Long-term exclosure of sheep-grazing from an ancient wood: vegetation change after a sixty-year experiment

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Prepared by Ian Rotherham & Ondřej Vild

The area in front of the fence has been heavily grazed at least since the 18th century. The area behind the fence was protected from domesticated stock since 1955. Yarncliff Wood, Peak District National Park, England. Photo credit: Ondřej Vild

Many European landscapes were partly shaped by grazing animal impacts over millennia. In mid-20th century Britain, recognition of detrimental over-grazing impacts of domestic stock on woodland regeneration has triggered conservation initiatives, including fencing exclosures.

One of these sites was the Yarncliff Wood, semi-natural, oak-birch woodland in the English Peak District National Park. Since at least 1852, there had been almost no tree or shrub regeneration. Then, in 1955, in order to investigate if sheep-grazing is the reason why there was a lack of woody regeneration in the site, an enclosure of 1.12 ha was erected. The other possibility considered was regeneration being blocked by extremely unfavourable soil conditions and harsh climate.

The experiment was one of the first of its kind and therefore attracted attention from local researchers. The most detailed surveys up to the 1980s were led by Dr Donald Pigott, with details described in Pigott (1983). After sheep exclusion, he observed quick recovery of Deschampsia flexuosa, Vaccinium vitis-idaea and Vaccinium myrtillus. Other species, including woodland specialists such as Luzula pilosa, gradually appeared. By 1981, the dominant trees, oak (Quercus petraea agg.) and birch (Betula pendula, Betula pubescens and hybrids), were regenerating along with a few saplings of previously-absent woody species, mostly beech (Fagus sylvatica) and rowan (Sorbus aucuparia). Together with soil development, these trends clearly indicated further successional trajectory as observed in continental Europe with light-demanding oak and birch replaced by shade-tolerant beech.

The old enclosure (left) and the unenclosed part (right) with a fence separating these areas (middle). While the unenclosed area is characteristic by heavily grazed turf with no regeneration, abundant tree regeneration is visible in the protected area. Yarncliff Wood, Peak District National Park, England. Photo credit: Ondřej Vild

In this study, we aimed to examine the recent development of the site. We recorded ground flora and woody regeneration in the (i) old enclosure, established in 1955, (ii) new enclosure, established in the early 1980s and (iii) an adjacent grazed area, used as a control. This mix of enclosures of different ages and still unenclosed woodland provided an almost unique opportunity to study these long-term successional processes.

We observed a gradual change in tree species composition in the old enclosure, with declining dominance of light-demanding oak and birch, and increasing shade-tolerant rowan and beech. However, change was slower than predicted, especially considering the rapid initial change observed by Pigott (1983). Interestingly, beech struggled to establish in either enclosure although it is native and common in south-east England and probably occurred in northern England too. The ground flora of the old enclosure changed very little over the past thirty years except for a dramatic decline of co-dominant, Pteridium aquilinum. Due to the combination of light-demanding species which survived and colonizing shade-tolerant woodland plants, the new enclosure herb layer was the richest. Outside the enclosures, tree recruitment was prevented by sheep-grazing and the herb flora dominated by light-demanding and disturbance-tolerant species.

Whilst this detailed study is from a single site, the findings indicate that historically high levels of grazing may have strongly influenced the present state of such upland woods. Further monitoring of this unique experiment may help resolve issues of whether the succession is blocked or only slowed because of poor soils and harsh climate.

Crooked branches of oaks (Quercus spp.) through a wheel stone. Yarncliff Wood, Peak District National Park, England. Photo credit: Ondřej Vild

References:

This is a plain language summary for the paper of Ondřej Vild & Ian Rotherham published in Applied Vegetation Science (https://doi.org/10.1111/avsc.12543).