| dc.description.abstract | Tropical forests possess exceptional levels of tree species richness but explaining this diversity has presented a long existing challenge. Habitat niche partitioning provides a hypothesis for species co-existence, whereby species avoid competitive exclusion by partitioning demands on multiple resources within an environment. However, limited understanding concerning how tree function is influenced by multiple environmental variables has limited the support for this hypothesis. This knowledge gap also limits our ability to predict how tropical forest tree communities will respond to environmental change, given multiple dimensions of a species’ niche are likely to be affected. In this thesis, I investigate the role of niche partitioning in supporting co-existence of species and the turnover of species across edaphic gradients, as well as how long-term changes to the environment from selective logging and drought affect niche space of tropical tree species. I use species distribution models and measurements of leaf physiological traits to determine the key dimensions of tree species’ niches in primary forests. In chapter 2 I demonstrate niche partitioning is strong within tropical forests with at least 60-86% of abundant species occupying their own unique niche. Species partition a wide range of abiotic environments, including soil nutrient, topographic and light environments, with greater environmental heterogeneity enhancing the scope for niche partitioning. Building on this, in chapter 3 I find that variation in nutrient availability explains more variation in leaf physiology and habitat preferences than light availability of species from the Dipterocarpaceae family that dominates South-East Asian forests. This highlights the importance of edaphic environments in structuring tropical forest communities. I also find different leaf nutrients are related to photosynthetic capacity in different forest types, revealing that multiple different nutrients may limit productivity and affect species distributions in tropical forests. Many tropical forest tree species are highly specialised with limited ability to adjust their traits between environments, underlining their potential vulnerability to environmental change. In chapter 4 I show seedlings from selectively logged Bornean forests have different community weighted mean trait values, with greater belowground investment in logged forests. These adaptations are sufficient to overcome soil stress and to maintain foliar nutrient concentrations. However, I show seedlings of species that dominate old-growth forests are less able to adapt their traits and experience elevated mortality rates in logged forests. I attribute this to greater soil nutrient limitation as they are unable to maintain leaf nutrient concentrations. Selective logging will therefore likely drive shifts in species composition towards greater dominance of earlier-successional species that have traits capable of surviving in disturbed environments. This could result in local-scale reductions in species diversity and functional diversity, which could reduce long-term resilience to environmental change. In contrast, in Chapter 5 I demonstrate small trees in Amazonian forests are able to respond to changes in their environment following long-term drought conditions. Following mortality of large canopy trees, small trees can respond to increased light availability even under reduced water availability by adjusting resource allocation and by increasing nutrient use efficiency. Despite evidence of resilience to long-term drought conditions, hyper-dominant species show a greater capacity to respond, which may further enhance the dominance of these species under future climates. In conclusion my results highlight the importance of habitat niche partitioning in structuring tropical forest tree communities and identify key environmental variables that determine species distribution and tree function. My results have important implications for the conservation and restoration of tropical forests under environmental change. Avoidance of environmental homogenisation and changes to as few environmental conditions as possible is likely to be important in maintaining high species diversity in tropical forests and to avoid increased dominance by few generalist species. Many current conservation and restoration projects focus on recovering vegetation, but my research highlights the additional need to maintain and restore soil environments, especially for the long-term persistence of highly specialist species. | en_GB |
