Light conditions and the evolution of heteroblasty (and the divaricate form) in New Zealand

Heteroblasty, changes in vegetative phenotype during ontogeny, is unusually common in the New Zealand flora. Some feature(s) unique to the New Zealand situation must have influenced the evolution of this strategy. Similarities were examined between the ontogenetic changes in phenotype and growth strategy in Elaeocarpus hookerianus, Carpodetus serratus and Pseudopanax crassifolius. Variation in hypothesised light capture efficiency of juvenile and adult forms can be related to changes in the light environment that these growth forms experience. Heteroblasty is hypothesised to have evolved as a result of the change from a low light intensity environment below the canopy of high altitude conifer/broad-leaved forests, to a high light intensity environment above the canopy. The differences in architecture between juvenile E. hookerianus and C. serratus on the one hand, and P. crassifolius on the other hand, are likely to be related to their adaptation to heterogeneous moderate light intensity and homogeneous low light intensity, respectively. The divaricate form characteristic of many New Zealand shrubs may have arisen following the development of heteroblastic trees with a divaricate juvenile, and the subsequent loss of the adult state. This paper provides hypotheses which future research can scientifically test.