| dc.description.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. | |
