• Energy Research

AbstractGrowing evidence suggests that liana competition with trees is threatening the global carbon sink by slowing the recovery of forests following disturbance. A recent theory based on local and regional evidence further proposes that the competitive success of lianas over trees is driven by interactions between forest disturbance and climate. We present the first global assessment of liana–tree relative performance in response to forest disturbance and climate drivers. Using an unprecedented dataset, we analysed 651 vegetation samples representing 26,538 lianas and 82,802 trees from 556 unique locations worldwide, derived from 83 publications. Results show that lianas perform better relative to trees (increasing liana‐to‐tree ratio) when forests are disturbed, under warmer temperatures and lower precipitation and towards the tropical lowlands. We also found that lianas can be a critical factor hindering forest recovery in disturbed forests experiencing liana‐favourable climates, as chronosequence data show that high competitive success of lianas over trees can persist for decades following disturbances, especially when the annual mean temperature exceeds 27.8°C, precipitation is less than 1614 mm and climatic water deficit is more than 829 mm. These findings reveal that degraded tropical forests with environmental conditions favouring lianas are disproportionately more vulnerable to liana dominance and thus can potentially stall succession, with important implications for the global carbon sink, and hence should be the highest priority to consider for restoration management.

Lianas, or woody vines, are integral components of tropical forests that have substantial impacts on the dynamics and functioning of these important ecosystems. There is growing evidence that liana competition with trees is threatening the global carbon sink by hindering the recovery of forests from disturbances. A recent hypothesis, built on local and regional evidence, suggests that their competitive success over trees (liana dominance) is facilitated by interactions between forest disturbance and climate. Despite their increasing competitiveness with trees at the global scale, robust measurements of liana aboveground biomass (AGB) have been limited. Moreover, it is unclear how forest disturbance interacts with liana–tree ratios (LTRs), climate, topography and soil properties to shape tree dynamics and the trajectories of succession in tropical forests. This thesis aims to improve understanding of the role lianas play in the recovery of tropical forests from disturbance, through the use of innovative sampling approaches to investigate forest dynamics and relationships with disturbance, liana–tree trade-offs and environmental factors.

Tropical forests store 40-50 per cent of terrestrial vegetation carbon1. However, spatial variations in aboveground live tree biomass carbon (AGC) stocks remain poorly understood, in particular in tropical montane forests2. Owing to climatic and soil changes with increasing elevation3, AGC stocks are lower in tropical montane forests compared with lowland forests2. Here we assemble and analyse a dataset of structurally intact old-growth forests (AfriMont) spanning 44 montane sites in 12 African countries. We find that montane sites in the AfriMont plot network have a mean AGC stock of 149.4 megagrams of carbon per hectare (95% confidence interval 137.1-164.2), which is comparable to lowland forests in the African Tropical Rainforest Observation Network4 and about 70 per cent and 32 per cent higher than averages from plot networks in montane2,5,6 and lowland7 forests in the Neotropics, respectively. Notably, our results are two-thirds higher than the Intergovernmental Panel on Climate Change default values for these forests in Africa8. We find that the low stem density and high abundance of large trees of African lowland forests4 is mirrored in the montane forests sampled. This carbon store is endangered: we estimate that 0.8 million hectares of old-growth African montane forest have been lost since 2000. We provide country-specific montane forest AGC stock estimates modelled from our plot network to help to guide forest conservation and reforestation interventions. Our findings highlight the need for conserving these biodiverse9,10 and carbon-rich ecosystems.

AbstractThermophilization is the directional change in species community composition towards greater relative abundances of species associated with warmer environments. This process is well-documented in temperate and Neotropical plant communities, but it is uncertain whether this phenomenon occurs elsewhere in the tropics. Here we extend the search for thermophilization to equatorial Africa, where lower tree diversity compared to other tropical forest regions and different biogeographic history could affect community responses to climate change. Using re-census data from 17 forest plots in three mountain regions of Africa, we find a consistent pattern of thermophilization in tree communities. Mean rates of thermophilization were +0.0086 °C·y−1 in the Kigezi Highlands (Uganda), +0.0032 °C·y−1 in the Virunga Mountains (Rwanda-Uganda-Democratic Republic of the Congo) and +0.0023 °C·y−1 in the Udzungwa Mountains (Tanzania). Distinct from other forests, both recruitment and mortality were important drivers of thermophilzation in the African plots. The forests studied currently act as a carbon sink, but the consequences of further thermophilization are unclear.

People living in conflict-affected areas are particularly vulnerable to climate-related impacts. However, few comparative studies have examined differences in adaptation practices across different conflict-affected mountain areas in Africa. This study focuses on 2 mountain areas, the Bamboutos Mountains (western Cameroon, affected by sectarian conflict) and the Itombwe Mountains (eastern Democratic Republic of the Congo, affected by political instability). Semistructured interviews were conducted with 282 smallholder farmers living in these 2 mountainous areas. Farmers in both areas reported climatic changes and impacts on crops, animals, and human health. Some adaptation strategies were used across sites (eg increasing use of improved seeds and changing planting dates), but some differed (eg using inputs) in relation to differences in impacts observed, conflict characteristics, and farmers' cultural backgrounds. For example, in the Itombwe Mountains, herding was preferred over crop production (as cows could be moved when insecurity increased), whereas in the Bamboutos Mountains, crop production was preferred over rearing large animals (as these could be easily stolen by the separatists). We discuss the perceived major barriers to adaptation and their implications.