Environmental Engineering Reference
In-Depth Information
with preventative options, in which attempts are made to screen for and restrict the spread of poten-
tially invasive species. However, predicting which species will be invasive is a significant chal-
lenge and remains an imprecise science (Hulme 2006). For example, in conifers traits such as seed
size (<50 mg), seedling establishment, juvenile survival, length of the juvenile period (<10 years),
and the periodicity of large seed crops are proven indicators of invasive potential (Richardson and
Rejmanek 2004; Buckley et al. 2005). However, these traits are not infallible given that other exter-
nal factors such as propagule pressure can be significant (Rouget and Richardson 2003; Richardson
and Rejmanek 2004). Indeed, complex genotype-specific screening methods are now being sug-
gested to reduce the risk of future invasive weed problems (Barney and Ditomaso 2008).
6.3.4 p
ErEnnial
p
olyculturES
Although considerable emphasis has been placed on the use of monotypic plantings of short rota-
tion woody plants and perennial grasses, a growing body of evidence suggests that diverse plantings
may convey greater energetic efficiency, stability, and habitat value (Tilman 1996; Tilman et al.
1996, 2006; Lehman and Tilman 2000; Vilà et al. 2007). For example, Tilman et al. (1996) have
shown that ecosystem productivity increases with species diversity in grasslands dominated by
native warm-season grasses and tall-grass prairie forbs. An additional advantage of using diverse
plantings of native species is that fertilizer and water inputs are lower than those required for many
monocultures (Tilman et al. 2006). In fact, low-input/high-diversity grasslands may actually be car-
bon negative because their potential biofuel yield and ecosystem carbon sequestration well exceeds
the fossil CO
2
emitted during their production (Tilman et al. 2006). Although data are lacking
from forested systems, recent work in Mediterranean forests suggests wood production may also be
positively associated with tree species richness (Vilà et al. 2007). In the eastern United States, for-
est productivity has been shown to be positively correlated with invertebrate biomass and songbird
reproductive rates (Seagle and Sturtevant 2005). Additionally, animal species diversity is strongly
correlated with habitat structural diversity (Tews et al. 2004), which can be enhanced in plantings
by combining species with contrasting growth forms. For example, arthropod diversity tends to
increase with plant diversity in restored grasslands (Siemann et al. 1998).
To some extent, design elements based on the relationship between species and structural diver-
sity and plant and animal productivity could be incorporated into short rotation woody crop plan-
tations in addition to the possibility of using diverse plantings of natives or well-managed native
vegetation. For example, planting a patchwork pattern of various species would promote vertical and
horizontal heterogeneity across the planting site. Species would need to be paired based on compat-
ible growth and harvesting characteristics. The concept of area control from even-aged silviculture
could also be incorporated at a spatial scale relevant to focal wildlife species to provide a diversity
of age classes across the planting unit. In area control, harvesting is done in strips or blocks at a
rate that allows the entire unit to be harvested over several years so that by the time the last patch is
harvested, the first patch is ready to be harvested again.
In forested ecosystems, treatments that enhance structural and compositional diversity and con-
serve and create biological legacies (e.g., cavity trees and down dead wood) may also be used to
achieve many of the same ends as perennial polycultures of native prairie species or short rotation
woody crops. Possible techniques include precommercial and commercial thinning in even-aged
stands, variable retention and/or legacy tree retention within even-aged treatment blocks, and
uneven-aged management.
6.4 other consIderatIons
Our review has highlighted that it is important to compare the merits of biofuels in terms of net car-
bon sequestration with other land uses and land-use change and to weigh up all components of carbon
sequestration as well as emission. Ideally, this should be done by way of a life-cycle assessment to
