Effects of long-term alleviation of nutrient limitation on shoot growth and foliar phenolics of Empetrum hermaphroditum

Alpine tundra ecosystems are characterised by low productivity, due in part to low nutrient availability. These ecosystems are often dominated by 'stress tolerant' species such as Empetrum hermaphroditum, which contribute to stress by producing and releasing biologically active phenolic compounds into the environment. In a nine-year field experiment in alpine tundra, we investigated changes in growth and the levels (concentrations and contents) of foliar redox-active phenolics of current-year shoots of E. hermaphroditum in response to nine long-term environmental manipulation treatments. The treatments were aimed at reducing ecological stresses commonly present in high-latitude ecosystems, primarily stresses associated with low availability of N and other nutrients. Treatments included additions of various forms of N (single and combined applications of NH4+ and NO3-, inorganic N as a component of a full nutrient treatment, and protein as a source of organic N), and additions of glucose, activated carbon, and lime. Shoot growth and levels of foliar phenolics varied greatly between years, but the variation was not clearly explained by the inter-annual variation in macroclimate. Addition of inorganic N generally stimulated growth (especially stem biomass) and increased levels of leaf phenolics. The responses were, however, slow, and varied both between years and between individual inorganic N treatments. Compared to the other treatments, application of inorganic N as a component of a full nutrient treatment had the most consistent positive effect on shoot growth and phenolic content, but it did not affect the concentration of phenolics, suggesting that the treatment did not affect the net rate of phenolic production per unit shoot biomass. During some years of the experiment, the combined application of NH4+ and NO3- resulted in increased production of phenolics per unit biomass accumulation. In contrast to inorganic N fertilisation, application of organic N generally reduced both shoot biomass and phenolic content. Non-N treatments had no substantial effects on either the growth or levels of foliar phenolics of E. hermaphroditum. The observed long-term responses of E. hermaphroditum to environmental manipulation treatments may be important for evaluating potential effects of variation in phenolics production and interference potential of this species under conditions of environmental change and for predicting long-term responses of nutrient-poor communities to environmental changes.