Degradation of tropical forests is a major driver of the global extinction crisis. A key question is understanding the role of evolution history during forest succession in the context of forest restoration for maintaining ecosystem function and stability. This study was conducted in a fragmented forest landscape in the central highlands of Vietnam. We sampled living trees with diameters at breast height of ≥6.0 cm in nineteen 0.25 ha plots to evaluate forest community structure changes over two early successional stages (<10 years and 10–20 years old) after abandonment and old-growth. We used both statistically metric and nonmetric analyses to examine correlations of community composition during successional stages and along elevational gradients. We found that (i) significant differences existed in the structural compositions between early successional forests and old-growth forests, but did not exist within early successional forests; (ii) the phylogenetic structure shifted from overdispersion to clustering with increasing successional ages; and (iii) above-ground biomass (AGB), representing ecosystem functioning, significantly increased from early-to-late successional stages, but did not correlate with phylogenetic diversity or elevation. Our results revealed that the forest community structure was strongly affected by degradation, particularly AGB and phylogenetic structure. These findings have clear implications for sustaining biodiversity persistence and ecosystem functioning in human-modified landscapes in the study region.

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Forest ecosystems play an integral role in climate regulation through carbon sequestration and storage. Tropical forests in Laos have undergone major degradation which threatened the standing biomass and carbon sequestration potential of these forests, apart from altering the dynamics of the ecosystem. In this study, species diversity and forest structure were assessed through 32 0.25-ha study plots representing 3 major forest types in Phou Khao Khouay Nation Park, Laos. The findings found a total of 5,477 individuals, 188 species belonging to 57 families. H. pierrei was the most dominant tree species (IVI =9.29%) among 138 species in DEF; A. grandis and L. fenestratus were the most co-domimant species (IVI=8.57%) among 126 species of MDF and P. merkusii covered the grestest IVI (20.02%) among 54 species in MCF. Individual tree distribution was inversed J-shape in all forest types suggesting good regeneration and recruitment potential. Significant differences of taxonomic and structural between 3 forest types showed through Kruskal-Wallis test with p-value < 0.05. Above ground carbon biomass decreased with decreasing species richness, basal area and volume through forest types, specifically 184.00±66.79 Mg/ha in DEF; 107.57±7.90 Mg/ha in MDF and 110.99±7.69 Mg/ha in MCF. Taxonomic and structural attributes contributed positive effects on above ground carbon biomass. Biodiversity conservation should be a key component of the UN Reducing Emission from Deforestation and Degradation strategy (REDD+).

Chapter 3 Changes in forest structure and tree species diversity after seven and eight years in tropical rainforests of Vietnam 3.1 Forest structure 3.1.1