Long-term deer exclusion in yew-wood and oakwood habitats in southwest Ireland: Natural regeneration and stand dynamics

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Abstract

Woodland dominated by Taxus baccata is one of the rarest of European woodland types and has priority habitat status under Annex I of the EU Habitats Directive, yet little is known about its stand dynamics or the effects of long-term grazing. The abundance of naturally regenerating seedlings and saplings was monitored over a 32-year period in exclosures in a yew-wood and a neighbouring oakwood in the Killarney National Park, southwest Ireland. Both woods are heavily grazed by introduced sika deer (Cervus nippon). Mortality, recruitment and growth of adult trees were monitored over a 20-year period. Comparison was made with unfenced plots adjacent to each of the exclosures. Regeneration in the yew-wood exclosures was chiefly by Ilex aquifolium, Sorbus aucuparia and Fraxinus excelsior. No Taxus baccata saplings were recorded and seedlings of this species were very rare throughout the duration of the experiment. Taxus baccata trees which died during the 20-year monitoring period were significantly smaller than those which survived, indicating that self-thinning is occurring and the wood in its present form may be of relatively recent origins. Changes in adult tree species composition suggest that yew woodland in Ireland may develop from a yew-hazel scrub woodland sere. A variety of species regenerated in the oakwood exclosures, including Taxus baccata, and dense holly thickets formed in several areas. Quercus petraea failed to regenerate beneath the oak canopy. We conclude that chronic heavy grazing in the Killarney woodlands strongly influences the natural regeneration of several tree species. Research into the scientific manipulation of grazing levels in temperate woodlands is required. However, grazing intensity is not the only factor affecting regeneration, canopy conditions in particular are also likely to be of high significance, and this should be reflected in management plans.

Introduction

Grazing animals can have major impacts on the vegetation of semi-natural woodlands (Mitchell and Kirby, 1990). Natural regeneration of tree species can be impeded or precluded by the eating of vulnerable seedlings and saplings (e.g. Linhart and Whelan, 1980, Pigott, 1983, Putman et al., 1989, Latham and Blackstock, 1998) although susceptibility to browsing can vary considerably between species (Gill and Beardall, 2001). Established trees may be damaged by activities such as bark stripping (e.g. Larner, 1977) and, in the long term, the effects of grazing can impact markedly on the stand structure and composition of woodlands (e.g. Peterken and Tubbs, 1965). Heavy grazing pressure is a feature of many types of woodland in Ireland, particularly within the rugged landscapes of the National Parks (Kelly, 2000, Higgins, 2001). This is cause for serious concern given the very limited area of semi-natural woodland remaining in Ireland, estimated at covering 1.1% of the country (Higgins et al., 2004). The Killarney National Park, Co. Kerry, possesses some of the best remaining examples of semi-natural woodland in Ireland (Kelly, 1981, Kelly and Iremonger, 1997), including two woodland types of international importance: yew woodland and acidophilous oak woodland. Kelly (1981) highlighted that a conspicuous feature of these woods was the extreme scarcity of natural regeneration of any tree species.

Woodland dominated by Taxus baccata L. (common yew)1 is one of the rarest of European woodland types and is largely restricted to southern England and western Ireland. The unique nature of these woodlands has afforded them priority habitat status under Annex I of the EU Habitats Directive (Fossitt, 2000). Irish yew-woods differ markedly from those on the chalk downlands of southern England studied by Watt (1926) and Tittensor (1980), in that they are found on areas of karst limestone pavement rather than chalk. They thus have more geologically in common with the yew-woods on Magnesian limestone found in the coastal denes of Co. Durham, northeast England (Hulme, 1996). A striking feature of yew-woods is the extreme paucity of the field layer and the scarcity of shrubs or regenerating saplings, which Rodwell (1991) attributes in English woods to the heavy shade cast by the dense canopy. It has been unclear to what degree grazing is influential in this regard in the case of Irish yew-woods as little is known about their structural dynamics. The reasons for the dominance of yew are also unclear as in most woodland on such terrain in Ireland (e.g. the Burren, Co. Clare), Corylus avellana is dominant, Fraxinus excelsior is abundant and T. baccata is rare (Kelly and Kirby, 1982).

