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AsTask Leader for assessment of biomass harvest impacts on forest health and water resources in California, Phillip M. Rury wrote the following Chapter 7 of the California Energy Commission report on the Costs and Benefits of a Biomass-to-Ethanol Production Industry in California
7.0 Impacts of Biomass Harvest on Forest Health and Water Resources
7.1 Assumptions and Biomass Harvest Scenarios
Due to the diversity of forest and other plant communities that may be targeted for
biomass removal to support ethanol production, and the wide range of harvesting
methods and frequencies available, the nature of potential adverse and beneficial
ecological impacts to forests and associated water resources will vary significantly from
site to site. To provide concisely summarize potential watershed-level impacts on forest
health, water quality and biological communities, key assumptions must be made for a
narrowly-defined biomass harvesting scenario, regarding the forest management practices
to be used and compliance with environmental regulations governing those practices.
Since the 13 million acres of commercial timberland in California that might benefit from
thinning is dominated by coniferous tree species, this assessment of watershed-scale
ecological impacts of biomass harvesting focuses exclusively on these coniferous forests.
7.1.1 Chaparral Harvest
Mature chaparral vegetation, which consists mostly of scrubby, slow-growing, evergreen
shrubs about 1-3 meters tall, occurs in arid, fire-prone areas. Fire plays a critical role in
the nutrient cycle of chaparral, because it accelerates the return of plant-bound nutrients
and organic carbon into the soil, thus promoting new growth and support of the native
chaparral flora while eliminating invasive, introduced plant species that are not fire-
adapted. Since recovery of nutrient cycles to pre-harvest levels also likely would require
several decades, biomass removal would undermine the food chain and put these fragile
habitats at risk of being displaced by invasive species of introduced weeds. While fire
suppression in populated areas of chaparral may be desireable to protect property, the
chaparral depends on fire to maintain its unique biological composition (i.e., “fire
climax” communities), which often includes protected species of plants and wildlife
native to chaparral, that also depend on the fire cycle to maintain suitable habitat.
Because the net primary productivity (NPP) of chaparral is both nutrient and water
limited, it is too low to support sustainable yield harvesting of biomass, which would
rapidly deplete what few nutrients are found in the nutrient poor soils typical of
chaparral. As noted by Chabot and Mooney (1985), the net carbon assimilation of this
flora is about half that of deciduous shrubs growing under the same climatic regime. For
example, a 20 year old chaparral stand in San Diego County, CA had a standing biomass
of only 4.6 kilograms per square meter (kg/sq.m.) and NPP yielded annual above ground
biomass production of 130 grams/sq.m./year. Also, biomass from these ecosystems is
unattractive from both an economic and processing perspective, since chaparral plants
contain resins and chemicals (e.g., terpenes) that are undesireable for ethanol production.
Impacts of biomass harvesting from chaparral ecosystems are not evaluated further in the
following discussions, since this appears not to be an economical, technically feasible,
ecologically acceptable, or politically viable source of biomass for ethanol production.
7.1.2 Forest Harvest and Slash Removal Assumptions
Current and projected demand for forest biomass for ethanol production can be met by
slash created during only a fraction of the ongoing timber harvesting in California, so that
ecologically responsible removal of slash from logged areas can be targeted at a subset of
the most accessible, economic, and least ecologically sensitive forest units being
harvested on normal rotations of 20 years or more. We assume that slash removal for
ethanol production will be integrated with slash removals for the purposes of fire
prevention and forest sanitation (i.e., disease and insect control). We also assume that
logging and slash treatment will conform to California’s Forest Practice Rules (Title 14,
California Code of Regulations, Chapters 4 and 4.5) and that the principles of ecological
and water resources protection incorporated into these rules also will be used to focus
selective removal of slash on the least vulnerable sites and forests. Thus, slash removals
would not occur in nutrient-limited forests, on steep slopes or other highly erodible soils,
in riparian zones and headwaters of salmon/trout streams, nor in habitats of any protected
species whose habitat or food supply may be damaged or undermined by slash removals.
