Kari Klanderud

Plot-scale evidence of tundra vegetation change and links to recent summer warming

Temperature is increasing at unprecedented rates across most of the tundra biome(1). Remote-sensing data indicate that contemporary climate warming has already resulted in increased productivity ov ...

Recent increases in species richness and shifts in altitudinal distributions of Norwegian mountain plants

Opportunities for observing long-term changes in natural biota are rare. Observations on the distribution and frequency of vascular plants were performed on 23 mountains situated along a west–east gradient in Jotunhei men, central Norway, where detailed site descriptions and species lists exist from ad 1930–31. The sites were resurveyed during the summer of 1998, to examine possible changes in species richness and species distributions along the altitudinal gradient during a 68-year period. Increased species richness was found on 19 of the mountains and was most pronounced at lower altitudes and in the eastern areas. Lowland species, dwarf shrubs and species with wide altitudinal and ecological ranges showed the greatest increases in abundance and altitudinal advances since the 1930–31 study. Species with more restricted habitat demands, such as some hygrophilous snow-bed species, have declined. High-altitude species have disappeared from their lower-elevation sites and increased their abundance at the highest altitudes. Climatic warming occurring in the last 100 years might have allowed the invasion of lowland and lee-slope species. Increased competition at sites where such species have invaded may have led to a decreased abundance of the less competitive species and a concentration of high-altitude species on the highest ridges. Natural succession since the ‘Little Ice Age’, increased deposition of nitrogen during recent years and changes in grazing and tourism might have in‘ uenced some of the species turnovers, but recent climatic changes are considered to be the most likely major driving factor for the changes observed.

SIMULATED CLIMATE CHANGE ALTERED DOMINANCE HIERARCHIES AND DIVERSITY OF AN ALPINE BIODIVERSITY HOTSPOT

Alpine and arctic ecosystems may be particularly vulnerable to climate change. We know little about alpine plant community responses to the predicted abiotic changes, or to possible changes in the biotic environment caused by climate change. Four years of experimental warming and nutrient addition altered dominance hierarchies, community structure, and diversity of an alpine biodiversity hotspot in south Norway. The previously dominant dwarf shrub Dryas octopetala was replaced by graminoids and forbs under nutrient addition and warming with nutrients. Community diversity declined due to decreased bryophyte and lichen richness and abundances, and dwarf shrub abundances. In controls and in plots with only warming, where Dryas maintained dominance, the relationships between changes in Dryas cover and changes in community parameters were negative, suggesting that Dryas controls community processes. Under nutrient addition, bryophyte and lichen diversity decreased with decreasing Dryas cover, probably due to increased competition from graminoids and forbs. The shift in dominance hierarchies changed community structure and dynamics through increased biomass, vegetation height, and competition for light. Community diversity dropped primarily because changes in the abiotic environment modified biotic interactions, highlighting that species interaction must be considered in climate change experiments and in models predicting climate change effects.

Local temperatures inferred from plant communities suggest strong spatial buffering of climate warming across Northern Europe

Recent studies from mountainous areas of small spatial extent (<2500 km(2) ) suggest that fine-grained thermal variability over tens or hundreds of metres exceeds much of the climate warming expected for the coming decades. Such variability in temperature provides buffering to mitigate climate-change impacts. Is this local spatial buffering restricted to topographically complex terrains? To answer this, we here study fine-grained thermal variability across a 2500-km wide latitudinal gradient in Northern Europe encompassing a large array of topographic complexities. We first combined plant community data, Ellenberg temperature indicator values, locally measured temperatures (LmT) and globally interpolated temperatures (GiT) in a modelling framework to infer biologically relevant temperature conditions from plant assemblages within <1000-m(2) units (community-inferred temperatures: CiT). We then assessed: (1) CiT range (thermal variability) within 1-km(2) units; (2) the relationship between CiT range and topographically and geographically derived predictors at 1-km resolution; and (3) whether spatial turnover in CiT is greater than spatial turnover in GiT within 100-km(2) units. Ellenberg temperature indicator values in combination with plant assemblages explained 46-72% of variation in LmT and 92-96% of variation in GiT during the growing season (June, July, August). Growing-season CiT range within 1-km(2) units peaked at 60-65°N and increased with terrain roughness, averaging 1.97 °C (SD = 0.84 °C) and 2.68 °C (SD = 1.26 °C) within the flattest and roughest units respectively. Complex interactions between topography-related variables and latitude explained 35% of variation in growing-season CiT range when accounting for sampling effort and residual spatial autocorrelation. Spatial turnover in growing-season CiT within 100-km(2) units was, on average, 1.8 times greater (0.32 °C km(-1) ) than spatial turnover in growing-season GiT (0.18 °C km(-1) ). We conclude that thermal variability within 1-km(2) units strongly increases local spatial buffering of future climate warming across Northern Europe, even in the flattest terrains.

