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

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.