Environmental Engineering Reference
In-Depth Information
generic resource (such as “biomass”) can be higher in perennial mixtures of native species than
monocultures of exotic species (Tilman et al. 2006), although these findings have been controversial
(Russelle et al. 2007; Tilman et al. 2007).
The same traits that identify exotic species as the most useful bioenergy crops will also raise
additional ecological questions if large-scale planting proceeds, e.g., the invasive potential of exotic
species (Keane and Crawley 2002; Colautti et al. 2004; Raghu et al. 2006; Barney and Ditomaso
2008). However, invasiveness is not exclusively the realm of exotic species because political regions
are frequently disjunct relative to a species' actual biogeographic range. Thus, species native to one
political district may actually represent an exotic species to the ecosystem in question.
6.3.3.1 case study: exotic conifers as Biofuel crops and Invasive Weed risks
Conifers are currently being considered as a biomass feedstock for cellulosic conversion to fuels
(Scion 2008). However, previous experience with such species suggests that additional widespread
planting of exotic conifers may result in significant negative consequences for biodiversity. Exotic
conifers were introduced for forestry and revegetation purposes in the southern hemisphere and
have subsequently become a significant invasive weed in South Africa, Australia, and New Zealand
(Ledgard 2004; Richardson and Rejmanek 2004), with emerging issues in South America (Pena et al.
2008; Richardson et al. 2008). The extent of these invasions is not trivial, with at least 2000 km
2
affected in New Zealand (Ledgard 2004) (Figure 6.3), whereas in South Africa one species alone,
Pinus pinaster
, has spread over approximately 3256 km
2
and has severely affected the native fyn-
bos vegetation (Richardson et al. 1994). The family
Pinaceae
exhibits strong invasive tendencies
(28 of 225 species are known invasives) of conifers, and the
Cupressaceae
(6 of 66 species known
invasives) are also of concern (Richardson et al. 1994). The species that have invaded the largest
areas are
P. contorta
,
P. halepensis
,
P. nigra
,
P. patula
,
P. ponderosa
,
P. pinaster
,
P. radiata
, and
P. sylvestris
(Richardson et al. 1994; Ledgard 2004). Interestingly, non-native conifers have yet to
yield many naturalization events, and they represent very few invasive populations in the northern
hemisphere as compared with southern hemisphere countries despite wide-scale planting of exotic
species (Mortenson and Mack 2006).
The simplest solution to preventing unwanted invasive species problems from new bioenergy
forests is to plant native species. However, in many instances exotic species will be preferred when
they produce greater biomass in shorter periods than available native species. Thus, we are left
FIGure 6.3
Corsican pine (
Pinus nigra
) has spread from a single farm shelter belt to cover Mount Barker
in New Zealand's Southern Alps. The tail end of the wilding conifer spread is now 10 km from the original
plantings. (Courtesy of Nick Ledgard, Scion.)
