Limited effect of increased atmospheric CO. concentration on ombrotrophic bog vegetation

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1 Limited effect of increased atmospheric CO Blackwell Science Ltd 2 concentration on ombrotrophic bog vegetation Marcel R. Hoosbeek 1, Nico van Breemen 1, Frank Berendse 2, Philippe Grosvernier 3, Harri Vasander 4 and Bo Wallén 5 1 Department of Environmental Sciences, Laboratory of Soil Science and Geology, Wageningen University, PO Box 37, 6700 AA Wageningen, The Netherlands; 2 Department of Environmental Sciences, Terrestrial Ecology and Nature Management, Wageningen University, Bornse Steeg 69, 6708 PD Wageningen, The Netherlands; 3 Natura, Applied Biology and Environmental Eng. Le Saucy 17, 2722 Les Reussilles, Switzerland; 4 University of Helsinki, Department of Forest Ecology, PO Box 24, FIN Helsinki, Finland; 5 Department of Plant Ecology, Ecology Building, Lund University, S Lund, Sweden Summary Author for correspondence: Marcel R. Hoosbeek Tel: Fax: marcel.hoosbeek@ bodeco.beng.wag-ur.nl Received: 8 August 2000 Accepted: 21 January 2001 Boreal and subarctic peatlands contain 20 30% of the world s soil organic carbon, and if growing, they constitute sinks for atmospheric. We hypothesized that even in the nutrient-poor bog environment, elevated would stimulate Sphagnum growth more than vascular plant growth, thereby improving Sphagnum s competitive 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 concentration (560 ppm on volume basis) had no significant effect on Sphagnum or vascular plant biomass at either site. This research suggests that, just as with other nutrient-poor ecosystems, increased atmospheric concentrations will have a limited effect on bog ecosystems. Key words: elevated, free-air enrichment (FACE), carbon sequestration, bog, peat, Sphagnum. New Phytologist (2001) 150: Introduction Since the industrial revolution, the atmospheric concentration has been increasing exponentially. However, the rate of increase of atmospheric -C during the 1980s (3.2 ± 0.2 Gt C yr 1 ) is smaller than expected when considering burning of fossil C (5.5 ± 0.5 Gt C yr 1 in the 1980s), which is in part because of feedbacks involving terrestrial ecosystems ( fertilization: 1.0 ± 0.5 Gt C yr 1 ) (Schimel, 1995). About 60 Gt of -C is cycled annually along the terrestrial photosynthesis-decomposition pathway, 20 times more than the annual net addition to the atmosphere (Goudriaan, 1992). So, even small changes in net primary productivity or in decomposition of soil organic carbon (SOC) could significantly influence the net increase of atmospheric. Because elevated may strongly influence net primary productivity, species abundance and community composition (Mooney et al., 1999) and the chemical and physical composition of plant material, and therefore the decomposability of plant litter, strong feedbacks to the pools of SOC are expected. In this regard, northern peatlands are particularly important, because they contain 20 30% of the world s SOC (Gorham, 1991), and if growing, they constitute a sink for atmospheric (Van Breemen, 1995). Through their anatomical, morphological, physiological and organo-chemical properties, Sphagnum species create waterlogged, nutrient-poor, acidic conditions, thereby outcompeting most vascular plants (Van Breemen, 1995). We hypothesized that even in the nutrient-poor bog environment, elevated will stimulate plant growth (Silvola, 1985; Jauhiainen et al., 1992, 1994). We expected growth of Sphagnum to be stimulated more than that of vascular plants because increased growth lowers N concentration, which affects vascular plants much more than Sphagnum, because growth of Sphagnum is less nutrient limited than that of co-occurring vascular plants (Sveinbjörnsson & Oechel, 1992; Jauhiainen et al., 1998). Increased peat growth (due to lower decomposition rates of Sphagnum vs. vascular plant material), which positively feeds back to Sphagnum itself further depresses vascular plants. This New Phytologist (2001) 150:

