Western North American Naturalist Western North American Naturalist

Volume 71 Number 2 Article 13

8-12-2011

Effects of wildfire and postfire floods on stonefly detritivores of Effects of wildfire and postfire floods on stonefly detritivores of

the Pajarito Plateau, New Mexico the Pajarito Plateau, New Mexico

Nicole K. M. Vieira Colorado State University, Fort Collins and Colorado Parks and Wildlife, Wildlife Research Center, Fort Collins, Colorado, nicole.vieira@state.co.us

Tiffany R. Barnes Colorado State University, Fort Collins, tiffanyraebarnes@hotmail.com

Katharine A. Mitchell Colorado State University, Fort Collins, kam_9654@hotmail.com

Follow this and additional works at: https://scholarsarchive.byu.edu/wnan

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Recommended Citation Recommended Citation Vieira, Nicole K. M.; Barnes, Tiffany R.; and Mitchell, Katharine A. (2011) "Effects of wildfire and postfire floods on stonefly detritivores of the Pajarito Plateau, New Mexico," Western North American Naturalist: Vol. 71 : No. 2 , Article 13. Available at: https://scholarsarchive.byu.edu/wnan/vol71/iss2/13

This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Western North American Naturalist by an authorized editor of BYU ScholarsArchive. For more information, please contact scholarsarchive@byu.edu, ellen_amatangelo@byu.edu.

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1Department of Fish, Wildlife and Conservation Biology, Colorado State University, Fort Collins, CO 80523.2Colorado Parks and Wildlife, Wildlife Research Center, 317 West Prospect Rd., Fort Collins, CO 80526.3E-mail: nicole.vieira@colostate.edu

Allochthonous inputs of organic materialare a primary source of energy in low-orderwoodland streams (Allan 1995). Riparian leaflitter and woody debris represent critical re -sources for invertebrate detritivores, who playan important role in both function and structureof lotic food webs. Functionally, detritivoresare responsible for up to 25% of leaf litterdegradation in streams (Petersen and Cummins1974; see review in Al lan 1995). Not surpris-ingly, experimental removal of detritivorous

invertebrates can result in lower decomposi-tion rates of terrestrial organic matter (Wallaceet al. 1982). Furthermore, shred der detritivoresbreak down coarse organic material (CPOM)into finer materials, thereby increasing foodavailability to invertebrates who are collectorfilterers and gatherers (Cummins et al. 1973,Short and Maslin 1977, Wallace and Webster1996). Thus, changes in quantity, quality, ortiming of terrestrial litter inputs as a result ofdisturbance may negatively impact detritivore

Western North American Naturalist 71(2), © 2011, pp. 257–270

EFFECTS OF WILDFIRE AND POSTFIRE FLOODS ON STONEFLYDETRITIVORES OF THE PAJARITO PLATEAU, NEW MEXICO

Nicole K. M. Vieira1,2,3, Tiffany R. Barnes1, and Katharine A. Mitchell1

ABSTRACT.—Wildfires alter the quantity and quality of allochthonous detritus in streams by burning riparian vegetationand through flushing during postfire floods. As such, fire disturbance may negatively affect detritivorous insects thatconsume coarse organic matter. We assessed how 2 crown fires impacted stonefly detritivores in streams of the PajaritoPlateau, New Mexico. We documented stonefly populations before and after the fires and postfire floods, and comparedrecovery trajectories among unburned, lightly burned, and severely burned reaches. We also conducted experiments to assessburned detritus as a food resource for Pteronarcella badia Hagen. Specifically, we characterized microbial conditioning, nutri-ent content, and breakdown rates of burned and unburned deciduous leaves and pine needles. We compared colonizationof P. badia in field-placed leaf packs and growth of P. badia in a microcosm experiment on burned and unburned treatments.Detritivorous stoneflies in Plateau streams survived wildfire, but were extirpated from burned reaches after severe postfirefloods in both Capulin and Guaje canyon. In Guaje Canyon, Amphinemura banksi Baumann and Gaufin was more resilientto flood disturbance than P. badia and recolonized soon after floods abated, whereas recolonization of A. banksi was delayedin Capulin Canyon. Experiments revealed that detritus quality did not explain slow recovery; despite reduced microbial con-ditioning and decomposition rates, P. badia colonized and grew well on burned detritus. Instead, postfire floods removedshredder stoneflies and their detrital resources; and traits such as body size, voltinism, and dispersal likely interacted with thepostfire landscape to shape recovery trajectories in burned streams.

