Phenological study on a tropical secondary forest in West Sumatra, Indonesia

Abstract

We made litter fall obser vations in a West Sumatran tropical rain forest ever y 2 weeks through 3 years (including the drought year, 1997). Seasonal changes in long-term average rainfall were bimodal, but even in the two dr y seasons average monthly precipitation was > 200 mm. Leaf-fall rates in a year with normal rainfall peaked in the first dr y season; leaves made up one-third of mean annual litter fall. Larger trees had highly synchronized leaf fall during the peak shedding period. Smaller trees shed simultaneously during severe drought. Twigs < 0.5 cm diameter tended to fall in proportion to the 3/2 power of leaf-fall rates; therefore, we considered autonomous twig fall in relation to the pipe model theor y. Variances in reproductive organ shedding rates (seasonal and annual) were larger than those of leaves and branches, possibly because of the higher susceptibility of reproductive parts to drought. Reproductive fall rates in the year with nor mal weather peaked in May (flowers) and August (fruits). Seasonal patterns of flowering and f r u i t i n g w e r e c l a s s i f i e d i n t o t h r e e g r o u p s . Understor y species had an extended flowering pattern. We discuss phenological patterns found in this ver y high humidity climate and the impacts of increasingly frequent droughts.