PSEUDOTSUGA MENZIESII (MIRB.) FRANCO : ITS POLLEN DISPERSAL AND LATE QUATERNARY HISTORY IN THE PACIFIC NORTHWEST

Abstract

The theoretical dispersal distance of Pseudotsuga menziesii pollen is short ; over 90% of its pollen falls within a distance of 800 m from the source forest. The diagnostic (D) factor [relative pollen frequency divided by the pollen deposition number for individual species, or the reciprocal number of the total pollen influx (gr cm^<-2>yr^<-1>)] of Pseudotsuga is higher in PI time than later in PII and PIII because the postglacial forest vegetation has been successively developed by filling empty niches in the Pacific Northwest. By employing the conversion (C) factor (the averaged D value in each pollen zone), percentage diagrams published earlier are converted to approximate absolute deposition curves. It then clearly shows that Pseudotsuga stands were sparsely distributed in the Willamette Valley and the southern Puget Lowland during the full-glacial period. These scattered populations became the source of modern Pseudotsuga forests in the northern Pacific coastal region. It spread almost over the Pacific Northwest by 10,500 years ago. Immediately after its arrival in the Puget Lowland, Pseudotsuga then increased logistically, and the intrinsic rate of the population growth is extremely high (0.00606-0.01033 yr^<-1>), a typical value for pioneering arboreal species. Refugia of the interior population are not clear, yet one can conclude that Pseudotsuga reached northern Idaho by 10,000 years B.P. from southern sites and then migrated westwards.