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.