• Energy Research

To evaluate the effects of climate change on boreal forest ecosystems, both atmospheric CO2 (to 560 ppmv) and air temperature (by 3°–5°C above ambient) were increased at a forested headwater catchment in southern Norway. The entire catchment (860 m2) is enclosed within a transparent greenhouse, and the upper 20% of the catchment area is partitioned such that it receives no climate treatment and serves as an untreated control. Both the control and treatment areas inside the greenhouse receive deacidified rain. Within 3 years, soil nitrogen (N) mineralization has increased and the growing season has been prolonged relative to the control area. This has helped to sustain an increase in plant growth relative to the control and has also promoted increased N export in stream water. Photosynthetic capacity and carbon–nitrogen ratio of new leaves of most plant species did not change. While the ecosystem now loses N, the long-term fate of soil N is a key uncertainty in predicting the future response of boreal ecosystems to climate change.

When perennial herbs face the risk of being outcompeted in the course of succession, they are hypothesized to either increase their biomass allocation to flowers and seeds or to invest more in vegetative growth. We tested these hypotheses in a 3-year garden experiment with four perennials (Hypochaeris radicata, Cirsium dissectum, Succisa pratensis and Centaurea jacea) by growing them in the midst of a tall tussock-forming grass (Molinia caerulea) that may successionally replace them in their natural habitat. In all species except for the short-lived H. radicata, costs of sexual reproduction were significant over the 3 years, since continuous bud removal enhanced total biomass or rosette number. To mimic succession we added nutrients, which resulted in a tripled grass biomass and higher death rates in the shorter-lived species. The simulated succession resulted also in a number of coupled growth responses in the survivors: enhanced plant size as well as elevated seed production. The latter was partly due to larger plant sizes, but mostly due to higher reproductive allocation, which in turn could be partly explained by lower relative somatic costs and by lower root-shoot ratios in the high-nutrient plots. Our results suggest that perennial plants can increase both their persistence and their colonization ability by simultaneously increasing their vegetative size and reproductive allocation in response to enhanced competition and nutrient influxes. These responses can be very important for the survival of a species in a metapopulation context.

Soil respiration is an important pathway of the C cycle. However, it is still poorly understood how changes in plant community diversity can affect this ecosystem process. Here we used a long-term experiment consisting of a gradient of grassland plant species richness to test for effects of diversity on soil respiration. We hypothesized that plant diversity could affect soil respiration in two ways. On the one hand, more diverse plant communities have been shown to promote plant productivity, which could increase soil respiration. On the other hand, the nutrient concentration in the biomass produced has been shown to decrease with diversity, which could counteract the production-induced increase in soil respiration. Our results clearly show that soil respiration increased with species richness. Detailed analysis revealed that this effect was not due to differences in species composition. In general, soil respiration in mixtures was higher than would be expected from the monocultures. Path analysis revealed that species richness predominantly regulates soil respiration through changes in productivity. No evidence supporting the hypothesized negative effect of lower N concentration on soil respiration was found. We conclude that shifts in productivity are the main mechanism by which changes in plant diversity may affect soil respiration.

Summary Boreal and subarctic peatlands contain 20–30% of the world’s soil organic carbon, and if growing, they constitute sinks for atmospheric CO2. We hypothesized that even in the nutrient‐poor bog environment, elevated CO2would stimulateSphagnumgrowth more than vascular plant growth, thereby improvingSphagnum’scompetitive strength and enhancing carbon (C) sequestration. Free‐air carbon dioxide enrichment (FACE) experiments took place on predominantly ombrotrophic peatbog‐lawns in Finland (FI), Sweden (SW), The Netherlands (NL), and Switzerland (CH). After 3 yr of treatment, increased CO2concentration (560 ppm on volume basis) had no significant effect onSphagnumor vascular plant biomass at either site. This research suggests that, just as with other nutrient‐poor ecosystems, increased atmospheric CO2concentrations will have a limited effect on bog ecosystems.

Climatewarming is known to increase the aboveground productivity of tundra ecosystems.Recently, belowground biomass is receiving more attention, but the effects of climate warming onbelowground productivity remain unclear. Enhanced understanding of the belowground componentof the tundra is important in the context of climate warming, sincemost carbon is sequesteredbelowground in these ecosystems. In this study we synthesized published tundra belowgroundbiomass data from36 field studies spanning amean annual temperature (MAT) gradient from−20 °C to 0 °C across the tundra biome, and determined the relationships between different plantbiomass pools andMAT. Our results show that the plant community biomass–temperaturerelationships are significantly different between above and belowground. Aboveground biomassclearly increased withMAT, whereas total belowground biomass and fine root biomass did not showa significant increase over the broadMATgradient. Our results suggest that biomass allocation oftundra vegetation shifts towards aboveground in warmer conditions,which could impact on thecarbon cycling in tundra ecosystems through altered litter input and distribution in the soil, aswellas possible changes in root turnover.

AbstractSphagnum cuspidatum,S. magellanicumandS. rubellumare three co‐occurring peat mosses, which naturally have a different distribution along the microtopographical gradient of the surface of peatlands. We set out an experiment to assess the interactive effects of water table (low: −10 cm and high: −1 cm) and precipitation (present or absent) on the CO2assimilation and evaporation of these species over a 23‐day period. Additionally, we measured which sections of the moss layer were responsible for light absorption and bulk carbon uptake. Thereafter, we investigated how water content affected carbon uptake by the mosses. Our results show that at high water table, CO2assimilation of all species gradually increased over time, irrespective of the precipitation. At low water table, net CO2assimilation of all species declined over time, with the earliest onset and highest rate of decline forS. cuspidatum. Precipitation compensated for reduced water tables and positively affected the carbon uptake of all species. Almost all light absorption occurred in the first centimeter of theSphagnumvegetation and so did net CO2assimilation. CO2assimilation rate showed species‐specific relationships with capitulum water content, with narrow but contrasting optima forS. cuspidatumandS. rubellum. Assimilation byS. magellanicumwas constant at a relatively low rate over a broad range of capitulum water contents. Our study indicates that prolonged drought may alter the competitive balance between species, favoring hummock species over hollow species. Moreover, this study shows that precipitation is at least equally important as water table drawdown and should be taken into account in predictions about the fate of peatlands with respect to climate change.