Effects of canopy gap size on the recruitment of Shorea robusta Gaertn. F. seedlings in the Terai forests of Nepal

Gap dynamics is a crucial ecological process in which canopy openings created by disturbances influence structure, regeneration and growth pattern in forest ecosystems. The canopy gaps (an area > 25 m² opened by the removal of canopy trees) in the forests are usually caused by both anthropogenic and natural stressors, such as logging, treefalls, windstorms, and fires. Though the canopy gaps play an important role in shaping recruitment of the seedlings and hence the forest ecosystem assemblages, little is known about how the gaps have affected the vegetation establishment in the forest ecosystems, particularly in Terai, Nepal. The present study was carried out in core and buffer zone forests of two lowland national parks of Nepal, viz., Parsa National Park and Bardiya National Park, to assess the influence of gap size on the Shorea robusta seedlings’ recruitment and establishment. Altogether 120 gap sites were sampled ranging in size from 71 to 914.5 m², which were categorized into small gaps (< 200 m²), medium gaps (200–400 m²), and large gaps (> 400 m²). The mean density of S. robusta seedlings was significantly higher in small (1.98 ind. m⁻²) compared to the medium-sized (1.37 ind. m⁻²) and large-sized (0.54 ind. m⁻²) canopy gaps in both core areas and buffer zones. The small gaps provided the most favorable conditions for seedling recruitment in both locations, possibly due to optimum light and nutrient availability. Moreover, regression analysis showed a significant inverse relationship between seedling density and the size of the canopy gaps. These results indicate that the larger gaps are not favorable for the assemblage and recruitment of S. robusta seedlings, which might be due to the feeble seed dispersal ability of the species for longer distances. Forest managers, therefore, are suggested to maintain small gap sizes to foster the natural regeneration of S. robusta in the forest stands of Terai, Nepal.