Monique M. P. D. Heijmans

Climatic modifiers of the response to nitrogen deposition in peat-forming Sphagnum mosses: a meta-analysis

Peatlands in the northern hemisphere have accumulated more atmospheric carbon (C) during the Holocene than any other terrestrial ecosystem, making peatlands long-term C sinks of global importance. Projected increases in nitrogen (N) deposition and temperature make future accumulation rates uncertain. Here, we assessed the impact of N deposition on peatland C sequestration potential by investigating the effects of experimental N addition on Sphagnum moss. We employed meta-regressions to the results of 107 field experiments, accounting for sampling dependence in the data. We found that high N loading (comprising N application rate, experiment duration, background N deposition) depressed Sphagnum production relative to untreated controls. The interactive effects of presence of competitive vascular plants and high tissue N concentrations indicated intensified biotic interactions and altered nutrient stochiometry as mechanisms underlying the detrimental N effects. Importantly, a higher summer temperature (mean for July) and increased annual precipitation intensified the negative effects of N. The temperature effect was comparable to an experimental application of almost 4 g N m(-2)  yr(-1) for each 1°C increase. Our results indicate that current rates of N deposition in a warmer environment will strongly inhibit C sequestration by Sphagnum-dominated vegetation.

Spatial and temporal performance of the miniface (free air CO2 enrichment) system on bog ecosystems in northern and central Europe

The Bog Ecosystem Research Initiative (BERI) projectwas initiated to investigate, at five climaticallydifferent sites across Europe, the effects of elevatedCO2 and N deposition on the net exchange ofCO2 and CH4 between bogs and the atmosphere,and to study the effects of elevated CO2 and Ndeposition on the plant biodiversity of bogcommunities. A major challenge to investigate theeffects of elevated CO2 on vegetation andecosystems is to apply elevated CO2concentrations to growing vegetation without changingthe physical conditions like climate and radiation.Most available CO2 enrichment methods disturb thenatural conditions to some degree, for instance closedchambers or open top chambers. Free Air CO2Enrichment (FACE) systems have proven to be suitableto expose plants to elevated CO2 concentrationswith minimal disturbance of their natural environment.The size and spatial scale of the vegetation studiedwithin the BERI project allowed the use of a modifiedversion of a small FACE system called MiniFACE. Thispaper describes the BERI MiniFACE design as well asits temporal and spatial performance at the five BERIfield locations. The temporal performance of theMiniFACE system largely met the quality criteriadefined by the FACE Protocol. One minute averageCO2 concentrations measured at the centre of thering stayed within 20% of the pre-set target for morethan 95% of the time. Increased wind speeds werefound to improve the MiniFACE systems temporalperformance. Spatial analyses showed no apparentCO2 gradients across a ring during a 4 day periodand the mean differences between each sampling pointand the centre of the ring did not exceed 10%.Observations made during a windy day, causing aCO2 concentration gradient, and observations madeduring a calm day indicated that short term gradientstend to average out over longer periods of time. On aday with unidirectional strong winds, CO2concentrations at the upwind side of the ring centrewere higher than those made at the centre and at thedownwind side of the ring centre, but the bell-shapeddistribution was found basically the same for thecentre and the four surrounding measurement points,implying that the short term (1 sec) variability ofCO2 concentrations across the MiniFACE ring isalmost the same at any point in the ring. Based on gasdispersion simulations and measured CO2concentration profiles, the possible interferencebetween CO2-enriched and control rings was foundto be negligible beyond a centre-to-centre ringdistance of 6 m.

The response of Arctic vegetation to the summer climate: relation between shrub cover, NDVI, surface albedo and temperature

Recently observed Arctic greening trends from normalized difference vegetation index (NDVI) data suggest that shrub growth is increasing in response to increasing summer temperature. An increase in shrub cover is expected to decrease summer albedo and thus positively feed back to climate warming. However, it is unknown how albedo and NDVI are affected by shrub cover and inter-annual variations in the summer climate. Here, we examine the relationship between deciduous shrub fractional cover, NDVI and albedo using field data collected at a tundra site in NE Siberia. Field data showed that NDVI increased and albedo decreased with increasing deciduous shrub cover. We then selected four Arctic tundra study areas and compiled annual growing season maximum NDVI and minimum albedo maps from MODIS satellite data (2000–10) and related these satellite products to tundra vegetation types (shrub, graminoid, barren and wetland tundra) and regional summer temperature. We observed that maximum NDVI was greatest in shrub tundra and that inter-annual variation was negatively related to summer minimum albedo but showed no consistent relationship with summer temperature. Shrub tundra showed higher albedo than wetland and barren tundra in all four study areas. These results suggest that a northwards shift of shrub tundra might not lead to a decrease in summer minimum albedo during the snow-free season when replacing wetland tundra. A fully integrative study is however needed to link results from satellite data with in situ observations across the Arctic to test the effect of increasing shrub cover on summer albedo in different tundra vegetation types.

