Author(s): Alexandra Reich, N Michele Holbrook, John J Ewel
The tropical emergent tree Hyeronima alchorneoides has large decreases in leaf size with tree age: 1200 cm2 at 1 yr, 900 cm2 at 3 yr, 200 cm2 at 11 yr, and 80 cm2 in old (>30 yr) individuals. We tracked leaf growth and physiological attributes on trees of three different ages (1, 3, and 11 yr) to determine the developmental basis and functional consequences of this variation. Leaves on young trees grew faster and sustained maximum rates of leaf expansion longer than leaves on older trees. Leaf mass per area (LMA) did not differ among age classes. Maximum photosynthetic rates reflected differences in leaf nitrogen concentration, in which leaves from the lower crown of younger trees outperformed those at a comparable crown position in older trees. One-year-old trees had the lowest stomatal conductance and the greatest instantaneous water use efficiency. Ontogenetic plasticity in mature leaf size, structure, and physiology may be a balance between the advantages conferred by rapid height growth when trees are young and the benefits derived from producing branches that increase light harvesting ability as trees reach the canopy.