Species‐specific responses of an alpine plant community under simulated environmental change

Abstract Question: How will warming and increased nutrient availability affect individual alpine plant species abundances (non-vascular and vascular) and community composition? Location: Dryas octopetala heath at alpine Finse, southern Norway. Methods: Four years with experimental warming (open top chambers) and nutrient addition. Detrended Correspondence Analysis and Redundancy Analysis were used to examine changes in community composition. GLM-ANOVA was used to examine treatment effects on individual species. Results: Warming alone decreased the abundance of some Carex and bryophyte species, but did not affect community composition. Nutrient addition and warming combined with nutrient addition increased the abundance of high stature species, such as grasses (Festuca spp., Poa alpina) and some forbs (e.g. Cerastium alpinum, Potentilla crantzii). Low stature forbs (e.g. Tofieldia pusilla), a lycophyte (Selaginella selaginoides) and most bryophytes and lichens decreased in abundance. After four years of warming combined with nutrient addition 57% of the mosses, 57% of the liverworts and 44% of the lichens had completely disappeared. Community composition changed significantly, with the largest shift when warming and nutrient addition was combined. Conclusions: Tall species may expand at the expense of low stature species in the alpine region if temperature and soil nutrient content increase. Contrasting responses between grasses and sedges, and species-specific responses within forbs, sedges and shrubs, within and across alpine and arctic sites, suggest that the use of functional types in environmental change research may mask important information on individual species responses. The response of one species within a functional type cannot predict the response of another. Nomenclature: Lid & Lid (1994) for vascular species; Hallingbäck & Holmåsen (1995) for bryophytes; Krog et al. (1994) for lichens.

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.

From facilitation to competition: temperature-driven shift in dominant plant interactions affects population dynamics in seminatural grasslands.

Biotic interactions are often ignored in assessments of climate change impacts. However, climate-related changes in species interactions, often mediated through increased dominance of certain species or functional groups, may have important implications for how species respond to climate warming and altered precipitation patterns. We examined how a dominant plant functional group affected the population dynamics of four co-occurring forb species by experimentally removing graminoids in seminatural grasslands. Specifically, we explored how the interaction between dominants and subordinates varied with climate by replicating the removal experiment across a climate grid consisting of 12 field sites spanning broad-scale temperature and precipitation gradients in southern Norway. Biotic interactions affected population growth rates of all study species, and the net outcome of interactions between dominants and subordinates switched from facilitation to competition with increasing temperature along the temperature gradient. The impacts of competitive interactions on subordinates in the warmer sites could primarily be attributed to reduced plant survival. Whereas the response to dominant removal varied with temperature, there was no overall effect of precipitation on the balance between competition and facilitation. Our findings suggest that global warming may increase the relative importance of competitive interactions in seminatural grasslands across a wide range of precipitation levels, thereby favouring highly competitive dominant species over subordinate species. As a result, seminatural grasslands may become increasingly dependent on disturbance (i.e. traditional management such as grazing and mowing) to maintain viable populations of subordinate species and thereby biodiversity under future climates. Our study highlights the importance of population-level studies replicated under different climatic conditions for understanding the underlying mechanisms of climate change impacts on plants.

