Long-term patterns and impacts of grey squirrel debarking in Lady Park Wood young-growth stands (UK)

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Abstract

Debarking of trees by American grey squirrels Sciurus carolinensis can be observed widely in British woodland. In this study, squirrel debarking and associated damage and mortality were assessed from 1977 to 2002 in mixed-deciduous stands in Lady Park Wood. Debarking mainly affected beech Fagus sylvatica stems sized 7.5–35 cm dbh, particularly those that were located in the upper stratum and growing most rapidly. Damage escalated during the study period: by 1983 24% of live beech stems in the vulnerable size-range had been badly damaged; this climbed to 48% by 1993, but reached only 54% in 2002, though a substantial number of beech had also been killed by debarking by 2002. Long-term mortality rates ranged from 2.3% to 5.4% per year and population half-life values from 13 to 30 years. The trigger for the initial debarking outbreak seemed to have been a severe drought in 1976. This opened the canopy and thereby released many, previously suppressed, beech trees. Debarking declined after 1993 as the canopy re-closed and beech growth generally declined. Over the same period, however, debarking to other species increased, suggesting it may have become habitual. Not all beech trees or stands where beech was prominent were seriously affected, and some badly damaged beech showed signs of recovery. Nonetheless, debarking had strongly impeded beech and thereby increased the long-term prospects for ash Fraxinus excelsior, lime Tilia cordata/platyphyllos and other tree species.

Introduction

The grey squirrel Sciurus carolinensis Gmelin has become a common sight in British woodland. It was introduced at various locations from America around the start of the 20th century and has since spread widely (see Shorten, 1954, Lloyd, 1983, Huxley, 2003). It has also become a serious pest, as it frequently debarks pole-sized trees: young stands with fast growing beech Fagus sylvatica L. and sycamore Acer pseudoplatanus L. are especially vulnerable, especially if they contain a high density of juvenile squirrels, some mature seed-bearing trees, and have a history of squirrel debarking (see Shorten, 1954, Shorten, 1957, Kenward, 1982, Kenward, 1983, Kenward, 1989, Rowe, 1984, Rowe and Gill, 1985, Kenward and Parish, 1986, Gurnell, 1987, Kenward et al., 1988a, Kenward et al., 1988b, Kenward et al., 1992, Kenward et al., 1996, Kenward and Dutton, 1996, Mountford, 1997, Mountford, 2004, Mountford and Peterken, 1999, Mountford et al., 1999).

Debarking can be observed widely in southern Britain. A recent estimate of damage in the Chilterns, an important forest region in southern England, found that 54% of beech and 44% of sycamore trees had some evidence of squirrel damage (Rayden and Savill, 2004). Another recent assessment of valuable broadleaved woodland in southern Britain found 100% of sycamore trees, 66% of beech, and 40% of oak Quercus petraea (Mattuschka) Lieblein, Q. robur L. had been debarked (Huxley, 2003). Conifers are also attacked, including larch Larix decidua Mill., Norway spruce Picea abies (L.) Karst., Scots Pine Pinus sylvestris L., and western red cedar Thuja plicata D. Don. Even though methods to control squirrel numbers and limit debarking damage have been developed, this is costly and requires diligence. Damage is already so severe in the Forest of Dean (which includes the site used in this study) that local forest officers consider its future economic timber production and conservation is seriously threatened. The associated costs for British private and state forestry sectors are likely to exceed one million pounds per year (see Huxley, 2003). Not only have grey squirrels spread throughout much of Britain often causing irreparable damage to broadleaved trees (Huxley, 2003), they have also recently gained a foothold in northern Italy from where they may spread across continental Europe (Bertolino and Genovesi, 2002).

Part of the existing research on grey squirrel debarking comes from a 10-year study based on permanent transects in mixed deciduous stands in Lady Park Wood (Mountford, 1997). This paper extends this research using new data from a survey conducted in summer 2002. It provides details of the long-term pattern and impact from data collected at a single site over a 25-year period. It examines what triggered the initial outbreak of debarking, how the level of debarking changed over time, which trees and parts of the wood were affected most, and how debarking affected the composition of this research natural area. For the first time, data is presented on the long-term survival rate of badly debarked trees.

Section snippets

Study site

This study was conducted at Lady Park Wood, details of which are given by Peterken and Jones, 1987, Peterken and Jones, 1989. This wood has been a research natural area since 1944, with much of the reserve having been set aside to develop with minimal intervention since this time. Subsequent recordings provide one of the best worldwide studies of natural stand development (see Peterken and Jones, 1987, Peterken and Jones, 1989, Mountford, 1994, Peterken and Mountford, 1995, Peterken and

Recording and analysis

The study was conducted in the young-growth stands at Lady Park Wood, which cover 21 ha and include a variety of stand, soil and terrain types. Changes in stand composition and grey squirrel damage were recorded on six 20 m-wide permanent transects that have an aggregated length of about 2200 m. They were spaced at intervals of about 150–250 m across the stands and collectively covered 2.04 ha (see Fig. 1 in Mountford, 1997). The transects were first recorded in 1944, but it was not until 1977 that

Change in the composition of the young-growth stands

The study area changed considerably in composition over the 1977–2002 study period (see Table 1, which also gives the scientific names of species). The main species present were birch, lime, ash, beech, hazel, sessile oak and field maple. The beech population initially ranged from 1–87 cm dbh. It included a few larger trees, which were standards retained at the last fellings in 1940–1943, after which the area was left to re-grow naturally without planting. Most stems had, however, grown up after

Escalation, patterns and causes of grey squirrel debarking

The permanent transect records from Lady Park Wood provide an invaluable insight into the long-term significance of grey squirrel debarking in mixed beech stands in lowland Britain. In the first few decades after grey squirrels first colonised the area, sometime around 1940 (Shorten, 1954), little debarking was recorded in the wood save for a file note saying that squirrels had debarked and killed a few birch in the young-growth in 1957/1958. Presumably, the density of squirrels was initially

Acknowledgements

This research was carried out as part of the NAT-MAN project – Nature-Based Management of Beech in Europe – for the European Community Fifth Framework Programme. The Forestry Commission, English Nature and Countryside Council for Wales have maintained Lady Park Wood as a minimum-intervention reserve and made such long-term research possible. Alan Orange carried out the 1977 transect recording, whilst George Peterken and colleagues did the recording in 1983. Anne Mountford and Graham Berry

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