Correspondence of pollen assemblages with forest zones across steep environmental gradients, Olympic Peninsula, Washington, USA

Abstract

The use of pollen records to document vegetation responses to climatic change in mountains relies on the ability of pollen assemblages to differentiate among elevationally stratified vegetation zones. Comparisons among modem pollen assemblages within mountainous areas provide a basis for assessing this potential. We examined relationships between pollen assemblages, forest vegetation and climate at 65 small lake sites distributed across five forest zones on the Olympic Peninsula, Washington. Sample sites spanned nearly the full range of climatic gradients on the Peninsula, c. 900-6000 mm annual precipitation and c. 17-8C July mean temperature (from 9 to 1981 m a.s.l.). The pollen percentages of most arboreal taxa showed a strong relationship with elevation, especially for taxa with poor dispersal potential (Abies and Picea) or high elevation ranges (Tsuga mertensiana and Alnus sinuata). Tsuga heterophylla was the only pollen taxon abundant across all forest zones. Both detrended correspondence analysis (DCA) and linear discriminant analysis (DA) arranged pollen assemblages along temperature and precipitation gradients approximating the actual distribution of vegetation along these gradients. DA classified 90% of the sites into their correct forest zones, but was very sensitive to sample size, suggesting that caution should be exercised when using DA for classifying fossil pollen assemblages. Pairwise comparisons using the squared-chord-distance metric (SCD) showed that the SCD threshold that best distinguishes neighbouring forest zones is affected by the number of samples in each forest zone and by the patchiness of species distributions within each zone, suggesting that SCD is also sensitive to sample size and that thresholds used with the modem analogue technique must be calibrated for each vegetation zone. This study indicates that pollen assemblages on the Olympic Peninsula record local forest zones despite steep environmental gradients and the close proximity of neighbouring forest zones.