2 460 sequence of events forms a strong positive feedback of increased sequestration of -C. The Bog Ecosystem Initiative (BERI) was funded by the European Commission and comprised nine research groups in five European countries. In each country, experimental field sites were installed on ombrotrophic bogs dominated by a mixture of peat mosses (Sphagnum spp.) and vascular plants. The hypothesis was tested with free-air enrichment (FACE) technology during the growing seasons of Materials and Methods Field sites The FACE experiments took place on predominantly ombrotrophic peatbog-lawns in eastern Finland (FI), southern Sweden (SW), The Netherlands (NL) and Switzerland (CH). The fifth BERI site in north-west England is not included in this study because Sphagnum biomass data were not available. Locations, climate summaries and atmospheric N deposition rates are provided in Table 1. The experiments in FI, SW and CH took place in situ, but in NL large peat cores (1.1 m diameter and 0.6 m depth) were collected in frozen condition and transferred to containers near Wageningen. The containers (same size as peat cores) were buried to a depth of 0.5 m in a grass lawn. An overview of the moss and vascular plant species composition of the field sites is provided in Table 2. FACE experiment At each site 10 FACE rings (Miglietta et al., 2001) with a diameter of 1 m (in NL 1.1 m) were randomly laid out on the Table 1 Location, climate summary, and atmospheric N deposition Location Mean daily coldest month Temp C warmest month Mean annual precipitation (mm y 1 ) Number of days with snow cover N deposition (g N m 2 y 1 ) FI SW NL CH Salmisuo, Ilomantsi, eastern Finland (62 47 N, E) Kopparåsmyren, southern Sweden (57 8 N, E) Peat from Dwingeloo (52 49 N, 6 25 E) transplanted to Wageningen (51 99 N, 5 70 E) < La Chaux-des-Breuleux, Swiss Jura (47 13 N, 7 3 E) Moss species % cover Vascular species % cover Table 2 Overview of the dominant moss and vascular plant species in 1996 FI Sphagnum balticum 61 Eriophorum vaginatum 14 Sphagnum papillosum 33 Vaccinium oxycoccos 4 Sphagnum magellanicum 4 Andromeda polifolia 2 Scheuchzeria palustris 2 SW Sphagnum magellanicum 69 Eriophorum angustifolium 8 Sphagnum papillosum 13 Vaccinium oxycoccos 3 Sphagnum balticum 7 Drosera rotundifolia 3 Sphagnum rubellum 7 Calluna vulgaris 2 Andromeda polifolia 2 NL Sphagnum magellanicum 97 Vaccinium oxycoccos 19 Sphagnum papillosum 1 Erica tetralix 9 Eriophorum angustifolium 4 Drosera rotundifolia 2 Calluna vulgaris 1 CH Sphagnum fallax 62 Carex nigra 3 Polytrichum strictum 37 Vaccinium oxycoccos 3 Eriophorum vaginatum 2 New Phytologist (2001) 150:

3 461 bog surface. In five rings the atmospheric concentration was kept at ambient levels (about 360 ppm on volume basis) while in the other rings the concentration was maintained at 560 ppm for 24 h d 1. The elevated rings were located at a distance of at least 6 m from ambient air rings to prevent pollution. To avoid edge effects, no data were collected in the outer 15 cm of each plot. Blowers next to each FACE ring supplied ambient air or -enriched air to circular tubes resting on the bog surface on which 72 small venting pipes were mounted. The venting pipes had small holes directed towards the centre of the ring at 6 and 12 cm height above the moss surface. Air was sampled in the middle of elevated rings at 7.5 cm above the moss surface and analysed for with an infrared gas analyser. Based on the measured concentration and wind speed, the supply was adjusted automatically via a PC and mass flow controllers to maintain the target concentration of 560 ppm. During the winter months the FACE system was turned off because of minimal biological activity. Calibration and test experiments were conducted to optimize and evaluate the performance of the FACE equipment. System performance was considered adequate when the 1-min average concentrations measured at the centre of the ring stayed within 20% of the preset target for more than 80% of the time. The temporal