Plant diversity and botanical composition in an Atlantic heather-gorse dominated understory after horse grazing suspension: Comparison of a continuous and rotational management

Abstract

Floristic diversity changes substantially depending on the type of land management. We evaluated the impact of a continuous (CG) and rotational (RG) grazing system on the dynamics of plant diversity and community composition in a heather-gorse understory after a two and six-year pause from horse grazing. Previously grazed sites had higher total and rare species richness and diversity than ungrazed (UN), regardless the type of grazing system. The positive impact of previous grazing on species richness was higher in CG and continued after six years in both grazing systems. Seven species of high conservation interest in heathlands were present in the studied areas. Most of them were associated to CG management and none was exclusively in RG. The reduction of the shrub component by previous horse grazing led to a decrease of gorse dominance, thus potentially reducing fire risk and increasing plant diversity. The extent of the positive effect of horse grazing controlling excessive accumulation of combustible material and favoring species richness differed when relative abundance of species, and not solely the number of species, was taken into account (Simpson’s dominance increased and Pielou’s evenness decreased after six years). Overall, the effects of previous rotational and continuous grazing on plant diversity in an Atlantic heather-gorse dominated plant community were still apparent 2 or 6 years after grazing interruption, although declined after the six-year grazing interlude. Rotational management revealed a slight advantage upon the control of gorse biomass long term, and assisted to maintain the relative abundance of species more evenly over time. The alpha and beta diversity indices confirmed that plant diversity similarity between rotational and continuous management increased after six years.