The Effects of Woody Biomass Removal and Fire on Soil and
Ecosystem Properties in the Pacific Northwest
NARA Soil Sustainability Group, University of Washington
Robert Harrison, Jason James, Stephani Michelsen-Correa, Marcela Menegale
Draft Literature Review and Report
June 30, 2013
The conversion of woody biomass to energy has become a prime national
policy objective in recent years. Bioenergy from forest residues is preferable to
ethanol made from corn because growth in the sector will not drive up food prices.
While the emergence of a bioenergy market from woody biomass could impact the
price of wood products, utilizing non-bole woody material can minimize these
effects. In the Pacific Northwest, one of the premier wood-producing regions in the
world, additional removal of woody material from harvest sites raises questions
about maintained site productivity and long-term sustainability. However, different
questions must be asked depending upon which side of the Cascade Range potential
harvest sites are located, due to differences in tree species, climate, and source of
non-bole woody biomass. The question of sustainability of additional biomass
removal from the East and West side of the Cascades will be tackled separately in
this document.
Forests West of the Cascades
Disturbance history plays an important role in the quality of ecosystem
services provided by our nation’s forests and must be taken into account by the
agencies charged with managing these landscapes. Of particular significance in the
Pacific Northwest is the role of fire. In Washington State, the forests on the west
side of the Cascades historically have relatively long fire return intervals. These
historical fire return intervals range from 217 years for Douglas fir stands to 937
years for cedar, spruce, and hemlock forests (Agee et al 1990; Fahnestock and Agee,
1983; Agee, 1993). When fires do burn on the west side of the Cascades they tend
to be high severity, stand replacing events. Federal and state agencies have been
highly effective in their fire suppression efforts, which has increased fuel loading.
In spite of these efforts and large amounts of resources directed towards
suppression there has been a recent spike in the average number of acres burned
each year (Stephens and Ruth, 2005). The shift from a suppression centric fire
management policy to one that recognizes fire as a “critical natural process, [that]
will be integrated into land and to resource management plans and activities”
(NWCG, 2001) has been a significant change to the decades long policy of excluding
fire from the landscape. Additionally, its significance lies in the documented
recognition of fire as an essential component of healthy ecosystems and that short
term benefits of fire exclusion are a detriment to the long term health of the forest
(USDA Forest Service, 1995).
Slash burning has been a common practice in Douglas-fir forests on the west side of
the cascades starting in the early 1900’s. Concerns were raised in the 1970’s about
smoke from these slash fires which was followed by a reduction in the slash burning
through the late 1980’s (Agee, 1993, Radke et al, 1990). Smoke and particulate
emissions from prescribed fire continue to be an issue. Smoke management plans
for prescribed burns need to follow the standards for clean air outlined by the
Environmental Protection Agency (EPA) National Ambient Air Quality Standard
(EPA, 1998). Despite these concerns prescribed burning is still an important
method for reducing hazardous fuel loads (Weisshaupt et al, 2005). Removal of
slash as opposed to burning on site and its effect on soil nutrient retention has not
been adequately addressed in literature for western Washington.
[This section is probably in the most need of more development]
Forests East of the Cascades
On the East side of the Cascade Range, many forest stands are in need of
restoration for a variety of attributes because of fire suppression and/or a lack of
harvesting. Compared to the wet, maritime influenced climate on the West side of
the Cascades, East-side forests are considerably drier, and historically had short fire
return intervals from 6 to 24 years (Hatten et al., 2005). Due to a century of fire
suppression, many stands have missed at least one or two fire cycles, leading to
increased stand densities, accumulation of understory/ladder fuels, and unnatural
buildup of course woody debris (CWD) on the forest floor (Schwilk et al., 2009;
Stephens et al., 2012). Consequently, stand-replacing wildfires in Eastern
Washington and Oregon have increased beyond their natural extent or frequency
(Wright and Agee, 2004; Stephens et al., 2012). The historical role of fire in these
forests was low-intensity, high frequency burns that maintained relatively low stand
densities of old Ponderosa Pine, Douglas-fir, Lodgepole Pine, and Engelmann Spruce
(Wright and Agee, 2004). Precommercial thinning (PCT) is an effective strategy to
restore these ecological conditions and reduce fire hazard by removing small,
understory trees, as well as some larger trees, to return the stand to historical
densities and fuel loads. If land managers use PCT as a source of biomass for energy
production, they may partially offset the costs of forest restoration (Page-Dumroese
et al., 2010). However, the effects of such thinning on soil quality, function, and
productivity should be carefully considered both in the context of severe wildfire
burns and sustainable supply of biomass for future harvests.