Habitat dependent nurse effects of the dwarf-shrub Dryas octopetala on alpine and arctic plant community structure

Abstract: Some species may increase community diversity by modifying the local environment for co-occurring species. This is most common in habitats of high abiotic stress. To explore whether the mat-forming dwarf shrub Dryas octopetala functions as a nurse plant in alpine and arctic plant communities of contrasting but generally stressful environmental conditions, we measured species diversity, richness, cover, and composition of different functional groups (vascular plants, bryophytes, and lichens) inside and outside Dryas mats at sites differing in environmental severity at Finse, alpine Norway, and in arctic Svalbard. Dryas appears to function as a nurse plant primarily for bryophytes, and mainly at the most severe sites in Svalbard. At the more benign sites at Finse, total diversity (Shannon’s index) and richness, and richness of all functional groups, were lower inside Dryas than outside, suggesting that Dryas has no nurse plant function on diversity parameters there. Species composition, however, differed inside and outside Dryas in all the communities, due to both presence/absence and abundance differences at Finse, and primarily due to differences in species abundances in Svalbard. This may suggest that Dryas functions as a nurse plant for individual species of all of the three functional groups in all the communities, even if competitive impacts of Dryas may overrule the positive effects on a whole-community level. Finally, our results show that Dryas may shift from being a nurse plant to being a competitor when its cover increases, but only under relatively low abiotic stress.

The importance of Biotic vs. Abiotic drivers of local plant community composition along regional bioclimatic gradients

We assessed if the relative importance of biotic and abiotic factors for plant community composition differs along environmental gradients and between functional groups, and asked which implications this may have in a warmer and wetter future. The study location is a unique grid of sites spanning regional-scale temperature and precipitation gradients in boreal and alpine grasslands in southern Norway. Within each site we sampled vegetation and associated biotic and abiotic factors, and combined broad- and fine-scale ordination analyses to assess the relative explanatory power of these factors for species composition. Although the community responses to biotic and abiotic factors did not consistently change as predicted along the bioclimatic gradients, abiotic variables tended to explain a larger proportion of the variation in species composition towards colder sites, whereas biotic variables explained more towards warmer sites, supporting the stress gradient hypothesis. Significant interactions with precipitation suggest that biotic variables explained more towards wetter climates in the sub alpine and boreal sites, but more towards drier climates in the colder alpine. Thus, we predict that biotic interactions may become more important in alpine and boreal grasslands in a warmer future, although more winter precipitation may counteract this trend in oceanic alpine climates. Our results show that both local and regional scales analyses are needed to disentangle the local vegetation-environment relationships and their regional-scale drivers, and biotic interactions and precipitation must be included when predicting future species assemblages.

Responses in leaf functional traits and resource allocation of a dominant alpine sedge (Kobresia pygmaea) to climate warming in the Qinghai-Tibetan Plateau permafrost region

Assessing the influence of warming on leaf traits, carbon, and nutrient concentrations above and below ground to understand how the dominant sedge Kobresia pygmaea (C. B. Clarke) C. B. Clarke may respond and adapt to extant and future climate in the alpine meadow of the Qinghai-Tibetan Plateau. A warming experiment was conducted in the permafrost region of the Qinghai-Tibetan Plateau from 2008 to 2009. Two 2-year warming treatments (T1, annual warming of 2.1°C; T2, annual warming of 4.4°C) were used, and responses of leaf traits and above- and belowground carbon, nitrogen, and phosphorus concentrations of K. pygmaea were examined. The results show that both moderate (T1) and more extensive (T2) warming decreased leaf mass, leaf thickness, and vascular bundle size, and increased the mass-based photosynthetic rate (Amass) and photosynthetic nitrogen use efficiency (PNUE). A moderate warming significantly decreased leaf carbon (C), nitrogen (N), and phosphorus (P), and root C and N concentrations of K. pygmaea. These decreases were even more pronounced under the more extensive warming. The decreases in leaf N and P were significantly larger than the decrease in leaf C concentration. Root P concentration increased more under the extensive than the moderate warming. The observed increase in leaf C:N ratio in the warming treatment indicates that enhanced temperature may increase the long-term nitrogen use efficiency of K. pygmaea leaves. This again suggests that K. pygmaea might adapt well to future climate warming, and that nitrogen might be a more important factor for K. pygmaea dominated alpine meadows under future climate warming.