PARENT MATERIAL DEPTH CONTROLS ECOSYSTEM COMPOSITION AND FUNCTION ON A RIVERSIDE TERRACE IN NORTHWESTERN ALASKA

Many studies have focused on factors that influence ecosystem composit ion and function, but little is known about the influence of varying q uantities of a single parent material without confounding effects of a ge or location. On a riverside terrace of the Agashashok River, the de pth of the cap of silt and sand over the gravel floodplain strongly in fluenced species composition, production, and response to additions of nitrogen (N) and water. Thin siltcaps (< 0.25 m) had vegetation domin ated by herbaceous species, whereas thicker siltcaps had a strong comp onent of shrubs. The depth of the siltcap accounted for about 50% of t he variation in the first principle-component of the variation in spec ies composition and cover. In situ net N mineralization increased with increasing siltcap depth, but net nitrification declined. Production by herbs increased by about 20% with water additions but not with N ad ditions, and the responses were strongest at the two intermediate silt cap depths. Shrub production increased by about 30% with N addition bu t not with water additions, with stronger responses on thicker siltcap s. The importance of the quantity of parent material may apply to othe r situations where the depth of fine-textured parent material is relat ively shallow. Greater attention to the role of quantitative differenc es in parent material appears warranted.