Structural and floristics characteristics of three typical successional stages of the tropical evergreen broadleaf forest in Kon Chu Rang Nature Reserve, Gia Lai Province, Vietnam

Background. Structural and floristic characteristics are a crucial aspect in proposing technical solutions for forest ecosystem restoration. The study was conducted in Kon Chu Rang Natural Reserve, Gia Lai, Vietnam. The differences in structure (such as density, tree size, abundance, diversity, species composition, etc.) between secondary and old-growth forests were shown in the several previous researches. However, within secondary forests, different histories of disturbance have resulted in very different stages of forest succession, despite the same length of time since human influence ceased and the same protection conditions. Secondary forests recovering after shifting cultivation, with directly light and fast-growing species, have higher density and abundance, while tree size indices and the number of dominant species are lower compared to secondary forests recovering after logging, which are mainly composed of shade-tolerant species. The research results provided a basis for group classification and the application of silvicultural measures to effectively promote forest recovery processes.

Purpose. To study the potential for natural successional recovery as a basis for proposing the application of silvicultural measures to rehebilitate the evergreen closed tropical rain forest in the Kon Ha Nung Biosphere Reserve.

Materials and methods. The subject of the study was the tropical evergreen broadleaf forest types in Kon Chu Rang Nature Reserve. In this study, satellite imagery (Landsat 8) was collected in the same season from 2013 to 2022 and Normalized Difference Vegetation Index was calculated to determine the forest successional stages of tropical evergreen broadleaf rainforest in Kon Chu Rang Nature Reserve. The results of classification combined with field survey based on the establishment of 09 permanent sample plots (50×50 m, 2,500 m²) to ensure the forest successional stages. These plots were established in each typical successional stage (secondary forest after logging, secondary forest after shifting cultivation, and old-growth forest). In each plot, all live woody stems with a diameter at breast height greater than 10 cm were measured, including tree diameter at breast height and tree species. All data collected in each plot were then used for data analysis using SPSS software. This research conducted an ANOVA (Analysis of Variance) with a Fisher's Least Significant Difference post hoc test to explore differences between multiple group means of tree density, number of trees distribution in each group of tree diameter, tree diameter, basal area and tree diversity. In addition, to investigate forest structure and diversity, the Impotance Value Index, Shannon-Wiener Index (He') and Simpson Index, and Jaccard's coefficient of similarity were calculated in this study.

Results. Tree density ranged from 347 to 763 stems per hectare and total basal area from 15.5 to 42.8 m² per hectare. No significant difference was observed among the three forest types for tree diameter classes from 10 to 25 cm, while for tree diameter classes greater than 25 cm, old growth forest had the highest tree density, significantly different from the others. A decrease in tree density was observed in all forest types except old growth, which had the highest tree density and basal area for tree diameter classes greater than 25 cm. Diversity was found to be significantly higher in the old-growth forest compared to the secondary forest, which may be due to the duration of the restoration process and the initial stage of disturbance cessation. A total of 31 to 43 tree species were identified in 28-38 genera and 19-22 families, with the lowest species richness observed in the secondary forest after logging and only 3-7 tree species calculated in the tree composition. The dominant species in the post-logging secondary forest were heliophilous and fast-growing tree species such as Machilus parviflora, Macaranga tanarius, Litsea elongata, Clausena sp. and Prunus arborea, whereas in the post-shifting secondary forest they were shade-tolerant such as Rehderodendron truongsonense, Cinnamomum mairei, Castanopsis pseudoserrata, Litsea elongata, Syzygium wightianum. In particular, the associations of Clusiaceae and Myrtaceae species in old-growth forests were a novel finding.

Conclusions. It can be concluded that the structure and diversity characteristics of these successional stages exhibited remarkable variation. The old-growth forest had greater tree density, basal area, tree diversity and evenness than those of in secondary forest, along with the differences in number tree distribution, tree composition and diversity. These differences may come from the regeneration time and site condition. These results suggest that long-term monitoring and research are essential to assess restoration success over time.