composition: control of grazing and browsing by domestic animals and deer,
burning, felling, and planting. More than any other habitat, management of
forests and scrub requires both a short, and long-term perspective. Whereas
management can relatively quickly alter the composition and structure of the
field layer and structure of the understorey, changes in dominant tree species
(except by selective felling) usually take much longer.
Species-richness of breeding and wintering birds and sometimes also overall
bird densities, usually increase with age of tree stand (e.g. Manuwal and Huff
1987; Buffington et al. 1997; Donald et al. 1997, 1998). In addition, in
Europe the proportion of tropical migrants (mainly warblers) is highest in
early successional forest and scrub, particularly in vegetation 1–4 m high,
while in eastern North America the proportion of tropical migrants increases
with vegetation height, and is greatest in vegetation over 10 m high
(Mönkkönen and Helle 1989). This is because most migrants in Europe are
scrub birds and most in eastern North America are forest birds. The proportion
of cavity-nesting birds also tends to increase with age of stand (e.g. Donald
et al. 1998), due to an increase in the number of suitable large and dead trees
(Newton 1998).
However, these broad changes in avifauna in relation to age of stand can still
be greatly influenced by management. In most countries outside the tropics vir-
tually all forests have been, and still are, managed for wood production, even if
not specifically planted for this (Section 14.7.4). Management for wood prod-
ucts aims to maximize production of unblemished wood from a limited range of
selected tree species. To this end, trees are harvested prior to maturity. Fallen dead
wood is usually taken for firewood, and dead trees and limbs are often removed
for safety reasons and to prevent the spread of disease. The overall effect is to
reduce tree species diversity, simplify structure, diminish the abundance of
mature trees and dead wood, and consequently lower the value for birds.
Structural diversity of such forests can be increased by thinning and creating gaps
(Section 14.7.5), cavity nest-sites can be provided by adding nest boxes while the
quantity of dead wood can be increased as described in Section 14.7.6. By con-
trast, old-growth forests that have had minimal or no management can support
a larger number of bird species including some that are rare or absent in managed
forests. These include species that require tree cavities or large-crowned trees for
nesting, old trees, standing dead trees (snags) and/or dead limbs and branches for
feeding (see Newton 1998; Imbeau et al. 2000; Poulsen 2002). Old-growth
forests do not require management, other than perhaps in some cases reintro-
duction and management of previously exterminated large herbivores and their
predators to facilitate more natural forest dynamics.
346 |Habitat management