Dry Heath Arctic Tundra Responses to Long-term Nutrient and Light Manipulation

Abstract

Long-term fertilization studies in several arctic ecosystems have demonstrated dramatic responses of plant community structure with concomitant changes in ecosystem properties. Although these results are well documented in moist tussock and wet sedge tundra, dry heath tundra has been less studied. In an Alaskan dry heath arctic tundra, we conducted a biomass harvest of plants that received additional nitrogen (N, 10 g m–2 yr–1) and/or phosphorus (P, 5 g m–2 yr–1) or reduced light (50% of ambient) for 8 yr. We expected responses to be similar to those of other arctic tundra communities with increased biomass resulting from added nutrients and species responding individualistically to generate the community-level response. However, total vascular biomass did not change in the dry heath tundra in response to any treatment, although individual species and functional group biomass differed from controls. Aboveground productivity, estimated using new apical growth, significantly increased in the N and N+P plots caused by significantly greater abundance of a tussock-forming grass, Hierochloe alpina. The lowest species richness was recorded in the N alone plots, where a deciduous shrub, Betula nana, had its greatest biomass, and richness also declined in N+P plots. Plots that received P alone had similar biomass and species richness to controls, although shrubs decreased in abundance. The shade treatment caused minor biomass differences, marginally less new apical growth, and slightly lower species richness compared to control plots. These results were similar to several ongoing studies in Alaskan moist tussock and wet sedge tundras where above-ground productivity increased in response to added N and/or P but biomass response lagged. This shift in the dry heath tundra from an evergreen shrub to a grass dominated system in the N and N+P plots may cause profound ecosystem function changes as woody biomass capable of long-term carbon storage is lost.