Maintenance of Variation in Cone Morphology in California Closed-Cone Pines: The Roles of Fire, Squirrels and Seed Output

In the Californian representatives of the Subsection Oocarpae (Pinus attenuata, P. muricata and P. radiata), there is variability in several cone characters. The cones range from being thin-scaled and attached to branches at relatively right angles, to having very thick scales and prominent apophyses on one side and attached to branches at acute angles. Thick-scaled cones provide more protection for the seeds from the high temperatures of fires than do thinscaled cones and may deter attacks from the squirrels Tamiasciurus and Sciurus. These cones carry fewer seeds per given weight of cone which is energetically less efficient for the squirrels to harvest. Squirrels are known to selectively feed on cones carrying more seeds per cone. Thin-scaled cones yield more seeds per given cone weight. This is energetically advantageous to reproductive success but may make them favorite targets for squirrel feeding. Consequently, individuals with thin-scaled cones can only be favored in areas where fires are not too hot and where squirrels are rare or absent. Such conditions seem to be met for pine populations on Santa Cruz, Santa Rosa, Guadalupe and Cedros Islands, and San Vicente and Ensenada, Baja California. It is precisely in these populations where cones with thin scales are most frequent. In areas where fires may be expected to be hotter and squirrels more common, individuals with thick-scaled asymmetric cones predominate. The cones of a number of pine species show a great deal of variability in cone shape, structure, scale thickness and angle of attachment to branches. This is true of jack pine Pinus banksiana Lamb. (Rudolph et al. 1957) lodgepole pine Pinus contorta Dougl. (Critchfield 1957) and the closed-cone pines of California, P. radiata D. Don P. attenuata Lemmon and P. muricata D. Don (Duffield 1951; Newcombe 1962; Forde 1964; and others). In his discussion of the genus Pinus, Shaw (1914) suggested that thick-scaled, serotinous cones, closely appressed to branches, were adaptations for protection of the