REMOVAL OF WIND-DISPERSED PINE SEEDS BY GROUND-FORAGING VERTEBRATES

Nine treatments involving three species of pine (Jeffrey pine, Pinus j effreri; ponderosa pine, P. ponderosa; and lodgepole pine, P. contorta ) seeds were used to test five hypotheses concerning the determinants of seed removal by animals in the field. Each treatment comprised 100 seeds. Winged seeds were tethered so they could not be blown away. See ds were checked daily for up to 16 d. Animals (primarily chipmunks) re moved most seeds within a few days. Winged Jeffrey pine seeds (large) disappeared significantly faster than ponderosa pine seeds (medium) wh ich disappeared faster than lodgepole pine seeds (small). Winged Jeffr ey pine seeds placed in closed-canopy Jeffrey pine forest disappeared significantly more slowly than similar seeds placed in nearby bitterbr ush (Purshia tridentata) shrubland. Animals removed winged Jeffrey pin e seeds placed under or between shrubs at similar rates. Winged Jeffre y pine seeds placed under plant litter disappeared much more slowly th an did seeds placed on the ground surface or buried in mineral soil so that about half of the wing was exposed. Animals gathered wingless Je ffrey pine seeds significantly more slowly than winged seeds. And rate of Jeffrey pine seed removal was not dependent on the background dens ity of naturally wind dispersed Jeffrey pine seeds. Seeds within treat ments were not removed at a constant rate. Instead, animals initially removed seeds rapidly during the first one or two days of the experime nt (initial seed removal rate) and then gathered seeds much more slowl y during the remaining days (long-term seed removal rate). This change in harvest rate is presumably a consequence of animals not using the habitat uniformly. Half-lives were used as a measure of seed removal r ate. Initial half-lives ranged from 18 h (winged Jeffrey pine seeds be tween bitterbrush shrubs) to 80 h (winged Jeffrey pine seeds hidden un der needle litter). Long-term half-lives for the same treatments were 64 and 351 h. These high rates of removal for experimental seeds indic ate that animals have the capacity to harvest most of the naturally-pr oduced pine seeds during the two-month period between seedfall and win ter. However, most of the seeds are harvested by seed-caching animals, which serve as important seed dispersal agents for Jeffrey and ponder osa pines.