The Relationship of Sheep Grazing and Soil Heterogeneity to Plant Spatial Patterns in Dune Grassland

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Abstract
(1) Plant spatial patterns and their relationship with sheep grazing and the underlying soil patterns were examined in a dune grassland at Newborough Warren, Anglesey, Wales. (2) In the absence of grazing a hummock-hollow system in the vegetation was evident. The hummocks were 10-40 cm diameter units formed from tussocks of the dominant perennial grasses in the grassland, Arrhenathrerum elatius, Poa pratensis and Frestuca rubra ssp. ruba. The majority of other species co-occurred on the hummocks, which respresented high density high density patches. In contrast, the hollows had few species present and a low plant density. (3) Grazing led to the break up of the hummock-hollow system, with A. elatius, in particular, declining in vigour. Other species such as Holcus lanatus and Anthoxanthum ordoratum invaded the hollows and spread throughout the grassland. A. elatius, P. pratensis and F. rubra maintained a concentration of dominance on the hummocks, but the latter two species were spreading into the hollows. (4) The underlying soil showed a strong heterogeneity that was related to the hummock-hollow microtopography. Soil under the hummocks had higher percentage roots and percentage organic carbon, was moister, more acidic, had higher conductivity and had higher levels of Ca, K, Mn, Mg, Na and PO4 than the soil under the hollows. After three years of grazing, this pattern of soil heterogeneity had not changed. (5) The heterogeneity that existed in the vegetation reflected the level of sheep grazing. Grazing led to rapid changes in the plant patterns, whereas soil patterns were more stable and hence less dynamic.