Abstract：Exploring the abiotic (e.g., topography, climate, and soil) and biotic (diversity, structure, and functional trait identity) drivers of aboveground biomass (AGB) is one of the hot topics in forest ecology, and hence remains highly debated across local, regional and continental scales. In this thesis, I attempted to explore the direct and indirect effects of various diversity facets (e.g., taxonomic, structure, phylogenetic, and functional trait diversity and identity) on AGB in tropical forests along a local scale topographical gradient. To do so, I analyzed biophysical tree data from 101 tropical forest plots (each size = 0.05 ha) along an elevation gradient ranging from 218 to 1850 m above sea level. I applied structural equation modeling (SEMs) to evaluate the direct and indirect effect of elevation and diversity indices on AGB.
First (in Chapter 3), I tested the joint direct and indirect effects of elevation, stand density, individualdiameter at breast height (DBH) and height variation, and taxonomic diversity facet, i.e., either species richness, species evenness, Shannon's species diversity or Simpson's dominance, on AGB. The result showed that the variation in AGB was best explained by higher stand density whereas species diversity declined AGB. Interestingly, tree DBH and height variation showed negligible contribution to AGB. Increasing elevation decreased stand density, species richness, and AGB but increased individual tree height variation. These results may exhibit the prominent role of the selection effect or the competitive exclusion or environmental filtering to regulate species diversity, stand structural complexity, and AGB in the studied tropical forests.
Second (in Chapter 4), I studied the linkage amongst elevation, species richness, individual tree DBH variation, phylogenetic diversity indices i.e., either phylogenetic species evenness, phylogenetic species variability, or phylogenetic species clustering, and AGB in the tropical forests (i.e., pooled data) and across three forest types, i.e., tropical evergreen and mixed hardwood forests, tropical evergreen, and deciduous forests, and sub-tropical coniferous forests. I found phylogenetically closely related species with different abundance were mainly responsible for the higher AGB in all forests and across forest types except for tropical evergreen and mixed hardwood which may be due to the selection effects. However, phylogenetically closely and distantly related diverse species contributed much to AGB in the tropical evergreen and mixed hardwood forests, implying the role of higher niche diversification.
Third (in Chapter 5), I tested the possible combined effects of functional trait diversity and trait identity, i.e., community-weighted mean (CWM) of trait values on AGB while considered the direct
and indirect effects of tree DBH variation along an elevational gradient. In the tested models, trait identity was represented by CWM of maximum height (CWMHmax), CWM of specific leaf area
(CWMSLA), CWM of leaf dry matter content (CWMLDMC), and CWM of wood density (CWMWD) whereas functional trait diversity was represented by functional richness (FRic), functional evenness (FEve), functional dispersion (FDis), and functional divergence (FDiv). I found that the strong positive effect of functional dominance, i.e., CWMHmax, overruled the effects of low functional trait diversity (i.e., a latent variable where FRic contributed positive but FDis and FEve contributed negative, and negligible contribution from FDiv) on AGB, indicating that functionallysimilar tall-stature trees promote AGB. However, I found that the acquisitive (CWMSLA) and conservative (CWMLDMC and CWMWD) functional strategies were relatively unimportant in shaping AGB in studied tropical forests.
In conclusion, the results in this thesis suggest that functionally similar and phylogenetically close species having highly compact stand density probably due to tall dominant trees could underpin AGB in the tropical forests of Nepal. Although it is generally believable that mass ratio and niche complementarity are mutually non-exclusive mechanisms, I found the strong role of the mass ratio
effect in shaping AGB as compared to the niche complementarity effect. Thus, competitive exclusion, ecological similarities, niche overlap played an important role in shaping the diversity, composition, and functioning of these tropical forests. Lastly, I suggest conserving tall-stature trees for enhancing carbon storage but it is also important to protect a diverse range of species and tree sizes for biodiversity conservation that underpins human well-being through several other forest services and functions.