Similarly, we assume that timber harvests would be planned to mitigate adverse impacts
of nutrient and biomass losses from slash removal and soil erosion by leaving even more
snags and fallen logs than the minimum extent required by the forest practice rules.
7.1.3 Forest Thinning Assumptions
As for slash removal, we assume that existing plans for pre-commercial and commercial
forest thinning need not be altered/expanded and thinning rotations need not be shortened
to a frequency greater than once a decade for any forest unit, to meet the current and
projected demand for thinning residues targeted for ethanol production. All of the same
assumptions made for harvest and slash removal, regarding conformance of silvicultural
operations with the California Forest Practice Rules and ecological protection efforts
such as minimizing or mitigating thinning impacts to the most sensitive forest units, are
made when evaluating potential impacts of forest thinning. We assume that removals of
thinning residues for ethanol production will be planned to serve the collateral goals of
enhancing timber yield and quality, insect/disease control, and fire prevention. We also
assume that biomass removal from pre-commercial thinning operations will be allowed
only in the most accessible watersheds, forests, and sites that already have roads and are
least vulnerable with respect to soil compaction, erosion, nutrient depletion, downstream
flooding from increased surface runoff, stream siltation and water quality degradation,
and associated damages to sensitive fisheries such as salmon and trout.
7.1.4 Compliance with California Forest Practice Rules
Many adverse watershed-level ecological impacts to forests and associated aquatic
ecosystems can and must be mitigated to comply with environmental laws and
regulations, such as the California Forest Practice Rules. These rules are designed to
avoid or minimize adverse impacts of silvicultural and timber harvest practices on soils,
carbon and nutrient cycles, forest productivity, biological diversity, wildlife and
endangered species habitat, site hydrology, downstream flooding, stream siltation, water
quality, and fisheries. We assume that biomass harvesting scenarios for which impacts
are evaluated will fully comply with these rules and be targeted at the least ecologically
sensitive forest habitats. Thus, the following impact assessment incorporates
considerations of the potential net beneficial impacts that might be expected when and if
adverse impacts are mitigated, to the maximum extent practicable, by conforming to the
silvicultural methods prescribed in the Forest Practice Rules for different types and site-
specific sensitivities of forests.
7.2 Potential Adverse Impacts and Mitigation
7.2.1 Soil Compaction and Erosion
The U.S. Forest Service and many forest ecologists agree that soil compaction and
erosion is an inevitable consequence of silviculture and timber harvesting, even when
conducted with the most careful methods available. Heavy machinery used in thinning,
harvesting, fire fighting operations, post-fire salvage, and the construction/presence of
roads, landings and skidder trails all compact, disturb, and scarify soils, while decreasing
soil porosity and increasing erosion (e.g., Beschta et al., 1995; Frost, 1995; Rhodes and
Purser, 1998). These soil damages decrease soil permeability, tree root growth, water
storage capacity, and infiltration or rain/snowmelt needed to recharge ground water and
maintain base flows of ground water and streams, especially during the dry season.
Impacts of partial cut logging with a feller/buncher can disturb 20-40% of the forest soils
and the soil compaction and associated reductions in forest productivity can persist for
40-70 years or longer (Rhodes and Purser, 1998). Removal of slash, which acts as mulch
when scattered over the logged surface, also exacerbates wind and water erosion of soils,
especially on steep slopes and in highly erodible soils.
7.2.2 Depletion of Soil Carbon and Nutrients
Many studies of forest nutrient cycles in relation to timber harvests and silviculture have
shown that biomass removals from forests, as pre-commercial thinnings, whole trees,
logs, and/or slash, can deplete soil organic matter/carbon and other nutrients essential for
plant growth (Jug et al., 1999; Olsson, 1999; Vesterdal et al., 1995; Jacobson et al., 2000;
Yanai, 1998). The severity of cumulative nutrient losses due to biomass removal varies
widely among sites and forest types, based on soil, topographic and hydrologic factors, so
that time scales for recovery of forest soil productivity also are highly variable. Nutrients
lost from biomass removals are replenished more slowly, for example, by coniferous
forests growing in soils with limited nutrients, due to the limited annual return of leaf
litter and dead branches to the soils from these evergreen trees. Nutrient losses from slash
removal also can reduce plant/tree growth and floristic diversity (Bergquist et al., 1999)
and reductions in growth of pine and spruce can persist for 10 years after whole tree
harvest (Jacobson et al., 2000).
