Isolating barriers between the Californian closed-cone pines

Abstract

The strength and nature of factors preventing the interspecific and intraspecific hybridisation of the Californian closed-cone pines, Pinus attenuata, P. muricata and P. radiata were examined in cultivated material in the Australian Capital Territory.

Differences in flowering times were evident among some provenances of the main species, but were not significant. Flowering times of the major species were sufficiently different to be important, and only a very small overlap in flowering times occurred between P. radiata and P. muricata, the earliest and latest species respectively.

Yields of germinable seeds from intraspecific crosses involving P. muricata were low; the seeds which were obtained came from crosses utilising southern provenances of the species. The northern provenances yielded no viable seed and they also gave very low yields of viable seeds when crossed with southern trees of the same species. There was no evidence of comparable behaviour in the widespread, ecologically-differentiated P. attenuata. Low seed yields were largely due to reproductive failure at or near the time of fertilisation.

Strong evidence was obtained that Cedros Is. pine presently described as a variety of P. muricata belongs under P. radiata.

Growth rates of interspecific hybrid seedlings appeared to be intermediate between those of seedlings of the parental -species.

Attempts to utilise properties of P. attenuata such as cold or drought resistance in any hybridisation programme with P. radiata should not be hampered by reproductive barriers. However the more attractive northern forms of P. muricata are so isolated that effective hybridisation would be very difficult.