performance of the BERI FACE systems largely met these quality criteria. When calculated over the entire season, and averaged over the five replicated rings, the percentage of time in which 1-min average concentrations deviated less than 20% from the preset 560 ppm target was 95% in FI, 96% in SW, 98% in NL and 96% in CH (Miglietta et al., 2001). Biomass measurements Plant cover was measured nondestructively using the pointquadrat method (Jonasson, 1988). The point-quadrat covered a permanently marked area of 25 by 37.5 cm and consisted of 150 points with a grid size of 2.5 cm. At each of the 150 points a needle was lowered and vegetation hits were recorded. Sphagnum cover was measured at the beginning of the experiment (May or June 1996) and thereafter at the end of each growing season. Vascular plant cover was determined each year at peak biomass. Sphagnum length growth was measured according to a modified cranked wire method (Clymo, 1970). In each ring four stainless steel wires with a small brush at the lower end were inserted into the peat and fixed at 2 cm below the surface. The distance between the moss surface and the top of the wire was measured each month during the growing seasons. At the end of the experiment Sphagnum was harvested at each wire in 7.5 cm diameter columns for conversion of length to biomass. The bulk density of the moss cover was determined for the sections 0 1 cm (capitula) and 1 3 cm below the moss surface by drying at 70 C to constant weight. Above ground vascular plant biomass was determined at the end of the experiment by clipping off all vascular plants at the moss surface in a cm 2 area per ring. All species were sorted into current year and older parts and dried at 70 C to constant weight. Below ground vascular biomass was determined in three 10-cm diameter and 30-cm deep columns per ring. After sorting out, all fractions were dried at 70 C to constant weight. Results Sphagnum biomass production under ambient ranged from 190 ± 43 in CH to 310 ± 33 g m 2 yr 1 in NL (Table 3). We did not find significant increased concentration treatment effects on Sphagnum biomass at either site (using ANOVA with site and treatment as factors). Below ground vascular biomass was about four times larger than above ground vascular biomass in FI and SW (Table 4). In NL and CH this difference was smaller. The above ground response to increased concentration was positive in FI, NL and CH, but negative in SW, while the below ground response was positive at all sites. However, none of these responses is statistically significant. Because neither Sphagnum nor vascular plant biomass production was affected by increased concentration in this 3-yr experiment, the hypothesis stating that elevated would stimulate the growth of Sphagnum more than that of vascular plants was rejected. Table 3 Sphagnum dry weight production (g m 2 years 1 ) after 3 yr of exposure to either ambient or increased concentrations in FACE rings (n = 5; n.s., not significant at P 0.05; SE, standard error) Sphagnum biomass production (g m 2 yr 1 ) and s.e. Treatment response Ambient (360 ppm) Elevated (560 ppm) Elev : Amb ratio ANOVA FI 256 ± ± n.s. SW 211 ± ± n.s. NL 310 ± ± n.s. CH 190 ± ± n.s. New Phytologist (2001) 150:

4 462 Table 4 Vascular plant above and below ground biomass (g m 2 ) in 1998 after 3 years of exposure to either ambient or increased concentrations in FACE rings (n = 5; n.s. = not significant at P 0.05) Vascular plant biomass (g m 2 ) and SE a = above ground, b = below ground Treatment response Ambient (360 ppm) Elevated (560 ppm) Elev : Amb ratio ANOVA FI a 73 ± 9 a 85 ± n.s. b 272 ± 55 b 300 ± n.s. SW a 67 ± 7 a 52 ± n.s. b 297 ± 61 b 383 ± n.s. NL a 227 ± 29 a 286 ± n.s. b 350 ± 71 b 413 ± n.s. CH a 122 ± 14 a 124 ± n.s. b 248 ± 59 b 311 ± n.s. Discussion The BERI FACE experiment has been the only enrichment study in bog ecosystems. Direct comparison with similar nutrient-poor wetland ecosystems is therefore not possible. Other nutrient-poor ecosystems, like the