Late quaternary dynamics of pollen influx at mineral lake,Washington

Pollen influx analysis at Mineral Lake, Washington, indicates that immediately south of the Puget Lobe of the Fraser Glaciation, tundra was a characteristic vegetation until 16,300 years ago. Invasion ofPinus contorta began 17,500 years B.P., and boreal climax conifers (Abies, Picea andTsuga mertensiana), 16,300, but was temporarily interrupted by the Vashon advance (14,500–14,000 yr B.P.).Pseudotsuga menziesii began to grow in population 10,750 years ago, and woodland was established within a time span of 1,000 years. Modern lowland coniferous forests began to form 7,000 years ago. Logistic analysis of pollen abundance changes show that the intrinsic growth rate,r (yr⁻¹), of pioneer species (e.g. 0.024–0.026 inPteridium aquilinum) is higher than that of climax species (e.g. 0.003 inThuja plicata).P. menziesii, a subclimax species, shows an intermediater value (0.013) between these two ecologically different taxa. The absoluter value ofP. contorta (−0.011) is only slightly lower than that ofP. menziesii, although their replacement began almost simultaneously. Thus competition between these species was intense before the inflection point ofPinus curve 10,100 years ago. At this time, forest gaps became abundantly available forPseudotsuga, as indicated by a peak of the diagnostic factor (the reciprocal of the pollen influx).