Effects of seed size of wind‐dispersed pines (Pinus) on secondary seed dispersal and the caching behavior of rodents

Abstract

Seeds and nuts dispersed by scatter‐hoarding animals are relatively large compared to propagules dispersed by other means. Possible selective forces in the evolution of large seed size include the selectivity of foraging animals and the ways that food‐storing animals treat seeds and nuts after harvest. Treatment by rodents, primarily yellow pine chipmunks (Tamias amoenus), of four species of pine seeds that vary in size was studied in the Carson Range of western Nevada. The pines, lodgepole pine (Pinus contorta, 8.7 mg), ponderosa pine (P. ponderosa, 55 mg), Jeffrey pine (P. jeffreyi, 157 mg), and sugar pine (P. lambertiana, 213 mg), produce winged seeds that are initially wind‐dispersed but are gathered by rodents and cached in the soil. Radioactive scandium‐46 was used to follow the fates of seeds of all fours species placed around three source trees during autumn 1998 to 2000. Rodents gathered the seeds of all four species, but they took fewer of the lodgepole pine seeds and only six lodgepole seed caches (n=2106 total caches) were found during the three years. Among the other three species, number of seeds per cache decreased with increasing seed mass. However, the product of number of seeds per cache and seed mass was similar for all species. Sugar pine seeds were cached slightly deeper than ponderosa and Jeffrey pine seeds. For the species examined, seed size appeared to have had little effect on several other attributes, including mean dispersal distance, substrate choice, and microhabitat choice. Large size decreases wind dispersibility of pine seeds, but secondary dispersal by scatter‐hoarding rodents compensates for poor wind dispersal so that total dispersibility of large‐seeded pines is not compromised.