Arctic tundra and Alpine grassland, also showed no or small increases of biomass under elevated (Körner, 1996). According to Oechel & Vourlitis (1994) nutrient availability in these nutrient-poor ecosystems must increase before plants are able to benefit from fertilization. The effect of elevated on a nutrient-rich wetland ecosystem was investigated by Drake (1992), who used open top chambers on a Chesapeake Bay marsh. In a mixed community of the C 3 sedge Scirpus olneyi and the C 4 grasses Spartina patens and Distichlis spicata, the biomass of the C 3 component increased over 100%, which was accompanied by a decrease in the biomass of the C 4 component of the community. In general, nutrient-rich ecosystems show an increase in biomass under elevated (Körner, 1996). Because of their anatomical, morphological, physiological and organo-chemical properties and the ability of Sphagnum species to create waterlogged, nutrient-poor, acidic conditions, thereby outcompeting most vascular plants, we hypothesized that under nutrient-poor conditions Sphagnum species would still be able to benefit from elevated, thereby improving their competitive strength. However, this research suggests that, just as with other nutrient-poor ecosystems, elevated will have a limited effect on bog ecosystems. Acknowledgements The European Commission funded this research (ENV4-CT ). The Swiss part of the project was funded by the Swiss Federal Office for Education and Science (project ). References Clymo RS The growth of Sphagnum: methods of measurement. Journal of Ecology 58: Drake BG A field study of the effects of elevated on ecosystem processes in a Chesapeake Bay wetland. Australian Journal of Botany 40: Gorham E Northern peatlands: role in the carbon cycle and probable responses to climatic warming. Ecological Applications 1: Goudriaan J Biosphere structure, carbon sequestering potential and the atmospheric 14 C carbon record. Journal of Experimental Botany 43: Jauhiainen J, Silvola J, Vasander H The effects of increased nitrogen deposition and on Sphagnum angustifolium and S. warnstorfii. Annales Botanici Fennici 35: Jauhiainen J, Vasander H, Silvola J Differences in response of two Sphagnum species to elevated and nitrogen input. Suo 43: Jauhiainen J, Vasander H, Silvola J Response of Sphagnum fuscum to N deposition and increased. Journal of Bryology 18: Jonasson S Evaluation of the point intercept method for the estimation of plant biomass. Oikos 52: Körner C The response of complex multispecies systems to elevated. In: Walker B, Steffen W, eds. Global change and terrestrial ecosystems. Cambridge, UK: University Press, 619. Miglietta F, Hoosbeek MR, Foot J, Gigon F, Hassinen A, Heijmans M, Peressotti A, Saarinen T, Van Breemen N, Wallén B Spatial and temporal performance of the MiniFACE. (Free Air Enrichment) system on bog ecosystems in northern and central Europe. Environmental Monitoring and Assessment 66: Mooney HA, Canadell J, Chapin FS, IIIEhleringer JR, Körner C, McMurtrie RE, Parton WJ, Pitelka LF, Schulze E-D Ecosystem physiology responses to global change. In: Walker B, Steffen W, Canadell J, Ingram J, eds. The terrestrial biosphere and global change, Cambridge, UK: Cambridge University Press, Oechel WC, Vourlitis GL The effects of climate change on land-atmosphere feedbacks in arctic tundra regions. Trends in Ecology and Evolution 9: Schimel DS Terrestrial ecosystems and the carbon cycle. Global Change Biology 1: New Phytologist (2001) 150:

5 463 Silvola J dependence of photosynthesis in certain forest and peat mosses and simulated photosynthesis at various actual and hypothetical concentrations. Lindbergia 11: Sveinbjörnsson B, Oechel WC Controls on growth and productivity of bryophytes: environmental limitations under current and anticipated conditions. In: Bates JW, Farmer AM, eds. Bryophytes and lichens in a changing environment, Oxford, UK: Oxford University Press. Van Breemen N How Sphagnum bogs down other plants. Trends in Ecology and Evolution 10: New Phytologist (2001) 150: