Spatial Patterns Of Tree Species And Coexistence Mechanisms In Tropical Evergreen Broadleaved Forests In Southern Vietnam

Complicated forest structure means continuous and long ecological functions.Plant diversity is the key factor for forest biodiversity and measuring forest community structure.Besides,the spatial distribution patterns of plant populations influence the forest community structure and its succession process.Therefore,the study on forest structure,diversity,and spatial distribution patterns of tree species has important significance for secondary forest management and its ecological function recovery.In this study,three evergreen broadleaved forest stands in three special-use forests located in three southern provinces of Vietnam,namely Binhchau-Phuocbuu Nature Reserve,Dongnai Culture and Nature Reserve,and Phuquoc National Park,were selected as the study area.Spatial and non-spatial structural characteristics were quantitatively analyzed based on data from three 2-ha study plots established in these three forests.The study used spatial structure indices(nearest neighbour indices)to analyze the overall structure of the forest stands.Besides,the point pattern analysis,the individual species-area relationship model,and species abundance distribution models were used in the study to clarify underlying processes and ecological mechanisms that maintain the coexistence of tree species.The study also discussed the relationship between the forest’s structural characteristics and species diversity.The current study was conducted to provide a theoretical basis for restoring degraded forests and conserving biodiversity in southern Vietnam;at the same time,the study also served as a suitable reference material for understanding the dynamics of tropical forests.The main results of the study are as follows:1.The density of three evergreen broadleaved forest stands in three study sites(Binhchau-Phuocbuu Nature Reserve,Dongnai Culture and Nature Reserve,and Phuquoc National Park)ranged from 1297 to 1967 stems per hectare.The mean diameter at the breast height of trees ranged from 11.1 to 13.3 cm,while the mean height ranged from 9.6 to 13.2 m.The stand’s total basal area and volume ranged from 22.06 to 25.55 m~2 per hectare and 117.22 to 212.98 m~3 per hectare,respectively.The species composition of the stands ranged from 62 to 81 species,with a significantly different number of species in the rich forest state(Dongnai Culture and Nature Reserve)compared to the medium forest state(Binhchau-Phuocbuu Nature Reserve and Phuquoc National Park).In the rich forest state,four species(Shorea guiso,Nephelium lappaceum,Syzygium cumini,and Diospyros venosa)were ecologically significant and shaped the dominant tree species group.In the medium forest state,the dominant tree species group consisted of five species(Vatica odorata,Knema pierrei,Garcinia merguensis,Nephelium lappaceum,and Syzygium cumini)in Binhchau-Phuocbuu Nature Reserve and eight species(Hopea pierrei,Archidendron quocense,Syzygium cumini,Syzygium oblatum,Drypetes perreticulata,Shorea roxburghii,Memecylon edule,and Diospyros venosa)in Phuquoc National Park.The similarity in species composition between forest stands belonging to the same forest status was higher than between different forest states.The dominant tree species had a wide distribution across the three study sites.The Meyer distribution best fits the tree diameter distribution,while the Weibull distribution best fits the tree height distribution in forest stands.The correlation between the total height and diameter at breast height can be simulated using four functions: S,logarithmic,power,and compound.However,the logarithmic function was best suited for the medium forest state,while in the rich forest state,the power function was best suited.The species diversity characteristics of forest stands were significantly different.The four dominant species of the three studied stands,Vatica odorata,Knema pierrei,Shorea guiso,and Hopea pierrei,showed strong competition compared to other dominant species.The other dominant tree species in forest stands often occur together and are ecologically related,showing 37-41% similarity on the Cluster Dendrogram.2.The study analyzed the spatial structure characteristics of the forest stand through the relationship between the target tree and its four nearest neighbour trees.The results indicated that most trees in forest stands were randomly distributed,well-mixed,and relatively dense.The overall structure of forest stands was moderately differentiated,and each neighbourhood comparison degree occupied nearly 20% of trees.Among bivariate,trivariate,and quadrivariate distributions,the most prominent characteristics of the spatial structure were as follows: no matter how the structural parameters were combined,most trees were randomly distributed or well mixed under different structural combinations.The study used four types of distribution of structural parameters,namely univariate,bivariate,trivariate,and quadrivariate distributions,to comprehensively and systematically describe the forest’s structure characteristics from different levels and angles.These four distributions provide prerequisite information for the precise regulation of forest structure and reconstruction of stand structure.3.The analytical results of the spatial distribution and diversity patterns of dominant species revealed that accumulators,repellents,and neutral species all play a role in maintaining species coexistence in forest communities in the study area,with varying degrees of importance depending on the spatial scale.The spatial distribution and diversity patterns of dominant species were influenced by ecological processes such as dispersal limitation,habitat heterogeneity,and species interactions.The results also showed that the niche and neutral theories were not mutually exclusive,and both theories could explain the underlying processes that maintain the diversity of evergreen broadleaved forests.4.The structural equation model showed that the model used in the study fully reflected the observed data,as evidenced by the goodness-of-fit test with good results,with CFI = 0.9782 and SRMR = 0.02.Forest stands’ spatial and non-spatial structures significantly affected species diversity indices(Species richness,Shannon diversity,Simpson diversity,and Pielou diversity).The spatial structure was found to have a greater effect on species diversity indices(Factor loading coefficient = 0.91)than the non-spatial structure(Factor loading coefficient = 0.08).There was a positive correlation between stand structure and species diversity indices.In contrast,species competition was found to have a negative correlation with species diversity indices.The thesis presents a reliable scientific basis for managers to develop silvicultural methods and solutions for the sustainable management,conservation,and development of forest resources.The spatial pattern and species diversity analytical results are theoretically meaningful and valuable for selecting and arranging tree species to restore or enrich southern Vietnam forests.The adjustment of the stand structure should be based on the spatial associations and diversity patterns of dominant tree species.