Effects of Holocene Alnus Expansion on Aquatic Productivity, Nitrogen Cycling, and Soil Development in Southwestern Alaska Feng Sheng Hu, 1 * Bruce P. Finney, 2 and Linda B. Brubaker 3 1 Departments of Plant Biology and Geology, 265 Morrill Hall, University of Illinois, Urbana, Illinois 61801, USA; 2 Institute of Marine Science, University of Alaska, Fairbanks, Alaska 99775, USA; and 3 College of Forest Resources, University of Washington, Seattle, Washington 98195, USA ABSTRACT Numerous pollen records provide evidence for the widespread range expansion of Alnus throughout Alaska and adjacent Canada during the middle Ho- locene. Because Alnus can fix atmospheric N 2 , this vegetational change probably had a profound effect on N availability and cycling. To assess this effect, we analyzed a sediment core from Grandfather Lake in southwestern Alaska for a suite of geo- chemical indicators, including elemental composi- tion, biogenic silica (BSi) content, and carbon (C) and nitrogen (N) isotopes of organic matter. These data, in conjunction with a pollen record from the same site, are used to infer biogeochemical pro- cesses associated with the mid-Holocene Alnus ex- pansion. The increase in Alnus pollen percentages from 10% to 70% circa 8000 –7000 BP ( 14 C years before present) suggests the rapid spread of Alnus shrub thickets on mountain slopes and riparian zones in the Grandfather Lake region. Coincident with this vegetational change, the mean value of the sediment BSi content increases from 20.4 to 106.2 mg/g, reflecting increased diatom productiv- ity within the lake as a result of Alnus N 2 fixation in the watershed soils and the associated N flux to the lake. Elevated aquatic productivity at this time is also supported by increased percentages of organic C and N, decreased C:N ratios, and decreased values of  13 C. Furthermore, the  15 N values of sediments increase substantially with the establishment of Al- nus shrub thickets, suggesting enhanced N availabil- ity and accelerated N cycling within the lake and its watershed. Superimposed on a general trend of soil acidification throughout the postglacial period, soil acidity probably increased as a result of the Alnus expansion, as can be inferred from decreasing ratios of authigenic base cations to allogenic silica (Si) and increasing ratios of authigenic aluminum (Al) to allogenic Si. The ultimate cause of these mid-Holo- cene ecosystem changes was an increase in effective moisture in the region. Key words: Holocene paleoecology; biogeochem- istry; Alnus; nitrogen cycling; aquatic productivity; long-term soil development; nitrogen isotopes; ele- mental chemistry; lake sediments; Alaska. INTRODUCTION Certain plant types can exert major influences on ecosystem functioning. For example, Alnus can in- crease nitrogen (N) availability and ecosystem pro- ductivity because it has the ability to fix atmo- spheric N 2 through a symbiotic relationship with the Frankia actinomycete. Comparisons of forest stands with and without Alnus show that its pres- ence significantly increases rates of primary produc- tion, carbon (C) accumulation, and N cycling (Bin- Received 21 July 2000; accepted 3 January 2001. *Corresponding author; e-mail: fshu@life.uiuc.edu Ecosystems (2001) 4: 358 –368 DOI: 10.1007/s10021– 001– 0017-0 ECOSYSTEMS © 2001 Springer-Verlag 358