Estimation of the Outcrossing Rate on Dryobalanops aromatica Gaertn. f. in Primary and Secondary Forests in Brunei, Borneo, Southeast Asia

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Dryobalanops aromatica is an emergent canopy tree species with hermaphroditic bee-pollinated flowers. This species is distributed in the lowland dipterocarp forests of Malaya, Sumatra, Rhio Archipelago and Borneo. The flowering of many different dipterocarp species was observed in some parts of Brunei in 1991, and all over Brunei in 1992. The outcrossing rates of D. aromatica were estimated in 1991 and 1992 in the secondary forest, and in 1992 in the primary forest. Selective harvesting of D. aromatica has occurred in the secondary forest about 20 years ago. Four enzyme systems with a total of six isozyme loci were used to estimate the single locus and the multilocus outcrossing rates. The multilocus outcrossing estimates ranged from 0.794 to 0.856, with a mean of 0.816. These estimates are similar to those for other tropical tree species. Although the flowering density of D. aromatica in the secondary forest was lower than in the primary forest, the multilocus outcrossing estimates were not significantly different. The variation between years in the secondary forest was also not significant. These findings may be largely the result of the high flowering tree density of this species, even in the secondary forest. In this study, the outcrossing rates of D. aromatica were maintained by high flowering density regardless of the size structure or the topography of the habitat.
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