How many seeds does it take to make a sapling?

Abstract

Tall canopy trees produce many more seeds than do understory treelets, yet, on average, both classes of trees achieve the same lifetime fitness. Using concurrent data on seedfall (8 years) and sapling recruitment (12 years) from a long-established tree plot at the Cocha Cashu Biological Station in Peru, we show that a 40-m canopy tree must produce roughly 13 times the mass of seeds to generate a sapling as a 5-m understory treelet. Mature tree height accounted for 41% of the variance in seed mass per sapling recruit in a simple univariate regression, whereas a multivariate model that included both intrinsic (seed mass, tree height, and dispersal mode) and extrinsic factors (sapling mortality as a surrogate for microsite quality) explained only 31% of the variance in number of seeds per sapling recruit. The multivariate model accounted for less variance because tall trees produce heavier seeds, on average, than treelets. We used "intact" (mostly dispersed) seeds to parameterize the response variable so as to reduce, if not eliminate, any contribution of conspecific crowding to the difference in reproductive efficiency between canopy trees and treelets. Accordingly, a test for negative density dependence failed to expose a relationship between density of reproductive trees in the population and reproductive efficiency (seed mass per recruit). We conclude that understory treelets, some of which produce only a dozen seeds a year, gain their per-seed advantage by failing to attract enemies à la Janzen-Connell, either in ecological or evolutionary time.