Factors affecting the natural regeneration of Quercus in Scottish oakwoods. I. Competition from Pteridium aquilinum.
In Scottish upland, semi-natural oakwoods, natural regeneration of oak (Quercus petraea and Q. robur) is frequently lacking. This threatens the long-term persistence of a habitat of considerable conservation importance. In the absence of excessive grazing pressure, large openings in the woodland canopy can provide suitable sites for regeneration, but these are often dominated by dense stands of bracken Pteridium aquilinum, which is thought to restrict woody growth severely. In this paper experiments carried out in a semi-natural oakwood in north-eastern Scotland are described which tested the hypothesis that shading by a Pteridium canopy in summer and smothering by the dying fronds in winter exert a detrimental effect on the growth of oak seedlings. Two separate types of Pteridium stand were selected: the first were more dense and growing in relatively large gaps in the oak canopy, and the second were less dense and growing in smaller gaps. In the large gaps, 3 treatments were used: Pteridium cut continuously all year, cut in winter only, and uncut all year; in the small gaps only the first and second treatments were used. Artificially raised seedlings (raised from acorns of a type intermediate between the 2 oak species) were planted in June 1990 at all the sites and protected from rabbit browsing by wire cages. The effects of a Pteridium canopy on photosynthetically active radiation (PAR) during the growing season were also measured. The presence of a Pteridium canopy during the growing season reduced PAR to 5.9% of full sunlight in the larger canopy gaps, and to 11.4% in the small gaps. Over two growing seasons (1991-92), significant reductions in accumulated oak seedling biomass were recorded in both large and small canopy gaps. Significant increases in specific leaf area, leaf area ratio, and a decrease in root:shoot ratio were also recorded. Smothering by dying Pteridium fronds in winter significantly reduced seedling biomass, but had no effect on biomass partitioning or seedling morphology. These results suggest that effective control of dense Pteridium stands is necessary to promote the successful regeneration of oak. However, this should be done in conjunction with measures to ensure the continued supply of early successional woodland habitats.