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 and this might operate through impacts on mechanisms of seed release. Therefore, the impact of freezing on samara release and seed germination in infructescences (cones) of Casuarina cunninghamiana was assessed. Cones at field moisture content were frozen (22 h) and thawed (2 h) though 0 to 5 cycles. Freezing impaired cone dehiscence and samara release (<1% samaras released with ≥2 freezing cycles) and reduced germination from samaras frozen while still in the cone (30 to 50% loss in total germination with 1 to 5 freezing cycles, respectively). Seed germination from a sample of air dried samaras was only mildly impacted (10% drop in total germination with 5 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 retained in the canopy during winter, and a potential contributory factor in species persistence and invasiveness.

Cold Sensitivity of Field-Grown Allocasuarina and Casuarina Tree Species

“…This study has confirmed the early work with containergrown C. cunninghamiana that demonstrated its cold sensitive (Riley and Saygı 2021b) and extended this to other Allocasuarina and Casuarina accessions in a field context. Notwithstanding this, the growth performance of the accessions tested in this work show that there is clear merit in accessing sheoaks more widely in Turkey in climate zones, e.g., along the Mediterranean and Aegean coasts, where extreme and/or extend subzero temperatures are unlikely to occur.…”

“…Therefore, the key finding of this study was that some species of sheoak have potential for high growth rates under harsh conditions as in Turkey but are not tolerant of the extreme cold of Central Anatolia. At the study location in Niğde, the soil does not normally freeze in winter, so the death of the shoots of the sheoaks was consistent with the results of Riley and Saygı (2021b) but the recovery of some plants from belowground indicates partial root survival. However, this recovery was weak and unlikely to result in the persistence of these species.…”

“…In the field, there is a single report of frost damage to C. cunninghamiana in plantations in California, USA (Merwin et al 1995). There have also been studies on the effect of freezing on seed release in a range of species (Riley and Saygı 2021a) and containerised sapling survival with C. cunninghamiana (Riley and Saygı 2021b). Therefore, for this study, plants of 10 accessions including six species of Allocasuarina (2 species) and Casuarina (4 species) were established in the field in Niğde in Central Anatolia, Turkey and their growth and survival monitored over 1.5 years, using the local species, Elaeagnus angustifolia (F. Elaeagnaceae), as a comparator.…”