Secondary forest comprises a large area in the tropical region, and it has increasingly believed that the future of tropical forests depends on the effective management of such second growth forests largely modified by human activity. In tropical secondary forests, shifting cultivation, swidden or slash-and-burn is a major land-use that has been attributed to causing large scale deforestation and forest degradation. This view has been embedded in many policy documents, although there are conflicting views within the literature as to the impact of shifting cultivation on secondary forest dynamics. This is largely due to the complex nature of this land-use that makes any generalisation difficult.
In the Philippines – a country overwhelmed with rich biodiversity and species endemism, shifting cultivation is locally known as kaingin and represents a dominant land-use in upland rural areas. Although not officially recognised, it has a major contribution to the livelihoods and food security of smallholder rural farmers. After post–logging secondary forests, post–kaingin secondary forests also form the largest group of secondary forests in the Philippines. This thesis based on an empirical study where secondary forest dynamics after shifting cultivation were investigated in an upland area in the Leyte Island, the Philippines.
Chapter 2 of my thesis is a synthesis that reports, spatial and temporal distribution of research on shifting cultivation using a systematic literature search protocol, and the key findings of research in conservation biology, plant ecology, soil nutrients and chemistry, and soil physics and hydrology. A bias towards research on anthropology and/or human ecology was found with most research reported from tropical Asia-Pacific region. A great variation in findings on selected forest/environmental parameters was also evident.
In Chapter 3, I report a field study where tree diversity, species composition and forest structure and their recovery in secondary forests after shifting cultivation was investigated along a fallow/shifting cultivation land-use gradient. Twenty-five individual sites representing four different fallow categories and old-growth forest as control were surveyed. Tree species richness was significantly higher in the oldest fallow sites while other diversity and forest structure indices were higher in the old-growth forest sites. A homogeneous species composition was found in older fallow sites and in old-growth forest. It was found that secondary forests disturbed by shifting cultivation recovers rapidly in terms of species richness and abundance, and that patch size is a strong predictor of this recovery.
Uncertainties in the aboveground biomass carbon in degraded secondary forests after shifting cultivation are one of the main issues hindering its inclusion in the current REDD+ negotiations. In Chapter 4, the distribution and recovery of biomass carbon along a fallow gradient were reported from my study sites. It was found that aboveground total biomass carbon and living woody biomass carbon was significantly higher in the old-growth forest sites while coarse dead wood biomass carbon was high in the new fallow sites. The high level of dead wood biomass carbon in new fallow sites was due mainly to high levels of coarse dead wood remaining as an artefact of clearing. Relatively higher biomass carbon recovery in oldest fallow sites than other fallow category with patch size again as an influential factor in explaining the variation in biomass carbon recovery in regenerating secondary forests after shifting cultivation was evident.
Shifting cultivation has been linked to an adverse impact on soil carbon and nutrients in the tropical region, albeit different conclusions from the studies. In Chapter 5, the distributions and recovery of soil organic carbon, soil nitrogen, phosphorus, and potassium in relation to shifting cultivation use in regenerating secondary forests were examined. Limited influence of shifting cultivation on soil carbon, nitrogen and phosphorus were found, although soil K recovery was low across the sites. Multivariate analysis revealed patch size together with slope and fallow age important in explaining the variation in soil carbon and nutrient recovery in different site categories and soil depths.
It has commonly believed that tropical landscapes after shifting cultivation are not suitable for biodiversity and carbon benefits. Drawing on my present study in the Philippines in Chapter 6, I demonstrate that regenerating secondary forests following shifting cultivation have the potential for inclusion in programs on forest conservation under REDD+ and CDM. I argue that regenerating secondary forests after shifting cultivation can be a cost-effective conservation, restoration, and forest management option not only in the Philippines but also in other countries where shifting cultivation is prevalent.
Overall, my thesis highlights the key biophysical and some policy aspects of shifting cultivation on tropical secondary forest dynamics. My results indicate that regenerating forests after shifting cultivation can act as a low-cost refuge for biodiversity conservation and an important source and sink of carbon that increases with abandonment age. A better understanding of the forest dynamics associated with shifting cultivation land-use, however, is important for the future of tropical forest management and conservation.