RESUMEN.—Los incendios cambian la cantidad y calidad de la materia orgánica alóctona en los ríos mediante la quemade la vegetación ribereña; además, las inundaciones después de los incendios se llevan materia orgánica río abajo. Como tal,la perturbación por incendio puede afectar negativamente a los insectos detritívoros que consumen la materia orgánica gruesa.Evaluamos cómo 2 fuegos de copas afectaron a los plecópteros detritívoros en arroyos del Altiplano Pajarito, NM. Documenta -mos las poblaciones de plecópteros antes y después de los incendios e inundaciones y comparamos las trayectorias derecuperación entre áreas no quemadas, levemente quemadas y gravemente quemadas. También llevamos a cabo experimentospara evaluar los detritos quemado como fuente alimenticia para Pteronarcella badia Hagen. Específicamente, caracterizamosel acondicionamiento microbiano, el contenido de nutrientes y las tasas de descomposición de hojas caducifolias y hojas depino quemadas y no quemadas. Comparamos la colonización de P. badia en paquetes de hojas colocadas en el campo, y sucrecimiento en un experimento de microcosmos, en detrito quemado y no quemado. Los plecópteros detritívoros en los arroyosdel Altiplano sobrevivieron a los incendios pero fueron extirpados de las áreas quemadas después de inundaciones severasque siguieron a incendios en los cañones Capulin y Guaje. En Guaje, Amphinemura banksi Baumann y Gaufin fue másresistente a la perturbación por inundación que P. badia y recolonizó poco después de que la inundación cedió, mientrasque la recolonización de A. banksi tardó más en Capulin. Los experimentos revelaron que la calidad de los detritos noexplican esta lenta recuperación; a pesar del menor acondicionamiento microbiano y las tasas de descomposición más bajas,P. badia colonizó y creció bien en detritos quemados. Es probable que las inundaciones después de los incendios sehayan llevado a los plecópteros y a los detritos que consumen, y que los rasgos tales como el tamaño del cuerpo, el voltinismo,y la dispersión hayan interactuado con el paisaje después del incendio para determinar las trayectorias de recuperación enarroyos quemados.

populations (Rounick and Winterbourn 1983)and result in bottom-up effects on aquatic foodwebs (Wallace et al. 1997).

Disturbances that alter terrestrial-aquaticecotones, such as agricultural practices (Delongand Brusven 1993, Townsend et al. 1997a),logging (Webster et al. 1990, Bilby and Bis-son 1992), and fire (Britton 1990, McIntyreand Minshall 1996, Minshall et al. 1997),often dramatically alter terrestrial subsidies ofleaf litter into streams. Wildfires also reducein-stream retention of detritus through post-fire flooding, removal of debris dams, andchanges in woody debris inputs (Bilby 1981,Minshall et al. 1997, Robinson et al. 2005). Lossof riparian subsidies after wildfire and poorretention of burned leaf litter have been corre-lated with functional shifts in benthic inverte-brate communities from spe cialist detritivores,such as shredders, to communities dominatedby generalist herbivore-detritivores (Mihuc etal. 1996, Minshall et al. 2001, Vieira et al.2004). For example, Vieira et al. (2004) foundthat the shredder stonefly Am phinemurabanksi Baumann and Gaufin (Ne mouridae)was strongly affected by wildfire and associ-ated postfire flooding and, despite being com-mon in prefire communities, did not recovereven after 6 years. These studies suggest thatwildfire may have long-term impacts on shred -der stonefly species in burned watersheds.

