Effects of holocene Alnus expansion on aquatic productivity, nitrogen cycling, and soil development in southwestern Alaska

Numerous pollen records provide evidence for the widespread range expansion of Alnus throughout Alaska and adjacent Canada during the middle Holocene. Because Alnus can fix atmospheric N-2, this vegetational change probably h ad a profound effect on N availability and cycling. To assess this effect, we analyzed a sediment core from Grandfather Lake in southwestern Alaska fo r a suite of geochemical indicators, including elemental composition, bioge nic silica (BSi) content, and carbon (C) and nitrogen (N) isotopes of organ ic matter. These data, in conjunction with a pollen record from the same si te, are used to infer biogeochemical processes associated with the mid-Holo cene Alnus expansion. The increase in Alnus pollen percentages from 10% to 70% circa 8000-7000 BP (C-14 years before present) suggests the rapid sprea d of Alnus shrub thickets on mountain slopes and riparian zones in the Gran dfather Lake region. Coincident with this vegetational change, the mean val ue of the sediment BSi content increases from 20.4 to 106.2 mg/g, reflectin g increased diatom productivity within the lake as a result of Alnus N-2 fi xation in the watershed soils and the associated N flux to the lake. Elevat ed aquatic productivity at this time is also supported by increased percent ages of organic C and N, decreased C:N ratios, and decreased values of delt a C-13. Furthermore, the delta N-15 values of sediments increase substantia lly with the establishment of Alnus shrub thickets, suggesting enhanced N a vailability and accelerated N cycling within the lake and its watershed. Su perimposed on a general trend of soil acidification throughout the postglac ial period, soil acidity probably increased as a result of the Alnus expans ion, as can be inferred from decreasing ratios of authigenic base cations t o allogenic silica (Si) and increasing ratios of authigenic aluminum (Al) t o allogenic Si. The ultimate cause of these mid-Holocene ecosystem changes was an increase in effective moisture in the region.