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Field-scale experiments simulating realistic future climate scenarios are important tools for investigating the effects of current and future climate changes on ecosystem functioning and biogeochemical cycling. We exposed a seminatural Danish heathland ecosystem to elevated atmospheric carbon dioxide (CO 2 ), warming, and extended summer drought in all combinations. Here, we report on the short-term responses of the nitrogen (N) cycle after 2 years of treatments. Elevated CO 2 significantly affected aboveground stoichiometry by increasing the carbon to nitrogen (C/N) ratios in the leaves of both co-dominant species (Calluna vulgaris and Deschampsia flexuosa), as well as the C/N ratios of Calluna flowers and by reducing the N concentration of Deschampsia litter. Belowground, elevated CO 2 had only minor effects, whereas warming increased N turnover, as indicated by increased rates of microbial NH 4 1 consumption, gross mineralization, potential nitrification, denitrification and N 2 O emissions. Drought reduced belowground gross N mineralization and decreased fauna N mass and fauna N mineralization. Leaching was unaffected by treatments but was significantly higher across all treatments in the second year than in the much drier first year indicating that ecosystem N loss is highly sensitive to changes and variability in amount and timing of precipitation. Interactions between treatments were common and although some synergistic effects were observed, antagonism dominated the interactive responses in treatment combinations, i.e. responses were smaller in combinations than in single treatments. Nonetheless, increased C/N ratios of photosynthetic tissue in response to elevated CO 2 , as well as drought-induced decreases in litter N production and fauna N mineralization prevailed in the full treatment combination. Overall, the simulated future climate scenario therefore lead to reduced N turnover, which could act to reduce the potential growth response of plants to elevated atmospheric CO 2 concentration.

Summary 1.Recent findings indicate that the interactions among CO 2 , temperature and water can be substantial, and that the combined effects on the biological systems of several factors may not be predicted from experiments with one or a few factors. Therefore realistic multifactorial experiments involving a larger set of main factors are needed. 2. We describe a new Danish climate change-related field scale experiment, CLIMAITE, in a heath/ grassland ecosystem. CLIMAITE is a full factorial combination of elevated CO 2 , elevated temperature and prolonged summer drought. The manipulations are intended to mimic anticipated major environmental changes at the site by year 2075 as closely as possible. The impacts on ecosystem processes and functioning (at ecophysiological levels, through responses by individuals and communities to ecosystem-level responses) are investigated simultaneously. 3. The increase of [CO 2 ] closely corresponds with the scenarios for year 2075, while the warming treatment is at the lower end of the predictions and seems to be the most difficult treatment to increase without unwanted side effects on the other variables. The drought treatment follows predictions of increased frequency of drought periods in summer. The combination of the treatments does not create new unwanted side effects on the treatments relative to the treatments alone.

In comparison with the effects of extended drought periods or severe nutrient stress, those of ozone are generally much milder, at least with respect to growth. However, there is substantial evidence from experiments, in the main using young saplings, that O $ does impose a stress on forest trees under European conditions. Decreased chlorophyll contents and photosynthetic rates, changes in carbon allocation, increased antioxidant activity, and reductions in biomass due to O $ have often been recorded, particularly in fast-growing species. Furthermore, O $ appears to weaken the trees' resilience to a range of biotic and abiotic stresses. Interactions between O $ and climatic stress, in particular drought and frost hardiness, are likely to result in potentially detrimental effects.A link between the occurrence of O $ and forest damage is not unequivocally established in Europe, and the problem remains of extrapolating and\or scaling up from studies on seedlings to predict responses to O $ of mature trees and forest stands, because we know so little about acclimation to O$ . An accurate assessment is also lacking of the magnitude of the O $ effect on European trees both in terms of the forest areas affected and its extent. In this review we suggest that C allocation is the key factor underlying the responses of trees to O $ . Stomata also play a key role, since the acquisition of C must be achieved while an effective control over water consumption is retained.Key words : Seedlings, mature trees, growth, drought, frost, ozone. Statistical evaluation of annual surveys of forest condition in Europe (UNECE, ICP Forests) over the last decade has shown that tree crown condition in the main damage areas cannot be wholly explained by the location, nature and climate of the site (Mu$ ller-Edzardz et al., 1997). This might imply an important role for external factors such as air pollutants in the decline of tree health. One quarter of the coniferous trees assessed were damaged ( 20 % defoliation) and damage was worst in central

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