Biomass dynamics in selectively logged forests in Vietnam

Abstract

Tropical forests play a key role in the global carbon cycle by storing about 45% of the carbon stored in all forest ecosystems worldwide. Changes in their carbon stocks and dynamics will have major implications for the global climate. The amount of carbon stored as biomass in tropical forests may differ considerably both between and within regions. In forest ecosystems, carbon is mainly stored in living biomass in standing trees (both above and belowground) and soil organic matter, while a relatively small amount is stored in woody debris (dead trees and tree parts). One of the major challenges in research on carbon dynamics in tropical forests involves the quantification of biomass and carbon stocks. Tree biomass is typically estimated using allometric models that use tree stem diameter (DBH) and often also tree height (H) and/or wood density (WD) as independent variables. The accuracy of these models is debated, particularly with respect to the use of models specific to local forests, versus regional or global models for which model parameters were not locally determined. Biomass dynamics in forests are driven by the amount of standing biomass on the one hand and per individual rates (species specific) of growth, recruitment and mortality on the other. These demographics vary greatly among forests, between and within regions. This is largely because of the large diversity that exists in environmental conditions and species composition both within and among tropical forest stands. Interspecific differences in growth and mortality rates for instance are strongly driven by differences in their functional traits but these trends and how they are affected by logging are poorly understood. In the thesis, Vu Thanh Nam developed local allometric equations for estimating AGB and RB based on destructive sample trees in an evergreen forest in Vietnam. He combined this model with detailed forest inventories to determine demographic rates and biomass dynamics of commonly occurring species (42 species) in two types of evergreen tropical forest plots: one set has been logged once around 1980 (once-logged plots) and the others were logged a second time around 2010 (twice-logged plots). At the species level he investigated correlations between demographic rates and wood density (WD, amount of wood dry mass per green volume). WD is considered a reasonable proxy for the functional type of a species (e.g. it being shade tolerant or light demanding) and it is a quality characteristic for logged wood. He also compared demographic rates and biomass dynamics of commercial and non-commercial species. Finally, he compared the potential economic benefit between logging and selling carbon credits (e.g. under a REDD+ initiative).