Effects of seedling size, El Nino drought, seedling density, and distance to nearest conspecific adult on 6-year survival of Ocotea whitei seedlings in Panama

We present an analysis of the long-term survival of two cohorts of seedling s of the tropical canopy tree Ocotea whitei (Lauraceae) on a 1-ha plot of m ature, lowland moist forest on Barro Colorado Island, Panama. In 1980, we c ounted an even-aged cohort of seedlings that germinated in 1979, then measu red and tagged survivors in 1981. We also measured and tagged a second, sma ller cohort of seedlings that germinated in 1981. We followed the subsequen t survival of all seedlings through 1985, Seedling mortality was phenotypic ally, temporally, and spatially non-random. Important correlates of nonrand om mortality included: (l)seedling size and age, (2) an El Nino drought, an d (3) biotic neighborhood. Larger and older seedlings survived better than smaller and younger seedlings, respectively, and the El Nino-related drough t of 1982-1983 was associated with elevated mortality rates. Seedling densi ty, which was strongly correlated with the proximity to the nearest conspec ific adult, increased mortality. The observed mortality patterns suggest th at processes consistent with the Janzen-Connell hypothesis operate during t he recruitment phase of O. whitei population dynamics. However, the process es causing the observed density- and distance-dependent mortality may vary with factors such as total seed number, seedling size, and climatic variati on, making it difficult to determine whether time-integrated seedling-to-ad ult spacing mechanisms other than self-thinning operate on a given plant po pulation. After 6 years in the hectare studied, survivors remained densest and most numerous underneath the adult trees. We conclude that only long-te rm demographic data, collected at a variety of scales on a variety of speci es, will ultimately answer the question: do Janzen-Connell effects contribu te substantially to structuring tropical forests?.