Impacts of horse grazing on botanical composition and diversity in different types of heathland

Plant dynamics under horse grazing was studied in a three-year field experiment in heather-gorse shrublands in NW Spain. The experimental design consisted of three vegetation types with four replicates (paddocks): (1) heather (Ericaceae)-dominated (H), (2) gorse (Ulex gallii Planch.)-dominated (G), and (3) co-dominated by gorse and heath-grasses (GG). Each paddock (1.2 ha) was grazed from May to October by two crossbred mature mares (310 +/- 52 kg bodyweight), one dry and one lactating (plus foal). Two other paddocks (one in H and one in GG) were excluded from grazing. Plant cover and height were sampled three times a year (spring, summer, and autumn) with a point-quadrat method. Floristic composition and diversity were annually sampled at 10 25-m(2) quadrats per paddock. Data were subjected to mixed models for repeated-measures and redundancy analyses. Horse grazing resulted in overall increases (P < 0.001) in heather and herbaceous cover (from 13% to 27%, and from 16% to 23%, respectively), and decreases (P < 0.001) in gorse cover and height (from 35% to 17%; from 30 to 25 cm), with scarce differences among vegetation types. Floristic diversity (species richness and Shannon index) increased more over time in grazed than in ungrazed paddocks. Redundancy analyses revealed that grazing effects on plant species assemblages depended on vegetation type, and that plant communities evolved differently between grazing treatments for both abundance and frequency data. Some herbaceous species characteristic of heathlands were favoured by horse grazing. Horse grazing reduced gorse dominance in G and GG vegetation, controlling excessive accumulation of combustible material and reducing fire risk, and promoted the presence of species of conservation interest, so it offers a promising management tool for the restoration of heathlands and their biodiversity.