Life on the edge for limber pine: Seed dispersal within a peripheral population

Interactions within populations at the periphery of a species' range may depart from those in populations more centrally located. Throughout its core range, limber pine (Pinus flexilis, Pinaceae) depends on Clark's nutcrackers (Nucifraga columbiana, Corvidae) for seed dispersal. Nuterackers, however, rarely visit the Pawnee National Grassland peripheral population of limber pine on the eastern Colorado plains. Using live mammal trapping, fluorescent pigment tracking of disseminated seeds, and limber pine seed germination experiments in the field, we tested the hypothesis that limber pine seeds in this peripheral isolate are dispersed by nocturnal rodents. Live trapping and tracking indicated that deer mice (Peromyscus maniculatus, Muridae) and Ord's kangaroo rats (Dipodomys ordii, Heteromyidae) are the likely seed dispersers in this population. Rodents cached seeds in surface caches on tree leaf litter, rock, or soil substrate or buried them under soil, tree litter, or plants. Seeds cached under soil and plants, as opposed to surface caches, accounted for the greater number of stored seeds. Numbers of seeds per cache for buried caches was significantly higher than numbers of seeds for surface caches. The largest caches on average were those buried under plants. Also, we found that some rodent caches contained one or more seeds up to 27 d after the caches were made. In experiments simulating observed cache types, we determined that most cache types, but especially buried caches, had some germination potential. Rodents disseminated seeds over shorter distances than do nutcrackers, possibly explaining previously observed genetic substructuring in the Pawnee population. Seed dispersal by rodents also precludes the metapopulation dynamics typical of limber pine in its core range, and may lead to the loss of peripheral populations over time.