Increasing atmospheric carbon dioxide: possible effects on arctic tundra

Effect of simulated warming on the functional traits of Leymus chinensis plant in Songnen grassland

Leymus chinensis grassland in Northeast China provides a natural laboratory for the investigation of climate change. The response of L. chinensis to experimental warming can provide insight into its regeneration behaviour and the likely composition of future communities under warmer climate. We used MSR-2420 infrared radiators to elevate temperature and examined soil organic carbon and nitrogen and soil total phosphorus and determined the growth and physiology of L. chinensis in response to manipulations of ambient condition and warming. Results showed that compared with the control, L. chinensis subjected to warming treatment showed increased soil organic carbon and soil total nitrogen, but no significant difference was observed in soil total phosphorus. Climate warming increased shoot biomass, ecosystem respiration, and ecosystem water-use efficiency and reduced net ecosystem CO2 exchange and evapotranspiration. This result implies that warming could rapidly alter carbon fluxes. The effect of warming treatment significantly increased the contents of glucose and fructose and significantly inhibited sucrose synthesis. However, the TCA cycle was enhanced when citric and malic acid contents further accumulated. The results implied that L. chinensis probably enhanced its warming adaption mechanism mainly through increasing glycolysis consumption when it was exposed to elevated temperature. These results provide an understanding of the fundamental evidence explaining the primary metabolism of L. chinensis in response to warming and suggest the future impact of the terrestrial carbon-cycle feedback on global climate change.

Shifting dominance within a montane vegetation community: results of a climate-warming experiment

In experimentally heated plots that each span a soil moisture gradient in a Rocky Mountain meadow, aboveground biomass of Artemisia tridentata (a sagebrush) increased in the drier habitat and that of Pentaphylloides floribunda (a shrub cinquefoil) increased in the wetter habitat relative to control plots. In contrast, aboveground forb biomass decreased in the wet and dry habitats of the heated plots. These results, combined with evidence for enhanced sagebrush seedling establishment rates in the heated plots, suggest that the increased warming expected under an atmosphere with a concentration of carbon dioxide twice that of pre-industrial levels could change the dominant vegetation of a widespread meadow habitat.

Nitrogen balance in forest soils: nutritional limitation of plants under climate change stresses

Forest ecosystems with low soil nitrogen (N) availability are characterized by direct competition for this growth-limiting resource between several players, i.e. various components of vegetation, such as old-growth trees, natural regeneration and understorey species, mycorrhizal fungi, free-living fungi and bacteria. With the increase in frequency and intensity of extreme climate events predicted in current climate change scenarios, also competition for N between plants and/or soil microorganisms will be affected. In this review, we summarize the present understanding of ecosystem N cycling in N-limited forests and its interaction with extreme climate events, such as heat, drought and flooding. More specifically, the impacts of environmental stresses on microbial release and consumption of bioavailable N, N uptake and competition between plants, as well as plant and microbial uptake are presented. Furthermore, the consequences of drying-wetting cycles on N cycling are discussed. Additionally, we highlight the current methodological difficulties that limit present understanding of N cycling in forest ecosystems and the need for interdisciplinary studies.

Arctic terrestrial ecosystems and environmental change

The impacts of environmental change on Arctic terrestrial ecosystems are complex and difficult to predict because of the many interactions which exist within ecosystems and between several concurrently changing environmental variables. However, some general predictions can be made. (i) In the sub-Arctic, subtle shifts in plant community composition with occasional losses of plant species are more likely than immigration of exotic species. In the high Arctic, colonization of bare ground can proceed and there are likely to be shifts in ecotypes. Major shifts in vegetation zones, such as the advance of the boreal forest, are likely to be slow and species specific responses will result in different assemblages of species in plant communities in the longer term. All changes in community structure, apart from species removal by direct extreme weather conditions (e.g. drought) will be slow because of the slow growth, low levels of fecundity and slow migration rates of plant species over large latitudinal ranges. (ii) Mobile mammals and birds can probably adjust to changes in the distribution of their food plants or prey in the Arctic, but vertebrate and invertebrate herbivores may face problems with changes in the quality of their food plants. Non-migratory animals could be severely affected by altered winter snow conditions which affect availability of food and shelter. (iii) Increases in primary production are uncertain and depend mainly upon the responses of soil microbial decomposer activity to changes in soil temperature, moisture and plant litter quality. Assumptions that climate warming will lead to warmer soils and increased nutrient availability to sustain higher productivity are uncertain as greater biomass may lead to reduced soil temperatures through insulation effects and increased nutrients released may be immobilized by soil microorganisms. (iv) Changes in environmental conditions are themselves often uncertain. There is particular doubt about changes in precipitation, growing season length, cloudiness and UV-B radiation levels while such environmental changes are likely to vary in magnitude and direction between different regions of the Arctic. (v) The large populations and circumpolar distributions typical of Arctic biota lead to a strong buffering of changes in biodiversity. Perhaps the greatest threats to Arctic biota will be imposed by the degradation of permafrost which may lead to either waterlogging or drought depending upon precipitation regimes.

EcoCELLs: tools for mesocosm scale measurements of gas exchange

We describe the use of a unique plant growth facility, which has as its centerpiece four 'EcoCELLs', or 5x7 m mesocosms designed as open-flow, mass-balance systems for the measurement of carbon, water and trace gas fluxes. This system is unique in that it was conceived specifically to bridge the gap between measurement scales during long-term experiments examining the function and development of model ecosystems. There are several advantages to using EcoCELLs, including (i) the same theory of operation as leaf level gas exchange systems, but with continuous operation at a much larger scale; (ii) the ability to independently evaluate canopy-level and ecosystem models; (iii) simultaneous manipulation of environmental factors and measurement of system-level responses, and (iv) maximum access to, and manipulation of, a large rooting volume. In addition to discussing the theory, construction and relative merits of EcoCELLs, we describe the calibration and use of the EcoCELLs during a 'proof of concept' experiment. This experiment involved growing soybeans under two ambient CO2 concentrations (approximately 360 and 710 micromoles mol-1). During this experiment, we asked 'How accurate is the simplest model that can be used to scale from leaf-level to canopy-level responses?' in order to illustrate the utility of the EcoCELLs in validating canopy-scale models.