Competition between heather and grasses on Scottish moorlands: Interactingeffects of nutrient enrichment and grazing regime

Considerable losses and degradation of heathlands (in moorlands and lowland s) have been reported across Europe, with Calluna vulgaris (heather) being replaced by other species, often grasses. Increasing atmospheric nitrogen d eposition and overgrazing have been suggested as the driving factors behind this change. This possibility was investigated in a study of the interacti ng effects of nutrient inputs and grazing on heather and three grass specie s (Nardus stricta, Deschampsia cespitosa and D. flexuosa) in the field, on a moorland in northeastern Scotland. In addition, the interacting effects o f increasing nutrients and Calluna canopy height on N. stricta and D. cespi tosa were studied using turves in an outdoor experimental area. In the fiel d, fencing had a larger effect than fertilizer on the growth of all species , except for N. stricta, the species most unpalatable to herbivores. Fencin g led to an increase in the height of the Calluna canopy, which may reduce light availability for the grasses. In the turf experiment, the height of t he Calluna canopy affected the diameter of the grass tussocks and percentag e of green matter (i.e. live leaf material), with plants under the more clo sed Calluna canopies being smaller. This study suggests that the slow-growing, evergreen Calluna is a more effe ctive competitor than the faster growing grasses when it has a tall, intact canopy, even at increased levels of nutrient supply. However, overgrazing promotes gap formation in the Calluna canopy, providing an opportunity for grasses to take advantage of increased nutrients. Thus the conservation of heather moorlands requires an understanding of the grazing level which allo ws Calluna to maintain sufficient canopy structure to outcompete grasses fo r light.