Structural variability of forest stands can be regarded as a measure of the number of different structural attributes and the relative abundance of each of these attributes (McElhinny et al, 2005). As already discussed above, the focus in this section is not how to maximize structural complexity in plantations, which are commonly managed as fairly uniform stands to grow a homogeneous product. Instead, the focus is on how structural complexity may be increased without substantially compromising plantation productivity or possibly even increasing it. In almost all situations, structural complexity in plantations will be lower than in comparable native forests (Moore and Allen, 1999). The importance of structural diversity in plantations is discussed mostly with regard to fauna biodiversity since it is largely the vegetation that provides the stand structure. Therefore we will focus on the relationships between structural variability in plantations and the biodiversity of fauna and how this relationship is influenced by management.
Many studies showed that plantations support biodiversity owing to the structural complexity or landscape heterogeneity (Kerr, 1999; Lanham et al, 2002; Thompson et al, 2003; Humphrey, 2005; Brown et al, 2006; Carnus et al, 2006; Lindenmayer et al, 2006 and others). Given the list of attributes in Box 5.1, it is obvious how stand structural complexity in plantations may be increased: through planting more species-diverse stands, using longer rotations, allowing understorey development, and the retention of residual trees and tree residues (see also Keeton, 2006; Bauhus et al, 2009).
The creation of more foliage layers through permitting development of the understorey provides more niches, which has already been discussed in the context of thinning responses. Two fauna groups that have been well studied with regard to vegetation structure are birds and invertebrates. Humphrey et al (1999) showed a positive correlation between vertical structure of pine and spruce stands and the diversity of syrphids (hoverflies) and carabids (ground beetles). Increasing the vertical structure of the lowest vegetation layer (0–2m) did support the diversity of carabid beetles but not of flies. Other authors who
Box 5.1 Attributes commonly used to characterize stand structure Both the total or average quantity of these elements as well as their variation in space is considered important:
Stand element Measured attribute
Foliage foliage height diversity,anumber of strata, foliage density
Canopy cover canopy cover, gap size classes, gap proportion
Tree diameter average DBH,bvariation in DBH, diameter distribution, number of
large trees
Tree height height of overstorey, variation in tree height, height class richness Tree spacing different aggregation indices, number of trees per ha
Stand biomass basal area, volume
Tree species diversity or richness, abundance of key species
Understorey total cover of understorey, herbaceous vegetation cover and its vegetation variation, species richness, shrub cover, shrub height,
regeneration density
Dead wood number, volume or basal area of stags, volume of coarse woody debris, log volume by decay or diameter class, variation in log density
a foliage height diversity is an expression of the number of vegetation strata within a vertical
stand profile and the relative density of these strata (MacArthur and MacArthur, 1961).
bDBH = diameter at breast height
studied the diversity of insects in relation to understorey vegetation have come to similar results (Jukes et al, 2001; Ohsawa, 2005; Oxbrough et al, 2005; Ohsawa and Nagaike, 2006; Oxbrough et al, 2006).
The relationship between bird species richness and vertical stand structure has been demonstrated in the classical study by MacArthur and MacArthur (1961). In particular, increased understorey cover and species richness appear to be important to provide foraging sites. Openings in Acacia falcataria plantations in Borneo assisted the development of an understorey that attracted birds and increased their diversity (Mitra and Sheldon, 1993). In addition, structural variability can be created through the retention of patches of native vegetation (Hartley, 2002), sometimes called understorey islands, if they are deliberately excluded from harvesting and site preparation (Ough and Murphy, 1998). Intermittent stand-tending practices such as thinning can have strong effects on structural variability, depending on their intensity and also the technique or equipment used (e.g. cable or ground based, manual or harvester) (Figure 5.3). Loumeto and Huttel (1997) found that species richness and the proportion of forest species increased with stand age but that this development may be slowed by disturbance such as fire, herbicide treatments or other weeding practices. Likewise, thinning appeared to set back succession by increasing early successional species (McQueen, 1973).
Figure 5.3 Disturbance of understorey vegetation through harvesting
equipment in Eucalyptus grandis plantation, New South Wales, Australia
Note: This disturbance could be reduced through careful planning of timber harvesting to keep the traffic of
machines within stands on extraction/access tracks
Cavities for nesting and dead wood as foraging sites are additional critical structural elements for many bird species and other species groups. Therefore bird diversity can be enhanced by retaining dead standing trees and also large living trees (Land et al, 1989; Kavanagh and Turner, 1994; Niemela et al, 1995). Large living trees eventually become dead wood and therefore snags, logs and other coarse woody debris in plantations. In addition, large living trees last longer than dead trees and provide cavities to those species which require such features in living trees. Dead wood on the ground, which is often pushed into windrows or piles following harvesting, is also an important structural element for birds and other fauna (Lindenmayer and Franklin, 2002). Large dead standing trees and logs are habitat for detritivores and decay organisms, provide shelter for forest-dependent vertebrates, hiding space for animals and are a long-term source of energy and nutrients. While it is generally known that retention of these structural elements (large trees, dead wood) benefits biodiversity, it is very difficult and has rarely been attempted to translate this into specific quantitative management goals for plantation forests. To advance the possible retention of structural elements in plantation forests, it is necessary to quantify the associated losses in productivity or the increase in operation costs. Bi et al (2002) and Palik et al (2003) provide examples for assessing the influence of retained mature trees on plantation productivity and seedling regeneration, respectively. The influence of these structural elements on plantation growth and operations may be reduced by not dispersing these elements over the entire area, but to keep them spatially aggregated, which is usually also easier to implement. One unresolved issue is whether aggregated retention of structural elements is as effective for habitat conservation as dispersed retention (Hartley, 2002) and what the consequences of the different patterns may be for other ecosystem structures and processes. For example, aggregated retention in Douglas fir forests caused a greater intensity of disturbance concentrated in a smaller portion of the harvested unit when compared to dispersed retention (Halpern and McKenzie, 2001).
In addition to the creation and maintenance of structural variability within plantations stands, the landscape setting and the matrix of plantation patches is also very important. At a large scale such as the landscape, spatial variability can be achieved through conserving remnant patches of native vegetation within a plantation matrix (Lamb, 1998; Lindenmayer and Franklin, 2002; Brown et al, 2006). Many studies have demonstrated the importance of patches of native vegetation for different species groups. For example, in Pinus
radiata plantations the diversity of bird species decreased with increasing
distance to native Eucalyptus forests and bird species richness was positively correlated with the width of natural forest strips within the plantation landscape (Tubelis et al, 2004). In an Acacia mangium plantation landscape on Sumatra, primates were only observed in retained patches of riparian forests that were connected to large natural forest patches in the landscape (Nasi et al, 2008). This indicates for this group of species that areas set aside from production should ideally be connected to the larger conservation areas or
natural forest. The size of such patches of remnant forest within plantations will also influence the kind of species that occur. Lindenmayer and Franklin (2002) make several suggestions about how structural diversity can be maintained at the landscape level within plantation estates. This includes the maintenance of natural water bodies, riparian forests and other corridors, vegetation retention after logging through the landscape and careful planning of forest roads. It points to the importance of the basic design of plantation estates in the landscape.