Leaf temperatures of divaricate and broadleaved tree species during a frost in a North Island lowland forest remnant, New Zealand

Abstract

The abundance of small-leaved, interlaced divaricate plants in New Zealand remains one of the most intriguing features of the local flora and vegetation. This growth form was first interpreted as a response to harsh Plio-Pleistocene climates, consistent with the contemporary prominence of divaricate plants in frosty and droughty environments; specific mechanisms, however, have proved elusive. The predicted effect of leaf size on leaf energy balance is potentially relevant to climatic explanations of the divaricate form: small leaves, such as those of divaricates, should be less vulnerable to chilling during radiation frosts than large leaves, because of the influence of boundary layer thickness on convective exchange with the surrounding air. We transplanted seedlings of three divaricate and three broadleaved species into replicated forest margin and understorey environments in a lowland forest remnant, and 10.5 months later used a thermal imaging camera to measure pre-dawn leaf temperatures on a clear winter night. Leaf width of divaricate plants responded more plastically to light environment than did that of broadleaved species. Thermal imaging indicated there was little interspecific variation in leaf temperatures in the understorey; however, on the forest margin, minimum leaf temperatures of broadleaved species were typically 0.4-0.6 °C lower than those of divaricate species. The ecological significance of these small differences in chilling is not clear, and more extensive studies of the effect of leaf size are needed, including a wider range of leaf sizes and more open sites.