Impacts of drought and nitrogen addition onCallunaheathlands differ with plant life‐history stage

Abstract

SummaryClimate change and atmospheric deposition of nitrogen (N) affect the biodiversity patterns and functions of ecosystems world‐wide. 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.Here, we present the results of a three‐year field and a two‐year glasshouse experiment, in which we assessed responses ofCalluna vulgarisheathlands to the single and combined effects of drought events (D) and N fertilization (D: 25% precipitation reduction in the field experiment and 20–50% soil water content reduction in the glasshouse experiment; N fertilization: 35 kg N ha−1 year−1).We examined the effects of D and N treatments on growth responses of the dominant dwarf shrubCalluna 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).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 onCallunain 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 youngCallunaplants. 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.Tissue δ13C signatures decreased and C:N ratios increased with plant age as a result of decreasing evaporative demands (per unit root biomass). N fertilization increases the shoot–root ratios and thereby the drought susceptibility ofCallunaplants.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.