Nutrient limitation of the long term response of heather [Calluna vulgaris (L.) Hull] to CO₂ enrichment

In a 27‐month CO₂‐enrichment experiment, Calluna vulgaris plants were grown on peat obtained from an upland heath in NE Scotland and given a nutrient supply which mimicked that in precipitation in the area. Three CO₂ concentrations were used; ambient, +100ppm and +200ppm. Calluna showed a negative growth response to increased CO₂ over the first year of treatment and a positive response by the end of the experiment. Final above–ground biomass was greatest in the enriched CO₂, treatments, showing an increase of 30 °₀ in + 100ppmCO₂. Determination of tissue nutrient concentration, and calculation of total nutrient uptake, demonstrated that nutrient uptake did not increase with increased growth, resulting in significant dilution of elements in leaf tissue. This suggests that, in its typical, nutrient poor habitats, the growth response of Calluna to CO₂ will be limited by nutrient deficiency, and will reach a maximum with a relatively small increase in CO, concentration. Flowering was advanced and extremely prolific in +100 ppm CO₂ grown plants, but the ecological significance of this is uncertain. The results highlight the need for long term studies of native species on their natural soils, using lower CO₂ concentrations than the usual ‘double CO₂’.