Ecological effects of several reforestation and rehabilitation treatments on abandoned shifting cultivation sites in Sarawak, East Malaysia

Abstract

The biomass, nutrient contents and soils of a chronosequence, including undisturbed primary forest (UPF), logged over forest (LOF), cleared and burned forest (for cultivation), abandoned cultivation sites, and four rehabilitation treatments were studied. Rehabilitation treatments included 6.5 year old Acacia mangium, Gmelina arborea and Leucaena leucocephala plantations and 7 to 10 year old fallow forest (natural regeneration). Plots were established on Red-Yellow Podzolic soils with an attempt to maintain similar soil conditions. Replicate biomass plots, 10 m x 20 m in size, were established for all treatments with additional one ha plots for each replicate of UPF and LOF plots. Clearing and burning of LOF for cultivation resulted in decreases in total N and total P, but in temporary increases in organic C, and exchangeable K, Ca and Mg. However, one and a half years after abandonment, C and nutrient levels had decreased to either the lowest levels in the chronosequence or to preburn levels. Carbon and nutrients in the soil increased during the four rehabilitation treatments, approaching UPF or LOF levels, although at somewhat different rates. Potassium levels in Acacia and fallow soils remained lower than levels 1.5 years after abandonment. Results indicate that aboveground biomass of undisturbed forest (476 t/ha) was reduced to 228 t/ha after logging. Clearing and burning ofLOF resulted in removal of all living biomass. Rehabilitation treatments resulted in aboveground biomass productions ranging from a low of 20 t/ha (Leucaena) to a high of 134 ^^a (Acacia). Contributions to aboveground biomass ranged from 1.4 to 17.8 % for litter; 0.7 to 40.3 % for undergrowth; and 41.8 to 96.9 % for overstorey. Within the overstories, stems comprised the largest amount of biomass ranging from 54.8 to 79.0 % followed generally in the order of large branches (3.6 to 9 %) or small branches (4.6 to 16.7 %), twigs (2.3 to 10.7 %), foliage (1.8 to 5.9), and fruit and flowers (0 to 0.5 %). Aboveground biomass of UPF immobilized the largest amounts of N, P, K, Ca and Mg; followed by LOF. Rehabilitation treatments immobilized different nutrients at varying rates. Overstories immobilized the largest amounts of nutrients except in the Leucaena plantation which contained larger amounts of nutrients in undergrowth. Amounts of nutrients immobilized in different vegetative components of the overstories varied with forest type and nutrient. Nutrients taken up by aboveground biomass ranged from 191 to 1271 kg/ha N; 9.1 to 75.4 kg/ha P; 225 to 1161 kg/ha K; 104 to 1624 kg/ha Ca; and 34 to 402 kg/ha Mg. The return of biomass during rehabilitation treatments was generally accompanied by a restoration of soil nutrients. Acacia, Gmelina and fallow were found to be satisfactory rehabilitation treatments for abandoned shifting cultivation sites, in terms of biomass production and restoring the protective function of a forest cover. There was some question as to the suitability of Acacia as a rehabilitation treatment due to early decline in growth rates and problems with disease and heart rot. Leucaena was found to have a negative effect on site recovery resulting in increased undergrowth vegetation which impeded establishment of tree species.