THE EVOLUTION OF BIRD-DISPERSED PINES

Twenty of approximately 100 species of pines (Pinus spp.) have wingless seeds, and 19 of these are in the subgenus Strobus. Eight of the wingless-seed Strobus pines are known to be dispersed by seed-storing corvids, particularly the nutcrackers (Nucifraga spp.), and the other 11 are presumed to be. The principal consequences of these nearly obligate bird-pine mutualisms include tree clustering and a population structure that differs from that of wind-dispersed pines. The wingless-seed Strobus pines in general are typified by ranges that include xeric conditions and/or high elevations, and large seeds, which are considered to be adaptive under either xeric or competitive conditions. The proposed evolutionary scenario for bird dependency begins with the distribution of ancestral Strobus pines into high elevation or semi-desert environments, sympatric with one or more seed-storing corvid forms, and an increase in seed size. We propose that dependency on birds for seed dispersal has occurred primarily in subgenus Strobus, because Strobus pines tend more towards winglessness and increased seed size in stressful environments than do Pinus pines. Seed winglessness and other bird-pine traits probably arose from a combination of genetic drift in small populations and selection by corvids. © 1990 Chapman and Hall Ltd.