The regeneration of Quercus petraea (sessile oak) in southwest Ireland: a 25-year experimental study

Abstract

The growth and survival of Quercus petraea saplings were monitored over a 25-year period, in an exclosure in a heavily-grazed oakwood on podzol soil in Killarney National Park. Seedlings were subjected to combinations of two levels of shading (under canopy and in an artificial clearing), two levels of competition (weeded and unweeded) and five fertiliser treatments (N, P, NP, Ca and control). Damage by Apodemus sylvaticus (long-tailed fieldmice) was confined to acorns and first-year seedlings, and was not a limiting factor. Invertebrate damage was unimportant. Seedling survival was greatly enhanced within the ungrazed exclosure, even under canopy. In the clearing, following a short-term breach of the fence in the second winter, Cervus nippon (sika deer) browsed 49% of oak seedlings in weeded plots but only 11% in unweeded plots, confirming that surrounding vegetation cover may exercise a protective effect. In later years, seedling survival was significantly higher in the weeded plots, presumably owing to reduced competition. Seedlings under canopy showed no significant response to fertilisation or weeding; the median proportion surviving per plot was zero within 8 years and all had died within 20 years. In the clearing, the median proportion surviving per plot was 0.33 after 8 years and 0.2 after 25 years. In a portion of the clearing with peaty soil and impeded drainage, seedlings showed reduced performance and significantly increased mortality. Microsphaera alphitoides (oak mildew) attacked seedlings mainly in the clearing; its impact appeared negligible. Fertilisation with P significantly enhanced performance in the clearing. No fertiliser treatment enhanced survival. Fertilisation with N alone led to sharply increased mortality in the clearing. I conclude that, in woodlands in western Ireland, successful oak regeneration is to be expected only in unshaded or lightly-shaded sites where grazing levels are low.