Along with reducing detritus quantity, firecan also reduce the quality of detritus by in -creasing charcoal content (Minshall et al.1997) and initiating moderate- to long-termchanges in riparian vegetation species (Brit-ton 1990). For example, Mihuc and Minshall(1995) assessed the nutritional value of ex -perimentally burned riparian vegetation as afood source for benthic macroinvertebratesand found that most taxa, including the gen-eralist detritivore stonefly Zapada columbianaClaassen (Nemouridae) could not grow onseverely (100%) burned detritus. However,Z. columbiana grew on coarse detritus thathad been “naturally” (35%) burned in a fire.It also grew on periphyton. This dietaryflexibility likely facilitated rapid recovery ofZ. columbiana in streams burned in the Yel-lowstone wildfires (Mihuc et al. 1996, Min-shall et al. 1997, Mihuc and Minshall 2005).Detritivores that specialize their feeding be -haviors, such as shredders that require or pre-fer coarse detritus, may be more adversely

affected by changes in detritus quality andquantity after a wildfire.

We had the unique opportunity to employa before-after approach to assess how detritivo -rous stoneflies (Pteronarcyidae, Capniidae,and Nemouridae) of the Pajarito Plateau (here-after “plateau”) near Los Alamos, New Mexico,re sponded to wildfire disturbance. Over thelast 3 decades, large-scale (>50 km2) crownfires have burned several canyon-bound drain -ages of the plateau, while other drainagesremained relatively unaffected. Burned canyonsexperienced extensive removal of riparianvegetation and multiple 100-year floods inthe year of the fires, followed by a progres-sive dampening of flood magnitudes as hillslopeand riparian vegetation recovered (Veenhuis2002, Gallaher and Koch 2004). Aquatic insectcommunities were dramatically altered bypostfire flooding, with notable declines ofspecialist feeding groups such as grazers andshredders (Vieira et al. 2004). Our researchinvestigated effects of the 1996 Dome Fireand the 2000 Cerro Grande Fire on detritivo-rous stoneflies in plateau canyons. To gaugerecovery, we compared stonefly responses afterthe fires and flooding to prefire populationsin lightly burned and unburned stream reaches.

In addition to our field study, we experi-mentally investigated how changes in detri-tus quality after the fire may have influencedpostfire re sponses of the shredder stoneflyPteronarcella badia Hagen (Pteronarcyidae).This species is common in most plateaustreams and is amena ble to handling in thelaboratory because of its large body size.First, we conducted an in situ field experi-ment to compare microbial activity and nutri-tional content of burned and unburned detritusrepresenting 2 common riparian re sourcesin plateau streams: narrowleaf cotton woodleaves (Populus angustifolia James; hereafterreferred to as “leaves”) and ponderosa pineneedles (Pinus ponderosa Douglas ex Law-son and C. Lawson; hereafter referred to as“needles”). We also compared colonizationof P. badia on these 4 leaf litter treatments.Secondly, we conducted microcosm experi-ments to compare growth of P. badia on burnedand unburned leaves and needles. We hypoth -esized that micro bial activity and nutrientcontent, and thus growth and colonization ofP. badia, would be highest on unburned leavesand lower on burned detritus treatments.

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METHODS

Survey of Detritivore Stonefly Populations

We surveyed the stonefly populations inPajarito Plateau streams of Bandelier NationalMonument and Santa Fe National Forest, nearLos Alamos, New Mexico (Fig. 1). The plateauslopes toward the Rio Grande in a series ofcanyons and mesas. First-order streams drainthese canyons and are similar in watershedarea (approximately 50 km2), flow permanence(e.g., spring-fed upper reaches, perennial mid-canyon segments, and ephemeral tributaries),gradient (changing from 3% to 7% with alti-tude), and baseflow (0.05 m3 ⋅ s–1). Riparianvegetation includes narrowleaf cottonwood and

ponderosa pine, as well as western boxelder(Acer negundo Linnaeus) and alder (Alnusoblongifolia Torrey). Plateau streams differ inwildfire history, where they were burned priorto 1980 (e.g., Frijoles Canyon), in the 1996Dome Fire (e.g., Capulin Canyon), or in the2000 Cerro Grande Fire (e.g., Guaje Canyon).

