Carbon and nitrogen cycles in European ecosystems respond differently to global warming☆
Introduction
Elevated atmospheric CO2 generated through human activities is predicted to affect the global climate by increasing the global mean surface temperature by 1.4–5.8 °C (IPCC, 2001). Different ecosystem processes and biological species may respond asymmetrically to climatic changes (Walther et al., 2002) and the overall effect on ecosystem functioning is therefore often highly complex and determined by the relative sensitivity of the different processes to climate change. Recently, significant changes in ecosystem functioning and structure for a wide range of biomes and ecosystems have already been demonstrated (Walther et al., 2002) as well as evidence for climate change related extended growth season, advancement of spring phenology and upward and northward movement of the plant species distributions (Myneni et al., 1997, Peñuelas and Filella, 2001, Root et al., 2003, Parmesan and Yohe, 2003).
Predictions from different types of models have indicated that global warming may accelerate due to C-cycle feedbacks (Agren and Bosatta, 1996, Cox et al., 2000, Cramer et al., 2001), i.e. decrease terrestrial carbon sequestration, but very few realistic measurements have been available to test these predictions. Understanding the effects of warming on the balance between changes in C vegetation uptake and the loss of carbon through respiration is essential if we are to reliably predict climate change effects on net ecosystem carbon sequestration. The confounding effects of nitrogen have also been discussed (Luo et al., 2004) although very few data are available or it has generally been limited to the impacts of nitrogen deposition (Nadelhoffer et al., 1999) rather than differential responses of the internal carbon and nitrogen cycles that we describe here.
The majority of previous studies of climate change effects on terrestrial ecosystems in the field face two limitations. First, studies are geographically unbalanced as the main focus has been on biomes from arctic, boreal or temperate regions (Van Breemen et al., 1998, Chapin et al., 1995) because ecosystem effects and feedback mechanisms have been expected to be largest in these regions and because global climate models predict greater temperature increases at higher latitudes (IPCC, 2001). Second, the methods used to experimentally warm ecosystems, which include soil electric heating cables (Van Breemen et al., 1998, Melillo et al., 2002), IR radiators (Harte and Shaw, 1995) and growth chambers (Chapin et al., 1995), are subjected to various artifacts and thus, concerns have been raised that they do not realistically simulate global warming (Schulze et al., 1999). Within two EU projects CLIMOOR and VULCAN (Beier et al., 2004) we studied effects of warming in so far underrepresented shrubland ecosystems at lower latitudes across Europe. We did this by combining a field scale experimental approach with a gradient (space for time substitution) approach. The combination of field-scale manipulations conducted along climatic gradients provides a tool to evaluate short-term (treatment) and long-term (gradient) effects and the relative sensitivity of different biomes to climatic perturbations. The aim of the study was to assess the short and long term effects of warming on key ecosystem processes and ecosystem functioning. The objectives of the present paper are:
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To synthesise the effects of warming on carbon and nitrogen sequestration and on the main release processes in shrubland ecosystems.
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To relate the short term response after 3 years of warming treatment to the long term changes represented by the temperature gradient across the sites.
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To compare the above- and below-ground responses.
Section snippets
Study sites and experimental design
The studies were carried out at 4 shrubland sites in Mols, Denmark (DK, 56°23′N 10°57′E, MAT 8.7 °C, MAP 750 mm year− 1, elevation 58 m); Oldebroek, the Netherlands (NL, 52°24′N 5°55′E, MAT 9.3 °C, MAP 1033 mm year− 1, elevation 25 m); Clocaenog, United Kingdom (UK, 53°03′N 3°28′W, MAT 7.0 °C, MAP 1607 mm year− 1, elevation 490 m) and Garraf, Spain (SP, 41°18′N 1°49′E, MAT 15.6 °C, MAP 456 mm year− 1, elevation 210 m). The soils cover an organic rich podzol in UK, nutrient poor podzols in DK and NL
Plants
Plant productivity in the control plots showed no temperature sensitivity among the sites since no significant differences were observed across the temperature gradient (ANOVA, Fig. 1a). On the other hand, warming increased total above-ground biomass production at the colder NL (68%, but not significant, t-test) and UK (156%, p < 0.05, t-test) sites (Fig. 1a). Warming had no effect on total above-ground biomass production at the warm Mediterranean ecosystem.
Soil carbon and nitrogen
Below-ground carbon loss measured as
Plants
The stronger response of plant growth to warming at the colder sites may seem surprising given the lag of response in N-mineralisation as well as the relatively small difference in plant growth across the sites despite big differences in ambient climatic conditions. On the other hand the lag of a warming effect on total above-ground biomass production at the warm Mediterranean ecosystem agrees with the expectation of these ecosystems being water limited with temperatures already close to the
Acknowledgements
We thank all the people involved in the extensive activities of running and maintaining the treatments and the measurements in the CLIMOOR and VULCAN projects. The work was financially supported by EU (Contracts ENV4-CT97-0694 and EVK2-CT-2000-00094) and the participating research institutes.
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This paper was presented at the 5th International Symposium on Ecosystem Behaviour, held at the University of California, Santa Cruz, on June 25–30, 2006.
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