Global plant trait relationships extend to the climatic extremes of the tundra biome.

The majority of variation in six traits critical to the growth, survival and reproduction of plant species is thought to be organised along just two dimensions, corresponding to strategies of plant size and resource acquisition. However, it is unknown whether global plant trait relationships extend to climatic extremes, and if these interspecific relationships are confounded by trait variation within species. We test whether trait relationships extend to the cold extremes of life on Earth using the largest database of tundra plant traits yet compiled. We show that tundra plants demonstrate remarkably similar resource economic traits, but not size traits, compared to global distributions, and exhibit the same two dimensions of trait variation. Three quarters of trait variation occurs among species, mirroring global estimates of interspecific trait variation. Plant trait relationships are thus generalizable to the edge of global trait-space, informing prediction of plant community change in a warming world.

Phenological response of tundra plants to background climate variation tested using the International Tundra Experiment.

The rapidly warming temperatures in high-latitude and alpine regions have the potential to alter the phenology of Arctic and alpine plants, affecting processes ranging from food webs to ecosystem trace gas fluxes. The International Tundra Experiment (ITEX) was initiated in 1990 to evaluate the effects of expected rapid changes in temperature on tundra plant phenology, growth and community changes using experimental warming. Here, we used the ITEX control data to test the phenological responses to background temperature variation across sites spanning latitudinal and moisture gradients. The dataset overall did not show an advance in phenology; instead, temperature variability during the years sampled and an absence of warming at some sites resulted in mixed responses. Phenological transitions of high Arctic plants clearly occurred at lower heat sum thresholds than those of low Arctic and alpine plants. However, sensitivity to temperature change was similar among plants from the different climate zones. Plants of different communities and growth forms differed for some phenological responses. Heat sums associated with flowering and greening appear to have increased over time. These results point to a complex suite of changes in plant communities and ecosystem function in high latitudes and elevations as the climate warms.

Increased plant biomass in a High Arctic heath community from 1981 to 2008.

The Canadian High Arctic has been warming for several decades. Over this period, tundra plant communities have been influenced by regional climate change, as well as other disturbances. At a site on Ellesmere Island, Nunavut, Canada, we measured biomass and composition changes in a heath community over 13 years using a point-intercept method in permanent plots (1995-2007) and over 27 years using a biomass harvest comparison (1981-2008). Results from both methods indicate that the community became more productive over time, suggesting that this ecosystem is currently in transition. Bryophyte and evergreen shrub abundances increased, while deciduous shrub, forb, graminoid, and lichen cover did not change. Species diversity also remained unchanged. Because of the greater evergreen shrub cover, canopy height increased. From 1995 to 2007, mean annual temperature and growing season length increased at the site. Maximum thaw depth increased, while soil water content did not change. We attribute the increased productivity of this community to regional warming over the past 30-50 years. This study provides the first plot-based evidence for the recent pan-Arctic increase in tundra productivity detected by satellite-based remote-sensing and repeat-photography studies. These types of ground-level observations are critical tools for detecting and projecting long-term community-level responses to warming.