exchange capacity and organic carbon were found to significantly influence tree stem recruitment and net change in abundance, although their effects on biomass remained unclear. Overall, the findings indicate that the recovery from disturbance in terms of numbers and biomass of tree stems may be predictable along environmental gradients. These insights have the potential for developing strategies to identify when and where tropical forests may require restoration intervention.