Date of Award

10-3-1991

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

Abstract

Patterns of seedling demography were studied in undisturbed wet forest at La Selva Biological Station, Costa Rica. In 1983, permanent transects were established at random locations, in which all seedlings $\le$0.5 m in height belonging to all species capable of reaching 10 cm dbh were tagged, identified, mapped, and measured in height. During the following 18 months, a total of 6,403 seedlings belonging to 194 species were tagged. All tagged seedlings were recensused at regular intervals.All but 302 tagged seedlings had died by June 1989, the total mortality rate was 95.3% for a period of 6 years. The mean one-year survivorship rate of all species of seedlings studied was 14%. The best survivorship was seen in Plot 3, and the lowest in Plot 2 swamp. In all areas, the highest survivorship occurred in cohorts that germinated during wet periods. Among the 75 most abundant species, half-life ranged from approximately 40 months to less than 0.4 months. In general, species survivorship rates were inversely related to seedling abundance. Half-life was found to be a good predictor of future cohort survivorship.Diversity was highest among seedlings germinating in Plot 3 (H$\sp\prime$ = 3.88), and was lowest in those from Plot 2 swamp (H$\sp\prime$ = 2.58). Change in diversity over time in 75% of 32 cohorts are consistent with random loss with respect to species. In the remaining cohorts, diversity was significantly higher or lower than expected by chance.Species composition patterns were analyzed using Detrended Correspondence Analysis. The ordination demonstrated a floristic trend among seedlings in relation to transect elevation, a pattern previously shown among adult taxa in the study area. Adult and seedling distributional patterns are weakly correlated on the species level. Species composition of newly germinated seedlings shows a weak seasonal cycle. The species composition of dead cohorts varies in relation to season and site elevation.

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