Freezing impact on cone dehiscence, samara release, and seed germination in Casuarina cunninghamiana (Casuarinaceae)

Abstract

Freezing as a climatic extreme can contribute to patterns of plant distribution by potentially impacting mechanisms of seed release. Therefore, the impact of freezing on samara release and seed germination in infructescences (cones) of Casuarina cunninghamiana Miq. was assessed. Cones at field moisture content were frozen (22 h) and thawed (2 h) through zero to five cycles. Freezing impaired cone dehiscence and samara release (<1% samaras released with ≥2 freezing cycles) and reduced germination from frozen samaras still in the cone (30% to 50% loss in total germination with one to five freezing cycles, respectively). Seed germination from a sample of air-dried samaras was only mildly impacted (10% drop in total germination with five freezing cycles). This vulnerability of C. cunninghamiana to freezing damage, particularly samara release, appears to be a novel finding for woody perennials with fruiting structures that are retained in the canopy during winter. Vulnerability to freezing damage also appears to be a potential factor in species persistence and invasiveness.