DISPERSAL OF SINGLELEAF PINYON PINE (PINUS-MONOPHYLLA) BY SEED-CACHING RODENTS

Pinon pines dominate millions of hectares in western North America, bu t the role of rodents as consumers and dispersers of seeds has been vi rtually ignored. Here I report the results of a 2-year study on the in fluence of four seed-caching rodents (Perognathus parvus, Dipodomys pa namintinus, Peromyscus truei, and Peromyscus maniculatus) on the dispe rsal of singleleaf pinon pine (Pinus monophylla) in western Nevada. Du ring the moderate seed crop of autumn 1993, 11% of the edible pinon se eds 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 experim entally placed on the ground below trees. Half-lives of seeds (a measu re 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 lab eled seeds were placed under each of five ''source trees,'' rodents qu ickly removed many and cached most of the seeds they removed. At two o f the source trees, rodents stored nearly all seeds in their burrow la rders. At the other three source trees, rodents scatter hoarded many o f 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 we re found under shrubs. During summer 1995, 69% of established seedling s occurred under shrubs, which served nurse plants for young pines. Al though pinon pines appear to be coevolved or coadapted with corvid (ja y and nutcracker) seed dispersers, rodents also serve as important age nts of seed dispersal.