Spatial variation in solar irradiance and the recruitment of rainforest trees at Khao Yai National Park, Thailand

Studies of tree distributions at the Mo Singto Forest Dynamics Plot (Khao Yai National Park, Thailand) suggest that the recruitment of some species may be limited by very recent climate change. While large individuals (≥ 10 cm DBH) of the wild rambutan Nephelium melliferum Gagnep. (Sapindaceae) are distributed over the entire 30 ha plot, small individuals (< 10 cm DBH) seem restricted to certain areas. N. melliferum seeds appear to be dispersed widely across the site (by white-handed gibbons, Hylobates lar), so we are investigating whether sites lacking saplings have become unsuitable for germination or seedling survival. These sites could have aspects and slope angles that orient them toward the sky, for example, in which case they would receive greater insolation and might be the first to become too hot and dry from climate change. We used models of topographic variation at Mo Singto and the sun’s daily trajectory across the sky to compute spatial variation in aspect, slope, and cumulative annual insolation across the plot. Sites occupied by adult and sapling N. melliferum do not differ significantly in aspect, but saplings occur on 11% steeper slopes (so, less-oriented to the sky) with 2% lower annual insolation than adults. The biological significance of receiving 2% less solar radiation each year is unclear, so we will follow with field measurements to determine the differences in ground temperature and soil moisture between sites occupied by saplings and adults. We will also conduct experiments to evaluate if germination success and seedling survival are lower on slopes occupied by adults, to determine if these higher-insolation sites have become too hot and dry for recruitment in this species.