Secondary dispersal of bigcone Douglas-fir (Pseudotsuga macrocarpa) seeds

Large-seeded pines (Pinus spp.) are known to be dispersed by seed-caching corvids (i.e. jays and nutcrackers) and rodents (e.g. chipmunks and mice), with a concomitant decrease in seed dispersability by wind. We tested the idea that seeds of bigcone Douglas-fir (Pseudotsuga macrocarpa), which are winged but larger than the seeds of other members of Pseudotsuga, are dispersed by a combination of wind and seed-caching rodents. We compared characteristics of seeds from P. macrocarpa in southern California (mean seed mass 132.6 mg) to seeds of a population of Pseudotsuga menziesii (Douglas-fir) in northern California (24.8 mg). We also tested whether rodents would cache seeds of P. macrocarpa. Seeds of P. macrocarpa had greater wing loadings (1.37 mg/mm2) and descent velocities (2.47 m/s) than those of P. menziesii (0.52 mg/mm2 and 1.28 m/s, respectively). These data indicate that the wind dispersability of P. macrocarpa is likely to be less than that of P. menziesii, but this loss of wind dispersability is partially compensated for by secondary dispersal of seeds by rodents, which readily gathered and cached the larger seeds of P. macrocarpa up to 34 m from source trees. Large seed size confers several advantages to P. macrocarpa, most importantly attracting seed-caching animals that effectively bury seeds.