BIOMASS AND CO2 FLUX IN WET SEDGE TUNDRAS - RESPONSES TO NUTRIENTS, TEMPERATURE, AND LIGHT

The aim of this research was to analyze the effects of increased N or P availability, increased air temperature, and decreased light intensi ty on wet sedge tundra in northern Alaska. Nutrient availability was i ncreased for 6-9 growing seasons, using N and P fertilizers in factori al experiments at three separate field sites. Air temperature was incr eased for six growing seasons, using plastic greenhouses at two sites, both with and without N + P fertilizer. Light intensity (photosynthet ically active photon flux) was reduced by 50% for six growing seasons at the same two sites, using optically neutral shade cloth. Responses of wet sedge tundra to these treatments were documented as changes in vegetation biomass, N mass, and P mass, changes in whole-system CO2 fl uxes, and changes in species composition and leaf-level photosynthesis . Biomass, N mass, and P mass accumulation were all strongly P limited , and biomass and N mass accumulation also responded significantly to N addition with a small N X P interaction. Greenhouse warming alone ha d no significant effect on biomass, N mass, or P mass, although there was a consistent trend toward increased mass in the greenhouse treatme nts. There was a significant negative interaction between the greenhou se treatment and the N + P fertilizer treatment, i.e., the effect of t he two treatments combined was to reduce biomass and N mass significan tly below that of the fertilizer treatment only. Six years of shading had no significant effect on biomass, N mass, or P mass. Ecosystem CO2 fluxes included net ecosystem production (NEP; net CO2 flux), ecosyst em respiration (R-E, including both plant and soil respiration), and g ross ecosystem production (GEP; gross ecosystem photosynthesis). All t hree fluxes responded to the fertilizer treatments in a pattern simila r to the responses of biomass, N mass, and P mass, i.e., with a strong P response and a small, but significant, N response and N X P interac tion. The greenhouse treatment also increased all three fluxes, but th e greenhouse plus N + P treatment caused a significant decrease in NEP because R-E increased more than GEP in this treatment. The shade trea tment increased both GEP and R-E, but had no effect on NEP. Most of th e changes in CO2 fluxes per unit area of ground were due to changes in plant biomass, although there were additional, smaller treatment effe cts on CO2 fluxes per unit biomass, per unit N mass, and per unit P ma ss. The vegetation was composed mainly of rhizomatous sedges and rushe s, but changes in species composition may have contributed to the chan ges in vegetation nutrient content and ecosystem-level CO2 fluxes. Car er cordorrhiza, the species with the highest nutrient concentrations i n its tissues in control plots, was also the species with the greatest increase in abundance in the fertilized plots. In comparison with Eri ophorum angustifolium, another species that was abundant in control pl ots, C. cordorrhiza had higher photosynthetic rates per unit leaf mass . Leaf photosynthesis and respiration of C. cordorrhiza also increased with fertilizer treatment, whereas they decreased or remained constan t in E. angustifolium. The responses of these wet sedge tundras were s imilar to those of a nearby moist tussock tundra site that received an identical series of experiments. The main difference was the dominant P limitation in wet sedge tundra vs. N limitation in moist tussock tu ndra. Both tundras were relatively unresponsive to the increased air t emperatures in the greenhouses but showed a strong negative interactio n between the greenhouse and fertilizer treatments. New data from this study suggest that the negative interaction may be driven by a large increase in respiration in warmed fertilized plots, perhaps in relatio n to large increases in P concentration.