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The tundra is warming more rapidly than any other biome on Earth, and the potential ramifications are far-reaching because of global feedback effects between vegetation and climate. A better understanding of how environmental factors shape plant structure and function is crucial for predicting the consequences of environmental change for ecosystem functioning. Here we explore the biome-wide relationships between temperature, moisture and seven key plant functional traits both across space and over three decades of warming at 117 tundra locations. Spatial temperature–trait relationships were generally strong but soil moisture had a marked influence on the strength and direction of these relationships, highlighting the potentially important influence of changes in water availability on future trait shifts in tundra plant communities. Community height increased with warming across all sites over the past three decades, but other traits lagged far behind predicted rates of change. Our findings highlight the challenge of using space-for-time substitution to predict the functional consequences of future warming and suggest that functions that are tied closely to plant height will experience the most rapid change. They also reveal the strength with which environmental factors shape biotic communities at the coldest extremes of the planet and will help to improve projections of functional changes in tundra ecosystems with climate warming.

Temperature is increasing at unprecedented rates across most of the tundra biome. Remote-sensing data indicate that contemporary climate warming has already resulted in increased productivity over much of the Arctic, but plot-based evidence for vegetation transformation is not widespread. We analysed change in tundra vegetation surveyed between 1980 and 2010 in 158 plant communities spread across 46 locations. We found biome-wide trends of increased height of the plant canopy and maximum observed plant height for most vascular growth forms; increased abundance of litter; increased abundance of evergreen, low-growing and tall shrubs; and decreased abundance of bare ground. Intersite comparisons indicated an association between the degree of summer warming and change in vascular plant abundance, with shrubs, forbs and rushes increasing with warming. However, the association was dependent on the climate zone, the moisture regime and the presence of permafrost. Our data provide plot-scale evidence linking changes in vascular plant abundance to local summer warming in widely dispersed tundra locations across the globe.

AbstractWet sedge tundra communities in the High Arctic are valuable sources of forage for several resident and migratory herbivores; however, the effects of grazing on these systems have been rarely studied. We simulated grazing in two wet sedge meadows at a site on Ellesmere Island that has not been affected by grazing. Over two summers, we clipped plots at four different frequencies and removed litter to assess effects on aboveground net primary production, availability of soil nitrogen, shoot concentrations of carbon and nitrogen, and soil temperature and moisture regimes. Available soil nitrate and ammonium were highest in plots with intermediate clipping frequencies. Shoot nitrogen concentrations were also greater at intermediate clipping frequencies in two of the four species studied. Aboveground net primary production decreased after clipping, regardless of frequency. Litter removal resulted in slightly increased soil moisture, but had no effect on aboveground net primary production. Soil temperature was not affected by any of our treatments. These results suggest that nitrogen cycling is stimulated by intermediate frequencies of simulated grazing, but clipping decreased aboveground net primary production in ungrazed high arctic wet sedge tundra.

AbstractWe used retrospective analysis of the widespread evergreen dwarf-shrub, Cassiope tetragona, to reconstruct average summer air temperature for Alexandra Fiord, Ellesmere Island, Canada. Retrospective analysis is a technique based on dendrochronological methods. In this study, chronologies are based on the morphological characteristics of the plant stems. Two growth and two reproduction chronologies, ranging from 80 to 118 years long, were developed from each of two populations at the High Arctic site. We used multiple regression models to develop a 100-year-long (1895–1994) reconstruction of July–September average air temperature that explained 45% of the climatic variance in the instrumental record. The reconstruction revealed an increase in summer temperature from ∼1905 to the early 1960s, a cooling trend from the mid-1960 to the 1970s, and an increase in temperature after 1980. These historical temperature patterns correspond well with those from other climate proxies from sites on Ellesmere and Devon Islands. As well, the similarity between our model and an arctic-wide proxy temperature time series suggests that the Cassiope-based reconstruction contains a large-scale temperature signal. There is great potential for the development of proxy climate data using Cassiope tetragonafrom sites throughout the Arctic.

The present work investigates C and N allocation patterns in two forage plants: a forb, Oxyria digyna, and a sedge, Eriophorum angustifolium, in subarctic Sweden and high arctic Canada. Short‐term changes in concentrations after simulated or natural herbivory (caused by Gynaephora groenlandicaon Oxyriain the high arctic habitat) were also investigated. There were no clear differences in concentrations of C and N between the high arctic and subarctic sites in either species. In Oxyriaof the subarctic habitat, the minimum N concentrations occurred at earlier phenological stages compared with plants in the high arctic habitat. Simulated herbivory increased the concentration of C in belowground tissues relative to those in control plants in Oxyriaat the subarctic site, which may indicate increased allocation of non‐C compounds to the growing shoots or daughter ramets. Herbivory by Gynaephora groenlandicacaterpillars increased the N concentrations of Oxyriaboth in aboveground and belowground tissues, possible indicating increased uptake of N in the high arctic habitat. Eriophorumdid not show clear trends in concentrations relating to habitat, phenology or simulated herbivory. The difference between Oxyriaand Eriophorumin their response to herbivory apparently resulted from contrasting growth habits between the species. Tiller death after reproduction and long leaf life span may be the main reasons for the lack of clear patterns in concentrations in Eriophorum.Compensation after herbivory may be attained by the early production of daughter tillers in Eriophoruminstead of the regrowth of the damaged ramets, as in Oxyria.Monitoring the responses for only one season is apparently a too short time period in these long‐lived plants.

We investigated population structure of Carex aquatilisssp. stans, Carex membranacea, and Eriophorum angustifoliumssp. tristein ungrazed and grazed (by muskoxen) high arctic sedge meadow vegetation. Age structure, density, biomass, growth, and reproduction of tillers were compared between sites in 1995 and 1996. Tillers were considerably older in ungrazed vegetation relative to grazed vegetation. The density and biomass of C. stanswere significantly greater in grazed vegetation than in ungrazed vegetation, whereas the density of C. membranaceaand E. tristewas the same but the biomass was lower relative to the ungrazed meadows. The grazed plants of all species reproduced earlier, but no seedlings were found in grazed vegetation, in contrast to ungrazed vegetation. Carex stansappears to gain competitive advantage over the other two species in grazed vegetation. This may be due to the larger belowground biomass, the high phenotypic plasticity in rhizome lengths and more persistent rhizome connections. E. tristemay be the least buffered against grazing due to its lower belowground biomass and the early disintegration of rhizomes. On the other hand, E. tristeallocates more to sexual reproduction than the two other species.