Understanding tropical forest carbon dynamics in Malaysian Borneo: logging disturbance, key drivers and methodology

Tropical forests play a critical role in the global carbon budget, yet their extent and functioning has been, and continues to be, threatened by increasing human disturbance and anthropogenic pressures, including deforestation and forest degradation. Consequently, logged and structurally degraded forests are fast becoming one of the most prevalent land-use types throughout the tropics, yet, logged forest landscapes have received little research attention compared to their old-growth counterparts. This thesis aims to quantify the carbon budget along a logging disturbance gradient, examine factors driving carbon fluxes and evaluate the accuracy of measurement methods. Using the heterogeneous mosaic landscape of Malaysian Borneo, this thesis captures a disturbance gradient from old-growth through to heavily logged forest.

First, the complete carbon budget is quantified across the disturbance gradient using ground-based biometric methods and validated with measurements collected independently from an eddy covariance flux tower. Similarities and differences in carbon allocation between logged and old-growth forests are explored to understand how tropical forests respond to logging. Secondly, the thesis explores what drives woody stem CO2 efflux at different spatial scales. Stems are the largest contributor to forest biomass, and so the respiratory consumption of stems provides a vital insight into forest metabolism and carbon allocation strategies, and ultimately ecosystem response to logging. Lastly, the thesis investigates how accurate the employed methods are for estimating woody stem CO2 efflux in the field with the intention of providing recommendations to mitigate potential biases. A case study approach is used to quantify vertical and diurnal variations in stem CO2 efflux along with terrestrial LiDAR scanning to investigate the accuracy of allometric equations to estimate total stem surface area.

Overall, this research makes a novel contribution to improving our understanding of how tropical forest carbon dynamics respond to logging activities. Given that these ecosystems face an insurmountable threat from anthropogenic climate change and continued pressure from logging and agricultural expansion, increasing our knowledge and understanding enables more informed decisions regarding tropical forest policy, management and protection.