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Climate change and nitrogen deposition affect biodiversity and ecosystem functioning, but interactive effects of these global change drivers are poorly understood. We analysed single and interactive effects of nitrogen (N) fertilisation and drought on the growth performance of Calluna vulgaris. We measured biomass production and allocation, tissue nutrient (N, phosphorus (P) and carbon (C)) concentrations, N allocation patterns (using 15N tracer) and plant's water status (using δ 13C signatures) as response variables in a 2-year greenhouse experiment. N fertilisation increased biomass production and biomass shoot:root ratios. 15N allocation patterns indicated an increasing aboveground N allocation following N fertilisation. Tissue δ 13C signatures were higher in N-fertilised plants. Plant responses to drought were weak. We found strong antagonistic interaction effects of N fertilisation and drought for biomass production. δ 13C values peaked when N-fertilised plants were subjected to drought, indicating that N fertilisation increased the evaporative demands of Calluna plants, likely due to increased biomass shoot:root ratios, which in turn resulted in higher drought susceptibility. As an important consequence, even slight drought events may weaken the competitiveness of Calluna when interacting with enhanced airborne N loads. Single-factor studies, thus, need to be complemented by multi-factor analyses to assess conceivable impacts of co-occurring global change drivers.

We investigated long-term responses (since 1850) of Fagus sylvatica (Luxembourg; central Europe) to shifts in temperature, precipitation, and nitrogen deposition by analyzing diameter at breast height (DBH) increment, basal area increment (BAI), and tree-ring stable isotopes (δ13C, δ15N). We compared stands on soils with contrasting water supply (Regosols and Cambisols with an available water capacity of ca. 40 and 170 mm, respectively) and of two different age classes (ca. 60 vs. 200 years). All stands showed a peak in DBH increment in the decade 1978–1987, but a decline in increment growth in subsequent decades. In addition, BAI declined in mature stands in the last two decades. Decreasing increment rates were attributable to an increasing drought limitation of stands, mainly induced by increasing temperatures in the last two decades. Contrary to our expectations, stands on Cambisols showed a similar susceptibility to shifts in temperature and precipitation as stands on Regosols, suggesting a strong adaptation of the respective ecotypes grown at dryer sites. This result was in line with long-term trends for tree-ring δ13C signatures, which did not differ significantly between stands on Cambisols and Regosols. Climate impacts on tree-ring δ15N signatures were low. High spring precipitation and temperatures caused increasing and decreasing δ15N values, respectively, but only in mature stands on Cambisols. Stands on Regosols tended to have lower δ15N values than stands on Cambisols. Decreasing δ15N values in recent decades suggest an increasing impact of allochthonous N loads with isotopically lighter N. We investigated long-term responses (since 1850) of Fagus sylvatica (Luxembourg; central Europe) to shifts in temperature, precipitation, and nitrogen deposition by analyzing diameter at breast height (DBH) increment, basal area increment (BAI), and tree-ring stable isotopes (δ13C, δ15N). We compared stands on soils with contrasting water supply (Regosols and Cambisols with an available water capacity of ca. 40 and 170 mm, respectively) and of two different age classes (ca. 60 vs. 200 years). All stands showed a peak in DBH increment in the decade 1978–1987, but a decline in increment growth in subsequent decades. In addition, BAI declined in mature stands in the last two decades. Decreasing increment rates were attributable to an increasing drought limitation of stands, mainly induced by increasing temperatures in the last two decades. Contrary to our expectations, stands on Cambisols showed a similar susceptibility to shifts in temperature and precipitation as stands on Regosols, suggesting a strong adaptation of the respective ecotypes grown at dryer sites. This result was in line with long-term trends for tree-ring δ13C signatures, which did not differ significantly between stands on Cambisols and Regosols. Climate impacts on tree-ring δ15N signatures were low. High spring precipitation and temperatures caused increasing and decreasing δ15N values, respectively, but only in mature stands on Cambisols. Stands on Regosols tended to have lower δ15N values than stands on Cambisols. Decreasing δ15N values in recent decades suggest an increasing impact of allochthonous N loads with isotopically lighter N.

