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www.fs.fed.us/psw

Pacific Southwest Research StationFOREST SERVICE, U.S. DEPARTMENT OF AGRICULTURE

Restoring Forest Health

Even after 140 years without a fire, mixed-conifer forest such as Teakettle’s Experimental Forest has adistinct patch pattern and complex structure. Researcher Malcolm North and colleagues examined thestructure and function of these ecosystems and their response to widely used restoration treatments.Collectively the studies found fire was essential to restoring many ecosystem processes and that thinning could be best used as a tool to influence burn intensity and extent.

Walk into many forests in the Sierra Nevada andyou will have to thread your way through denseclusters of small trees, step over a clutter ofwoody debris, and navigate under a closed,darkened canopy. Yet in the late 19th century,John Muir wrote of “the inviting openness of theSierra woods. Trees of all the species standmore or less apart in groves, or in small,irregular groups, enabling one to find a waynearly everywhere.” Like many forests of theWestern United States, Sierra Nevada forestshave significantly changed following a century offire suppression, land use, and climate change.Historically, fire burned through mixed coniferabout every 15 years, but now fire burns solittle forest acreage that some scientists haveestimated the fire-return interval is more than600 years. When wildfire inevitably occurs,accumulated needle litter and ladder fuels(intermediate-sized trees that can carry aground fire into the tree tops) often combine toproduce catastrophic crown fires that kill alltrees and can threaten homes. State and federalagencies have developed plans for reducing firerisk and restoring forest health, but these plansare controversial.

What exactly constitutes a healthy forest? After acentury of fire suppression, how can land mana-gers use methods such as fire and thinning torestore forests? Can thinning help restore forestsadapted to frequent fire, and if so, how shouldforests be thinned?

A major experiment in mixed-conifer forest at theTeakettle Experimental Forest is designed to shedlight on these questions. As the most extensiveforest type in the Sierra Nevada and a key sourceof timber, mixed conifer is the focus of researchled by plant ecologist Malcolm North of theUSDA Forest Service, Pacific Southwest ResearchStation (PSW). Teakettle, managed by the PSWResearch Station, is a 3,000-acre (1300hectare) old-growth forest on the western slopesof the Sierra Nevada 45 miles (75 kilometers)east of Fresno, California. At this site, a team ofmore than two dozen researchers examinedecosystem processes over a 7-year period. Theteam coordinated their studies by using the samedesign, and by sampling at common locations.“It’s said that trying to coordinate scientists fromdifferent disciplines is like trying to herd cats,”comments North, “but at Teakettle, researchersworked well together because everyone was

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Fire and thinning effects on mixed-conifer forests

Summer 2006

FrFrom Sciencom Sciencee......Major ThemesWater stress: In fire-suppressed forestswith high tree densities, limited soilmoisture strongly affects many eco-system processes.

Large tree linkages: Overstory treessignificantly modify forest microclimateand sustain food webs through fungal andseed production.

Fire and forest function: Many ecosystemprocesses can be “jump started” with fire.

Research ResultsPatch legacy effect: Vegetation patchesinteract with fire and thinning treatments tostrongly influence local posttreatmentecological processes.

Litter and slash: Thinning treatments reduceladder fuels, but if forest is left unburned,the additional slash and litter reduceunderstory diversity and regeneration, andcreate additional fire risk.

Historical conditions: Under an active fireregime in the 1860s, historical mixed-coniferstands had few trees, an equal number of allsizes, and more large trees than are presentin modern old-growth forests.

Why the change in the last 100 years? Bystudying tree rings and fire scars, whichtogether reveal when and how often historicalburns occurred, scientists have decipheredmuch of the fire history of forests in theWest. Before 1865, fires occurred atTeakettle at approximately 12- to 17-yearintervals. But after 1865, not only atTeakettle but also throughout much of theWest, fire has generally been scarce. In theabsence of fire, bark beetles and diseasebecome the main causes of mortality.However, unlike fire, they do not periodicallyremove young trees and leave mature canopytrees intact. Instead, pests and pathogensattack dense clumps of trees that havebecome moisture stressed, killing both theyoung saplings and old trees that survivedmany past fires.

When fire does occur, usually at longintervals, destructive conflagrations oftendevelop. The prolific growth of understoryvegetation, which carries flame from groundlevel to the canopy, has played a large role increating hard-to-control, hot fires thatconsume, rather than restore, forests.

During the last 60+ years, fire suppressionhas played a major role in understorysurvival and fuel buildup. Prior to that time,scientists can only speculate why firebecame scarce. It may have been thechange to a warmer, wetter climate,extensive grazing, or possibly fewer fires setby Native Americans, whose populationswere decimated by newly arrived settlers.