The acidophilous oakwoods of Ireland and Britain that are dominated by Quercus petraea (sessile oak) with an Ilex aquifolium understorey and Blechnum spicant in the field layer, also have Annex I status. They tend to occur in high rainfall western areas, particularly in the uplands, and are characterised by the variety and abundance of bryophytes, lichens and filmy ferns (Hymenophyllaceae) which include species with restricted Atlantic distributions (Ratcliffe, 1968). A general account of these woods in Ireland has been given by Kelly and Moore (1975) and of the Killarney oakwoods by Kelly (1981).

Fenced exclosures have often been used to examine the effects that grazing is having on natural regeneration in woodland (e.g. Latham and Blackstock, 1998, Sykes, 1992) and fencing is frequently used as a management tool in areas where grazing is a problem. This paper describes natural regeneration and changes in stand structure in relation to fencing in two semi-natural woodlands, a yew-wood and an oakwood, in southwest Ireland over a period of 32 years. There have now been several long-term ecological monitoring studies conducted in a range of woodland types in Britain, for example, Quercus roburBetula pubescens woodland (Mountford et al., 2000), Q. roburF. excelsior woodland (Kirby et al., 1996), Q. roburFagus sylvatica woodland (Putman et al., 1989), Q. petraeaB. pubescens woodland (Pigott, 1983), Q. roburF. excelsiorTilia spp. woodland (Peterken and Jones, 1987, Peterken and Jones, 1989) and Pinus sylvestris woodland (Gong et al., 1991, Sykes, 1992). However, there have been no previous reports on long-term studies in yew woodland except for Barkham's (1992) consideration of ground flora changes in a yew stand at Brigsteer Park Wood, Cumbria, over an 18 year period. Furthermore, there have been few other ecological studies of comparable duration in Ireland, the notable exception being the Tomies oakwood study of Kelly, 2000, Kelly, 2002. Our aims were to investigate the effects that long-term deer exclusion would have on the abundance and growth of seedlings and saplings, in particular those of the canopy dominants T. baccata and Q. petraea. In addition, we aimed to examine the changes in stand structure that occurred during this period with regards to recruitment and mortality.

Section snippets

Study area

The study was conducted in the woodlands of the Muckross Peninsula in the Killarney National Park, southwest Ireland (Fig. 1) between 1969 and 2001. The vegetation of these woods has been described in detail by Kelly (1981). The peninsula is low-lying with an altitude of 17–30 m. It is divided by a geological boundary, separating Carboniferous Limestone to the east from Devonian Old Red Sandstone to the west. The limestone area supports Reenadinna Wood (longitude 9°30′, latitude 52°1′), a Taxus

Reenadinna yew-wood

Between 1969 and 2001, the most frequent juveniles in the yew-wood exclosures were of Ilex aquifolium, Fraxinus excelsior and Sorbus aucuparia (Fig. 2). The density of I. aquifolium seedlings changed little during this period, whilst that of S. aucuparia declined slightly. F. excelsior seedlings increased significantly in density (P = 0.039, R2 = 0.17 ± 2.5, data for 1969–2001). I. aquifolium sapling density increased during the first 11 years of deer exclusion (P = 0.001, R2 = 0.10 ± 1.1, data for

Yew stand dynamics

Mitchell (1990a) showed via pollen analysis that a substantial T. baccata population has existed on the Muckross Peninsula for about the last 5000 years, but suggested that the yew-wood in its present form may have arisen following the cessation of agricultural and industrial activities on the peninsula in the late eighteenth century. The evidence from the present study, that self-thinning of smaller stems and individuals is still occurring within the yew population, supports the idea of

Conclusions

This 32 year experiment has shown that natural regeneration of several species can occur beneath a yew-wood canopy in the absence of grazing, although T. baccata itself is unable to do so. Natural regeneration of several species, including T. baccata, can occur beneath an oakwood canopy, although, as has been shown in other studies, Q. petraea is unable to do so. Chronic heavy grazing in and around the Killarney woodlands is strongly suppressing natural regeneration.

From the conclusions derived

Acknowledgements

The field work for this study has been funded throughout by the National Parks and Wildlife Service of the Department of the Environment, Heritage and Local Government and its predecessor organisations. We thank in particular Dr. J. Cross and Dr. A. Craig, who established some of the oakwood exclosures in 1974–1975. For support in Killarney, and especially the continued maintenance of the fences, we thank Mr. D. Kelleher, Mr. P. O’Leary, the other Wildlife Rangers and other staff of the

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