SHOOT BIOMASS, DELTA-C-13, NITROGEN AND CHLOROPHYLL RESPONSES OF 2 ARCTIC DWARF SHRUBS TO IN-SITU SHADING, NUTRIENT APPLICATION AND WARMINGSIMULATING CLIMATIC-CHANGE

As climatic change might induce ecophysiological changes in plants whi ch affect their long-term performance, we investigated responses in ab ove-ground biomass, delta(13)C, nitrogen and chlorophyll of two evergr een arctic dwarf shrubs, Cassiope tetragona and Empetrum hermaphroditu m, to 5 (biomass, N) or 6 years of shading, nutrient application and a ir/soil warming at a dwarf shrub dominated tree-line heath (450 m a.s. l) and a high altitude fellfield (1100 m a.s.l.) in Swedish Lapland. W arming enhanced the green biomass (equivalent to the last 3-4 years of leaf production) and the ratio of green to brown biomass of C. tetrag ona at the fellfield, and diluted the shoot N concentration. Fertilize r application led to higher shoot N concentration and larger green-to- brown biomass ratio at both sites, and fertilizer application and warm ing generally had an additive effect on the green biomass. We conclude that both warming and increased soil nutrient availability stimulated the growth of C. tetragona at the fellfield whereas at the heath ther e was a clear increase in production only if enhanced temperature was combined with nutrient application. Across treatments C. tetragona at the fellfield had 0.6 parts per thousand higher delta(13)C and 1.4 mg g(-1) more leaf N, and the soil organic matter delta(13)C was 1.0 part s per thousand higher at the fellfield than at the heath. However, an increase in shoot N concentration with altitude does not necessarily l ead to higher delta(13)C as no differences in delta(13)C were observed when leaf N of the two dwarf shrubs was increased by fertilizer appli cation C. tetragona in non-warmed plots had higher delta(13)C values t han those from warmed plots at the same altitude, which provides the f irst in situ experimental validation of the theory that temperature pa rtly is responsible for altitudinal trends in plant carbon isotope dis crimination. Increased biomass and chlorophyll concentration of C. tet ragona in warmed plots points to increased assimilation, at least at t he fellfield. As the delta(13)C-based and, therefore, time-integrated estimate of the ratio of CO, concentration in the leaf intercellular s paces to that in the atmosphere (C-i/C-a) also increased, warming prob ably enhanced the stomatal conductance relatively more than the C assi milation, which may be harmful if climatic change leads to reduced soi l moisture content and increased plant competition for water. At both sites C. tetragona and E. hermaphroditum responded to shade by increas ing the concentration of shoot N and photosynthetic pigments whereas b iomass production (and therefore also net photosynthesis) did not decl ine. Shade was accompanied by a 0.6-1.3 parts per thousand (E. hermaph roditum) or 1.2-2.2 parts per thousand (C. tetragona) decrease in delt a(13)C. This could be due to enhanced stomatal conductance with shadin g, and perhaps to shade reducing the ericoid mycorrhizal uptake of soi l organic C, a factor which has been overlooked as an influence on pla nt delta(13)C.