Responses of carbon and nitrogen concentrations in high arctic plants to experimental warming

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Abstract
We studied total carbon (C) and nitrogen (N) concentration in five arctic plant species, Cassiope tetragona (L.) D. Don (evergreen dwarf shrub), Dryas integrifolia Vahl (semi-evergreen dwarf-shrub), Salix arctica Pall. (deciduous, dioecious dwarf shrub), Oxyria digyna (L.) Hill (forb), and Carex stans Drej. (graminoid, sedge), after five growing seasons of experimental increase in temperature. The experiments were established in different habitats along a moisture gradient in a coastal lowland at Alexandra Fiord, Ellesmere Island (79ºN). Sampling was conducted three times during the growing season. The seasonal patterns of C and N concentration were similar across all investigated species, even though the absolute concentrations differed. Nitrogen concentrations were high early in the season during the active growth period but declined later in the season. Warming decreased the N concentration and increased the C/N ratio in the three woody species, whereas N concentrations in the forb and the sedge were not affected. Although not measured directly in our study, the differences in concentrations may be due to the slower response in nutrient uptake and growth in the dwarf shrubs but increased nutrient uptake and growth in the forb and the sedge. Among-site differences were only significant in C concentrations of old tissues of Cassiope tetragona and S. arctica. Male and female S. arctica did not differ in their responses to warming. Differences among plant species in their plastic responses may relate to survival during rapid environmental change. Slow-responding dwarf shrubs may be under increased risk of competition from more responsive and faster growing species, such as forbs and graminoids.Key words: allocation, climate change, dwarf shrub, forb, graminoid, ITEX, Ellesmere Island.