The Influence of Understory Vegetation Cover on Germination and Seedling Establishment in a Tropical Lowland Wet Forest

Abstract

Under closed canopy, vegetation cover varies as much as 20 percent at ground level at the lowland rain forest study site, Estacion Biologica La Selva, in Costa Rica. To determine the effect variation in understory cover has on seed germination and seedling establishment, we followed the fate of naturally germinating seeds in treatment plots in which the understory vegetation had been removed, and in neighboring control plots. The overstory canopy was left intact in all plots. Average cover values for treatment and control plots were 85.0 and 90.4 percent, respectively. After a 3.5-mo period, we found no differences due to the treatment in germination or survival of new seedlings, suggesting that variation in understory vegetation had little influence on establishment compared to that of the overstory canopy during this time period. STUDIES IN TROPICAL RAIN FOREST DYNAMICS have emphasized the extreme differences in light conditions between treefall gaps (light gaps) and the neighboring understory (Richards 1952; Poore 1968; Whitmore 1975, 1978; Hartshorn 1978; Bazzaz & Pickett 1980). Previous studies have shown that these extreme differences in light conditions between gap and understory have a direct effect on seed germination (Bell 1970, Guevera & GomezPompa 1972, Cheke et al. 1979, Vazquez-Yanes & Smith 1982), seedling establishment (Lebron 1979; Garwood 1982; Augspurger 1983, 1984), and sapling growth (Brokaw 1980). In the present study, we consider possible effects of intermediate differences in light availability at the forest floor. Present evidence (Denslow 1980), although limited, indicates that the majority of tropical tree species require an opening in the canopy (with a concomitant increase in light level) sometime in their early life in order to reach reproductive maturity. However, light quality is not homogeneous in either light gaps (see review in Denslow 1980) or the understory (Evans 1939, 1966; Whitmore & Wong 1959; Evans et al. 1960; Grubb & Whitmore 1967; Odum et al. 1970; Bjorkman & Ludlow 1972; Chazdon & Fetcher 1984a, b). Thus, a plant species' ability to establish may vary within each of these environments, depending upon the sensitivity of juvenile individuals to variation in light levels. In this study, we experimentally tested whether variation in understory vegetation cover in a tropical rain forest affects seed germination and seedling establishment. We followed seed germination and seedling establishment in treatment plots where incoming light levels were artificially increased by cutting understory plants, and in neighboring uncut plots. We hypothesized that germination and seedling survivorship would be higher in treatment low-cover plots than in control high-cover plots due to artificially produced differences in light levels.