Benefits of horses grazing heathlands revealed in study

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Horses have shown their value in the conservation management of heathlands in a Spanish study.

Atlantic heathlands are dwarf-shrub plant communities dominated by heather and are prized for their biodiversity, aesthetic and cultural value.

Their distribution includes Atlantic countries throughout Europe, from central Norway to northern Morocco. They can also be found in the Atlantic portions of Denmark, northwest Germany, The Netherlands, Belgium, Western France, Britain and the northwest Iberian Peninsula.

Many regions of Western Europe have lost up to 80 to 90% of their heaths over the last century, but Iberian heathlands still cover wide areas and represent almost a quarter of the total European coverage.

They are also the most biodiverse in Europe, occupying the boundary between the Mediterranean and Atlantic.

Their conservation is linked to some extent to human activity, such as cutting, cultural burning and grazing. However, in northwest Spain, a fall in rural populations and the abandonment of some management practices since the 1950s have contributed to large accumulations of highly flammable plant material.

Professor María Pilar González-Hernández and her colleagues, writing in the journal Global Ecology and Conservation, noted that free-ranging horses are compatible with heathland conservation management.

The study team from the University of Santiago de Compostela in Lugo, Spain, set out to evaluate the impact of horses under continuous and rotational grazing systems on plant diversity and community composition in a heather-gorse understory after a two and six-year pause from horse grazing.

The area used for the study comprised a pinewood stand with underlying gorse and heather at San Estevo de Parga in northwest Spain. The stand was established in 1970 and no thinning had been undertaken.

The initial understory vegetation was dense shrubland dominated by gorse (Ulex europaeus), which made up more than 80% of the plant cover, and heathers (Calluna vulgaris, Erica cinerea and Erica umbellata, among others).

The most common herbaceous species were grasses such as Agrostis curtisii and Agrostis capillaris.

Two fenced areas of six hectares apiece were used for the grazing experiment, with one area used for continuous grazing, the other for rotational grazing cycled through four fenced areas of 1.5ha each within the 6ha.

Each 6ha area was grazed by two mature mares of the protected Galician Mountain Horse breed.

Smaller areas next to these blocks were fenced and left ungrazed to serve as a comparison.

The researchers found that the previously grazed sites, regardless of the type of grazing system used, had higher total and rare species richness and diversity than heathland areas that were left ungrazed.

The positive impact of previous grazing on species richness was higher under a continuous grazing system and continued after six years in both grazing systems.

“Horse grazing increased species richness and diversity, as well as numbers of rare species in heathlands,” the study team reported. “Seven species of high conservation interest in heathlands were present in the studied areas.”

There was a decrease in gorse dominance, increasing plant diversity and potentially reducing fire risk.

“Overall,” they concluded, “the effects of previous rotational and continuous grazing on plant diversity in an Atlantic heather-gorse dominated plant community were still apparent two or six years after grazing interruption, although declined after the six-year grazing interlude.”

Rotational management revealed a slight advantage in the control of gorse biomass long term, and helped to maintain the relative abundance of species more evenly over time.

The researchers noted that, unlike heath, gorse is highly selected by horses in heathlands. “The reasons seem to be related to the greater protein content and absence of tannins for this legume, therefore with opposite nutritional attributes to heather species.”

Horses, they say, are the most efficient in reducing gorse dominance among the common domestic herbivores, and promoted grasses when shrub cover decreased in other Atlantic heathlands. Unlike sheep, they do not preferentially graze flower heads, and may be better in maintaining flower-rich swards.

They said that rotational and continuous horse grazing systems, as presented here, can be suitable management strategies to preserve Atlantic heathlands, at least for the periods of grazing and post-grazing studied by the team.

Plant diversity and botanical composition in an Atlantic heather-gorse dominated understory after horse grazing suspension: Comparison of a continuous and rotational management
M.P.González-Hernández, V. Mouronte, R. Romero, A. Rigueiro-Rodríguez, M.R.Mosquera-Losada
Global Ecology and Conservation, Volume 23, September 2020, e01134 https://doi.org/10.1016/j.gecco.2020.e01134

The study, published under a Creative Commons License, can be read here.

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