Light requirements of neotropical tree seedlings: a comparative study of growth and survival.

Eighteen species of wind-dispersed trees on Barro Colorado Island, Panama, are compared with respect to germination, survival, and growth of seedlings during 1 yr under sun and shade conditions in a screened enclosure. The species vary in their mean dry weight of seed from 1.9-686 mg. High and synchronous germination occurs in both sun and shade for 16 of the species. Most species have epigeal germination with leafy green cotyledons. Seedlings of 15 species survive better in sun than shade; none survive better in shade than sun. Shade tolerance varies widely and continuously among the species; it correlates with adult wood density, an indicator of growth rate and successional status, but not with the dry weight of seed reserves. The more shade-tolerant species have a lower proportion of seedlings dying from disease in the shade. Seedling height after 1 yr is greater in sun than in shade for all species. In the shade, number and length of leaves show little or no change with time; leaf turnover is negligible. In the sun, leaf number, increases and successively younger leaves mature at larger sizes. Leaf size is greater in the sun than in the shade. Senescence of the leafy cotyledons is more rapid in sun for all species. This study predicts that all species examined benefit from seed dispersal to light-gaps.-from Author