3. Ontogenetic phenology
Apart from the progression with size and/or age to reproductive competence, the process of maturation can entail radical differences between juvenile and adult habits, in a phenomenon called heteroblasty. This is common and often very pronounced in the New Zealand flora (Greenwood and Atkinson 1977). Elsewhere, less extreme differences between juvenile and adult habits are generally the norm for woody perennials. Even vegetatively, maturation can bring some changes in seasonal phenology, very often in a progressively stronger expression of the seasonal phenology (e.g. Wareing 1958; Norskov-Lauritsen 1963; Burdon 1994).
3.1 New Zealand flora
The heteroblasty in various New Zealand trees and shrubs has been postulated as a defence against vertebrate herbivory (Greenwood and Atkinson 1977). Specifically, various such plant species have a juvenile habit with a tangle of thin, wiry stems bearing very small leaves. This habit is seen as being unrewarding for the large, flightless birds that were the ground-dwelling browsers, or else resilient to browsing damage. Despite controversy, and difficulty of proof because those browsers are extinct, definite support for the postulate has been obtained. In a neighbouring island flora, with closely related taxa but historically lacking those birds, there is not the same heteroblasty (Greenwood 1992; Burns and Dawson 2009). Other, if less striking cases of heteroblasty conferring defences or resilience against ground-dwelling herbivores include the production of sharp spines on foliage or stems during a juvenile phase. Seedlings of the apple tree are a well-known example of spines being confined to the juvenile phase.