Effects of increased nitrogen deposition on the distribution of 15N-labeled nitrogen between Sphagnum and vascular plants

To elucidate the sensitivity of bog ecosystems to high levels of nitrogen (N) deposition, we investigated the fate of 15N-labeled N deposition in bog vegetation in the Netherlands, both at ambient and increased N deposition. We doubled N deposition by adding 5 g N m−2 y−1 as dissolved NH4NO3 during three growing seasons to large peat monoliths (1.1 m diameter) with intact bog vegetation kept in large outdoor containers. A small amount of 15N tracer was applied at the start of the second growing season, and its distribution among Sphagnum, vascular plant species, and peat was determined at the end of the third growing season. The 15N tracer was also applied to additional plots at the untreated field site to check for initial distribution. One week after addition, 79% of the total amount of 15N retrieved was found in the living Sphagnum layer and less than 10% had been captured by vascular plants. Fifteen months later, 63% of the total amount of 15N retrieved was still present in the living Sphagnum layer at ambient N deposition. Increased N deposition significantly reduced the proportion of 15N in Sphagnum and increased the amount of 15N in vascular plants. Deep-rooting vascular plant species were significantly more 15N enriched, suggesting that at higher atmospheric inputs N penetrates deeper into the peat. Our results provide the first direct experimental evidence for that which has often been suggested: Increased atmospheric N deposition will lead to increased N availability for vascular plants in ombrotrophic mires.

The effect of temperature on growth and competition between Sphagnum species

Peat bogs play a large role in the global sequestration of C, and are often dominated by different Sphagnum species. Therefore, it is crucial to understand how Sphagnum vegetation in peat bogs will respond to global warming. We performed a greenhouse experiment to study the effect of four temperature treatments (11.2, 14.7, 18.0 and 21.4°C) on the growth of four Sphagnum species: S. fuscum and S. balticum from a site in northern Sweden and S. magellanicum and S. cuspidatum from a site in southern Sweden. In addition, three combinations of these species were made to study the effect of temperature on competition. We found that all species increased their height increment and biomass production with an increase in temperature, while bulk densities were lower at higher temperatures. The hollow species S. cuspidatum was the least responsive species, whereas the hummock species S. fuscum increased biomass production 13-fold from the lowest to the highest temperature treatment in monocultures. Nutrient concentrations were higher at higher temperatures, especially N concentrations of S. fuscum and S. balticum increased compared to field values. Competition between S. cuspidatum and S. magellanicum was not influenced by temperature. The mixtures of S. balticum with S. fuscum and S. balticum with S. magellanicum showed that S. balticum was the stronger competitor, but it lost competitive advantage in the highest temperature treatment. These findings suggest that species abundances will shift in response to global warming, particularly at northern sites where hollow species will lose competitive strength relative to hummock species and southern species.

Long-term effects of climate change on vegetation and carbon dynamics in peat bogs

Abstract Questions: What are the long-term effects of climate change on the plant species composition and carbon sequestration in peat bogs? Methods: We developed a bog ecosystem model that includes vegetation, carbon, nitrogen and water dynamics. Two groups of vascular plant species and three groups of Sphagnum species compete with each other for light and nitrogen. The model was tested by comparing the outcome with long-term historic vegetation changes in peat cores from Denmark and England. A climate scenario was used to analyse the future effects of atmospheric CO2, temperature and precipitation. Results: The main changes in the species composition since 1766 were simulated by the model. Simulations for a future warmer, and slightly wetter, climate with doubling CO2 concentration suggest that little will change in species composition, due to the contrasting effects of increasing temperatures (favouring vascular plants) and CO2 (favouring Sphagnum). Further analysis of the effects of temperature showed that simulated carbon sequestration is negatively related to vascular plant expansion. Model results show that increasing temperatures may still increase carbon accumulation at cool, low N deposition sites, but decrease carbon accumulation at high N deposition sites. Conclusions: Our results show that the effects of temperature, precipitation, N-deposition and atmospheric CO2 are not straightforward, but interactions between these components of global change exist. These interactions are the result of changes in vegetation composition. When analysing long-term effects of global change, vegetation changes should be taken into account and predictions should not be based on temperature increase alone.

