CHANGES IN VEGETATION AND SOIL CHARACTERISTICS OF REGENERATING FOREST AT RANDENIGALA

Abstract

Victoria, Randenigala, Rantambe(VRR) sanctuary is the largest sanctuary in Sri Lanka. Certainareas in the Rantambe catchment was disturbed for the construction of Randenigala dam in 1987with no planned rehabilitation programme. This study was initiated to assess the changingpattern of vegetation and its influence on soil physical and chemical properties of disturbedforestland near Randenigala dam, relative to the surrc:unding undisturbed natural forest.

The site was located 1 km North of the Randenigala Dam in a hilly slope in the intermediatezone and was surrounded by relatively undisturbed natural vegetation of dry mixed evergreenforest. In the sampling site (about 100 ha) the vegetation and soil has been removed down toparental material for dam construction. Three rectangular transacts measuring 5mxlOOmweremarked in each natural and regenerating forest sites .AlI plants were identified, height anddiameter at breast height (dbh) over >2cm of woody plants were measured. Soil pits were cut inboth disturbed and undisturbed forest sites and profiles were described. Soil samples wereremoved from each horizons for the analysis of soil physical and chemical properties. In thedegraded sites, soil pits were cut in the open area (6m), in the canopy edge (3m), and 0.5m fromthe tree base of dominant pioneer tree species to observe the soil improvement.

Plant density of the undisturbed forest was three times higher than the disturbed forest andconsists of 17 trees, six shrubs, and four woody climber species. Five trees, six shrubs, onewoody climber, two grasses, and three creepy herbaceous species were found in the regeneratingforest. Common families present were Euphorbiaceae (38%), Sterculaceae (24%) and Rubaceae(12%) in the undisturbed forest and Hipocraraceae (40%), Leguminosae (26%) andApocynaceae (18%)in the disturbed forest.

Five and two major soil horizons were identified in the undisturbed and disturbed forest sites,respectively. Soil N, P, K and organic matter contents and hydraulic conductivity were greaterand the bulk density was lower in the undisturbed site. The mean weight diameter (MWD) of thesoil aggregate was greater in the soil horizons of undisturbed site (1.6-2.7mm) compared to thedegraded site (1.1 mm). Similar trends were also observed for wet aggregate stability in theundisturbed (90% remaining) and disturbed (71% remaining) in forest soils. However, MWD(1.5mm) and wet aggregate stability (83% remaining) gradually improved towards the tree baseof pioneer tree species. (i.e. under Bouhinia racemosat in the regenerating disturbed forest site.Increase in soil organic matter content stabilizes the soil aggregates and makes soil moreresistant to erosion by wind and water under the canopy of pioneer species. The retention of soilunder these trees contributes to the development of the soil profile, improvement of soil fertilityand subsequently, the vegetation diversity of degraded sites.