Testing Restoration TreatmentsThe changed structure of fire-deprivedWestern forests has left them prone todestructive fires. But does simply restoringthe appearance of the forest by thinningladder fuels rejuvenate the forest andmaintain a healthy ecosystem? And if so,how large of a tree can be removed to

cover some of the costs of the restorationtreatment? Or must low-intensity fire beused as well to restore the forest’s functionand health?

The first step to answering these questionsfor Sierra mixed-conifer forest was to betterunderstand its current fire-deprivedcondition, initiated through studies thatbegan at Teakettle in 1998.

Scientists carefully selected 18 plots withincontiguous old-growth mixed conifer—each plot with an equal proportion of thethree patch types (closed canopy, shrub,and gap) that make up mixed conifer. Then,scientists compared five potential fire andthinning management scenarios withuntreated forest. The scientists used threeplots for each scenario.

There were five treatments: (a) fire, (b)understory thinning (trees <30 inches[76 centimeters] in diameter), (c) shelter-wood thinning (which left 8 regularlyspaced large-diameter trees per acre [20trees per hectare]) (d) understory thinningplus fire, and (e) shelterwood thinningplus fire. The experiment also included ano treatment control.

In addition to understanding the effects ofrestoration on forest structure, thescientists also studied how the treatmentsaffected soil conditions, ecosystem pro-cesses, and food chains, all of which givesome sense of the health of the forest.

Forest Structure—Trees and PlantsDeveloping a reconstruction of Teakettle’sstand conditions in 1865 immediately afterthe last large-scale fire, Malcolm North andJim Innes, PSW, and Harold Zald, PacificNorthwest Research Station (PNW), USDAForest Service, evaluated how effectivelythe five alternative restoration treatmentshad returned mixed-conifer forests toconditions produced by an active fire

Pacific Southwest Research Station

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Before

Fire-suppressed stand at Teakettle.

After

A 10-acre (4-hectare) plot before and after an understory thinning treatment.

regime (i.e., what the stands were like in1865). The scientists determined thatcombined understory thinning and firetreatments worked most effectively to reducethe density of trees, while simultaneouslyincreasing the percentage of pines, historicallya larger component of mixed-conifer forests.Restoration treatments should be based onmore than simply modifying current foreststructure; the development of the regeneratingforest after these disturbances should also beconsidered. Andy Gray of the PNW ForestInventory and Analysis Program and Zald foundthat light and soil moisture increased withtreatment intensity, particularly in the thin andburn plots, because higher fuel loads increasedfire-induced mortality of small trees. Theyfound that pine recruitment is limited by a lackof large pines supplying seed and an abundanceof large white fir and incense-cedar with prolificseed rain. If managers wish to increase pinecomposition, they may need to considerplanting only pine seedlings, reapplyingprescribed fire, or reducing overstory white firand incense-cedar seed sources.

Over 50 percent of California’s plant speciesoccurs in the Sierra Nevada—and most of thisdiversity occurs as the herbaceous understorybeneath conifer forests. Rebecca Wayman,University of California, Davis, and MalcolmNorth found that the understory responsedepended on the interaction of the patch typeand the fire and thinning treatment so thatpretreatment vegetation had a strong legacyeffect on restoration conditions. Burn/thincombinations led to significant increases in thenumber and cover of herbaceous species.Thinning increased understory light and soilmoisture, burning reduced leaf litter and slash,and both treatments reduced shrub cover.Because prescribed fires were lit in the fall,after first precipitation, fires did not burnsufficiently hot to rejuvenate the herbaceouslayer without year-old slash from understorythinning to help fuel the fire.

Soils-Nutrients, Litter, and WaterNitrogen: Soil scientist Heather E. Erickson ofPNW Research Station, USDA Forest Service,tracked the key nutrient nitrogen duringTeakettle experiments. Plants can only take upavailable nitrogen in one of two forms: nitrateion (NO3-) and ammonium ion (NH4+). So,the form of nitrogen in forest soils is just asimportant as the quantity. Whitethornceanothus contains symbiotic bacteria in rootnodules that fix N2 in the air, converting it toammonia. Soils beneath patches of ceanothuscontained the highest levels of availablenitrogen, both before and after restorationtreatments. Nitrogen that plants could useduring forest recovery was considerably higherafter the burn/thin treatments.

