Dispersal of Whitebark Pine Seeds by Clark's Nutcracker: A Mutualism Hypothesis

(1) Clark's nutcrackers (Nucifraga columbiana) store a mean of only 3.7 whitebark pine (Pinus albicaulis) seeds per cache, which reduces competition for moisture and space. The mean depth at which seeds are stored, 2.0 cm, is compatible with germination requirements, and many sites selected appear suitable for seed germination. (2) One nutcracker stores about 32 000 whitebark pine seeds at subalpine elevations each year, which represent 3-5 times its energetic requirements. Although parent nutcrackers feed stored seeds to juveniles, in some years there is probably an excess of seeds stored by the population as a whole. (3) Experimental results suggest that seeds retrieved from nutcracker caches are as viable as seeds extracted by hand from cones and that seedlings originating in nutcracker caches have a good survival rate: 56% over the first year and 25% by the fourth year. (4) In comparison with Clark's nutcracker, alternative disperal agents, i.e. rodents and cone disintegration, disperse fewer seeds, disperse seeds shorter distances from parent trees, place seeds in sites less suitable for germination, and/or make large seed caches which lower reproductive potential. Nutcrackers disseminate seeds throughout the subalpine habitat and are, in part, responsible for the 'pioneering' status of whitebark pine. (5) The evolution of wingless seeds and indehiscent cones in the Cembra pine group was probably a consequence of seed dispersal by an ancestral nutcracker form. It appears that the Clark's nutcracker-whitebark pine interaction is both coevolved and mutualistic.