POPULATION STRUCTURE OF RHODODENDRON ARBOREUM SMITH, AND ASSOCIATED TREE SPECIES ALONG THE ALTITUDINAL GRADIENT IN A MIXED FOREST OF RAINASKOT, LAMJUNG, NEPAL

Abstract

Population structure is the number of individuals present in each of the definite girth classes of tree species. The main objectives of this study was to determine the population structure and regeneration of Rhododendron arboreum and its associated tree species along the three altitudinal range in a mixed forest of Rainaskot Lamjung. The study area was divided into three altitudinal range: lower (1200-1400masl), middle (1400-1600masl) and upper (1600 -1653masl). Altogether 75 circular quadrants, 25 from each altitudinal range were laid by using random systematic sampling method. Circular plot of 10m radius were laid for the study. Height and Diameter at breast height of the trees (>10cm dbh), associated trees (>10cm dbh), were recorded within main plot. Number of saplings and seedlings were recorded within main plot. Finally five soil samples were prepared from 25 soil samples collected from the center of the main plot within each altitudinal range. Total fifteen soil samples were taken for analysis such as Soil pH, organic matter, soil texture and 3 macro nutrients (NPK). A total of 13 tree species belonging to 11 different families were reported. The species richness in the forest increases with increasing altitude. The most frequent and dense tree species in the forest was R. arboreum with IVI value 77.7 and was present in 93.33% of the total sampling plots. The density of R arboreum was 257.52 individualha-1 with basal area 16.40 m2 ha-1 . In altitude wise comparison of density of R. arboreum with total tree density, upper altitudinal range had (393.28 individualha-1 ), middle altitudinal range (248.19 individualha-1 ), and the lower altitudinal range had (127.28 individualha-1 ). IVI of R. arboreum was quite lower (68.19%) in the lower altitudinal range with compared to middle (70.02%) and upper altitudinal range (105.52%). The R. arboreum shows reverse J-shaped population structure and fair regeneration status in overall forest having higher number of seedlings compared to saplings. However, the numbers of saplings were less than the adults except in lower altitudinal range. The height class distribution of R. arboreum shows a reverse J-shaped distribution with little deviation in the upper altitudinal range. The soil pH of forest was found to be acidic. The seedlings, saplings and adults density were positively correlated with the nutrients such as nitrogen, phosphorus and organic matter except pH, potassium, canopy cover and litter depth. The anthropogenic disturbance, ecological diversity, and soil characteristics are responsible for the variation in population and regeneration attribute of the forest.