7.2.3 Food Chain and Wildlife Impacts
Fallen trees, logs, snags (standing dead trees), and slash provide habitat for a variety of
forest plants, insects and wildlife, so that their removal for fire prevention, forest
sanitation, energy production, or even as salvage following forest fires can reduce the
amount and quality of wildlife habitat, in some cases posing risk to protected species.
Removals of this organic matter also reduces the duff and other substrate and nutrients on
the forest floor that otherwise are available to decomposers, such as soil microbes,
invertebrates, beneficial insects, and fungi such as edible mushrooms and mycorrhizae,
that form the foundation of forest nutrient cycles and the food chains that support local
wildlife.
7.2.4 Hydrology and Flooding
Soil compaction from harvesting/thinning machinery decreases rain and snowmelt
infiltration, thus increasing peak surface water runoff and downstream flooding, while
reducing soil water storage, ground water recharge and the gradual release of base flows
of ground water to streams and wetlands throughout the growing season. The loss of soil
structure and organic matter due to slash/thinnings removal reduces the water holding and
filtration capacity of soils, so that the quantity and quality of ground water recharge and
long-term release is reduced. Rhodes and Purser (1998) reported that a loss of one inch of
soil from the Sierra Nevada causes a loss of soil storage capacity of 1,500 cu. ft. per acre.
Logging road cuts also intercept shallow subsurface ground water flows, thus acting as
tributaries that greatly increase the diversion to streams of overland, sheet-flow runoff
that otherwise would infiltrate forest soils. When combined with increased surface runoff,
the resultant large increases in peak surface runoff both exacerbate downstream flooding
and increase scouring and erosion of stream banks and channels, leading to sediment
buildup and reduced flood retention capacity in reservoirs.
In contrast to these well documented, adverse hydrologic impacts of timber harvesting
and other silvicultural activities, there is very little evidence for increased water yield or
water quality from forest thinning. Based on an extensive review of pertinent literature on
watershed hydrology and water yield/quality in relation to silviculture, Rhodes and Purser
(1998) found no conclusive evidence that forest thinning had a net beneficial, lasting
effect on the annual yield and quality of surface runoff and baseflows from forested
watersheds. Rather, studies reviewed have shown that clearing of 25% or more of the
forest cover is needed to achieve a significant increase in base flows from a forested
watershed (e.g., CBEA, 1997; Rhodes and Purser, 1998). However, this degree of
clearing, whether achieved by clearcuts, selective logging, or thinning, had adverse soil
erosion impacts leading to increased siltation and water quality degradation of streams
and resultant damages to sensitive fish populations such as salmon and trout. Reduced
water inputs to the rooting zone also both retards tree growth and reduces the ground
water recharge needed to maintain baseflows of water as gradual releases into streams,
thus exacerbating drought stress of forests and associated wetland/aquatic ecosystems
during the summer.
7.2.5 Water Quality and Fisheries
Silvicultural operations and timber harvest/thinning lead to unavoidable increases in soil
erosion and stream sedimentation, resulting in some degree of water quality degradation
and adverse effects on sensitive fish populations and their habitats, especially salmonids
such as salmon and trout. Sedimentation of streams is largely caused by logging roads,
which can increase natural sedimentation by factors of 2 to 10, and is exacerbated by
careless harvests/thinning on slopes and other highly erodible sites. While these impacts
will occur with or without slash or thinning removals from the forest, the spatial extent
and severity of these impacts will vary according to site conditions such as soil
erodibility, so that additional incrementals of erosion-induced impacts to aquatic
resources, caused by biomass removals, will vary from site to site.
