Forest restoration on degraded lands in Laos
Deforestation and land-use change have been, and still are, major threats to biodiversity and ecosystem services in the tropics, and forest areas are being lost at high rates in many countries in Southeast Asia, including Laos. Progress in the development and application of forest policies and programs, coupled with increasing trends to end damaging practices associated with swidden cultivation have created opportunities to restore forests. However, there is little scientific knowledge pertaining to forest restoration in Laos. Thus, the studies presented in this thesis examined factors influencing the recovery of secondary forests on abandoned fallows and tested site-specific approaches to restore forests on fallows, former grazing lands and a logged-over mixed deciduous forest. The results showed that 29% of the species richness, 39% of the stem density, 18% of the total basal area, as well as 41% and 7% of the density and basal area of commercial tree species of the surrounding natural forest fragments were recovered within a 20-year fallow period. The major factors influencing recovery are the distance from the forest edge, the fallow history and competition from increased occurrence of bamboo. A mixed-species planting trial established on fallows showed that the use of a mixture of pioneer and later-successional species results in good ecological compatibility, as evidenced by > 70% survival rates and subsequent growth of all planted species. Further, the addition of rice husk biochar to improve soil fertility resulted in 1.2-fold increase in seedling and sapling growth compared to the control and its effect was comparable with that of inorganic fertilizer application. An experiment involving direct seeding of former grazing lands using four native species showed that seedling establishment success was better for later-successional species (59-65%) than pioneer species (3-13%), and the establishment success varied with methods of sowing, the nature of the seeds, the seeding rate and site factors. In addition, an enrichment planting trial, involving five early- and late-successional species and two planting methods, showed that the shade-tolerant dipterocarps had better survival and growth rates than the light-demanding legumes in gaps than planting lines. Overall, the studies provide an important contribution, representing a major advance in the evidence base for selecting appropriate methods to accelerate forest restoration in Laos and other seasonal tropical environments in order to meet key economic and environmental objectives.