Restoration ecology of mixed deciduous forest ecosystem with teak in Northern Thailand

Abstract

Forest degradation is proceeding at an unprecedented rate in Thailand, due to population growth, agricultural land demand, as well as timber production demand especially Teak. Large areas of forest have been disturbed leaving behind with a less production area. The implementation of the Modified Forest Village System which is a modified from Taungya system is an attempt to solve this problem in Thailand's northern regions, by established Teak plantation combined with various economic tree species. The future restoration expected goal is to maintain species diversity by converting degraded areas back to natural or near natural forest. Thus, development of restoration strategies must follow advances in research. The objectives of this study were to investigate the effects and casual factors of different mixed-species plantations on floral composition and diversity regenerating in the understorey. Seed input to the plantation via both seed dispersal andthe soil seed bank was studied. Finally, the study focused on the effects of five mixed plantation stands on survivorship and productivity of enrichment seedlings in restoration processes. This study have 5 stands of mixed-species plantation included i) Tectona grandis ii) T. grandis and Tamarindus indica iii) T. grandis and Gmelina arborea iv) T. grandis, T. indica and G. arborea and v) T. grandis, T. indica and Anacadium occidentale. The results of the study showed that the highest floristic diversity regenerating in the understorey of the five mixed plantation types was found in the three-species plantation. Diversity indices gradually decreased from three-species to single-species plantations when considering both woody and non-woody species. However, the density of herb (Imperata cylindica and Chromolaena odorata) in singlespecies plantations was higher than in the two and three-species plantations. This indicates that the three-species plantation enhanced species diversity and suppressed the growth ofgrass in the understorey better than the single-species plantation. Dominant and climax species were found in single as well as mixed-species plantations, but at a low density. This suggests that climax species can establish during early stages of succession, and is useful to enhance developing and adjusting restoration strategies. For this reason enrichment planting by selecting some of climax tree species has higher potential to enhance restoration mechanisms. Observations on seed inputs showed low seed dispersal and soil seed banks of native woody species being transferred to the mixed plantation area. The number of species and their density decreased from natural forest, through the ecotone to the mixed-species plantation. Observations on seed germination indicated that 65% of woody seeds species had a germination rate of less than 50% under nursery conditions. These factors, of low woody-seed input and low germination rate for some dominant tree species, may retard the recovery process in mixed tree species plantations. The high survival rate of pre-climax enrichment seedlings has proved the success of this enrichment-technique restoration strategy. Teak (Tectona grandis) and Gmelina (Gmelina arborea) showed synergistic interactions and encouraged higher productivity accumulation in enrichment seedlings than in single and three-species plantations. Results suggest that the mixed-species plantation of Teak and Gmelina is a good foster combination for restoration by planting techniques. In conclusion, mixed-species plantation has more effective to accelerating natural succession better than single-species plantation by enhancing species diversity in their understorey. Due to low primary native tree seedling recruitment and seed sources in mixed plantations, enrichment planting is a useful technique of the restoration strategy. In addition this research clearly indicates that enrichment planting of preclimax species has the capability to increase restoration mechanisms by accelerating successional processes.Moreover, it can be suggested that mixed-species plantations, especially Teak and Gmelina, can be managed sustainably and can be useful for restoring degraded land.