7.2.6 Wildlife and Endangered Species Habitat
Biomass removals as slash, thinnings and post-fire salvage can alter wildlife habitat
through reductions in plant species diversity (Bergquist et al., 1999) and removal of logs,
snags and living trees that provide habitat for wildlife, their prey, and also support the
nutrient cycles and local food chains upon which they depend for survival (e.g., Jug et al.,
1999; Olsson, 1999; Vesterdal et al., 1995; Jacobson et al., 2000; Yanai, 1998). Such
impacts on habitats of protected species of fish and wildlife are the most significant risks
posed by removals, especially if the removals and related silviculture operations do not
conform to the Forest Practice Rules designed to protect these endangered species (e.g.,
northern spotted owl). As with impacts to all ecological receptors, however, normal
forestry operations often put wildlife and endangered species at risk whether or not any
biomass removal is conducted. Forest Practice Rules are designed site-by-site, based on
the type and productivity of each watershed and forest unit affected, and prescribe
methods to minimize harvest/thinning impacts on soils, nutrient cycling, food chains,
water quality and other habitat quality attributes needed to support endangered species
and other wildlife. Adherence to these silvicultural and logging rules when harvesting
biomass, thus, can assure that impacts to habitats of endangered species and other
wildlife are minimized to an acceptable level.
7.2.7 Mitigation of Adverse Impacts
Many of the potentially most severe impacts of forestry operations can be mitigated, by
adherence to the Forest Practice Rules and through additional, voluntary efforts to apply
the ecological protection principles that these rules embody. These principles address the
major potential impacts that, in theory, could be increased incrementally by biomass
removals. Because potentially adverse impacts of silvicultural and timber harvest
operations are unavoidable, they will occur whether or not biomass is removed from the
forests for purposes of fire prevention, forest sanitation, and/or ethanol production. Thus,
harvest of slash and thinnings for ethanol production pose a small increment of additional
impact and risk to ecological resources, the significance of which will be greatest at
nutrient limited stands and other sites that are most sensitive/vulnerable to biomass
removal. Careful planning of forestry operations and biomass removals, based on the site-
specific ecological sensitivities of forest units, can mitigate most adverse impacts by
targeting biomass removals only on the least sensitive areas and by conforming to the
forest practice rules and best management practices established for each forest. If
adequately designed and sensitively implemented, post-thinning mitigation of impacts
from logging roads, soil scarification, biomass removal, and other habitat alterations will
offset potential adverse impacts of biomass removals on soil structure, stability, nutrient
cycling, forest productivity, food chains, aquatic resources, and wildlife habitat.
7.3 Potential Beneficial Impacts
7.3.1 Reduced Damage from Fires and Fire Fighting
Wildfire damages to forest health and associated aquatic ecosystems include direct
damage to flora and fauna and indirect fire impacts that are mediated by reduced soil
quality, death of soil organisms and seeds, floristic changes, habitat loss, and impaired
nutrient and water cycles (e.g., Chabot and Mooney, 1985; CBEA, 1997; Morris, 1998;
Neary et al., 1999). These fire impacts also are often exacerbated by fire fighting
activities (e.g., road cuts, fire breaks, stream water removals) and post-burn timber
salvage operations that further disturb the fire-damaged soils, forest hydrology and
habitat/water quality of associated aquatic habitats (e.g., Beschta et al., 1995; Frost, 1995;
Rhodes and Purser, 1998).
Beneficial impacts on forest health from removal of biomass harvested during logging
and thinning operations reasonably can be expected primarily as a result of reducing the
incidence, spatial extent, and severity of forest fires, insect damage and diseases. Forest
sanitation methods that remove the diseased trees and slash from the forest will have the
dual benefits of controlling forest diseases/insect infestations while removing fuel for
fires, so that any subsequent fires will be much less intense than if the biomass were left
in the forest. Since silvicultural methods such as slash removal can reduce fire intensity
without significantly disrupting nutrient cycles at all but the most nutrient-limited sites
(e.g., Stephens, 1998; Monleon and Cromack, 1996), the prevention of intense fires by
removals of slash, thinnings and diseased trees/snags/logs from fire-prone areas can have
the net beneficial effect of reducing direct and indirect damages to forested watersheds.
Thus, by reducing the frequency and intensity of fires through targeted biomass removal,
fires will result in much less severe damage to trees of all ages, soil organic matter and
nutrients, soil biota and seed banks, wildlife, aquatic habitats, water quality and fisheries.
