Kriangsak Sri-ngernyuang

Habitat differentiation of Lauraceae species in a tropical lower montane forest in northern Thailand

Dependency on topographical habitat was examined for Lauraceae tree species in a lower montane forest using a large-scale research plot established at Doi Inthanon National Park, northern Thailand. Twenty species of 10 genera of Lauraceae were recorded in a 7.5-ha part of the plot; Lauraceae accounted for 18% of the total basal area. Lauraceae was the most species-rich and most abundant family in the plot. In a cluster analysis based on the matrix of spatial associations between species, two clusters were recognized. Members of one cluster seemed to associate with lower-elevation habitats, and members of the other associated with habitats on ridges. By subdividing the study plot into 20 m × 20 m squares, a discriminant analysis could be applied to the presence–absence data for the 17 species that had sufficient population density. The predictor variables used were the relative elevation, slope inclination, slope direction (transformed to deviation from SSW) and slope convexity for each of the squares. The discriminant models were tested statistically by applying the random shift technique. The models were significant for 11 of the species (65% of the species examined) and were associated with the topographical condition of the habitat. Stepwise selection of the predictor variables for these 11 species revealed that relative elevation and slope convexity were the most important factors for predicting the presence or absence of the Lauraceae species. Both these variables were considered to indicate the hydrological condition of the habitat.

Habitat divergence in sympatric Fagaceae tree species of a tropical montane forest in northern Thailand

Spatial distributions of many tropical trees are skewed to specific habitats, i.e. habitat specialization. However, habitats of specialist species must be divergent, i.e. habitat divergence, to coexist in a local community. When a pair of species specialize in the same habitat, i.e. habitat convergence, they could not coexist by way of habitat specialization. Thus, analyses of habitat divergence, in addition to habitat specialization, are necessary to discuss coexistence mechanisms ofsympatric species. In this study, the habitat specialization and habitat divergence along topographic gradients of eight sympatric tree species of the Fagaceae were studied in a 15-ha study plot in a tropical lower montane forest in northern Thailand. A statistical test with torus shift randomizations for 9 6 73 trees of Fagaceae revealed significantly biased distributions for all of the species, for at least one of the four topographic variables used: elevation, slope inclination, aspect and convexity. Slope convexity was the most critical topographic variable, along which all but one species had significantly skewed distributions. Out of 112 possible combinations of species pairs and topographic variables, 18 (16%) and two pairs (1.8%) showed significant habitat divergence and habitat convergence, respectively. The observed habitat divergence alone could not completely explain the coexistence of the eight species. There was a gradation in the habitat position of each species, with relatively large overlaps between species distributed in similar habitats, and small overlaps between species associated with contrasting habitats, respectively. The gradual changes in the habitats of the species suggested that dividing the species into a small number of distinct habitat groups, such as ridge and valley specialists, would not be straightforward.

Intra- and interspecific variation in wood density and fine-scale spatial distribution of stand-level wood density in a northern Thai tropical montane forest.

Tropicaltreewooddensityisoftenrelatedtootherspecies-specificfunctionaltraits,e.g.size,growthrateand mortality. We would therefore expect significant associations within tropical forests between the spatial distributions of stand-level wood density and micro-environments when interspecific variation in wood density is larger than intraspecific variation and when habitat-based species assembly is important in the forest. In this study, we used wood cores collected from 515 trees of 72 species in a 15-ha plot in northern Thailand to analyse intra- and interspecific variationinwooddensityandthespatialassociationofstand-levelwooddensity.Intraspecificvariationwaslowerthan interspecific variation (20% vs. 80% of the total variation), indicating that species-specific differences in wood density, rather than phenotypic plasticity, are the major source of variation in wood density at the study site. Wood density of individual species was significantly negatively related to maximum diameter, growth rate of sapling diameter and mortalityofsaplings.Stand-levelmeanwooddensitywassignificantlynegativelyrelatedtoelevation,slopeconvexity, saplinggrowthrateandsaplingmortality,andpositivelyrelatedtoslopeinclination.East-facingslopeshadsignificantly lower stand-level mean wood densities than west-facing slopes. We hypothesized that ridges and east-facing slopes in the study forest experience strong and frequent wind disturbance, and that this severe impact may lead to faster stand turnover, creating conditions that favour fast-growing species with low wood density.

Survival and germination of an experimental seed bank population of two species of Lauraceae in a tropical montane forest in Thailand

The survival and germination traits of two tropical species of Lauraceae, Lindera metcalfiana and Litsea cubeba, were investigated in a 2-year burial experiment and subsequent germination tests of sequentially excavated seeds. Fresh seeds extracted from fruit were packed in aluminum mesh bags and buried 5 cm deep in a mature montane forest in northern Thailand (1700 m altitude). No seed mortality was recorded for Litsea, but Lindera experienced 26%–48% mortality after being buried for 122 days. These seeds did not germinate in the forest soil during the 2-year experimental period. We found that fresh seeds of these two species did not germinate even under open conditions, suggesting that the seeds are initially deeply dormant after dispersal. In contrast, 29%–60% of excavated Lindera seeds germinated under open conditions after being buried for 30–753 days, and excavated Litsea seeds began to germinate after being buried for 408 days. These findings suggest that seeds enter a state of enforced dormancy. The ratio of germinable seeds peaked 184 days after burial in Lindera (33%) and 930 days after burial in Litsea (91%). The longer period of deep dormancy in Litsea prior to enforced dormancy is concordant with the fact that for regeneration Litsea requires larger-scale disturbance than Lindera.