Independent contrasts reveal climatic relationships of divaricate plants in New Zealand

Plant species with divaricate forms are particularly common in New Zealand, where approximately 10% of all endemic woody species can be categorised as divaricate. A number of potential selective pressures have been proposed in order to explain this unusual feature of New Zealand flora. It has been suggested that divaricate architecture is a defensive adaptation that limits damage caused by either moa browsing or by climatic stresses. Climate-based hypotheses have proposed that the growth form confers a selective advantage in variable environments that are windy, droughty or frost-prone. One specific mechanism proposed for a climatic explanation is that the divaricate form might shield inner foliage from photoinhibition on cold sites with high winter solar irradiance. By determining which climatic factors are most associated with the occurrence of divaricate plants, we aim to evaluate these hypothesised climate relationships. This study uses organism occurrence records and logistic regression to compare the climate relationships of 12 divaricate species with those of larger-leaved congeners. Of the seven climatic variables that were considered, July minimum temperature was the variable that most consistently differentiated between divaricate species and their broad-leaved congeners, the former growing on sites that were significantly colder on average during this month. In general, divaricate plants were associated with lower winter light intensity than their broad-leaved congeners; although moderation analysis showed that this relationship was reversed on the coldest sites, the small size of this effect gives little support to the hypothesis that the divaricate form is especially advantageous in environments conducive to photoinhibition. However, the strong association with cold winters adds to a body of evidence that the divaricate form is favoured in frosty environments.