A number of Northern Hemisphere conifers from the family Pinaceae form the mainstay of much Southern Hemisphere plantation forestry (Lavery & Mead 1998). While Pinus radiata is perhaps the most widely planted species, a number of other species, including Pseudotsuga menziesii, are also used in commercial forestry plantations and several of these have become significant environmental weeds (Richardson 1998; Ledgard 2001; Bustamante & Simonetti 2005; Williams & Wardle 2005). Rapid growth rates and good seed production, facilitated by a lack of natural predators, have contributed to their success as biological invaders. In New Zealand, it has been estimated that invasive conifers infest, at varying densities, as much as 800 000 ha (Froude 2011), and considerable sums of money are now spent each year removing these infestations.
Exotic conifer plantations have long been viewed as ‘biological deserts’, being impoverished in terms of native biodiversity. However, more recent research has emphasized the many roles that these plantations can play in sustaining native biodiversity (Norton 1998; Brockerhoff et al. 2008), including as habitat for native species, through providing connectivity between adjacent areas of native habitat and through reducing harvesting pressure on native forests by providing an alternative timber resource. Becerra & Montenegro (2013) in this issue of Applied Vegetation Science highlight another potential role of exotic conifers, that of facilitating regeneration of native woody vegetation. They suggest that in their semi-arid study system in central Chile, invasive trees of P. radiata facilitate regeneration of native tree species. That an invasive tree species can facilitate regeneration of native species is not novel (Rodriguez 2006), but there has been little assessment of the potential role of invasive Pinaceae in this process. The Becerra & Montenegro paper provides an interesting example with P. radiata.
While the potential positive role of invasive P. radiata in facilitating the regeneration of woody species is highlighted by Becerra & Montenegro (2013), it is also important to acknowledge that there are a number of caveats around extrapolating this finding to other areas. In particular, the potential positive role of P. radiata or other invasive Pinaceae in the restoration of native biodiversity depends on several factors including:
- The presence of a native seed source in the vicinity and its successful dispersal to the invasive conifer stand;
- Any constraints placed on natural regeneration by the climate of the invaded area;
- The density of the infestation – dense Pinus stands typically have very low light levels and may well not be as conducive to regeneration as the scattered trees discussed in Becerra & Montenegro (2013);
- Potential impacts of exotic conifers on soil conditions – e.g. exotic conifers are known to result in a reduction in soil carbon (Tate et al. 2003) and can alter other soil attributes (e.g. pH and soil nutrient levels);
- The ability to control and remove the exotic conifers once the native species have become self-sustaining.
Invasive Pinaceae are also known to have a number of negative impacts on ecosystem services in the systems they invade. In particular, the role of invasive Pinaceae on reducing water yield from catchments has been reviewed by Mark & Dickinson (2008), although this is presumably only an issue when the catchment has never been dominated by woody vegetation as water yield from Pinus plantations is similar to that from native forest stands (Fahey & Jackson 1997).
While focusing on the potential role of invasive Pinaceae in facilitating restoration of native woody vegetation, mature pine plantations might also be important for facilitating native restoration, especially as these mature stands can also provide other services such as timber and carbon sequestration. Particular potential is presented in Pinus plantations where traditional commercial harvest is not contemplated. These non-harvest plantations, which are often established or retired for the purpose of carbon storage or soil conservation, can provide nursery sites where native plants establish and native forest succession is left to progress, leading to provision of multiple services. Research into the role that mature pine stands can serve in long-term provision of multiple environmental services is underway at the School of Forestry, University of Canterbury, NZ.
It may well be that invasive conifers such as P. radiata can be part of the conservation solution for restoring lost native woody ecosystems, so long as they do not become long-term residents themselves. Invasive stands of exotic conifers can also have other benefits, for example as habitat for otherwise absent native fauna (Fig. 1), thus helping facilitate their establishment back into areas from which they have been lost. However, the value of invasive conifers for restoration and conservation will depend on the local situation, and while the Becerra & Montenegro paper describes an interesting example of this process, it should not be assumed that the facilitative role would apply in all areas where invasive conifers are establishing into degraded native ecosystems.