Succession of fallows after shifting cultivation in Sungai Sarawak basin, Kuching, Sarawak

Shifting cultivation is the simplest and very widespread method of agriculture. It is estimated that about 3.2 million hectares (ha) of land has been cleared for shifting cultivation. It is basically developed from the traditional way of hunting and gathering by the rural people. In Sarawak, the people are mainly lban, Bidayuh, Kayan, Kenyah, Kelabit, Murut and other indigenous communities. They traditionally plant hill padi, interplanting with tapioca (Manihot sp.), maize (Zea mays), cucumber (Cucumis sp.), Brassica vegetables, sorghum etc. and then these areas are abandoned for the next cycle. This practice is often considered to be one of the major causes of extensive deforestation and environmental degradation. The abandonment has normally led to the formation of various stages of fallows or secondary forests. Forests are usually classified based on their structure (physiognomy). The secondary forests offer a distinct contrast to the primary forests, both structurally and floristically. The common successional categories in the survey area are the grassland, Temuda (3 to 8 years after shifting cultivation) and Belukar (8 to 20 years after shifting cultivation). The grassland becomes established approximately one year after shifting cultivation of hill paddy planting. Grassland is dominated by herbs, grasses and sedges. Shrubs and small trees are also present. The young Temuda I develops after approximately 3 to 4 years and is dominated by herbs, sedges, ferns and young shrubs. Trees generally grow to 3 meters (m) in height during this successional stage. Temuda II represents the successional stage following Temuda I. This successional stage develops between 5 to 8 years after shifting cultivation. Emerging shrubs and trees are still very common and characterize the vegetation of this successional stage. Trees may reach 6 m in height but the canopy is still rather open with pockets of shade from taller trees. Canopy closure takes place at the beginning of the Belukar I stage after about 8 to 15 years old. This stage of succession is conspicuously dominated by Palaquium sericeum (Importance value (Iv) = 13.33), Helicia petiolaris (Iv = 12.578), Artocarpus elasticus (Iv = 11.088), Adinandra acuminata (Iv = 11.075), Anisophyllea corneri (Iv = 9.187) and Premna integrifolia (Iv = 7.590). The fast-growing trees of the genera Ficus and Macaranga grow gregariously, notably Ficus uncinata, Ficus grossulariodes, Macaranga beccariana, Macaranga triloba, Macaranga pruinosa, Macaranga hypoleuca, Ficus geocharis, Macaranga costulata, and Macaranga gigantea. Other common species are Artocarpus nitidus, Artocatpus elasticus, Artocarpus anisophyllus, leban (Vitex pubescens), Dillenia sp., terbulan (Endospermum malaccense) and Callicarpa. Macaranga pruinosa, M gigantea and M beccariana are occasionally established in pure stands. Light-demanding herbs, grasses and sedges of the Temuda forest have largely been shaded out, except Blechnum orientale, some species of ginger and Nephrolepis biserrata. Licuala is quite commonly found among the sparser ground cover. The estimated above-ground biomass of Belukar vegetation is 181.8 tons/hectare (t/ha). The highest contribution of biomass is from Palaquium sericeum followed by Helicia petiolaris and Artocarpus elasticus. In Belukar II (more than 20 years old), the dominance of the fast-growing pioneer trees is ending. This vegetation is a transition to mixed dipterocarp forest. Shade-tolerant species have become established and are gradually gaining dominance in the overstorey. This type of vegetation occupies a small area along the edge of the mixed dipterocarp forest.