We investigated the response of the perennialgrass Molinia caerulea (L.) Moench to combinedeffects of fertilization (N, P) and drought events. Wehypothesized that N fertilization increases, anddrought decreases productivity, but that N additionstrengthens negative effects caused by drought.Methods Within a full-factorial 2-year greenhouseexperiment we measured biomass productivity andallocation, tissue nutrient concentrations and nitrogenallocation patterns using 15N as a tracer.Results N fertilization caused a strong increase inproductivity, but effects of drought were almostinsignificant. However, we found strongly interrelated,non-additive effects of fertilization and drought,expressed by a strong increase of necrotic tissue.Dead aboveground biomass showed the highestvalues for N and 15N.Conclusions Accelerated productivity of abovegroundtissue under N fertilization resulted in increased evaporativedemands and thus higher drought susceptibility.In addition 15N allocation patterns showed thatfertilization-drought treatments disenabled plants’control of their N allocation. Molinia was unable towithdraw leaf N during the dieback of abovegroundtissue. Due to the lack of an adaptive strategy to thecombined effects of fertilization and drought, increasingsummer drought may weaken the competitiveperformance of species with traits comparable tothose of Molinia in N-fertilized environments. We investigated the response of the perennialgrass Molinia caerulea (L.) Moench to combinedeffects of fertilization (N, P) and drought events. Wehypothesized that N fertilization increases, anddrought decreases productivity, but that N additionstrengthens negative effects caused by drought.Methods Within a full-factorial 2-year greenhouseexperiment we measured biomass productivity andallocation, tissue nutrient concentrations and nitrogenallocation patterns using 15N as a tracer.Results N fertilization caused a strong increase inproductivity, but effects of drought were almostinsignificant. However, we found strongly interrelated,non-additive effects of fertilization and drought,expressed by a strong increase of necrotic tissue.Dead aboveground biomass showed the highestvalues for N and 15N.Conclusions Accelerated productivity of abovegroundtissue under N fertilization resulted in increased evaporativedemands and thus higher drought susceptibility.In addition 15N allocation patterns showed thatfertilization-drought treatments disenabled plants’control of their N allocation. Molinia was unable towithdraw leaf N during the dieback of abovegroundtissue. Due to the lack of an adaptive strategy to thecombined effects of fertilization and drought, increasingsummer drought may weaken the competitiveperformance of species with traits comparable tothose of Molinia in N-fertilized environments.

In the present study we analysed whether airborne N pollution may constitute one important driver for the encroachment of Molinia caerulea in dry heathland ecosystems. Based on full-factorial field experiments (in 2006 and 2008) and complementary greenhouse experiments (in 2008), we quantified growth responses of Molinia caerulea to N and P fertilisation (50 kg N ha(-1) year(-1), 20 kg P ha(-1) year(-1)). Aboveground biomass production of Molinia caerulea was limited by P in 2006, but by N in both experiments in 2008. In the greenhouse experiment, N addition caused a sixfold increase of the biomass of vegetative tillers, and in all experiments the biomass and numbers of flowering tillers showed a significant increase due to fertilisation. Our experiments indicated that growth of Molinia caerulea was primarily limited by N, but in dry heaths the kind of nutrient limitation may be mediated by other factors such as water availability during the vegetative period. Shifts in biomass allocation patterns resulting from N fertilisation showed that Molinia caerulea encroachment in dry heaths is not only attributable to increased leaf biomass, but also due to higher investments in reproductive tissue that allow for increased seed production and thus accelerated encroachment of seedlings in places where the dwarf shrub canopy has been opened after disturbance.

There is growing evidence from different sources that prolonged high N deposition causes a shift from nitrogen (N) limitation to nitrogen and phosphorus (P) co-limitation or even P limitation in many terrestrial ecosystems. However, the number of ecosystems where the type of limitation has been directly tested by longer-term full-factorial field experiments is very limited. We conducted a 5-year fertilization experiment with N and P in the Lüneburger Heide (NW Germany) to test the hypothesis that, following decades of elevated atmospheric N inputs, plant growth in dry lowland heaths may have shifted from N to N-P co-limitation or P limitation. We also tested whether the plant tissue N:P ratio reflects the type of nutrient limitation in a continental lowland heathland. Experimental plots dominated by Calluna vulgaris received regular additions of N (50 kg N ha -1 y -1), P (20 kg P ha -1 y -1), a combination of both, or water only (control) from 2004 to 2008. Over the whole study period, a highly significant positive N effect on shoot length was found, thus indicating N limitation. We conclude that a clear shift from N limitation to N-P co-limitation or P limitation has not yet occurred. Tissue N:P ratios showed a high temporal variability and no relationship between tissue N:P ratio and the shoot length response of Calluna to nutrient addition was found. The N:P tool is thus of limited use at the local scale and within the range of N:P ratio observed in this study, and should only be used as a rough indicator for the prediction of the type of nutrient limitation in lowland heathland on a larger geographical scale with a broader interval of N:P ratio.