A 6000-year soil pollen record of subalpine meadow vegetation in the Olympic Mountains, Washington, USA

1 Subalpine meadow communities are influenced by edaphic and microclimatic gradients, and should be affected by climate change that affects these local controls. We used pollen preserved in meadow soils to investigate the long-term interaction of these factors in a 1-ha subalpine meadow in the Olympic Mountains, Washington, USA. 2 To describe the spatial scale at which soil pollen is related to neighbouring vegetation, pollen in 46 soil surface samples was compared with plant cover within concentric circular plots (0.5-1 m radii). Regression analysis of percentage pollen with plant cover at different distances from the surface sample had the highest correlation with radii < 1 m, confirming that entomophilous meadow species have very small relevant pollen source areas. 3 We analysed pollen records in soil columns at three sites representing a range of growing season lengths and community types within the meadow. Soils had aggraded by the deposition of eolian silt. Radiocarbon dates of pollen concentrates were similar to or older than radiocarbon dates on charcoal pieces at the same depth, suggesting negligible downwards movement of pollen in the soil profile. 4 The oldest soil pollen profile was from the wettest microsite, currently dominated by a snowbed Carex nigricans community. This site was occupied by a dry Juniperus community prior to c. 6000 BP (before present), when it shifted to more mesic communities dominated by Poaceae and Polemonium. The first appearance of a snowbed Calex nigricans community at this site, c, 2500-1500 BP suggests a change to cooler and/or wetter regional climate. 5 High levels of Polygonum bistortoides at all sites indicated a shift to long snow-free periods and mesic summer conditions during an interval corresponding to the Medieval Warm Period (c. 1200-700 BP). After 500 BP (during the Little Ice Age) Carex nigricans re-established in the wet microsite, while relatively little change occurred at the other two sites. Overall, the greater magnitude of change at this microsite than at the longer growing-season sites indicates that, in these subalpine meadows, short growing season sites are most sensitive to regional climate change.