The Unusual Life History of Alseis Blackiana: a Shade-persistent Pioneer Tree?

Classifications of tree species into regeneration guilds or functional groups are often based on a limited number of life history characteristics indicative of requirements for early establishment, with the assumption that these traits are correlated with growth requirements of larger individuals. We tested the validity of this assumption for Alseis blackiana, a common canopy tree in central Panama, by examining its seed germination, seedling growth, and sapling distribution, growth, and mortality. We found that the early life history of Alseis is characteristic of pioneer species: the seed germination rate was much higher in forest gaps than in the forest understory, small seedlings were absent from the understory, and seedlings showed strong plastic responses to variation in light. Seedling relative growth rate, leaf area production rate, and maximum carbon assimilation rates of Alseis are much more similar to those of the pioneer tree Miconia argentea than to those of the non-pioneer tree Tetragastris panamensis. However, in contrast to Miconia, established seedlings of Alseis are capable of remarkable persistence in the understory; in a 1ha plot on Barro Colorado Island, the mortality rate for 722 saplings ,1 cm dbh was ,4% per year between 1986 and 1998. We suspect that shade tolerance in Alseis is achieved by both physiological and morphological adaptations, e.g., efficient light interception by thin, nonoverlapping leaves, tolerance to litterfall damage, and a high resprouting capacity. These results show that existing functional group classifications do not account for the full range of seedling trait combinations found in tropical trees and indicate that a gap requirement for seedling establishment need not limit the abundance of adult individuals in a stand.