Understory trees and shrubs that once were destroyed by frequent, low-intensity fires have survived and grown, resulting in dense stands of trees and shrubs. For- Harvesting forest biomass est fires can climb these “fuel ladders” into the crowns of larger trees. (Shown here is a 1988 prescribed burn in the reduces wildfire fuel Santa Monica mountains.)

Gary Nakamura

Years of drought, tree mortality destroyed by frequent, low-intensity ground, where it exacerbated the fuel due to insects and the successful fires have survived and grown, result- and fire hazard while attenuating the suppression of forest fires over ing in dense stands of trees and intertree competition for water and the past century have created shrubs, which can serve as fuel lad- nutrients. Given its high cost and dense stands of trees and shrubs. ders into the crowns of larger trees. mixed benefits, precommercial thin- This buildup is potentially danger- While fires on or near the ground are ning has not been widely applied. ous as a reservoir of fuel for un- controllable, fires in the tree crowns Harvesting small trees became eco- controllable wildfires in California are intense and destructive. The pres- nomically feasible in the mid-l980s, ence of such hazardous fuel levels, forests. The advent of biomass when a market developed for wood combined with an increased risk of power plants has made removal fuel for electric power plants. The Pub- fire starts due to increasing develop- lic Utilities Regulatory Policy Act of of excess plant material from the ment in the wildlands and frequent 1978 encouraged the use of domestic forest economically feasible, re- hot, dry, windy conditions, can turn and renewable resources such as forest ducing wildfire hazard and pre- controllable fires into uncontrollable, biomass for electrical power genera- paring fire-adapted forests for catastrophic wildfires such as the 1992 tion. By 1990,975 megawatts of power reintroduction of prescribed fire. Fountain fire, 1992 Cleveland fire and had been produced using 8 million 1994 Cottonwood fire. bone-dry tons (BDT) of biomass annu- The health of California’s interior forests Fuel levels and fire hazard can be ally, of which 2.25 million BDT came is threatened by the presence of too alleviated by thinning small trees from from forests and the forest products many trees in too many places, created the forest, reducing wildfire intensity industry. in part by successful fire-suppression ac- from one that requires bulldozers to tivities in fire-adapted ecosystems such one that can be controlled with hand Forest biomass as the Sierra Nevada mixed-conifer tools. Until recently, small trees were Forest biomass refers to all above- forest. At risk are the habitats of many not processed into any commercially ground plant material. It can include wildlife species, including the Califor- valuable product and thinning repre- small or dead trees, shrubs, and the nia spotted owl, as well as homes, wa- sented a net cost of a few hundred dol- tops, foliage, or limbs of large com- tershed, health and the aesthetic val- lars per acre. Thinning consisted of mercially valuable trees. Because ues associated with forests. Under- cutting the main stem and lopping off these materials are typically left on the story trees and shrubs that once were branches, leaving the material on the forest floor to decompose, biomass

CALIFORNIA AGRICULTURE, MARCH-APRIL 1996 13 Top /eft,mixed-conifer forest stand in 1988, typical of stands that would be con- sidered for biomass harvesting to reduce fuel levels and fire hazard, and improve the health of the thinned stand. The bio- mass in trees 7 inches in diameter at breast height (DBH) and smaller is 26 bone-dry tons per acre.

Middle /eft,view of stand immediately fol- lowing a biomass harvest of trees 7 inches DBH and smaller. Foresters considered tree condition, species, size, and spacing before marking “reserve trees” with blue paint.

Lower left,view of stand 7 years after har- vest, 1994. Note the growth of the incense cedar in the lower right and the extent to which the forest has filled in with foliage and understory plants.

harvesting represents a broadening of forest materials that can be used. Op- erationally, biomass harvesting is equivalent to whole-tree harvesting. Fuel from sawmills (sawdust, trim- mings) and agricultural operations (prunings, nutshells) are residues ob- tainable at low or no cost other than hauling, whereas forest biomass is the most expensive fuel. In 1992, prices of $35 to $40 per BDT just covered the cost of harvesting, chipping and haul- ing from the forest to the power plant, bringing little or no profit to the forest landowner. Although paper-pulp chips represent a higher value prod- uct, they require more processing and usually longer hauls to pulp mills. Whether a financial gain or loss is real- ized from biomass harvesting depends upon the tons per acre available for harvest, site conditions (slope, tree sizes), haul distance to power plant and current markets for wood chips. Biomass harvest operations The mechanization of biomass har- vesting has made it both economically feasible and silviculturally useful - useful in reducing intertree competi- tion and improving the growth and condition of trees. Feller-bunching ma- chines travel to each tree and sever the stem at ground line with hydraulic shears, much like large pruning shears with 14- to 20-inch-diameter capacity. These machines gather several trees into bunches, which are then laid on the ground to dry. This ability to

