Cattle exclosure and vegetation dynamics in an ancient, Irish wet oakwood

Abstract

The effects of a 10-year period of cattle exclosure on the ground flora and tree regeneration of an Irish ancient lowland wet oakwood (Corylo-Fraxinetum deschampsietosum) community are assessed. The approach was to record changes in a quadrat sample located in a National Nature Reserve and to model change by multivariate statistical analysis using Principal Components Analysis and Redundancy Analysis ordination. Exclosure gave significant increases in the abundance of regenerating ash Fraxinus excelsior and holly Ilex aquifolium. Non-native tree species establishment also occurred. In the ground flora, there were significant decreases in the abundance of ruderal species and grasses and significant increases in graze-resistant and graze-sensitive species usually restricted to broadleaf woodland habitats. Changes were similar under low and high canopy sites, but greater under a high canopy. In Irish lowland wet oakwood, managed historically to favour oak and currently grazed by cattle, exclosure promotes the succession of ash to the canopy and holly to the understorey. With time, this is likely to lead to a shift from oak dominance. In a conservation context this, together with an increased risk of non-native tree species establishment, can be ecologically damaging, particularly in small sites without natural dynamics, as is the case in the hedged agricultural landscapes of western Europe. We conclude that site-based conservation objectives should be prioritised when implementing generic, landscape-scale management strategies, such as rotational exclosure.