Seed dispersal and seedling recruitment in the light-demanding tree Choerospondias axillaris in old-growth forest in Thailand

Abstract

As seeds are the only movable stage in the plant life-cycle, their dispersal determines the chances of recruitment and survival away from the parent trees and the distribution of the next generation. We studied the light- demanding tree, Choerospondias axillaris (Anacardiaceae), whose pyrenes (seeds) are dispersed mainly by deer and gibbons, to determine if seed dispersal is random with respect to parent trees and preferred germination and recruitment sites. Research was carried out during 2004 and 2005 on the Mo Singto 30-ha forest dynamics plot, Khao Yai National Park, in which all trees down to 1 cm in diameter are mapped and measured. The species has a relatively large number of adults but very few juveniles 1-10 cm in diameter and appears not to be replacing itself in this old-growth forest. Dispersal into gaps is essential for recruitment, but gaps make up less than 10% of the area. Dispersal by sambar (Cervus unicolor), barking deer (Muntiacus muntjak), and gibbons (Hylobates lar) is almost random over the plot and not directed to gaps. In the 2 years of the study, less than 1% of seeds found in sub-quadrat samples were in gaps, but in the following year, approximately half of 1-year old seedlings were found in gaps. While 4.5% of the 750 sub-quadrats contained seedlings, the percentage of sub-quadrats in gaps containing seedlings was 11% and 23% in the 2 years of census. Only seedlings in gaps survived to the second year. In this population, recruitment is limited by a shortage of gaps in which seedlings can grow. Both the size distribution of adults and the dependence of seedling recruitment on light in gaps suggest that this tree population may depend on episodic forest disturbances large enough to create much larger gaps in order to maintain itself. If so, this species cannot maintain itself in mature forest and may be in perpetual disequilibrium.