Selective Forces Influencing the Evolution of Divaricating Plants

Diels (1897) first put forward a climatic hypothesis for the origin of divaricating plants. This hypothesis has been elaborated by subsequent authors and is further extended herein. In contrast, Greenwood and Atkinson (1977) proposed that the divaricating shrub form evolved as a protection against moa-browsing. These two hypotheses are critically compared and it is concluded that the climatic hypothesis better explains the ecology, distribution and morphology of divaricating plants. Divaricating plants are adapted to existing non-forest and forest margin habitats and it is suggested that they arose during the harsh, near-treeless glacial periods of the Pleistocene. The main function of the divaricating form is to protect growing points and leaves from wind abrasion, desiccation and frost damage. Also, a favourable microclimate is maintained inside the shrub and this may permit higher rates of photosynthesis during periods of adverse weather. Distribution and ecology of divaricating plants are related to these factors. We consider divaricating plants evolved in New Zealand as one of the responses of a sub-tropical flora, isolated from sources of artic-, alpine-, and desert-adapted plants, to the onset of harsh glacial climates.