Author : Nguyen Tien Phong

Summary : The study analyzed the forest dynamics of the evergreen broadleaf forests in the Northern mountainous region of Vietnam. Specifically, the study (1) analyzed the change in the overstory tree structure (tree composition, density, biodiversity indices, DBH and HT frequency distribution, DBH-HT regression, forest layers, vertical and horizontal structure), the change of forest regeneration structure (regeneration composition and density, relationship of regenerating trees with overstory trees, regenerating tree distribution in height class, as well as quality and origin); (2) analyzed the change of physical and chemical properties of soil under the forests; (3) predicted the recruitment, transition and mortality of the forests; and (4) recommended some conservation strategies to the forests in view of climate change.
Nine 1-ha national permanent plots (225 quadrat plots) were located in three different provinces with various elevations and forest types. Data collection from each quadrat plot for two survey periods (2015-2020 and 2016-2021), include species name, number of species, number of individuals, DBH, HT, crown width, number of mortality and recruitment trees, basal area, volume and diversity indices in each survey, and soil properties for 225 quadrats plots. All the parameters were calculated and analyzed statistically (Nonparametric test, Kruskal-Wallis, x2, ANOVA) for all National Permanent Plots.
The number and density of trees significantly changed between two surveys and among the different National Permanent Plots (NPPs). NPPs with restored evergreen broadleaf forest (REBF) type have stable species number, even distribution across size classes, high biodiversity, increasing species richness, the highest average number of trees, followed by those with the average evergreen broadleaf forest (AEBF) type with fluctuating mortality and recruitment rates, low species diversity, uneven size distribution, many small trees, and the bamboo-timber tree mixed forest on soil mountain forest (BTTMF) type with low biodiversity, unstable, uneven distribution of individuals. Over-story tree composition of the forests tend to replace light demanding, fast-growing tree species with native, canopy-tolerant species. The dominant species formulating the over-story trees composition was low, from 5 species for BTTMF to 11 for REBF and AEBF. The number of trees and diameter at breast height (N-DBH) and number of trees and total height (N-HT) frequency distribution in the NPPs had a decreasing distribution pattern, with an inverse J-shape and saw-tooth curve shapes, respectively. The logarithmic equation was the most suitable to present the correlation between HT and DBH. The over-story tree layer mainly belonged to the A2 layer. The density and number of regenerating trees in the forest were average, and with a high number in the REBF and AEBF type and good soil conditions. The regenerating and overstory trees had a closed relationship (70-88.9%). The regenerating trees were mainly good in quality in the forest, and REBF types had the highest number of regenerating trees. The soil depth increased in the second survey, leading to enhanced water-holding capacity. Soil chemical properties varied between forest states and altitudes, influencing nutrient content. The highest mortality rate was 5.68% to 8.76%/year to the forest types with AEBF and high elevation, followed by BTTMF and REBF. The cubic equation can be used to predict the recruitment and transition rate of the forest according to the diameter classes, while the inverse equation can be used to predict the mortality rate in diameter classes.
Management strategies were tailored to each forest type. REBF requires protection and monitoring, AEBF needs enrichment planting in canopy gaps, cutting vines and shrubs, and BTTMF should focus on native tree planting and removing fast-growing species.