Water and litter: In the fire-suppressed forestat Teakettle, scientists found that soil moisturecontrolled many of the ecological processes—understory diversity, tree regeneration, soilrespiration, nutrient cycling and decom-position, and soil invertebrate abundance toname a few. Because Sierran summers are sodry, the amount of soil moisture depends onsnowpack depth and distribution. Annualclimate, particularly El Niño events, has astrong influence on plant diversity andestablishment, and many ecosystemprocesses. After treatments, soil moisture wasstill important to many processes, but anotherforest condition also became influential—theabsence of litter and slash. Many processespositively responded to the burn’s removal ofthe thick litter layer that had accumulated withfire suppression. Thinning alone, because it putmore litter and slash on the soil surface,tended to stall many ecosystem processes.

Carbon: With global climate change aworldwide concern, forest management mustemphasize carbon storage over its release asCO2, a greenhouse gas that contributes towarming trends. Amy Concilio and JiquanChen, University of Toledo, and Siyan Ma,University of California, Berkeley, and theircolleagues, followed soil carbon levels afterrestoration treatments by measuring levels ofrespiration, the biological process wherebyliving organisms release CO2. In plots wherethinning had taken place, no change wasfound at 1 year, but by 2 yearsposttreatment, CO2 release

significantly increased.Although fire initiallyreleases a pulse ofcarbon into theatmosphere, increasesin root and tree bolegrowth contribute tocarbon storage.These early findingssuggest that in thelong run, thinning maycontribute more toelevated CO2 andglobal climate changethan prescribed fire.

Food webs: Smallmammal response toburning and thinningtreatments differed bydietary habit.Researcher Marc D.Meyer, University ofCalifornia Davis, andcolleagues followedhow chipmunks,which have an adaptable generalist diet, andflying squirrels, which are truffle and lichenspecialists, responded to treatments.Chipmunks altered their diet depending on theabundance of different food sources, but theirnumbers did not change. At Teakettle, near thesouthern-most extent of their range, flyingsquirrels were concentrated in riparian areaswith almost 90 percent of their nest treeslocated within 500 feet (150 meters) of aperennial creek. Because of this concen-tration, their habitat was not extensivelyaffected by treatments; however, their mainfood source, truffles, decreased in abundanceand diversity as fireand thinningintensity increased.A greater abundanceand diversity oftruffles and lichensnear streams and agreater density of allsmall mammalssampled suggestthe importance ofriparian habitat inseasonally dryforests.

3Pacific Southwest Research Station

White fir regenerating in a gapsurrounded by large pine.

Climbing into the crown of a 200-feet (60-meter) tall red fir to sample lichens.

Northern flying squirreleating a truffle.

...T...To Manago Managementement

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A Conversation With Malcolm North

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Pacific Southwest Research Station

What do Teakettle findings tell us about various restoration treatments?Which treatments work most effectively to restore forest structure?It’s not realistic to try to move forests backward to a condition found prior toEuropean settlement because Native American ignitions, grazing intensity,and climate have all changed. However, we do know the ecologicalprocesses in these forests depend on frequent fire; it was their evolutionaryenvironment. In our study, the combination of understory thinning, followedby burning 1 year later, proved most effective in moving the forest toward acondition produced by a frequent fire regime. These thinning treatmentsfollowed specifications originally designed for spotted owl protection.However, the combined treatment needs to be further adjusted to optimizerestoration. Many more small-diameter trees that contribute to moisturestress should be removed, particularly those around large, old-growth trees.Some intermediate-size trees, trees with a diameter between 20 and 30inches (50 to 76 centimeters), need to be left to provide the nextgeneration of large, old trees. Thinning prescriptions should differ by speciesand rarely should pines or hardwoods be cut.

How about maximizing understory diversity?Again, the combination of understory thinning and burning proved mosteffective because it increased soil moisture, reduced litter and slash, anddecreased shrubs that are strong competitors for light and soil moisture.Neither burning nor thinning alone achieved these ends.

What do Teakettle studies tell us about managing mixed-conifer forests forthe northern flying squirrel and its foremost predator, the spotted owl?Postrestoration studies showed that all treatments reduced theflying squirrel's main food source, truffles, but treatmentintensity made a difference. Understory, as opposed toshelterwood cutting, and lower intensity fires best retain truffles.Protecting larger trees and decaying logs leaves fungal banksproviding a source of spores after restoration treatments.Additionally, the finding that 90 percent of squirrel nests arenear creeks has important implications. Retaining large trees,especially those close to streams may be essential for flyingsquirrels and the food on which they depend.

Overall, how should forest restoration be approached? Fire is the most important tool in forest restoration, but often itmay need to be supplemented by understory thinning to reduceladder fuels and to burn sufficiently in late fall when control burnsare typically carried out. The slash produced by thinning addsenough fuel for fire to burn hotenough to recycle nitrogen to thesoil, open up forest stands, andrestore many ecosystemprocesses. Thinning allowsheavier removal of white fir andincense-cedar, which need to besignificantly reduced. Restorationof forest health cannot be donethrough thinning alone; rather,any logging treatments should bedesigned to optimize restorationthrough fire.

