Forests dominated by broad-leaved trees support a distinctly different
avifauna from those dominated by conifers, while mixtures of the two support
an intermediate one containing species from both. The effects of individual tree
species composition are less well known, although tree species are known to
differ in their suitability for foraging (e.g. Peck 1989) and nesting in (e.g.
Hågvar et al. 1990). Some tree species produce seeds that are much favored by
particular bird species: for example, Oaks Quercus spp. by Jays Garrulus
glandarius, Hornbeams Carpinus betulusby Hawfinches Coccothraustes coc-
cothraustes, and Birch Betula pendulaby Common Redpolls Carduelis flammea,
and other finches.
14.7.2 Grazing and browsing
Grazing and browsing can be used to influence the structure and composition
of the field layer and the structure of the understorey of forests in the short or
medium term. In the longer term, grazing and browsing affect regeneration, and
thereby also influence the composition of tree and shrub species. Browsing also
removes scrub and leaves from lower branches.
The specific effects of grazing and browsing depend largely on the herbivore
involved and its density. High densities of large herbivores reduce the density
of vegetation within reach of whichever animals are being used, and encourage
low-growing grasses and bryophytes at the expense of taller grasses and forbs.
This tends to decrease the densities of birds that forage or nest in low scrub, that
nest on the ground or which feed on seeds of herbaceous vegetation. Conversely,
high levels of grazing/browsing tend to increase the densities of birds that require
an open understorey for feeding, for example, in grazed oakwoods in western
Britain, Redstarts Phoenicurus phoenicurus, Pied Flycatchers Ficedula hypoleuca,
Wood Warblers Phylloscopus sibilatrix, and Tree Pipits Anthus trivialis(see review
by Fuller 2001). By reducing the height of herbaceous vegetation, high grazing
levels may also reduce the densities of small mammal prey for raptors and owls.
Levels of grazing/browsing by wild deer can be reduced by culling and erecting
deer-proof fences to prevent re-colonization.
Grazing and trampling produce gaps in which tree seedlings can establish, but
prevent them from growing into saplings. These seedlings can be “released” by
reducing or excluding grazing, but this tends to create a dense, even-aged under-
storey of saplings. Regeneration of tree seedlings is often poor once grazing levels
have been reduced, because of lack of gaps for germination and because of
competition with seedlings from tall grasses and forbs. Medium levels of grazing
and variations in grazing levels are most likely to produce patchy and periodic
regeneration of trees (Mitchell and Kirby 1990).
Managing forests and scrub| 347