Reproductive Phenology of the Oak Family (Fagaceae) in the Lowland Rain Forests of Borneo

Abstract

Seventeen individual trees of 12 species of Castanopsis, Lithocarpus, and Quercus were studied at 2-4 week intervals for most of a year, and reproductive phenology of numerous other individuals and species was recorded over a 3-year period. Inflorescence primordia were macroscopically visible near the end of the wetter season and anthesis of all flushes occurred in the drier season. There was general synchrony of reproduction among reproducing individuals, but a few trees were sterile throughout the observation period. Some had multiple reproductive flushes at 3-5 week intervals. Anthesis in the drier season by Castanopsis and Lithocarpus, which are entomophilous, maximizes pollination because that is the season of insect population peaks. Quercus is anemophilous and therefore is unaffected by insect population cycles. THE RECENT INCREASE IN KNOWLEDGE OF PHENOLOGY of tropical plants, especially those of the seasonal neotropics (Opler et al. 1976, Bawa 1983, Borchert 1983), is the most significant information to accumulate on tropical phenology. In Borneo and other paleotropical areas that are less seasonal, the phenologies are less understood, although definite seasonal rhythms are known for some tree species (Holttum 1940, 1968; Koriba 1958). According to Whitmore (1975), flowering and fruiting of most lowland Far Eastern tropical trees is periodic but regular and often annual; the Dipterocarpaceae, which are dominant over large areas, are noted for their infrequent but gregarious flowering. In his detailed taxonomic treatment of the Fagaceae of Malaysia, Soepadmo (1972) gave flowering and fruiting months for most species, but no dear patterns of regularity or synchrony emerge from that information. However, such patterns might appear locally. Observers in north-temperate dimates are familiar with the annual synchronous flush of leaves and flowers of Quercus (oak) and Fagus (beech) in the spring and Castanea (chestnut) in the summer; sometimes the fertile flush is followed by one or more sterile flushes. Phenology of the paleotropical Fagaceae is more complex. STUDY AREA AND METHODS In Borneo, the Fagaceae occur from sea level to about 3000 m and the largest number of species is found in the lower montane forests from 1200 to 1800 m elevation. The fagaceous trees are often gregarious in the mountains but are mostly scattered in the lowland forests. The three most extensive forest types in the Sarawak lowlands (our study area), are edaphically determined: mixed dipterocarp forest on silty day soils; semi-xerophilous heath forest on relatively infertile, sandy soils; and peat-swamp forests (Ashton 1965, Richards 1965, Briinig 1973). They contain 14, 21, and 5 fagaceous species, respectively (Briinig 1967). The mixed dipterocarp forest is the tallest, has the most tree species, and has irregular surface physiognomy; the Fagaceae occur as trees of the middle storey. The heath forest and peat-swamp forests are lower, less rich in species, and their canopies are more even (Anderson 1964; Briunig 1970, 1973). In them, the Fagaceae occupy mostly the middle and upper storeys. There is little seasonal temperature variation in the Kuching area of Sarawak (1?29'N, 1 10?10'E), the site of our observations. Daily maxima and minima average 310 and 22?C (Fogden 1972), and the annual variation in daily mean temperature is only about 2?C (Briinig 1969, 1971). The relative humidity seldom drops below 60 percent and is usually close to 100 percent in the forests (Fogden 1972). Day length varies by only 10 minutes throughout the year. Annual rainfall at Kuching is about 4000 mm, and all months have rain (Walter and Lieth 1967, Briinig 1969, Fogden 1972). The area has Sarawak's most variable rainfall and thirty-day periods of relative drought (with less than 100 mm) can occur from March until September (Briunig 1969, 1971). The heaviest rainfall is brought by the northeast monsoon from December through February (Fig. 1A), when all-day rains can occur; at other seasons rain is mostly from convectional afternoon I Received 27 March, revision accepted 26 September 1984. BIOTROPICA 18(1): 51-55 1986 51 This content downloaded from 207.46.13.120 on Wed, 14 Sep 2016 05:47:06 UTC All use subject to http://about.jstor.org/terms