CHANGES IN VEGETATION AND SOIL CHARACTERISTICS OF REGENERATING FOREST AT RANDENIGALA

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.

Soil water extraction pattern of coconut (Cococs nucifera L.) in relation to soil compaction

Effect of soil compaction on the gowth and activity of coconut roots in two soil series, namely, Andigama series (gravelly soil) and Madampe series (sandy loam soil) was studied. The effective root zone of coconut in relation to water sbsorption process under natural soil compaction were also investigated. Soil compactin and soil moisture absorption ability was determined using cone penetrometer and neutron scattering methods, respectively. Coconut root distributution in Andigama and Madampe series showed that 75%-80% of roots of adult coconut palms were localised in a depth ranging from 20 cm to 100 cm. About 5% of the roots were located beyond the 100 cm depth and 15% -20% was confined to the top layer (0-20 cm) of soil. Root growth of coconut in the two series did not show any significant differences. However, the root activity in the Madampe series, with respect to moisture absorption was higher that that of the Andigama series due to low compaction of the former. Results also showed that soil compaction higher than 250N/cm restricted the activity of coconut roots in the Andigama series for moisture absorption up to a distance of 2m away horizontally from the base of the tree and the highest moisture extraction was observed at m distance. High moisture extraction by coconut roots was confined to a depth ranging of 20 to 120 cm and of 20 to 250 cm in soils of Andigama and Madampe series, respectively, due to differences in soil compaction levels. It was condluded that soil compaction limits the water absorption ability of coconut roots vertically from the base of the tree, rather than coconut root growth and penetration