Factors Affecting Size Distribution and Sapling Occurrence of Podocarpaceae at Khao Yai National Park, Thailand

Abstract

The objective of this study was to examine factors affecting the occurrence of saplings (height 0.15-1.35 m) of Podocarpaceae at Khao Khiew (Khiew Mountain) in Khao Yai National Park, Thailand. Four species of Podocarpaceae were studied: Podocarpus neriifolius, Nageia wallichiana, Dacrycarpus imbricatus, and Dacrydium elatum. The total area encompassed 20 hectares, with 70 systematically placed plots and 15 subjectively placed plots. We expected that sapling occurrence is influenced by a combination of factors, including soil nutrients, canopy cover, local slope, and disturbance. We therefore tested a series of candidate models for each species. Akaike’s Information Criterion (AIC_c) was used to compare the weight of evidence for each model. We found the highest support for models varied among species. The combination of soil available phosphorus (P) with soil depth had most support in explaining the occurrence of P. neriifolius saplings, while for N. wallichiana the depth of the soil showed the most support. D. imbricatus sapling occurrence was best supported by the pH model. A positive relationship was found between soil nitrogen (N), local slope, soil depth and sapling occurrence for all species surveyed. P. neriifolius and D. imbricatus sapling occurrence declined with increasing soil P, but increased for N. wallichiana. This research provides basic information useful for the conservation and management of Podocarpaceae, a little studied family in Southeast Asia.