SEED SIZE, GERMINATION AND SEEDLING RELATIVE GROWTH RATES IN THREE TROPICAL TREE SPECIES

Abstract

The rates of germination and seedling relative growth rates (under laboratory conditions) were related to seed size within and between species for three light-demanding tree species from dry semideciduous and/or dry forest in Ghana (Afzelia africana, Milicia excelsa and Pericopsis elata). The three species illustrated three potential mechanisms for early colonisation and dominance of canopy gap sites: rapid germination (Pericopsis), fast relative growth rates (Milicia) or relatively large seed size (Afzelia). The high relative growth rates of Milicia seedlings resulted from their high specific leaf area and leaf mass ratio rather than rapid rates of net assimilation per unit leaf area, but the differences between the two other species were determined by differences in unit leaf rate. Within Afzelia, larger seeds germinated less quickly but seedlings from larger seeds had higher relative growth rates; within Pericopsis, larger seeds possessed a higher N concentration in the embryo-cum-endosperm fraction but did not germinate or grow faster than small seeds. We conclude that differential seed size, germination and seedling relative growth rates must all be considered among the mechanisms determining colonisation of regeneration sites by different species. Differences in trends of germination and seedling relative growth rates with seed size may stabilise seed size variation within a species.