7.3.2 Optimized Carbon Assimilation and Growth
Forest thinning to remove inferior quality trees and undesireable species of trees and
shrubs may not increase total forest productivity or biomass, but in many situations will
enhance the photosynthetic rates, nutritional health, growth and quality of wood/timber in
the mature trees and other regenerative age/size classes of tree species that are retained
for future harvest. Photosynthesis, carbon assimilation, and growth of the desired/retained
trees results from their sudden release from competition with vegetation being removed,
for sun, water and soil nutrients (e.g., Bergquist et al., 1999; Burgess and Wetzel, 2000;
Cain, 1995; Rytter, 1994; Wang et al., 1995). Since the competition for soil nutrients is
significantly reduced by pre-commercial and commercial thinning, the slight incremental
increase in nutrient losses from the forest caused by the removal of slash or thinned tree
trunks is negligible. In most situations, thinning will enhance carbon gain and tree growth
despite the removal of additional nutrients as biomass used for ethanol production.
7.3.3 Improved Timber Yield and Quality
When coupled with the increased spacing that allows trees to attain their full genetic
potential for optimal wood quality, thinning normally leads to greater light availability,
increased wood production, and optimal timber quality (e.g., Baldwin et al., 2000;
CBEA, 1997; Morris, 1998; Rytter, 1994). Thus, if biomass removal is targeted only at
forest stands that are not so nutrient limited as to suffer a decline in wood production due
to even very small removals of nutrients on a 10 year or longer cycle, both reduced risk
of fire and disease as well as improved per tree yield and quality of timber can be realized
from thinning efforts that yield biomass for ethanol production.
7.3.4 Better Forest Sanitation and Insect/Disease Control
Pine and spruce bark beetles and other insects damage trees directly and/or may transmit
tree diseases, such as pitch canker disease in pines (from the fungus, Fusarium), live just
beneath the bark of trunks and branches (see Dallara et al., 1995; Sanborn, 1996; Weslien
and Schroeder, 1994). Managed spruce forests have been shown to be less susceptible to
infestation by spruce bark beetles than unmanaged stands and have smaller numbers of
dead trees in which the insects can reproduce and emerge to infest healthy trees (Weslien
and Schroeder, 1994). Dallara et al. (1995) also showed that infestations of pitch canker
disease increased the susceptibility to bark beetle attacks in eleven species of pine native
to California. Since the only known control for this fungal disease is forest sanitation, by
controlling the inoculum and its insect vectors (i.e., bark beetles), removals of infested
slash and other biomass for use in ethanol production can help reduce these disease
outbreaks. Forest sanitation methods used widely to control or prevent forest infestations
by these insects and diseases often focus on the treatment of slash, dead trees (logs and
snags), and infected green trees that harbor these pests in their inner bark. One common
in-field methods are to debark, chip or lop, and spread slash and infested trees in the sun
to kill these insects through dessication. Another is to burn these waste materials, which
also may be achieved by prescribed burning for fire hazard reduction.
While the chipping /lopping and spreading of slash and diseased trees over the ground
surface is commonly prescribed as forest sanitation to control these insect pests and other
diseases, this does not also reduce the risk or intensity of wildfires enough to prevent
severe ecological damages to forests. In contrast, removals of the infested slash and other
biomass from the forest is an effective means of reducing both the fire risk and severity
of any fires that do occur, while also effectively controlling the spread of forest diseases.
Because insect and other disease infestations and epidemics are exacerbated by drought,
which can render even health trees susecptible to infection (Sanborn, 1996), slash and
other forest biomass removal needed for the combined purposes of disease control, fire
prevention, and ethanol production should be timed to precede drought conditions.
7.3.5 Control of Invasive Plant Species
Thinning of forests can remove undesireable species of trees, shrubs and herbs, including
introduced species of noxious, invasive weeds, while enhancing the growth of native
species of shrubs and herbs that do not significantly compete with the timber trees for
root space, water, light, or nutrients. Thinning of undesired trees can also increase the
natural, multi-storied regeneration of desired tree species, such as seedlings, saplings and
poles of Douglas Fir (Nailey and Tappeiner, 1998) and other timber trees, that provide for
future timber harvests while also preventing invasive weed colonization of the forest.
