Light Conditions and the Evolution of Heteroblasty (and the Divaricate Form) in New Zealand

New Zealand Journal of Ecology (1998) 22(1): 43-54 ©New Zealand Ecological Society juvenile with a divaricate form (Philipson, 1964). This has prompted a search for some feature(s) peculiar to the New Zealand situation that may have influenced evolution. Here it is proposed that New Zealand’s forests provided light conditions where heteroblastic development is favoured. The current theories for the evolution of heteroblasty and the divaricate habit are outlined in Table 1. Other than some of the climate and microclimate hypotheses, the theories make a priori assumptions of adaptive strategy: the suggested mechanism for evolution relies purely on speculation about the interaction between selection pressure and phenotype. These theories are difficult to test because the conditions proposed to give rise to evolutionary change can not be repeated. An alternative approach combines the concepts of ontogenetic and phylogenetic contingency (Watson, Geber and Jones, 1995). Plant phenotype depends on the interaction between genotype and environment during ontogeny (environment is envisaged in its broadest sense including climatic and edaphic conditions as well as influence from other biota: competition, herbivory, parasitism, etc.). Plant genotype or more precisely constraints on development depends on the interaction between Introduction