Environmental Engineering Reference
In-Depth Information
higher nutrient concentrations than the heartwood that comprises the bulk of long-rotation stem
only harvesting (Kimmins 1997). By reducing rotation length, nutrient losses are magnified by the
increased harvest intensity induced by short rotations and the compounding effect of higher nutri-
ent concentrations in young trees. In addition to purpose-grown short-rotation bioenergy forests,
the extraction of wood residues from traditional logging is often the first step used by wood-based
lignocellulosic refineries because it is comparatively simple to utilize such a “waste” product (Scion
2008). Although residue removal does not change the rotation length per se, it does increase the
utilization intensity because whole-tree harvesting removes more nutrients than traditional stem-
only harvesting for timber (Kimmins 1997). Furthermore, removing residual woody biomass from
forests may create an ecological trap, in which dead-wood dependent species are “mass trapped” by
the delayed removal of woody debris (Hedin et al. 2008).
Bioenergy crops may in some cases increase rotation lengths (e.g., the replacement of annual
row crops with short-rotation coppice crops of willow and poplar) or perennial grasslands. Annual
row crops have been shown to deplete soil organic matter (most within the first few years of culti-
vation; Bowman et al. 1990) and thus require substantial fertilizer inputs to maintain productivity.
However, expansion of new cellulosic bioenergy crops will shift rotations from annual to perennial
crops, allowing soils to rejuvenate and facilitating increases in soil organic matter and fertility
(Cook and Beyea 2000; Rowe et al. 2009). For example, below-ground biomass is reportedly much
higher in
P. virgatum
(switchgrass; 7.74 Mg/ha) than cultivated cropland (4.35 Mg/ha) (Liebig et
al.
2005). Similar increases in soil organic matter have been reported after a shift from annual
cropping to fast-growing woody biomass crops such as poplars (Hansen 1993; Rowe et al. 2009),
although temporary losses as a result of erosion and mineralization are known to occur before
canopy closure, and in some cases no increases have been recorded (Grigal and Berguson 1998).
6.3.2.1.2 Wildlife Habitat
Old-growth forests sustain high levels of biodiversity, often with species unique to ancient forests,
and the large-scale conversion of virgin forest to short-rotation woody biomass crops would be highly
detrimental to forest biodiversity. Wind, fire, disease outbreaks, and other natural disturbance factors
initiate natural regeneration in parts of forests, replacing older trees and forest communities with
assemblages of disturbance-adapted colonizers and young trees. Over time, the process of succession
(Connell and Slatyer 1977) leads to a gradual return of a forest community with shade-tolerant under-
story plants and canopy trees and the associated fauna, with a considerable exchange of species. This
recovery of natural communities takes decades or centuries, depending on a wide range of factors,
and is also apparent in modified production ecosystems such as plantation forests (e.g., Brockerhoff
et al. 2003). On land where the natural land cover is (or was) forest, the habitat becomes increas-
ingly suitable for forest species and communities as the time span between disturbances (or harvest-
ing) increases. The same applies to disturbance effects in grassland and other nonforest ecosystems.
Therefore, short-rotation cropping systems offer generally little opportunity for the establishment of
a diverse range of species, except for disturbance-adapted specialists. In forests currently managed
for timber production, a shift to shorter rotations or away from management activities that attempt to
emulate natural disturbance regimes will further decrease their habitat suitability for forest-specialist
species and could conceivably cause localized extinctions of some species, although in some cases
shorter forest rotation times may be beneficial because they increase the proportion of early succes-
sional habitat that is important for some species. For example, Eycott et al. (2006) found the greatest
diversity of understory plants in young stands (<10 years) and concluded that shorter rotations pro-
vide greater biodiversity benefits for heath-land landscapes. However, short rotation bioenergy forests
are likely to have much higher stocking rates than traditional timber forests, which could suppress
understory plant diversity. Increasing crop rotation lengths by planting perennial species rather than
annual crops has significant potential to enhance habitat for biodiversity, particularly if ecosourced
native crop species are used (Cook and Beyea 2000). Perennial short-rotation woody biomass crops
that replace row crops in formerly forested areas have been shown to benefit forest-dependent bird
