Environmental Engineering Reference
In-Depth Information
Down dead wood and standing snags provide important habitat for small and large mammals
(Zollner and Crane 2003; McCay and Komoroski 2004), birds (Rosenberg et al. 1988), amphib-
ians (Butts and McComb 2000), arthropods (Jonsell and Weslien 2003; Jabin et al. 2004; Ulyshen
et al. 2004; Brin et al. 2009), and a host of microorganisms (Allen et al. 2000). Down wood is also
an important substrate for the regeneration of many species of forest trees (Noguchi and Yoshida
2004; O'Hanlon-Manners and Kotanen 2004) and other plants (Santiago 2000). Down wood may
provide transient refugia for tree seedlings and herbaceous plants in areas with high abundances of
native or exotic ungulates (Casabon and Pothier 2007; de Chantal and Granström 2007). Large and
highly decayed downed dead wood and snags are significantly less abundant in second-growth and
managed forests than in areas of primary or old-growth forest (e.g., Goodburn and Lorimer 1998;
Fridman and Walheim 2000). Increasingly, forest managers have recognized the importance of this
material, and its conservation and creation are often codified in management plans (Hunter 1990).
Consequently, an inherent danger in the rush to increase the utilization of forest residues is the
potential further depletion of this foundational habitat element of forested ecosystems.
The use of nonmerchantable living shrubs and trees that are undesirable as future growing stock
might incentivize activities that are currently unprofitable, such as timber stand improvement, inva-
sive species control, and fuel reduction treatments. However, provisions should be made for the
retention of some low-value cull trees with cavities or other desirable habitat features as well as
native species that are poorly represented but add structural or compositional diversity.
Ecologically sustainable residue harvesting will require a thoughtful assessment of the critical
thresholds of these materials that are needed to maintain site productivity, reduce erosion hazard,
and support biodiversity. Further consideration should also be given to the role of this material as
a source and sink for atmospheric CO
2
. In the case of forest residues, for the first few years after
harvest, logging slash is a net emitter of CO
2
as a result of microbial respiration associated with
the decay of fine materials and surface tissues (Ganjegunte et al. 2004; Birdsey et al. 2006).
However, depending on the recalcitrance of the species, temperature, and fragment size, the gener-
ally slow decay rates associated with coarse woody debris may make them an important carbon sink
and long-term source of nutrients (Ganjegunte et al. 2004).
6.3.2 B
ioEnErgy
p
lantationS
and
d
EdicatEd
c
ropS
/t
rEES
Traditional forest management has favored long rotation times that often exceed 75 years for tim-
ber (as low as 26 years in New Zealand) but can be as low as 5-10 years for intensive pulp or
charcoal production in tropical or subtropical regions. In contrast, arable row crops are frequently
harvested on an annual basis, require more intense management than forestry, and consequently
place greater demands on ecosystem services. To date, most “first-generation” biofuel feedstocks
have been annual crops [e.g., corn and rapeseed (
Brassica napus
) oil], which have replaced exist-
ing croplands or displaced native habitat. New “second-generation” feedstocks, such as cellulosic
conversion of wood residues and perennial grasses, are likely to increase dramatically in the near
future in response to the renewable energy targets of many countries. Cellulosic bioenergy has
the potential to dramatically alter current land-use patterns, with flow-on effects that change crop
rotation patterns throughout the landscape. Studies that examine different levels of logging residue
removals on soils with different potential erodability would provide valuable guidance for future
growth of this sector (EEA 2006).
6.3.2.1 Implications of changing crop rotation lengths
6.3.2.1.1 Soils and Nutrient Cycling
The phenomenon of harvest-induced site nutrient depletion has been of concern to foresters for
more than 50 years, and a shift to shorter rotation lengths for bioenergy production will exacer-
bate this effect. Young trees have a greater proportion of sapwood and crown material, which has
