Buried Seeds, Newly Dispersed Seeds, and the Dynamics of a Lowland Forest in Malaysia

Abstract

The ways by which light gaps in the canopy are formed and the importance of buried seeds in treefall gap regeneration were studied in lowland forest in Pasoh Forest Reserve, Peninsular Malaysia. In the top 10 cm of soil under the mature forest canopy were 131 seeds/M2, as determined by germinating seeds from the soil. This estimate of the buried seed population in Pasoh Forest Reserve is similar to estimates from several other Paleotropical forests but is lower than most buried seed population estimates reported for Neotropical forests. The density of buried seeds was significantly lower in 2to 5-year-old treefall gaps than under the canopy of mature forest. Flats of sterilized soil and flats of soil containing a natural complement of buried seeds were set out in recent treefall gaps to compare the rates of seedling emergence from buried seeds and from seeds that arrived after the gaps were formed. Although less abundant than in many other tropical forests, buried seeds give rise to seven times more seedlings during the first nine months than freshly dispersed seeds. Pioneer trees contributed a smaller fraction of the buried seeds (29%) than observed in other lowland tropical rain forests. We believe this results from the relative scarcity of pioneer trees in Pasoh Forest Reserve and the relatively large size of their seeds compared, for example, with seeds of pioneer trees in the mature forest on Barro Colorado Island, Panama. The number of pioneer trees is relatively low at least in part because most canopy trees in Pasoh Forest Reserve have small crowns and because many of the trees that die remain standing (45%); trees with small crowns and trees that die standing open small canopy gaps when they fall. TREE DEATH AND REPLACEMENT patterns in tropical forests have recently attracted much attention (e.g., Bazzaz 1984, Brokaw 1985). Knowledge of how canopy trees are replaced in natural forest in Malaysia may help to further refine already sophisticated silvicultural systems based on selective logging and natural regeneration (e.g., WyattSmith 1963; Tang 1974, 1980). Our lack of a detailed understanding of tree-by-tree replacement patterns in Malaysian forests prompted us to ask the following questions: (1) How abundant are viable seeds buried in the soil under the mature forest canopy? (2) What species and growth forms are represented in the buried seed pool? (3) What do seeds present in the soil prior to gap formation and seeds arriving subsequently contribute to the vegetation of recent treefall gaps? (4) How do the ways trees die influence forest composition? We cite comparative data from forests in other parts of the tropics to highlight some of the similarities and differences in forest processes in Malaysia. Although the seeds of many tree species in Malaysia have little dormancy, others take several months to germinate even when planted in the open and watered regularly (Ng 1980). Seeds that germinate rapidly when well watered and exposed to light and temperature conditions characteristic of recent treefall gaps may remain dormant for extended periods of time if buried in the shade of the forest canopy (e.g., Whitmore 1983, Vazquez-Yanes & Orozco Segovia 1984). Many light-demanding, short-lived (pioneer) tree species, in particular, appear to characteristically employ this buried seed strategy. Seeds of these species remain dormant in the soil until some disturbance to the covering vegetation leads to changes in light, temperature, soil oxygen, and other factors that favor their germination and growth (Vazquez-Yanes & Orozco Segovia 1984). To gauge the potential importance of this life history strategy, we determined the population density and species composition of buried seeds under mature forest in Pasoh Forest Reserve. If many seeds are dispersed into treefall gaps, the contribution to regeneration by seeds present in the soil prior to gap formation may be small relative to that of newly arriving seeds. Certainly dispersal closely coincident with gap formation is a prerequisite for the establishment of species that lack both seed dormancy and the ability to persist in the understory as suppressed seedlings. Although some viable seeds are buried in lowland forest soil in Malaysia (Symington 1933, Liew 1973, Ashton 1982), there are reasons to expect that a major source of many treefall gap colonists is newly dispersed seeds. For example, high seed input after gap formation was suggested by Whitmore (1975) to account for the single-species stands I Received 11 September 1985, revision accepted 9 April 1986. 326 BIOTROPICA 19(4): 326-333 1987 This content downloaded from 207.46.13.176 on Mon, 20 Jun 2016 06:17:01 UTC All use subject to http://about.jstor.org/terms of pioneer trees and differences in species composition among gaps formed at different times of year. To compare the importance of buried seeds and newly dispersed seeds in the colonization of recent treefall gaps, we compared seedling emergence from seed-free forest soil and soil with a natural complement of buried seeds. The conditions in this experiment approximated those of disturbed soil around the bases of uprooted trees, the only sites in even recently formed treefall gaps where there is necessarily both complete exposure to overhead sun and reduced root competition. Elsewhere in the tropics, pioneer trees are concentrated in such areas (Putz 1983). In many tropical forests the seeds of pioneer trees dominate the population of buried seeds (e.g., Whitmore 1983, Vazquez-Yanes & Orozco Segovia 1984). Production of seeds by pioneer trees and the longevity of these seeds in the soil depend on numerous biotic and abiotic factors. We discuss both the effect of seed size on the number of seeds buried in the soil and the effect of treefall gap size on the density of seed-producing pioneer trees. We suggest that treefall gaps in Pasoh Forest Reserve are generally smaller and pioneer trees consequently rarer than, for example, on Barro Colorado Island in Panama because the gaps in Pasoh are often caused by canopy trees with small crowns that open small gaps when they fall and by trees that die standing and disturb little of the canopy as they fall apart piecemeal and eventually topple over.