Establishment of exotic plantations is one of the most effective ways to induce natural regeneration for the restoration of degraded lands, as it has the potential to improve soil properties and creates favorable microclimates. This study aims to determine the effects of stand structure and composition as well as environmental factors under the canopies of three exotic plantations in northeastern Thailand on the regeneration of native species. For each plantation, we conducted three 10 m × 150 m transect lines with fifteen 10 m × 10 m subplots along a forest remnant gradient. The canonical correspondence analysis (CCA) was used to identify the environmental factors responsible for the differences in natural regeneration among these stands. Three exotic plantations had different stand structure in terms of number of tree species, basal area, and tree density but similar dominant native tree species. Across all stands, 74 native tree species, 60 genera, and 30 families were observed. Some physical and chemical properties in the topsoil were significantly different between species but similar among stands within a species. On the other hand, differences in environmental factors such as RLI were significant among species and stands within a species. The CCA ordination identified that the soil particles, soil pH, and light intensity were key factors influencing the native species composition, which could be categorized into three groups: drought-tolerant pioneer species; light-demanding pioneer species; and shade-tolerant climax species. However, most of the climax species were incapable of regenerating and maturing along the forest edge gradient to plantation stands. To accelerate the restoration process by converting these old exotic plantations to a dry evergreen forest, further research is required to determine the appropriate canopy management and/or dominant climax species for planting beneath their canopies.
In order to validate the important role of Acacia mangium plantation in mining rehabilitation, a study was conducted on the soil properties and tree composition in a 27-year-old A. mangium plantation growing on sandy (S27) and clay (C27) soil types, as well as a mixed plantation (MP) growing on clay soil type. The results were compared with those grown on an abandoned mining area (AB), a secondary forest (SF) and a primary forest (PF), at the Phangnga Forestry Research Station, Thailand. Three 40 x 40 m permanent plots were established and soil samples were randomly collected from depths of 0-10, 10-20, 20-30, and 30-50 cm, in each of the S27, C27, MP, AB, SF, and PF. The bulk density values in the S27, C27, and MP were lower than that in AB and was similar to those in SF and PF, particularly, the top soil. Total nitrogen, available phosphorus, organic matter, exchangeable potassium, and magnesium contents in the S27, C27, and MP were higher than that in AB, but were lower than those in SF and PF, indicating that the soil development in the S27, C27, and MP was slower than in the SF and PF. This lower bulk density values and higher soil nutrient contents were positively contributed by the A. mangium plantation. The Shannon-Wiener index obtained for S27 (1.43), C27 (2.51), and MP (2.77) were lower than that for the SF (3.86). The similarity indices of the tree species found in S27, C27, MP, and PF were low, ranging from 5.83-8.00, indicating that the development of the forest community was slow compared to SF (31.03). Enrichment planting with poorly dispersed shade tolerant trees has increased the diversity and improved the forest structure in the mined out areas and other similarly degraded lands.
Dominant Dipterocarp trees hold a significant importance in the ecology and economics of the declining tropical forests of Asia. Of equal importance is the restoration of these Dipterocarp forest using the technique of matching species with silvicultural practices. The study aimed to investigate the effect of the Acacia mangium nurse trees on the survival and growth of six Dipterocarp species planted on abandoned mining areas in the Phangnga Forestry Research Station, Thailand, namely; Dipterocarpus alatus Roxb., Dipterocarpus gracilis Blume, Hopea odorata Roxb., Shorea gratissima (Wall, ex Kurz) Dyer, Shorea roxburghii G. Don, and Parashorea stellata Kurz. The approximately 1.5-year-old Dipterocarp seedlings were planted at a 6-year-old A. mangium plantation cover and at open plots. The survival rate, diameter at root collar (Do), total height (H) and the relative growth rate (RGR) of Do and FI of the seedlings were compared. Soil samples at the open and the A. mangium plots were collected and its physical and chemical properties were analyzed. The results indicated that the survival rates of the Dipterocarps planted at the A. mangium plot were higher than those at the open plots. The survival rates (75-100%) of the 1 to 3-year-old D. alatus, H. odorata, S. gratissima, S. roxburghii, and P. stellata were relatively high at the A. mangium plot, but not for D. gracilis. The Do of S. roxburghii, D. alatus, H. odorata, and P. stellata were higher at the A. mangium plot. However, the Do and H of S. roxburghii, D. alatus, and H. odorata at the open plot were high and similar to that of the A. mangium plot. RGRDO and RGRH of S. roxburghii, D. alatus, and H. odorata were high both at the A. mangium and the open plots. Generally, the Dipterocarp seedlings were growing better under the A. mangium nurse trees, thus suggesting the possibility of their improved survival and growth in mined out areas. Moreover, the findings also suggest that S. roxburghii, D. alatus, and H. odorata can be planted at open areas with poor soil conditions and extreme environments. Lastly, thinning interventions are also needed to improve the growth of Dipterocarp trees.
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