Soil Effects of Severe Wildfire
The effect of fire on soil is a function of fire intensity, which is defined by the
maximum temperature of the fire and the time that temperature remains at a
certain point (Bento-Goncalves et al., 2012). Burn severity, on the other hand,
describes the response of the ecosystem to fire, including effects on soil properties,
hydrological dynamics, flora and fauna, the atmosphere, and society (BentoGoncalves et al., 2012). In general, stand replacing fires have the most deleterious
effects due to sustained high intensity and consequent severity of the burn. This also
informs the importance of the shift from high frequency, low intensity fire in the dry
forests east of the Cascade Range to high intensity, stand replacing burns.
The most intuitive change in soil properties due to fire is the combustion of
the forest floor and loss of organic matter. Depending on the severity of the fire, the
impact can be a small volatilization of minor constituents, charring, or complete
oxidation (Certini, 2005). One year after severe wildfire on the Eastern slopes of the
Cascades, Baird et al. (1999) observed 10-30% (7-25 Mg ha-1) decrease in soil C and
13-46% (0.4-3.0 Mg ha-1) decrease in soil N relative to unburned forest. Subsequent
surface erosion removed an additional 280-640 kg C ha-1 and 14-22 kg N ha-1.
Despite the inevitable loss of organic matter immediately following fire, Johnson and
Curtis (2001) demonstrated an increase of ~8% in C in the A horizon 10 years or
more after fire. The authors suggested three reasons for this increase: (I)
incorporation of unburned residues in mineral soil; (II) the transformation of OM to
more recalcitrant molecular forms; and (III) the entrance of N-fixing species, which
can significantly enhance C sequestration (Johnson and Curtis, 2001). Incomplete
oxidation of biomass leads to the creation of pyrolysis compounds that can
significantly impact C dynamics in soil, particularly the accumulation of C in stable
pools (Gonzalez-Perez et al., 2004). Volatilization of organic N during fire can
decrease soil N reserves, depending on burn severity (Certini, 2005). Low to
moderate intensity fires result in a flush soil ammonium, which dissipates quickly
(Weston and Attiwill, 1990). High severity fires, on the other hand, can result in
complete loss of the organic N stock and nitrogen deficiency on the site (Certini,
2005). The recovery of the soil C and N stocks depend at least partly on the burn
severity: more complete oxidation of OM leads less unburned material to be
incorporated in the soil and less char. In the most severe burn areas, complete loss
of the forest floor and volatilization of N can result in long-term nutrient deficiencies
and alter both above- and below-ground species composition on the site (Neary et
al., 1999).
Fire can result in an enrichment of available soil P, but this also dissipates
quickly. Other nutrients, including Ca, K, Mg, B, and trace metals show only
ephemeral changes due to fire. Fire denatures organic acids in soil, leading to
inexorable rise in soil pH. However, large increases only occur at temperatures in
excess of 450-500°C because complete combustion of fuel releases base cations to
the soil (Arocena and Opio, 2003).
The effect of fire on soil physical properties is likewise tied to burn severity.
In low to moderate intensity fires, structure is often enhanced by the formation of a
hydrophobic film on aggregate surfaces, while organic cements and hydrophobic
substances are disrupted or decomposed at high temperatures (Badia and Marti,
2003; Mataix-Solera and Doerr, 2004). More complete combustion of surface fuels
as well as disruption of hydrophobic substances in severe fires can lead to
significant threat of surface erosion, particularly sheet and rill erosion (Baird et al.,
1999). Nutrient loss due to accelerated erosion and ash entrainment in smoke
columns is of most concern following severe fires (Neary et al., 1999).
The shift to high severity wildfire on the East side of the Cascade Range can
significantly affect subsequent soil productivity. In a bioassay using lettuce, Baird et
al. (1999) found increased growth in soils following low to moderate severity fire,
but decreased growth in soils burned by high severity fire relative to an unburned
control soil. Widespread fuel reduction in Eastern Washington and Oregon is a
necessary step to prevent the rise of high severity fire and loss of productivity.
However, the costs of such measures (including PCT and controlled burning) have
been prohibitive for widespread application. The creation of a biofuel market
capable of utilizing thinning and fuel reduction biomass could provide a means for
landowners to recoup expenses for forest restoration (Page-Dumroese et al., 2010).
