Dynamics of forest growth in tropical forests of Ghana

Forzia Ibrahim

Doctoral thesis

Abstract:

African tropical forests are crucial reservoirs of biodiversity and carbon storage, yet the ecological processes shaping their composition, function and resilience remain poorly understood. Predicting forest responses to global change requires an understanding of how abiotic and biotic conditions regulate community structure, biomass accumulation and species interactions. In particular, the extent to which forest physiognomy and function are regulated by nutrient supply, species functional attributes, and the non-additive effects of vapour pressure deficit (VPD) and soil moisture remains largely unexplored. This study addresses these knowledge gaps using a multi-pronged approach to: (1) quantify how moisture availability, nutrients, and VPD shape community diversity and conservation value; (2) investigate the effects of climatic and biotic factors on whole-tree biomass growth; and (3) assess how the functional traits of species together with soil moisture mediate competitive interactions between trees. The analyses draw on a comprehensive 10-year dataset from a network of permanent sampling plots spanning wet evergreen, moist semi-deciduous, and dry semi-deciduous forests in Ghana, West Africa. Forest inventory data were combined with high-resolution climate reanalysis data and soil chemistry measurements to evaluate environmental controls on forest composition, diversity and growth. Tree diversity was quantified using Hill numbers, while conservation value was derived from species rarity and global extinction risk scores. Mixed-effects models were used to estimate the effects of environmental conditions on species diversity indices and aboveground biomass growth. Nonlinear non-parametric models were fitted to examine how biological and abiotic factors modulate competitive interactions between species and how those interactions scale up to influence stem diameter growth. A total of 3,471 trees representing 242 species were surveyed during the 10-year census interval, of which 17.4% were classified as threatened or near-threatened according to global extinction risk criteria. Species composition, richness and Shannon diversity varied across forest types and were primarily structured by moisture availability and VPD. Diversity generally increased with soil moisture but declined with higher VPD. Nutrients, particularly calcium and phosphorus, significantly influenced the composition of the moist forest zone. However, evidence for a potential mitigation of VPD impacts by greater soil moisture availability was also detected, leading to a stabilisation of biodiversity levels in more mesic environments. At-risk species were more common in high-rainfall regions relative to drier environments. Community-level biomass production was highest in the moist forest and increased with structural diversity. However, growth responses to VPD and soil moisture varied among forest types: in dry forests, growth remained suppressed and showed modest declines to increasing VPD; in moist forests, high soil moisture partly buffered against VPD stress; and in wet forests, growth peaked under intermediate moisture conditions. Species-level analyses revealed a consistently positive effect of tree size, and highly taxon-specific responses to competition and the non-additive effects of environmental factors. Variation in the wood density of species modified competitive outcomes, with dense-wooded species exhibiting stronger innate competitive abilities, particularly under moisture-limited conditions. Larger individuals were less sensitive to moisture-dependent competitive pressure, reflecting divergent life history strategies that differentially shape species fitness under varying environmental conditions. Collectively, these findings demonstrate that tropical forest structure, biodiversity, and productivity are shaped by complex interactions among environmental conditions, species traits, and competitive dynamics. The findings also suggest that a

Abstract:

ggregating data and analysing processes at a community-level can obscure or mask important species-level variation, underscoring the importance of integrating both species and community-level perspectives to understand forest responses to environmental gradients. The higher concentration of threatened species in wetter forests, coupled with the dynamic nature of competitive interactions, suggests that more prevalent and intense dry season drought episodes, which are forecast for tropical Africa, may disproportionately affect the physiognomy, function, and biodiversity of these communities. By linking demographic performance, biodiversity and environmental drivers, this thesis highlights the ecological trade-offs that regulate tropical forest carbon dynamics and the critical role of species diversity, functional attributes, and environmental buffering in sustaining forest productivity under changing climatic conditions.

Keywords

Moisture supply, Tree size, Vapour pressure deficit, Wood density, West Africa, Aboveground biomass growth, Biodiversity, Competition

Language used: English

Date on which the thesis was submitted / produced: 8. 1. 2026

Thesis defence

• Supervisor: doc. Ing. Radim Matula, Ph.D.
• Reader: Václav Treml, externi, Martin Šrámek, Bc., externi

Czech University of Life Sciences Prague

Faculty of Forestry and Wood Sciences

Doctoral programme:
Global Change Forestry