Silvicultural options to increase plantation biodiversity

In the previous section we have seen that biodiversity at the level of tree species richness has the potential to enhance plantation ecosystem functioning. As a result the related ecosystem goods and services may be provided at a higher level when compared to mono-specific plantations. While the relationships between tree species diversity and the provisioning and regulating functions may not in every case be positive, there are only a few examples where these may be detrimental. The examples above indicate that there may be economic incentives for plantation owners to incorporate higher levels of biodiversity to improve productivity, to sequester more C or to reduce the susceptibility to pests, pathogens or fire. However, at other trophic levels, and with regard to herbivores or predators, the maintenance of biodiversity may not be of direct benefit to plantation owners and may even come at a cost, if the provision of habitat for these organisms requires additional or more intensive silvicultural operations or reduces yield. In this section, we will review the current knowledge about silvicultural options to maintain or enhance plantation biodiversity.

Silviculture is the manipulation of forest structure and dynamics at the stand level with the specific aim of producing certain goods or services. While silvicultural systems operate at the stand level, they have ramifications for the higher levels of forestry planning and management (Nyland, 2002). Likewise, silvicultural decisions at the stand level are influenced by or have to conform with management goals at the higher levels of management such as the estate or the landscape. For example, the establishment of a potentially invasive exotic tree species in some plantation stands can have effects on biodiversity at the landscape scale, or the management of short rotations in plantation stands may provide the highest financial return, but may impact negatively on catchment water run-off (Nyland, 2002).

There is a range of silvicultural options that forest managers can apply at the stand level to achieve different management goals. They can be grouped into the following five categories:

1 site preparation and residues management;

2 tree species selection and mixtures;

3 stand density management (spacing and thinning);

4 structural complexity creation;

5 rotation length.

Silvicultural options to establish, maintain or enhance biodiversity values exist at the different planning levels (see Kerr, 1999, for a discussion of the options). Habitat or habitat components can be influenced at the tree neighbourhood level (Coates and Burton, 1997), stand level (e.g. Geldenhuys, 1997; Hartley, 2002; Lindenmayer and Franklin, 2002; Watt et al, 2002; Montes et al, 2005; Brown et al, 2006) as well as the landscape level (e.g. Lindenmayer and Hobbs, 2004; Tubelis et al, 2004; Montes et al, 2005), while the matrix and corridor function of plantations and the buffering of native ecosystems is clearly an issue for estate or landscape planning (Bone et al, 1997; Tubelis et al, 2004; Nasi et al, 2008). Therefore, silvicultural prescriptions are just one element in an overall plan to achieve certain biodiversity goals in plantations (Figure 5.2).

Plantations may harbour a surprisingly high proportion of native plant biodiversity. For example, Keenan et al (1997) found over 300 plant species beneath tropical timber plantations of the exotic Pinus caribaea and the natives Araucaria cunninghamii, Flindersia brayleyana and Toona ciliata in Northern Queensland. In some situations, much of the understorey flora of plantations comprises widespread and weedy species, including exotics (Michelsen et al, 1996). However, in other situations, plantations may contain

Figure 5.2 Multi-scaled plans for forest biodiversity conservation

Source: adapted from Lindenmayer and Franklin, 2002

Figure 5.2 Multi-scaled plans for forest biodiversity conservation

Source: adapted from Lindenmayer and Franklin, 2002

a large part of the native understorey vegetation: Newmaster et al (2006) described how nearly half of the species found in comparable native forests could be found after 50 years in conifer plantations in Ontario.

However, in most cases, biodiversity in plantations cannot be restored to an 'original level' found in native forests of the same locality, regardless of the silvicultural approaches used (Bone et al, 1997; Lamb, 1998). Even where plantations are established with native species, species richness remains below that found in native forests of the same tree species, as was found for example for invertebrates in eucalypt plantations (Cunningham et al, 2005) and in teak plantations (Tangmitcharoen et al, 2006). However, silvicultural measures may enhance biodiversity, even without substantial losses in timber production rates (Hartley, 2002).

Studies dealing with silviculture and biodiversity cover a wide range of ecosystem components like different vegetation layers and taxonomic groups such as mammals, reptiles, birds and invertebrates. However, few studies have sampled a wide range of taxonomic groups at the same time and in the same place. In a large tropical landscape composed of eucalypt plantations and lowland rainforest, Barlow et al (2007) demonstrated, through intensive sampling of a range of different taxonomic groups, that these different taxonomic groups respond in very different ways to plantations and the structures within them. Similar findings were obtained from the Tumut Fragmentation Experiment (Lindenmayer and Franklin, 2002), where one group of species benefited from the mix of contrasting environments provided by exotic radiata pine plantations and remnant native eucalypt forests. A second group of species was able to persist in the landscape owing to the matrix provided by plantations, whereas the plantation matrix was inhospitable to a third group of species. In conclusion it is very difficult to interpret studies that investigated the effects of plantations on specific species or ecosystem components, especially when these studies covered only a short period of time. It is therefore not surprising that many contradictory results have been reported from such studies with limited scope (see also Bawa and Seidler, 1998).

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Homeowners Guide To Landscaping

Homeowners Guide To Landscaping

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