Dispersal of singleleaf pinon pine (Pinus monophylla) by seed-caching rodents

Pinon pines dominate millions of hectares in western North America, but the role of rodents as consumers and dispersers of seeds has been virtually ignored. Here I report the results of a 2-year study on the influence of four seed-caching rodents (Perognathus parvus, Dipodomys panamintinus, Peromyscus truei, and Peromyscus maniculatus) on the dispersal of singleleaf pinon pine (Pinus monophylla) in western Nevada. During the moderate seed crop of autumn 1993, 11% of the edible pinon seeds fell to the ground, and, during the heavy seed crop of autumn 1994, 57% of the seeds fell. Rodents gathered ca. 80% of the seeds experimentally placed on the ground below trees. Half-lives of seeds (a measure of seed-removal rates) placed below productive pinon pines was 6.6 h in autumn 1993 and 18.6 h in autumn 1994. When 200 radioactively labeled seeds were placed under each of five ''source trees,'' rodents quickly removed many and cached most of the seeds they removed. At two of the source trees, rodents stored nearly all seeds in their burrow larders. At the other three source trees, rodents scatter hoarded many of the seeds, with most seeds buried 3-29 mm deep. Dispersal distances of scatter-hoarded seeds ranged up to 38.6 m, and 36% of the caches were found under shrubs. During summer 1995, 69% of established seedlings occurred under shrubs, which served nurse plants for young pines. Although pinon pines appear to be coevolved or coadapted with corvid (jay and nutcracker) seed dispersers, rodents also serve as important agents of seed dispersal.