Nigel Symes (The RSPB)
Plate 1 Cessation of management is likely to lead to loss of structural diversity.
Woodland management for birds: Birds and woodlands in Wales
5 Habitat structure in woodland
The importance of woodland structure
One of the main factors determining the distribution and abundance of woodland birds, in particular woodland types, is its structure. The Repeat Woodland Bird Survey (RWBS) and subsequent research has revealed that change in woodland structure is the most likely driver for many woodland bird declines. The impact of management on structure is variable and many woodlands have been altered significantly by centuries of management. The structure in managed woodland has developed in response to the scale and type of management, and where management has ceased, the time-scale since it ceased.
This chapter provides an overview of woodland structure considerations. When a wide range of structural conditions is present, a diverse and abundant woodland bird community can occupy a woodland. Each element provides resources for the birds, although the mature, dying and dead trees are usually the most directly useful. Non-managed woodland in a natural or near-natural state usually contains a range of structures in balance, ie it is not dominated by any one structural type. The management planning process should focus on replicating this more natural state within the constraints of the overall woodland management objectives.
In general terms, the main structural aspects that benefit woodland birds include:
a canopy layer of mature trees with an understorey of future canopy trees
a shrub layer of low-growing scrub species up to 5 m high
a field layer of forbs, ferns, grasses and bryophytes
deadwood in each strata, including lying on the ground
mature trees with dead snags, hollows and other small-scale features that develop with age
clearings and glades within the woodland
a well-structured woodland edge.
Canopy layer and understorey
A healthy canopy layer with an understorey of dense immature trees is an essential component for many woodland birds; good crown development means more foliage, and gaps in the canopy allow understorey, shrub and field layer development which increases invertebrate abundance and provides nesting opportunities. A well-structured canopy of dense foliage also protects humidity levels.
When a canopy becomes closed a simplified woodland structure follows with fewer tree species and lower structural complexity, eg a lack of shrub