Plateau streams are snowmelt-driven inmid-spring and typically show flashy responsesto monsoon precipitation in summer (Beesonet al. 2001). Monsoonal flood magnitudes in -crease significantly following wildfires in burneddrainages. After the Dome and Cerro Grandefires, annual peak flows increased from a pre-fire average of less than 1 m3 ⋅ s–1 to 100-yearfloods with magnitude greater than 80 m3 ⋅ s–1

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Fig. 1. Map showing recent crown fires and burn severity in canyons of the Pajarito Plateau, near Los Alamos, New Mexico,including the study streams Capulin Creek, Guaje Creek, and Frijoles Creek (Rito de los Frijoles). Black circles indicatethe study reaches.

in the first postfire year (Veenhuis 2002, Galla-her and Koch 2004). Severe flooding and de -bris flows in burned canyons dramaticallyaltered stream geomorphology (see Fig. 2),sedi ment transport, and bed substrate and sta-bility (Cannon and Reneau 2000, Cannon etal. 2001, Vieira et al. 2004). Postfire flood mag-nitudes progressively dampen each year from100-year flooding to 10–20 m3 ⋅ s–1 in the nextyear after a fire, to 2–5 m3 ⋅ s–1 in the secondand third postfire years (e.g., Fig. 2h), and ulti-mately back to prefire levels by 5 years after afire. Hydrological recovery depends on the loca-tion and intensity of monsoonal storms andalso on the reestablishment of riparian andhillslope vegetation, especially at severelyburned sites (see Fig. 2c).

Previous taxonomic surveys in streams of thePlateau and adjacent Jemez Mountains showthat resident stonefly species are typical of theRocky Mountains and the Southwest (Jacobiand Baumann 1983, Jacobi et al. 2005, Vieira etal. 2009). Plateau stoneflies considered to bedetritivores, as determined from the NorthAmerican Aquatic Invertebrate Database (Vieiraet al. 2006), include A. banksi, P. badia, severalspecies of Capnia (Capniidae), Paraleuctra occi-dentalis (Banks) (Leuctridae), and the nemouridsMalenka coloradensis (Banks), Zapada cinctipes(Banks), and Zapada haysi Ricker. The 2 com-mon species of the region, A. banksi and P.badia, differ significantly in species traits knownto influence resilience to hydrologic distur-bances, including body size, voltinism, larvaldrift, and adult dispersal (Townsend et al. 1997b,Vieira et al. 2004). Amphinemura banksi is asmall, univoltine species with moderate driftpropensity of nymphs and weak adult disper-sal abilities. By contrast, P. badia is a large-bodied stonefly with a semivoltine life cycle,minimal nymph drift, and moderate (but stillrelatively local) aerial dispersal of adults (Vieiraet al. 2006).

We collected benthic invertebrates in Capu -lin, Guaje, and Frijoles canyons as described inVieira et al. (2004). Simply stated, we collectedinvertebrates with a 750-μm mesh Surber sam-pler (0.093 m2 area) from randomly selectedriffles (n = 6–9) in each study reach and identi-fied organisms to the genus or species level.Individuals were enumerated and densities (in -dividuals per 0.093 m2) of A. banksi, P. badia,and total stonefly detritivores were calculatedfor each sample event. Sample sites chosen to

document the impacts of the Dome Fire in -cluded severely burned reaches in Capulin(no riparian vegetation) and unburned reachesof Frijoles. Sample events included the spring(April–May), summer (July–August), and fall(September–October) before the Dome Fire(1994–1995); 2 weeks after the first postfire100-year flood (July 1996); and in spring, sum-mer, and fall of postfire recovery years (1997–2002). To document im pacts of the CerroGrande Fire, 2 elevations were sampled inGuaje Canyon. Lower reaches were severelyburned (Fig. 2c), and upper reaches were onlylightly burned (Fig. 2d). Samples were col-lected before the Cerro Grande Fire in spring1998 and in spring, summer, and fall 1999;immediately after the fire but before flooding(June 2000); 3 weeks after the first postfire100-year flood (September 2000); and inspring and fall of postfire recovery years(2001–2002).

