The influence of logging on understorey plant communities in tropical lowland rainforest in Borneo

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The world’s tropical forests continue to be logged and converted into heavily modified, highly managed or degraded landscapes at high rates. This loss of extent and reduction in quality are having severe impacts on tropical forest ecosystems as habitat for many species, and on the conservation of biodiversity and maintenance of ecosystem functioning. The capacity of ecosystems to respond or remain resilient to such impacts is strongly dependent on the re-assembly trajectories of understorey plant communities following anthropogenic disturbances. This study provides evidence that logging affects the trajectories of successional change in understorey plant communities of tropical lowland rainforest by facilitating the invasion of exotic plant species and by filtering for particular plant functional groups. The resulting shifts in community composition and structure may have potentially long-term impacts on forest regeneration and the resilience of disturbed tropical forests to other drivers of global environmental change as well as implications for conservation management in human-modified tropical landscapes.

The world’s tropical forests continue to be logged and converted into heavily modified, highly managed or degraded landscapes at high rates. This loss of extent and reduction in quality are having severe impacts on tropical forest ecosystems as habitat for many species, and on the conservation of biodiversity and maintenance of ecosystem functioning. The capacity of ecosystems to respond or remain resilient to such impacts is strongly dependent on the re-assembly trajectories of understorey plant communities following anthropogenic disturbances.

In this thesis, I investigate how logging influences the trajectories of understorey plant community assembly and ecosystem resilience, in a tropical lowland rainforest landscape in Malaysian Borneo. I evaluate the importance of different ecological processes in this context and consider the implications of changes to forest community composition and structure. The majority of data analysed for this thesis was collected as a part of the Stability of Altered Forest Ecosystems (SAFE) project in Sabah.

First, I test whether logging facilitates the invasion of exotic plant species and whether logging and exotic plants interact to drive understorey plant community change. My findings indicate that exotic plant species richness, above-ground biomass and leaf area index (LAI) are predominantly influenced by changes in local forest cover loss and increase along a logging disturbance gradient. Moreover, there exists an additive effect between logging and the influence of exotic plants on native plant communities.

Next, I investigate whether semi-quantitative assessment of Braun-Blanquet vegetation cover scores provides a reliable proxy for direct quantitative measures of LAI and above-ground biomass of differing plant growth forms (PGF) within tropical forests. Results indicate that the Braun-Blanquet OTS provides a remarkably simple and accurate logarithmic scaling of LAI, but care should be taken in applying scaling rules uniformly across PGFs. In contrast, the Braun-Blanquet OTS shows a more complex relationship with plant above-ground biomass.

Finally, I explore whether plant species richness, functional diversity and phylogenetic diversity respond differently to logging. I found no significant effect of localor landscape-scale forest cover loss or configuration of logging roads on plant species richness. By contrast, both the trait functional dispersion index (FDis) and the net relatedness index (NRI) for phylogenetic dispersion showed strong gradients from clustered towards more randomly assembled communities at higher logging intensity. All functional traits showed significant phylogenetic signal, indicating strong, albeit not perfect concordance between functional and phylogenetic dispersion.

The overall results of this study provide evidence that logging affects the trajectories of successional change in understorey plant communities of tropical lowland rainforest by facilitating the invasion of exotic plant species and by filtering for particular plant functional groups. The resulting shifts in community composition and structure may have potentially long-term impacts on forest regeneration and the resilience of disturbed tropical forests to other drivers of global environmental change as well as implications for conservation management in human-modified tropical landscapes. The findings of this study provide a strong departure point, from which more experimental studies on the mechanisms underlying plant community assembly in tropical lowland forest following habitat modification can be initiated.