1. Climate change and atmospheric deposition of nitrogen (N) affect the biodiversity patterns and functions of ecosystems worldwide. While many single-factor studies have quantified ecosystem responses to single global change drivers, less is known about the interaction effects of these drivers on ecosystem functions. 2. Here, we present the results of a three-year field and a two-year greenhouse experiment, in which we assessed responses of Calluna vulgaris-heathlands to the single and combined effects of drought events (D) and N fertilisation (D: 25% precipitation reduction in the field experiment and 20-50% soil water content reduction in the greenhouse experiment; N fertilisation: 35 kg N ha−1 yr−1). 3. We examined the effects of D and N treatments on growth responses of the dominant dwarf shrub Calluna vulgaris (in terms of biomass production and allocation, tissue δ13C signatures and C:N ratios) in relation to two plant life-history stages and different ‘ecotypes’ (sub-Atlantic vs. subcontinental heathlands). 4. Plant responses varied strongly with life-history stage, and the interaction of N and D showed lower effects than would be expected based on additive responses to single factors. While D treatments had no effects on Calluna in the building phase (ca. ten-year-old plants), seedlings (particularly one-year-old plants) were highly susceptible to drought. Differences in response patterns were attributable to the high shoot-root ratios typical of young Calluna plants. These ratios decreased with progressing life-history as a result of increasing below-ground biomass investments. Below-ground biomass production and shoot-root ratios differed between plants from the different heathland sites. 5. Tissue δ13C signatures decreased and C:N ratios increased with plant age as a result of decreasing evaporative demands (per unit root biomass). N fertilisation increases the shoot-root ratios and thereby the drought susceptibility of Calluna plants. 6. Synthesis. Our findings suggest that plant responses to global change are difficult to anticipate by means of single-factor studies or by focusing on a single life-history stage. This highlights the need for global change research to include multiple factors and life-history stages when assessing an ecosystem's susceptibility to shifts in environmental conditions.

Data field and greenhouse experimentsThe excel file contains the datasets from our field and greenhouse experiments for all response variables and study years used in this publication: dry weight, shoot-root ratios, C:N ratios and delta13C signatures for aboveground and belowground biomass.Data.xlsx