Burned Detritus Experiments

Narrowleaf cottonwood leaves and ponderosapine needles were gathered prior to abscissionand dried at 56 °C for 24 hours. These needlesand leaves were then burned in a muffle fur-nace at 450 °C to simulate a fire. We modifiedthe 100% burn treatment as described by Mihucand Minshall (1995) such that burned materi-als were brittle with a charcoal residue butwere still available as CPOM to shredders.More severe burn methods were undesirablebecause leaves and needles disintegrated intoash. Each of 4 detritus treatments, burned andunburned leaves and needles, were weighedinto 8-g dry-weight samples. Artificial leaf packswere constructed by placing leaves or needlesin 2.27-kg citrus bags (Cady Bag County, Pear-son, GA) with 5 × 5-mm mesh openings. Leafpacks were transported in plastic bags to pre-vent leaf or needle loss prior to the study.

Ninety-six leaf packs (24 per treatment) wereplaced at 2320 m elevation in the Cache LaPoudre River, Fort Collins, Colorado, on 22October 1999 (4.6 °C, 10.6 mg DO ⋅ L–1, 7.3pH, and 50 mS conductivity; DO = dissolvedoxygen). A maximum of 10 leaf packs, includ-ing at least 2 from each detritus treatment, wererandomly allocated to each of 10 wooden racks(1.5 m long) and tethered with 36-cm cableties. Racks were placed in 2 rows (n = 5 each)in the center of a wide riffle (15 m), perpen-dicular to the current and at a depth of 0.5 m.

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Fig. 2. Photos depicting the impacts of intense crown fires and postfire floods on streams of the Pajarito Plateau, near LosAlamos, New Mexico. Photos show (A) lower Guaje Canyon before the 2000 Cerro Grande Fire; (B) Frijoles Canyon (burnedin 1977); (C) lower Guaje after the fire; (D) upper Guaje postfire, with a spot fire on the right bank; (E) Capulin Canyon fol-lowing the first 100-year flood after the 1996 Dome Fire (flood scars evident); (F) lower Guaje after the first 100-year flood;and (G) upper Guaje after moderate postfire flooding (widened, shifting gravel); and (H) a 3 m3 ⋅ s–1 flood in Capulin (1998).

Discharge was stable and at low-flow condi-tions (approximately 0.7 m3 ⋅ s–1) throughoutthe study period. Leaf packs were collectedafter 27 days (n = 12) and 50 days (n = 12).On each date, leaf packs were sampled in astratified random fashion, where at least oneof each treatment was sampled from each rack.Leaf packs (including invertebrates) were col-lected with a 250-μm mesh dipnet, placed inlarge plastic bags filled with stream water, andtransported in coolers to the laboratory.

All invertebrates from each leaf pack werepreserved in 80% ethanol and P. badia individu -als were enumerated. After all invertebrates wereremoved, leaf packs collected after 27 and 50days were rinsed with water, dried at 56 °C for24 hours, and weighed to the nearest 0.1 mg toobtain final dry mass. For 50-day samples, 1 g ofdried leaves or needles was randomly selectedfrom 5 leaf packs from each treatment andpooled for nutritional analysis. A CHN-1000carbon hydrogen nitrogen analyzer (LECO,St. Joseph, MI) was used to measure percentcarbon and percent nitrogen (an indication ofprotein content). Total lipid (% solvent extract)was determined from the sample with chloro-form-methanol extraction.

We measured changes in dissolved oxygenacross detritus treatments, in which microbialrespiration was used as a surrogate for micro-bial colonization and conditioning. Water forthe experiment was obtained from HorsetoothReservoir, Fort Collins, Colorado, and was fil-tered (0.45 mm) to remove organics and othermicrobes. Three leaves or needles from eachof the 4 treatments (unburned leaves, burnedleaves, unburned needles, and burned needles)were randomly selected from 50-day leaf packsand were randomly allocated to each of twelve50-mL beakers (n = 3 per treatment). Fouradditional beakers, 2 with unburned leaves and2 with burned needles, were used as rangefinders to terminate the study before 100%oxygen loss occurred. Two beakers withoutdetritus were included as a positive control toaccount for residual microbial activity in thereservoir water. At time zero, dissolved oxygen(mg ⋅ L–1) was measured in each beaker.Beakers were then sealed with Parafilm® toprevent exchange with atmospheric oxygen,placed on magnetic stir plates to maintain watercirculation, and incubated at 6 °C. After 18hours, DO was measured again and wet weights(g) of leaves or needles in each beaker were

recorded. This experiment was immediatelyrepeated in a second trial.

