Anna Stenström

Responses of tundra plants to experimental warming : Meta-analysis of the international tundra experiment

The International Tundra Experiment (ITEX) is a collaborative, multisite experiment using a common temperature manipulation to examine variability in species response across climatic and geographic gradients of tundra ecosystems. ITEX was designed specifically to examine variability in arctic and alpine species response to increased temperature. We compiled from one to four years of experimental data from 13 different ITEX sites and used meta-analysis to analyze responses of plant phenology, growth, and reproduction to experimental warming. Results indicate that key phenological events such as leaf bud burst and flowering occurred earlier in warmed plots throughout the study period; however, there was little impact on growth cessation at the end of the season. Quantitative measures of vegetative growth were greatest in warmed plots in the early years of the experiment, whereas reproductive effort and success increased in later years. A shift away from vegetative growth and toward reproductive effort and success in the fourth treatment year suggests a shift from the initial response to a secondary response. The change in vegetative response may be due to depletion of stored plant reserves, whereas the lag in reproductive response may be due to the formation of flower buds one to several seasons prior to flowering. Both vegetative and reproductive responses varied among life-forms; herbaceous forms had stronger and more consistent vegetative growth responses than did woody forms. The greater responsiveness of the herbaceous forms may be attributed to their more flexible morphology and to their relatively greater proportion of stored plant reserves. Finally, warmer, low arctic sites produced the strongest growth responses, but colder sites produced a greater reproductive response. Greater resource investment in vegetative growth may be a conservative strategy in the Low Arctic, where there is more competition for light, nutrients, or water, and there may be little opportunity for successful germination or seedling development. In contrast, in the High Arctic, heavy investment in producing seed under a higher temperature scenario may provide an opportunity for species to colonize patches of unvegetated ground. The observed differential response to warming suggests that the primary forces driving the response vary across climatic zones, functional groups, and through time.

Genetic variation and clonal diversity in four clonal sedges (Carex) along the Arctic coast of Eurasia

We studied the structure of genetic variation (at both ramet‐ and genet‐level) and clonal diversity within and among populations in the four closely related arctic clonal sedges Carex bigelowii, C. ensifolia, C. lugens and C. stans by use of allozyme markers. Compared to other sedges and arctic plants, the studied taxa all had high levels of genetic variation, both within populations and taxa. These taxa contained most of the total gene diversity (HT) within populations and a small part of the diversity among populations (GST ranged 0.05–0.43). Carex bigelowii had genetic variation (HS = 0.173, mean for populations) at a comparable level to other outbreeding arctic plants and to other widespread, rhizomatous and mainly outbreeding Carex species. In contrast, C. ensifolia (HS = 0.335), C. lugens (HS = 0.339) and C. stans (HS = 0.294) had within‐population variations that were higher than in most other studied Carex species and for arctic plants in general. Genetic variation was not related to any tested environmental variable, but it was lower in areas deglaciated only 10 000 years bp compared to areas deglaciated 60 000 years bp or not glaciated at all during the Weichselian. All the populations were multiclonal, except for two populations of C. stans that were monoclonal. In contrast to genetic variation, clonal diversity decreased with latitude and did not differ between areas with different times of deglaciation. In accordance with previous studies, C. bigelowii and C. lugens were found to be outbreeding, while C. ensifolia and C. stans had mixed mating systems.

Genetic and environmental effects on morphology in clonal sedges in the Eurasian Arctic.

We studied the variation in morphological characters of importance for resource acquisition and storage in 21 populations of four clonal sedge taxa in arctic Eurasia, Carex bigelowii, C. ensifolia subsp. arctisibirica, C. lugens, and C. stans, and the response to transplantation to a common garden in Tromsø, Norway. The morphology of C. stans was distinct from the other three taxa, all of which belong to the C. bigelowii species complex. However, differences among populations within taxa were even greater than differences among taxa, and environmental variables explained 40-50% of the among-population variation in the morphological characters. Stomatal size decreased with temperature while stomatal density increased. Shoot height and leaf width were smaller at peak lemming population phase, while rhizome length was shorter at higher longitudes. Transplantation to a common garden affected stomatal density in all taxa, stomatal size and shoot height only in some taxa, while leaf width was not affected. We found a weak, but highly significant correlation between geographical, morphological, and genetic distances. We concluded that although genotypic differentiation in arctic rhizomatous Carex species is reflected in their morphology, they are also capable of plastic morphological responses to the environment and that these responses are specific for each taxon.