Losing large trees: Insects and pathogens havereplaced fire as the main source of mortality,and during droughts they can kill many old-growth trees.

Restoring “health”: Fire is needed to restore mostecosystem functions. Thinning without burningmay reduce ladder fuels, but the additional slashand litter stall ecosystem recovery.

Fire-focused silviculture: Thinning may be a toolthat is best used to increase the ecological “work”done by fire, particularly when prescribed burnsare lit during cooler conditions in spring or fall.

Flexible thinning prescriptions: Thinning rules shouldbe flexible to leave most shade-intolerant pines andretain many intermediate-size (20- to 30-inch,[50- to 76-centimeter] diameter) trees.

Forest trajectory: To maintain an open stand andincrease pine, repeated prescribed burns andplanting pine seedlings may be needed in mixed-conifer stands.

Highly diverse understory of mixed-conifer forest.

A plot treated with understory thinning and prescribed fire.

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Malcolm P. North, Ph.D.Sierra Nevada Research CenterPacific Southwest Research StationUSDA Forest Service2121 2nd StreetSuite A-101Davis, CA 95616

Phone: 530-754-7398e-mail: mpnorth@ucdavis.eduhttp://www.psw.fs.fed.us/snrc

“I’ve always been interested in howdisturbances affect forests” says plantecologist Malcolm North. “In particular,how disturbance changes forest structure,species composition, and reverberatesthrough ecosystem functions.”

Studying how northern spotted owls usedwind- and fire-disturbed old growth in thePacific Northwest was the start of North’sresearch work. This also kindled aninterest in food webs and how owls, flyingsquirrels, and truffles are connected andinfluenced by forest structure. In 1995,North moved to California and startedworking with the USDA Forest ServiceForestry Sciences Lab in Fresno on nestconditions of the California spotted owl. Itwas shortly after this that the “SierraNevada Ecosystem Project Final Report”was published summarizing what wasknown about Sierra forests and whatareas particularly needed research. “It wasa wonderful synopsis of some excitingresearch ideas. One recurrent theme inthe report was the lack of informationabout how ecosystems respond to fire

and thinning treatmentscommonly used torestore fire-suppressedforests.”

In 1997 North beganthe Teakettle experi-ment to investigate thisquestion using areplicated fieldexperiment witheighteen 10-acre plots.More than two dozenscientists and graduatestudents joined theexperiment to measurea wide array of eco-

system processes for 3 years before andafter thinning and burning treatmentswere applied. The effort helped developa multidisciplinary understanding ofsome of the key functions in mixed-conifer forest, how they’ve been affectedby fire suppression, and how theyrespond to management practices.

North is especially interested indeveloping robust field experiments thataddress conservation and restorationissues of forest managers in the SierraNevada. He is now part of the SierraNevada Research Center in Davis and hasan affiliate appointment in the Depart-ment of Plant Sciences at University ofCalifornia Davis. With more than a dozenyears backpacking, climbing, andbackcountry skiing in the Sierra, North’slong-term goal is toprovide research thatcan improve stewardshipof the Sierra Nevada’secosystems.

North received his Ph.D.in forest ecology fromthe University ofWashington in Seattle.He also has an M.S. inforestry from YaleUniversity, and a B.A. inEnglish from VassarCollege.

Scientist Profile

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Pacific Southwest Research Station

excited by the possibility of discoveringlinkages between their different researchdisciplines.” This led to some interestingfindings and an awareness of commonthemes. “As scientists, we were looking forcomplex answers; suites of mechanismsthat would each influence different eco-logical processes. In the pretreatment forest,however, as complicated as we wanted tomake it, it usually came down to one limitingfactor, water.”

After the fire and thinning treatments wereapplied in 2000 and 2001, however, theecological responses were more com-plicated. “The treatments reduced thenumber of trees taking water up from thesoil, and this appears to have relaxed amoisture constraint,” says North. Wateravailability was still important, but otherfactors became influential such as theamount of litter and slash, and the kind ofvegetation patch (closed-canopy forest,shrub, or gap) that was on a site beforetreatment. “Although the focus and con-troversy have always been on whether theforest should be thinned and what size oftree can be removed, in the end, mostTeakettle scientists felt thinning was betterviewed as a tool that served the burn. It’sthe fire that does the real work of restoringthese forest’s health.”

