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.