Gap characteristics and their effects on regeneration, dominance and early growth of woody species
Swedish University of Agricultural Sciences, Southern Swedish Forest Research Centre, Tropical Silviculture and Seed Science Group, PO Box 49, SE-230 53 Alnarp, Sweden
* Corresponding author: Swedish University of Agricultural Sciences, Southern Swedish Forest Research Centre, Tropical Silviculture and Seed Science Group, PO Box 49, SE-230 53 Alnarp, Sweden. E-mail: isapkota{at}gmail.com
Aims: We aim to examine the canopy gap characteristics and evaluate their influence on regeneration, dominance and the early growth of woody species in seasonally dry Shorea robusta forests (Sal forests).
Methods: Sixty canopy gaps were surveyed in six randomly located transects belts in seasonally dry subtropical Sal forests of central Nepal. Each transect belt was followed until 10 gap sites were encountered. The equation for the area of an ellipse was used to calculate the size of canopy gap, measuring the longest axis and its perpendicular shorter axis. Number, sizes, ages and causes of tree falls creating canopy gaps along with number and sizes of border trees were identified and recorded. Detailed gap inventories were carried out using square 25-m2 quadrats placed in the middle of each gap. All individuals >2 m in height within the quadrat were identified at the species level and their diameter at breast height was measured. We assigned a nested 4-m2 quadrat to the corner of each 25-m2 quadrat, within which all woody individuals >10 cm tall were identified at the species level, and counted them and their regeneration mechanisms were identified. The height and collar diameter of the tallest individuals were measured. Descriptive statistics was calculated for the variables of interests and Pearson correlation, linear regression, independent-sample t-test and chi-square test were used to relate them and to test for their associations.
Important Findings: The study found mean gap size of 283 m2 and 
50% gaps of 10–15 years old. Gaps created by natural single-tree falls were significantly more numerous, and their mean size was significantly smaller than those resulting from artificial causes or multiple-tree falls. Gap size correlated with the basal area of felled trees, but it did not correlate with the number of tree falls. While tree fall basal area was significantly positively correlated to the seed-originated seedling to resprout ratio, it was negatively correlated, along with gap area and the basal size of retained trees, to seedling growth. The relative seedling density of Terminalia alata increased with increases in gap areas, while that of S.robusta decreased with increases in tree fall basal area, thereby lowering the plot-level dominance. However, the relative seedling densities of Eugenia operculata and Syzigium cumini were negatively and positively correlated, respectively, with tree fall basal area.
Keywords: canopy gap • seedling density • seedling growth • basal area • Sal forests • Nepal