Anthropogenic disturbance produces divergent effects in the community structure and composition of tropical semi-evergreen forests in the Eastern Himalaya

A comparative field study of six tropical semi-evergreen forest sites in Arunachal Pradesh found that increasing anthropogenic disturbance was associated with different responses in the tree, shrub and herb layers. Tree and shrub species richness and mean tree density were lower in the most disturbed stands, whereas herb richness and diversity peaked at intermediate disturbance. The study also recorded shifts in species composition, tree basal area and the size structure of trees across the disturbance gradient.

The researchers surveyed six sites in Aalo Forest Division, West Siang District, within India’s Eastern Himalaya. The sites extended from 28°20′50″N to 28°40′21″N and from 94°21′42″E to 94°42′32″E, at elevations of 450–950 m. The area receives an average of 2,300 mm of monsoon rain annually, with temperatures of 18–22°C and relative humidity of 77–98%. Sampling took place in March–April, July and October over three years.

Vegetation was measured with nested quadrats: 10 × 10 m plots recorded trees with circumference at breast height above 30 cm, 5 × 5 m plots recorded shrubs and saplings with circumference of 10–30 cm, and 1 × 1 m plots recorded herbs below 10 cm. Species-area curves determined the number of quadrats needed at each location. The researchers analysed species richness, density, abundance, frequency, dominance, Shannon-Wiener diversity, Pielou evenness, importance value and tree population structure.

Disturbance was represented by a cumulative score based on four locally predominant pressures: proximity to roads, proximity to habitation, tree cutting or lopping, and browsing or grazing by domestic cattle. Tatogito was classified as least disturbed, Roying, Poma Basti and Tagurshit as moderately disturbed, and the two BB Camp sites as highly disturbed. The study was a comparison among six locations along an assumed gradient, not a longitudinal assessment of change through time, and the disturbance score was a surrogate based on field indicators rather than a direct experimental manipulation.

A total of 160 plant species was recorded across the six sites, comprising 54 tree, 30 shrub and 76 herb species. Mean tree species richness declined from 3.2 ± 1.03 in the least disturbed site to 1.9 ± 0.57 in highly disturbed sites, while shrub richness declined from 2 ± 0.82 to 1.5 ± 0.53. Herb richness differed: the moderately disturbed site had the highest mean value, 3.7 ± 1.2. Mean tree and shrub richness differed significantly among sites, as did mean density in all three vegetation classes.

Diversity and evenness measures did not change in parallel. Tree Shannon-Wiener diversity was highest at the least disturbed site, with a value of 2.79, and lowest at the highly disturbed BB Camp Basti site, where it was 1.65. Herb diversity was highest at Tagurshit, a moderately disturbed site, at 2.96, while shrub diversity reached its maximum at the highly disturbed BB Camp Bamboo bridge area, at 2.04. Tree evenness was greatest in the highly disturbed BB Camp Basti site, at 0.24, whereas shrub and herb evenness reached their maxima in the least disturbed Tatogito site.

Density responses also varied among vegetation layers. Mean tree density was highest in the least disturbed forest at 3.4 ha⁻¹ ± 0.5 and lowest in the highly disturbed sites at 2.55 ha⁻¹ ± 1.2. Shrub density ranged from 6.30 ha⁻¹ ± 0.8 at Tatogito to 8.40 ha⁻¹ ± 1.7 at the highly disturbed BB Camp Bamboo bridge area. Herb density was lowest in the least disturbed site, at 6.70 ha⁻¹ ± 1.1, and highest in the highly disturbed BB Camp Basti area, at 23.3 ha⁻¹ ± 2.7. Thus, higher disturbance was associated with fewer trees but more shrubs and herbs at the surveyed sites.

Species composition and tree population structure also varied across the gradient. Tree-family richness fell from 18 families at the least disturbed site to 13 at the highly disturbed site, and Anacardiaceae was the only recorded family present at every site. Alnus nepalensis, Engelhardtia spicata and Albizia odoratissima were the dominant trees in the least, moderately and highly disturbed sites, respectively. At all six sites, trees were most abundant in the 30.1–90 cm circumference classes and least abundant in the 150.1–210 cm classes. Principal component analysis of tree basal area separated the six sites, with the first two axes accounting for 51.92% of the variation in species composition.

Together, the observations indicate that disturbance in these semi-evergreen forests did not produce a uniform response across vegetation layers. The study associated the decline in tree richness, density, basal area and large-tree representation with increasing disturbance, while some shrub and herb measures were greatest at moderate or high disturbance levels. The exotic herb Ageratum conyzoides was absent from the least disturbed forest but dominated highly disturbed sites, and the authors linked disturbed sites with proliferation of disturbance-tolerant shrubs and exotic herbs. Because the research covered only six sites and measured one regional disturbance gradient, the results characterize local associations rather than demonstrating causal or long-term changes throughout the Eastern Himalaya.

Subject of Research: Comparative field study of disturbance effects on tree, shrub and herb communities in six tropical semi-evergreen forests in Arunachal Pradesh, India

Article Title: Anthropogenic disturbance produces divergent effects in the community structure and composition of tropical semi-evergreen forests in the Eastern Himalaya

Article References: Nautiyal, D. C., & Manish, K. (2024). Anthropogenic disturbance produces divergent effects in the community structure and composition of tropical semi-evergreen forests in the Eastern Himalaya. BioRisk, 22, 1-15. https://doi.org/10.3897/biorisk.22.120802

Image Credits: AI Generated

DOI: 10.3897/biorisk.22.120802

Keywords: anthropogenic disturbance, Eastern Himalaya, Arunachal Pradesh, semi-evergreen forests, plant diversity, species richness, community composition, vegetation structure, forest ecology, Anthropogenic, disturbance, produces