The role of frost tolerance in defining the "kauri line" floristic boundary

Floristic boundaries, where the range limits of multiple species coincide, are frequently recognised ecological phenomena for which underlying mechanisms are often unclear. Plant species range limits are often determined by their tolerance to climatic conditions. Therefore, the positions of floristic boundaries are also likely related to climatic conditions. Seedlings are a vulnerable life stage and are often highly susceptible to adverse climatic events such as frosts, and frost susceptibility varies among plant species. Few studies have focussed on the relationship between measured frost tolerances of different plant species and their distributions relative to a floristic boundary. Our study looked at interspecific variation in frost tolerance in a suite of native tree species relative to the kauri line, a floristic boundary recognised at approximately 38°S in Aotearoa New Zealand. We exposed seedlings of four species with range limits near the kauri line and three species with more widespread distributions to one-off simulated frost events and monitored their health for eight weeks after the event. We also excised, froze, and conducted electrolyte leakage tests on leaf samples from each species. The frost tolerances of the kauri line species were similar to each other and were generally lower than those of the widespread species. The outcomes of the electrolyte leakage tests were broadly consistent with the whole seedling frost tests, although the frost tolerance of Pseudopanax lessonii, a kauri line species, was under-estimated. Our study highlights that frosts could play a role in the ranges of kauri line species, especially species whose seedlings are common in early successional habitats, such as Agathis australis. The convergence of the frost tolerance of A. australis with those of other kauri line species is not consistent with the Pleistocene ratchet hypothesis, which suggests a decoupling of plant species current and potential ranges due to rapid climatic changes. Climate change is likely to reduce the incidence of frosts in New Zealand and may facilitate the southward shift of kauri line species, changing the nature of this boundary.