The cooling capacity of mosses : controls on water and energy fluxes in a siberian tundra site

Arctic tundra vegetation composition is expected to undergo rapid changes during the coming decades because of changes in climate. Higher air temperatures generally favor growth of deciduous shrubs, often at the cost of moss growth. Mosses are considered to be very important to critical tundra ecosystem processes involved in water and energy exchange, but very little empirical data are available. Here, we studied the effect of experimental moss removal on both understory evapotranspiration and ground heat flux in plots with either a thin or a dense low shrub canopy in a tundra site with continuous permafrost in Northeast Siberia. Understory evapotranspiration increased with removal of the green moss layer, suggesting that most of the understory evapotranspiration originated from the organic soil layer underlying the green moss layer. Ground heat flux partitioning also increased with green moss removal indicating the strong insulating effect of moss. No significant effect of shrub canopy density on understory evapotranspiration was measured, but ground heat flux partitioning was reduced by a denser shrub canopy. In summary, our results show that mosses may exert strong controls on understory water and heat fluxes. Changes in moss or shrub cover may have important consequences for summer permafrost thaw and concomitant soil carbon release in Arctic tundra ecosystems.

Field Simulation of Global Change: Transplanting Northern Bog Mesocosms Southward

A large proportion of northern peatlands consists of Sphagnum-dominated ombrotrophic bogs. In these bogs, peat mosses (Sphagnum) and vascular plants occur in an apparent stable equilibrium, thereby sustaining the carbon sink function of the bog ecosystem. How global warming and increased nitrogen (N) deposition will affect the species composition in bog vegetation is still unclear. We performed a transplantation experiment in which mesocosms with intact vegetation were transplanted southward from north Sweden to north-east Germany along a transect of four bog sites, in which both temperature and N deposition increased. In addition, we monitored undisturbed vegetation in control plots at the four sites of the latitudinal gradient. Four growing seasons after transplantation, ericaceous dwarf shrubs had become much more abundant when transplanted to the warmest site which also had highest N deposition. As a result ericoid aboveground biomass in the transplanted mesocosms increased most at the southernmost site, this site also had highest ericoid biomass in the undisturbed vegetation. The two dominant Sphagnum species showed opposing responses when transplanted southward; Sphagnum balticum height increment decreased, whereas S. fuscum height increment increased when transplanted southward. Sphagnum production did not differ significantly among the transplanted mesocosms, but was lowest in the southernmost control plots. The dwarf shrub expansion and increased N concentrations in plant tissues we observed, point in the direction of a positive feedback toward vascular plant-dominance suppressing peat-forming Sphagnum in the long term. However, our data also indicate that precipitation and phosphorus availability influence the competitive balance between Sphagnum, dwarf shrubs and graminoids.

Response of a Sphagnum bog plant community to elevated CO2 and N supply

The response of plant growth to rising CO2 levels appears todepend on nutrient availability, but it is not known whether the growth of bogplants reacts similarly. We therefore studied the effects of elevatedCO2 in combination with N supply on the growth ofSphagnum mosses and vascular plants in ombrotrophic bogvegetation. Because the growth of Sphagnum is lessnutrient-limited than that of vascular plants, we hypothesized thatSphagnum would benefit from elevated CO2. In ourgreenhouse experiment, peat monoliths (34 cm diameter, 40cm deep) with intact bog vegetation were exposed to ambient (350ppmv) or elevated (560 ppmv) atmosphericCO2 combined with low (no N addition) or high (5 g Nm−2 yr−1 added) N deposition for twogrowing seasons. Elevated atmospheric CO2 had unexpected deleterious effectson the growth of Sphagnummagellanicum, the dominant Sphagnumspecies. Growth was greatly reduced, particularly in the second growing seasonwhen, regardless of N supply, the mosses looked unhealthy. The negativeCO2 effect was strongest in the warmest months, suggesting a combinedeffect of elevated CO2 and the raised temperatures in the greenhouse.High N deposition favored Rhynchosporaalba, which became the dominant vascular plant speciesduring the experiment. Biomass increased more when N supply was high. There wereno significant effects of elevated CO2 on vascular plants, althoughelevated CO2 combined with high N supply tended to increase theaboveground vascular plant biomass. As Sphagnum is the maincarbon-sequestrating species in bogs and rising atmospheric CO2levels are likely to be followed by increases in temperature, there is an urgentneed for further research on the combined effects of elevated CO2 andincreased temperature on Sphagnum growth in bog ecosystems.

Effects of elevated CO2 and N deposition on CH4 emissions from European mires

[1] Methane fluxes were measured at five sites representing oligotrophic peatlands along a European transect. Five study plots were subjected to elevated CO2 concentration (560 ppm), and five plots to NH4NO3 (3 or 5 g N yr(-1)). The CH4 emissions from the control plots correlated in most cases with the soil temperatures. The depth of the water table, the pH, and the DOC, N and SO4 concentrations were only weakly correlated with the CH4 emissions. The elevated CO2 treatment gave nonsignificantly higher CH4 emissions at three sites and lower at two sites. The N treatment resulted in higher methane emissions at three sites (nonsignificant). At one site, the CH4 fluxes of the N-treatment plots were significantly lower than those of the control plots. These results were not in agreement with our hypotheses, nor with the results obtained in some earlier studies. However, the results are consistent with the results of the vegetation analyses, which showed no significant treatment effects on species relationships or biomass production.