Morphological differentiation of Alnus (alder) pollen from western North America

Increasing the taxonomic resolution of fossil pollen identification is critical for advancing Quaternary paleoecology to a point where species-specific ecologies can be addressed in the fossil record. Here, we determine the critical morphological features that permit species-level differentiation of Alnus pollen, an abundant pollen type in Quaternary records from western North America. We examined over 21,000 pollen grains from the region's three common alder species: Alnus viridis subsp. sinuata Regel, Alnus incana subsp. tenuifolia Nuttall and Alnus rubra Bongard. Modern pollen samples were collected from 27 to 35 individual plants from across the range of each species. Nine morphological traits were measured on 30 pollen grains from each plant, and the number of pores was determined for an additional 200 pollen grains from each individual. Nested ANOVA analyses suggest that for individual Alnus plants, pollen morphology appears relatively stable, compared to variation between species. Statistically significant differences exist between the pollen of all three alder species in most morphological traits, but there is a high degree of within-species variability and between-species overlap in pollen morphology. Since no morphological trait on its own was sufficient for pollen identification to species, classification and regression tree (CART) analysis was used to derive multi-trait classification models. CART analyses show that A. rubra and A. viridis subsp. sinuata pollen can be differentiated into two distinct morphotypes, analogous to species separation, based on annulus width, arci strength, exine thickness and overall diameter. The intermediate pollen morphology of Alnus incana subsp. tenuifolia prevents identification of Alnus pollen to species when all three species are present in the pollen source area. This research lends support to paleoecological studies in western North America that have differentiated Alnus pollen into two morphotypes and revealed distinct postglacial histories that are masked when Alnus pollen are not differentiated.

► We examine the morphology of over 21,000 pollen grains from three species of Alnus.
► There is a high degree of intraspecific variability in Alnus pollen morphology.
► Overlap in pollen morphology prevents differentiation of all three Alnus species.
► CART models allow differentiation of A. rubra and A. viridis subsp. sinuata pollen.