These restoration and thinning practices, however, have their own effects on soil
properties, and must be carefully considered.
Soil Effects of Woody Biomass Removal
Similar to wildfire, a primary effect of PCT and fuel reduction treatments is
reduced organic matter, particularly on the forest floor as coarse woody debris
(CWD). Organic matter is essential for maintaining many ecosystem functions,
including soil C and N cycling, regulating gas exchange and water availability, and
supporting biological diversity (Jurgensen et al., 1997; Page-Dumroese and
Jurgensen, 2006). Periodic stand disturbances due to thinning could have negligible
short-term impacts (Sanchez et al., 2006), or more significant impacts depending on
soil type, ecosystem, and climatic regime (Grigal, 2000; Grigal and Vance, 2000).
Organic matter removal can induce changes to soil physical properties that may be
more important for soil productivity than nutrient removal. For example, loss of
forest floor and mineral soil organic matter lowers soil moisture retention, cation
exchange capacity, and subsequent tree growth in coarse-textured soils (Ginter et
al., 1979). While noting that no thinning studies had specifically examined the effect
of organic matter removal in the Western U.S., Page-Dumroese et al. (2010)
speculated that repeated thinnings over the life of a stand might impact some soils,
particularly those with poor water- and nutrient-holding capacities.
Compaction is the most likely effect of thinning activities on soil physical
properties. Compaction can have a number of impacts, including decreased soil
porosity, reduced movement of air, water and nutrient through soil, and reduced
microbial activity (Thibodeau et al., 2000; Bulmer and Simpson, 2005; von Wilpert
and Schaffer, 2006; Beylich et al., 2010). Rock content, soil texture and bulk density
can all affect the susceptibility of soil to compaction (Page-Dumroese et al., 2006;
Powers, 2006). Compared to clear-cut harvesting, thinning operations are more
likely to concentrate compaction on harvest traffic lanes (McIver et al., 2003). Other
potential impacts include: puddling through smearing of soil pores, which alters soil
structure and prevents infiltration; churning; loss of the top mineral soil layer
(Heninger et al., 2002); and rutting, which can disrupt hydrological cycles by
creating preferential overland flow paths (Curran et al., 2005). Page-Dumroese et al.
(2010) recommend using a risk-rating system for evaluating site sensitivity to each
of these impacts, since these disturbances can be more detrimental to tree growth in
localized areas than compaction. Such a risk-rating system has been proposed by
Wall (2012) based on a meta-analysis of 86 studies that examined the effects of
stem-only, whole tree, and thinning treatments.
The effects of nutrient removal due to thinning or biomass-to-fuel harvests
depend upon the total and available nutrient stores in the soil before treatment.
Knowing the size of the soil pools and the total amount of nutrients removed in a
thinning operation would facilitate management planning to replace or retain
nutrients (Page-Dumroese and Jurgensen, 2006). However, this information is costly
to obtain and rarely available for a specific site; estimates based upon biomass
removal levels and soil data within the literature should be used in concert with
best management practice (BMP) guidelines.
A primary concern of additional removal of woody biomass from a site is the
effect of reduced CWD and organic matter (OM) on microbial communities in soil.
Nitrogen is often the most limiting nutrient for tree growth in western forest soils
(Chappell et al., 1991), and plant availability of N (as NH4 and NO3) depends upon
soil microbial activity. Loss of surface OM and canopy cover generally raises soil
temperature, which increases decomposition and N mineralization rates (Smethurst
and Nambiar, 1990). On the other hand, if large amounts of logging slash are left
after harvest, microbes can immobilized most soil N in their biomass until the
residues are decayed (Thibodeau et al., 2000). The degree and extent to which these
changes to N mineralization affect a site depend on how much of the stand biomass
(thinning intensity) was removed from the site (Page-Dumroese et al., 2010). Grady
and Hart (2006) found that in stands thinned of about 40% of their basal area,
mineral soil mineralizable N amounts were significantly less than unharvested
stands.
While the potential for productivity decline due to nutrient and organic
matter deficiencies and negative impacts to soil physical properties certainly is
present for biomass-to-energy thinning treatments, these effects pale in comparison
to stand-replacing fire. Indeed, in forests that have been excluded from both
harvesting and fire for many decades, returning them to historical stand densities
and stand dynamics should be a management priority. Utilizing harvest residues
from these stands for bioenergy, while maintaining best management practices,
provides a viable path forward for ecosystem health, economic feasibility, and
security of human infrastructure in forests of the Eastern Cascades.
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