14 CALIFORNIA AGRICULTURE, VOLUME 50, NUMBER 2 gather and handle trees in bunches Harvesting larger trees for higher harvested, soil compaction is a concern. makes it economically feasible to re- value products is often combined with However, compaction can be mitigated move biomass for processing into fuel break-even or uneconomical biomass by driving equipment over slash (tree chips or other products. If the trees harvesting, essentially investing some limbs, foliage)or forest floor litter and were simply cut or killed, but not re- of the value of the commercial harvest duff (decaying plant material), rather moved from the forest, fuel and fire in thinning and improving the health than bare soil. Designated skid trails and hazard would increase along with in- of the stand. limits on traffic over wet soils in the sect activity and the treatment would The quantity of biomass that makes early spring and fall also could reduce not be as beneficial silviculturally. such a harvest economical is also the areas of sigruficant compaction. Following a week or two of drying level at which fuel loading makes Removal of whole trees raises con- to reduce moisture content, bunches wildfire suppression difficult. Biomass cerns about depleting soil nutrients. are dragged by tractor to an in-woods harvesting can reduce the fuel load to Unlike tropical forest ecosystems, chipping machine for processing into 5 BDT / ac, where a fire could be con- which retain the major portion of eco- fuel chips. The momentum imparted trolled with hand tools because inten- system nutrients in the standing bio- by the rotating chipper head hurls the sity and flame length are low. mass, temperate forest ecosystems re- chips into vans for transport to power Using current equipment, efficient tain 80 to 90% of their total nutrients plants. and safe biomass harvesting is usually in the soil, with 10 to 20% evenly di- Feller-bunching machines harvest done on slopes less than 30%. Equip- vided between standing biomass and whole trees, including branches, stem ment exists for steep slopes, but oper- leaves and branches on the forest floor. and leaves, removing their organic ating costs are higher, and it is more Table 1presents the biomass and nutri- matter and nutrients from the forest. difficult to control damage to crop ent distribution for a typical mixed- Variations include delimbing heads, trees that are not removed. conifer forest prior to biomass harvest- which cut trees, then remove the limbs In addition to reducing competition ing - 850 trees per acre less than 7 and foliage at the stump to be recycled and fire fuel, biomass harvesting can inches in diameter; 150 trees per acre into the soil. Feller-bunching heads affect the type and extent of under- greater than 7 inches in diameter; a can be mounted on booms, which can story vegetation and concomitant forest floor 2 to 3 inches thick, consist- reach for trees, obviating the need to changes in animal habitat. Thinning ing of leaves, twigs, and branches; and drive up to each tree. Forwarders that trees to open the understory to light a productive forest soil 3.3 feet deep. pick up logs and load them onto wag- and to reduce competition for soil Biomass harvesting would remove ons reduce the road network necessary moisture can promote development of most of the trees less than 7 inches in for harvest operations. grass and shrub species and the ani- diameter, and some of the larger trees, mals that use them. depending upon plans for growing the Forest stand considerations thinned stand and future regeneration. Densely stocked stands are suitable Potential adverse impacts Thinning intensity could vary, leaving candidates for biomass harvesting for Because feller-bunching machines denser spots for wildlife cover and three reasons: (1)reducing intertree must travel to each tree that is to be thinner areas where fire risk is greater. competition will improve the vigor of the residual trees and their resistance to insects and drought; (2) there is suf- ficient biomass per acre to make har- vesting and hauling economically fea- sible; and (3)such levels of biomass constitute a severe fire hazard. For an operation to be economical, 25 green tons (approximately 12.5 BDT) of biomass are required from each acre. Trees harvested for biomass range in size from 2-inch DBH (diam- eter breast height measured at 4.5 feet above the ground surface) and a height of 15 feet, to 8-inch DBH and 40 feet high. These trees, prior to the mechanization of biomass harvesting, were not economical or practical to treat. Larger trees are left to grow or be harvested for higher value products such as lumber or paper-pulp chips.

