Freezing under contrasting seasonality: Dry season survival as a stepping stone to freezing tolerance

Background/Question/Methods

There is strong cellular-level evidence that plants that can resist drought are also better able to resist freezing. We explored whether such a link could act as an exaptation at continental scales, by measuring whether exposure to freezing temperatures is less damaging to plants from seasonally dry than perennially moist environments.

We tested this hypothesis in an ecological context, using seven clades found under contrasting climatic conditions in Australian rainforests: wet tropical, seasonally dry tropical, and wet temperate. Leaves were harvested during midwinter, and freezing tolerance was assessed under controlled conditions, at steps from 1ºC to -10ºC.

Results/Conclusions

Plants from the seasonally dry site were significantly more resistant to freezing than their relatives from the wet tropical site, at lowest temperatures of both -5 and -10ºC (P < 0.001, Tukey HSD). Plants from the wet temperate site were also significantly more resistant to freezing than plants from the wet tropical site at both temperatures (P < 0.001, Tukey HSD). However, there was no statistical difference in resistance to freezing between plants from the seasonally dry and wet temperate environments (P = 0.756 and 0.966 for -5 and -10ºC, respectively.) There were also significant differences between clades independent of site. Fabaceae and Pittosporaceae showed relatively little injury, while severe damage occurred in members of Myrtaceae. Seasonal exposure to drought could facilitate adaptation to cold climates, potentially providing an adaptive intermediate between tropical climates (e.g., Late Cretaceous–Early Tertiary) and seasonally cold climates (e.g., post-Oligocene).