Regeneration failure in Atlantic oakwoods: the roles of ungulate grazing and invertebrates

Abstract

The effects of browsing by ungulates on the regeneration of Atlantic oakwoods (a habitat of European conservation importance) were studied at five sites in western Scotland. The principal objective was to determine the densities of ungulates that will allow regeneration of the woods. Use of the sites by red deer, roe deer and sheep was estimated by counting pellet groups throughout the year. Estimated summer and winter total herbage offtakes, derived from published species-specific seasonal defecation rates and dry matter intake, were used as indices of grazing pressure. Sapling growth and ungulate and invertebrate damage to saplings were recorded by repeat monitoring. The incidence of browsing of saplings varied considerably between sites and seasons from a summer minimum of around 20% to a winter maximum of over 70%. In summer, rowan and birch were more likely to be browsed than oak and hazel, but there was no overall significant difference between tree species in winter. Browsing incidence increased with estimated offtake during winter, but not during summer. Browsed saplings lost height on average, but growth of saplings that escaped browsing was also poor, and most unbrowsed saplings (especially oak) did not increase in height. Invertebrate damage affected the majority of oak saplings and could be having a significant effect on sapling growth under a mature oak canopy. The abundance of oak and other hardwood saplings present at the sites indicates that the Atlantic oakwoods have the potential to regenerate, but even complete removal of ungulates is unlikely to result in widespread regeneration, owing to the limited growth of saplings located under the mature tree canopy. Forest management that allows light to reach the saplings and enhance growth rates may be required before this potential will be realised. As browsing incidence was only weakly related to estimated ungulate grazing pressure, blanket recommendations applicable to any site based upon a threshold ungulate density which will allow regeneration cannot be made. Rather, deer management plans for sites should be made on an individual basis, and apply not just to the oakwood itself, but to the full range of the deer which use the wood.