CALIFORNIA AGRICULTURE, MARCH-APRIL 1996 15 Based on table 1, biomass harvesting for electric power has made biomass of all trees less than 7 inches diameter power uncompetitive. To maintain re- would remove about 125 pounds ni- newable resource energy generators trogen per acre, 19% of the total in the like wind, geothermal, solar, hydro biomass component but only 2% of the and biomass, Assembly Bill 1202 pro- ecosystem total. poses that no less than 1.5%of the Table 2 shows the rates at which electricity supplied and consumed in nutrients are added to an ecosystem, the state come from biomass power replenishing nutrients that are re- plants. This represents the biomass moved during biomass and other har- power production in early 1994. Indi- vest activities. Concentration and rect benefits of biomass power pro- quantity of precipitation vary with air duction are improved air quality quality. Smoggy air produces precipi- from reduction of open air burning; tation higher in nitrogen and sulfur agricultural waste management; less oxides. The air over western U.S. for- clusion. Trees killed by drought be- severe forest fires; preservation of ur- ests is relatively clean, as reflected in come fuel for the next wildfire so we ban landfill space by redirecting wood the rates. Weathering is the dissolution must reduce these fuels in some man- waste to power plants. of primary rock minerals such as feld- ner, possibly using fire itself. We can- Mechanization of harvesting and spar and biotite into their elemental not, however, safely and controllably processing has made biomass harvest- components. Mineralization is the bio- reintroduce fire to forest ecosystems ing economically and operationally logical decomposition of soil organic given the current levels of fuels built feasible, hastening the day when tasks matter and forest litter by insects, up from decades of fire exclusion. For- like timber falling will be replaced by fungi and bacteria into compounds est biomass harvesting represents a high skill, higher paying jobs required and elements that can be absorbed by method for reducing fuel levels and for mechanized harvesting. plants. fire hazard for some sites and stand As increasing urbanization, auto As presently practiced, biomass conditions. emissions and industry in rural and harvests to reduce fuel and fire hazard The cost of removing biomass can forested areas degrade air quality, for- would occur infrequently, every 15 to be offset by selling it as fuel, paper est managers are becoming more re- 20 years. In this way, nutrients re- pulp chips or other products. The Pub- stricted in the use of prescribed bum- moved during a harvest can be replen- lic Utilities Regulatory Policy Act of ing to reduce fuel levels. Harvest and ished before the next harvest, except 1978 encouraged development of bio- removal of forest biomass would re- possibly at sites where soils are thin mass power plants by locking in for 10 duce the amount of material that and infertile and a higher proportion years the price paid for power at the would require prescribed burning. of the ecosystem’s nutrients are in the “avoided cost” of building new power As a renewable energy source, bio- standing vegetation. plants in 1978 when petroleum prices mass also may offer opportunities to The potential impacts on wildlife were high. Biomass power is inher- displace carbon emissions from fossil from biomass harvesting are no more ently more expensive than petroleum fuel combustion. Carbon dioxide re- or less severe than any other harvest or natural gas energy - especially for- leased by burning or decay of plant or vegetation management practice. est biomass, which entails a good deal material can be “recycled” through Whole-tree removal reduces fuel load- of processing and handling - so pric- photosynthesis as forests grow and ing, lessening the impact of fire, but ing biomass power at this peak price store cellulose and other organic mate- also presents a potential hazard for made it economically feasible. Natural rials. Biomass plantations of woody wildlife and site productivity. Like gas is currently very inexpensive, and herbaceous plants could be grown any powerful and effective tool, the making the “avoided cost” price for to sequester carbon, reducing carbon costs and benefits of this method must biomass energy too low to pay much, dioxide input to the atmosphere until be weighed. if anything, for biomass harvested it is burned as fuel for power genera- from the forest and hauled up to 50 tion. It is sobering, however, to con- Outlook and perspective miles. Thus, removing biomass from sider that it would require 70 million Fire is beginning to be appreciated the forest becomes a net cost to the acres (California is 100 million acres) as a necessary part of healthy forest landowner, reducing the number of of productive land to grow sufficient ecosystems in California. The occur- acres receiving this treatment. biomass to offset 20% of current (1992) rence of large, catastrophic wildfires in As of April 1996, about 30% of bio- U.S. fossil fuels emissions, about 4.4% recent years attests to our inability to mass power production capacity has of global fossil fuel emissions. control them. The tree mortality been lost through plant closures. In caused by the recent drought reflects a some cases, sawmills and their associ- conversion to less drought-hardy spe- ated cogeneration plants have closed; G. Nakamura is Area Forestry Specialist, cies in some forests due to total fire ex- in other cases, the low price being paid UC Cooperative Extension, Redding.

16 CALIFORNIA AGRICULTURE, VOLUME 50, NUMBER 2