Continued from front page

Teakettle Experimental Forest

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For Further Reading

The U.S. Department of Agriculture (USDA) prohibits discrimination in all its programs and activities onthe basis of race, color, national origin, age, disability, and where applicable, sex, marital status, familialstatus, parental status, religion, sexual orientation, genetic information, political beliefs, reprisal, orbecause all or part of an individual's income is derived from any public assistance program. (Not allprohibited bases apply to all programs.) Persons with disabilities who require alternative means forcommunication of program information (Braille, large print, audiotape, etc.) should contact USDA'sTARGET Center at (202) 720-2600 (voice and TDD). To file a complaint of discrimination, write toUSDA, Director, Office of Civil Rights, 1400 Independence Avenue, S.W., Washington, D.C. 20250-9410, or call (800) 795-3272 (voice) or (202) 720-6382 (TDD). USDA is an equal opportunityprovider and employer.

Pacific Southwest Research StationUSDA Forest ServiceP.O. Box 245Berkeley, CA 94701

What’s NextTo make this information readily accessible to managers, students, and anyinterested members of the public, an interactive DVD with a film about theexperiment is in postproduction. The film will summarize the experiment, fiveshort special feature films will provide greater detail about specific components ofthe research, a library of publications provide research results, and a Web site andquery function will provide an active question-and-answer resource.

A second phase of the Teakettle experiment is in planning, in which theexperiments’ findings will be used to test new thinning and burning prescriptionsat the watershed scale. Different prescribed burn applications (varying seasonand burn intensity) will be compared, and thinning prescriptions will be modifiedso that they differ by tree speciesand retain a target level of trees indifferent diameter sizes.

Work is also underway to expand thescope of the research by startingprojects in the central, northern, andeastern Sierra to examine howecosystem processes and response tothese treatments differ betweengeographic locations and forest types.

Published byPacific Southwest Research Station, USDA Forest Service

James R. Sedell, Station Director800 Buchanan Street, Albany, CA 94710 • 510-559-6300 • http://www.fs.fed.us/psw

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Erickson, H.; Soto, P.; Johnson, D.; Roath, B.; Hunsaker, C. 2005.Effects of vegetation patches on soil nutrient pools and fluxes within amixed-conifer forest. Forest Science. 51(3): 211-220.

Gray, A.; Zald, H.; Kern, R.; North, M. 2005. Stand conditionsassociated with tree regeneration in Sierran mixed-conifer forests. ForestScience. 51(3): 198-210.

Ma, S.; Chen, J.; North, M.; Erickson, H.; Bresee, M.; Le Moine, J.2004. Short-term effects of experimental treatments on soil respiration inan old-growth, mixed-conifer forest. Environmental Management. 33(1):148-159.

Meyer, M.D.; North, M.P.; Kelt, D.A. 2005. Short-term effects of fire andforest thinning on truffle abundance and consumption in the southern SierraNevada of California. Canadian Journal of Forest Research. 35: 1061-1070.

North, M.; Oakley, B.; Chen, J.; Erickson, H.; Gray, A.; Izzo, A.;Johnson, D.; Ma, S.; Marra, J.; Meyer, M.; Purcell, K.; Rambo, T.; Roath,B.; Rizzo, D.; Schowalter, T. 2002. Vegetation and ecologicalcharacteristics of mixed-conifer and red-fir forests at the TeakettleExperimental Forest. Gen. Tech. Rep. PSW-GTR-186. Albany, CA: U.S.Department of Agriculture, Forest Service, Pacific Southwest ResearchStation. 52 p.

North, M.; Chen, J.; Oakley, B.; Song, B.; Rudnicki, M.; Gray, A.; Innes,J. 2004. Forest stand structure and pattern of old-growth westernhemlock/Douglas-fir and mixed-conifer forest. Forest Science. 50(3):299-311.

North, M.; Oakley, B.; Fiegener, R.; Gray, A.; Barbour, M. 2005.Influence of light and soil moisture on Sierran mixed-conifer understorycommunities. Plant Ecology. 177(1): 13-24.

North, M.; Hurteau, M.; Fiegener, R.; Barbour, M. 2005. Influence of fireand El Niño on tree recruitment varies by species in Sierran mixed conifer.Forest Science. 51(3): 187-197.

Smith, T.; Rizzo, D.; North, M. 2005. Patterns of mortality in an old-growth mixed-conifer forest of the southern Sierra Nevada, California.Forest Science. 51(3): 266-275.

Web Resourcehttp://teakettle.ucdavis.edu

Film shoot at Teakettle.

Early draft of this issue of Science Perspectives by Anne M. Rosenthal,is a science writer based in the San Francisco Bay Area.