In Lao PDR, which is located at the northern edge of the tropics, several distinct forest types, including both deciduous and evergreen forests, occur in close proximity with one another. Evergreen coniferous forest dominated by Pinaceae often occur at higher elevations. However, at lower elevations other deciduous and evergreen forest types occur in very close proximity. We sampled two forest types at Phou Khao Khoauy National Park, 40 km NE of Vientiane in central Laos, to investigate the factors determining tree species distribution. Eleven 0.25 (50 m x 50 m) were set up between 300-450 m elevation and all trees ≥10 cm were enumerated. Plot locations were randomly assigned after stratifying by forest type (mixed deciduous forest, dominated by Fagaceae, and dry evergreen diperocarp forest) and 47 families, 70 genera and 123 species were identified. Soil samples were collected from two locations per plot, each sample was collected at two depths, between 0-10 cm and 10-20 cm depth and analyzed for basic soil parameters (pH, organic matter (OM), nitrogen (N) and phosphorus (P), potassium oxide (K2O), particle size and texture). We ran cluster analysis based on similarity using abundance data for plant community group identification. Next, we ran non-metric multidimensional scaling (NMDS) and a post-hoc non-parametric tests to examine the influence of soil parameters on plant distribution. Cluster analysis identified two significantly (P<0.05) different groups of plots. . Phosphorus (p = 0.021), K20 (p = 0.029), sand (0.007) and clay (0.008) were significantly associated with variation in species composition among plots. Our results confirmed that in Laos, which located on the northern edge of the tropics, substantial turnover in tree species composition sometimes occurs over short distances, although tree species may be shared among forest types. Moreover, turnover in species composition appears to be associated with soil conditions.