Since invasive weeds more easily invade a forest after high intensity fires have removed
undergrowth, disturbed soils and killed seeds of native species, slash removals for use as
biomass can reduce the risk of fire-potentiated invasions of forests by non-native plants.
7.3.6 Hydrology, Flooding and Water Quality
It also is conceivable that reductions in vegetative cover and soil organic matter could
increase ground water recharge and base flows in forested watersheds due to greater
infiltration of rainfall and snowmelt, at least temporarily, until the roots of the remaining
timber trees colonize the soils in which the removed vegetation had been rooted. This
theory underlies the simplified mathematical models of thinning-enhanced water yield,
developed by the CBEA (1997) for use in forested watersheds of the Sierra Nevada.
While these models have serious weaknesses, such as ignoring the increased uptake of
water by the remaining trees and by recovering vegetation in the thinned areas of the
forest, it is conceivable that sufficient thinning followed by vegetation control in these
areas could marginally increase water yield. However, there currently is little empirical
evidence for nor consensus regarding the potential for thinning to increase water yields.
Rhodes and Purser (1998) found no conclusive evidence that forest thinning had a net
beneficial, lasting effect on the annual yield and quality of surface runoff and baseflows
from forested watersheds. However, they did identify methods of mitigating adverse
impacts on water quality from forest management practices, including approaches that
will increase the annual yield and quality of base flows in forested watersheds, such as
avoidance of disturbances to highly erodible soils/sites during harvest/thinning and post-
harvest removal of logging roads. If integrated with biomass removals during timber
harvests and forest thinning, such mitigation measures can enhance forest hydrology and
ground/surface water quality, while reducing peak runoff, stream sedimentation, damage
to fisheries, and flooding in downstream ecosystems.
By reducing the risk and severity of forest damages from fires and insect/disease
infestations, periodic removals of infected and combustible biomass also will reduce
erosional and combustion losses of soil organic matter and nutrients, which collectively
reduce the water holding capacity of soil and undermine soil stability, thus increasing
peak surface runoff rates and volumes and exacerbating downstream flooding,
sedimentation, and water quality degradation in streams and other surface waters. For
example, the CBEA (1997) reported that 25 cu. ft. of sediments are released to
streams/reservoirs from each acre of intensely burned forest in the Sierra Nevada, which
leads to water quality degradation, fisheries damages, and downstream flooding. Thus,
any efforts to reduce the frequency and intensity of forest fires, such as biomass removal,
can have a net beneficial impact on forest hydrology, flood control, water quality, and the
health of aquatic ecosystems and their resident fish communities.
7.4 Summary of Potential Net Ecological Benefits
Despite the controversy surrounding theories about the ecological benefits of forest
practices, such as thinning and slash removals, in many situations available evidence
often justifies removals of biomass for the dual purposes of fire prevention and disease or
insect pest control. When compared to the normal ecological impacts of these forest
management and harvesting activities, the very slight additional increments of impact
from biomass removals of slash and thinnings are relatively insignificant. While the
chipping /lopping and spreading of diseased slash and trees over the ground surface is
routinely prescribed as a forest sanitation method to control insect pests and diseases, this
does not also reduce the risk or intensity of wildfires enough to prevent severe ecological
damages to forests. Thus, removals of the infested biomass from the forest is an effective
means of reducing both the fire risk and severity of any fires that do occur, while also
effectively controlling the spread of forest diseases. By targeting biomass removals on
high fire/disease risk forest stands, where thinning and slash treatment efforts are needed
anyway, ethanol production can be supported with net beneficial effects on forest health.
Intense forest fires and disease epidemics often cause damages to forest and aquatic
ecosystems that far exceed the slight, incremental impacts on these resources from
infrequent biomass removals. Therefore, when conducted in conformance with the
California Forest Practices Rules and employing ecological impact mitigation measures
designed for site specific conditions, such biomass removals will have a net beneficial
impact on the health of forested watersheds, especially if they are targeted only on those
forest units that are least sensitive/vulnerable to the adverse effects of biomass removal.