ABSTRACT: Up to about 15 years ago a deciduous forest in the Nam Wa sub-watershed was increasingly encroached for agricultural purposes. Then, about 15 years ago, the local people and the local authority in Lai Nan District cooperated to conserve this forest area through natural succession. This study was carried on the quantitative floristic characteristics, soil organic matter and soil nutrients with the aim of identifying the impact of the disturbance. The trees’ database in the area and land use history were used to classify the area into five degrees of disturbance as follows; i) very high (orchard); ii) high (abandoned crop field with 15 years of natural succession); iii) medium (most trees were logged 15 years or more ago and then left to succession), iv) low (some trees were logged 15 years or ago and then left to succession); and iv) very low (small number of trees were logged). The results indicate that the density and basal area is the lowest in the very high disturbance area and the highest in the low disturbance area. Indeed the measured characteristics of the soil differ significantly among the disturbance areas (ANOVA: P<0.05), and therefore, that the quantitative characteristics of this disturbed forest have not completely recovered after 15 years of natural succession. The soil chemistry at two soil depths: 0 - 20 cm and 20 - 40 cm does not vary for most soil chemical properties measured. The pH, organic matter, total nitrogen, available phosphorus and exchangeable potassium show no statistically significant difference between each of the sites with different degrees of disturbance nor between the two soil depths at each site. It is therefore possible that soil organic matter and some soil nutrients are recovered after 15 years of forest succession.
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