Species richness, diversity and density of understory vegetation along disturbance gradients in the Himalayan conifer forest

Abstract

We investigated whether species richness, diversity and density of understory herbaceous plants differed along logging (gap) and grazing (primarily by cattle) disturbance gradients, and sought to identify drivers of richness, diversity and density of understory vegetation of logged sites. A factorial experiment was conducted in the mixed conifer forest of Gidakom in Western Bhutan. Levels of the logging treatment included small (0.15 – 0.24 ha), medium (0.25 – 0.35 ha) and large (0.36 – 1.31 ha) gaps. The grazing treatment included grazed (primarily by cattle) and ungrazed (where herbivores were excluded by a fence) plots nested within each gap. Data were collected from 12 gaps (4 replicates at each level of logging) using the point intercept method. Shannon Weaver Diversity and Margalef’s indices were used to estimate species diversity and describe species richness, respectively. Soil samples were analyzed for pH and nutrients. The interaction effect of logging and grazing was significant (p≤0.001) only on species diversity. Relative to ungrazed areas, species diversity was significantly higher (0.01≤p≤0.05) in medium grazed gaps. Under grazed conditions, soil P was negatively correlated with gap size and species diversity. While species diversity was positively correlated (0.01≤p≤0.05) with soil N in grazed plots species richness was positively correlated (0.001≤p≤0.01) with soil N in ungrazed plots. Relative density of Yushania microphylla and Carex nubigena were higher under ungrazed conditions. Our study suggests that the combined effect of cattle grazing and logging results in higher species diversity of understory vegetation in medium and grazed gaps in mixed conifer forests of Bhutan,whereas increase or decrease in relative density of major species is determined primarily by the independent effects of grazing and logging. From management perspective, forest managers must refrain from creating large gaps to avoid loss of nutrients (mainly P and N), which may eventually affect tree regeneration. Managers intending to maintain understory vegetation diversity must consider the combined effects of grazing and logging, ensuring low to moderate grazing pressure.