Pteronarcella badia Growth Experiment

Live P. badia (length: x– = 7.46 mm, SD =0.89, mid-instar stage) were collected in October1999 from the Cache La Poudre River, belowwhere the leaf packs were deployed. Organismswere transferred to stream microcosms at Colo-rado State University, Fort Collins, Colorado,and allowed to acclimate for 24 hours withoutfood. Microcosms were oval (76 × 46 × 14 cm)flow-through fiberglass tanks with a water vol -ume of 13 L and a current of 30 cm ⋅ s–1 gen-erated by a paddle wheel. Unfiltered water wassupplied from Horsetooth Reservoir (11.7 °C,7.7 mg DO ⋅ L–1, 7.7 pH and 60 μS conductiv-ity), and microcosms were housed in a green -house to provide a natural light regime. Treat -ment cages were constructed with Tupperware®

containers (15.5 × 11 × 11 cm). We drilled 6-cm-diameter holes into each side of the cagesand covered the holes with 250-μm mesh toallow water flow without losing detritus. Fourcages were placed in each of the 10 microcosmchambers.

Detritus treatments were randomly allocatedamong the 4 cages in each of 10 stream micro-cosms (n = 10 cages per detritus treatment).Leaves and needles in the cages were overlainwith a piece of plastic mesh (6 × 6 cm, 1-mm2

opening) and a small cobble was placed on topto anchor the mesh and to provide cover forP. badia. Flow-through holes in the cages werekept clean of debris and algae. Individual P. ba -dia were immobilized with carbonated water,placed on a wet sponge, and measured forlength with digital calipers. Then 3 individu alswere randomly assigned to each of the 40cages. Test organisms fed ad libitum on thetreatments for 12 days, after which they weremeasured again using the same methodologyfor live insects. Individuals were then sacri-ficed, and both body length and head capsulewidths were measured to validate live mea -surement techniques. Mean length of live indi-viduals per cage was used to compare growthbefore and after the feeding experiment.

Statistical Analysis

Recovery of total density of stonefly detri-tivores in plateau streams and a comparisonof how A. banksi and P. badia responded tofire and postfire floods were investigated

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graphically and with descriptive statistics. Toassess colonization across leaf pack treatments,we compared the number of P. badia per gramof detritus in each leaf pack at the time of col-lection (27 days and 50 days). Leaf pack break-down rates (k) after 27 days and 50 days werecalculated as the percent loss of dry mass perday relative to the initial dry mass at time zero.Microbial respiration was calculated as thepercent loss in DO (mg ⋅ L–1 per g detritus)after 18 hours, after being adjusted for DOchanges in water-only beakers, compared toDO at time zero. Pteronarcella badia growthwas calculated as the percent change in meanlength per treatment cage relative to the meaninitial length of the same cage.

We employed ANOVA and Tukey’s HSDtest to assess differences among the 4 treat-ments in leaf pack breakdown rate, P. badiacolonization, microbial respiration, and P. badiagrowth. In initial ANOVA models, we includedblock effects due to microcosm, sample time(27 days vs. 50 days), trial (DO), position ofwooden racks in the stream, and all 2-wayinteractions. However, if blocks and covariateswere highly insignificant (P > 0.5), they wereremoved for a simplified collapsed model ofburn treatment, detritus type, and burn × detri-tus type interactions. Tukey’s tests correspondedto the collapsed models. Assumptions of nor-mality, heterogeneity, and independence oferrors were checked with diagnostic tests avail-able in SAS statistical software (SAS Institute,Inc. 1996), and transformations were performedwhen indicated.

RESULTS

Stonefly Population Surveys

The dominant stonefly detritivores in CapulinCanyon prior to the Dome Fire included Ne -mouridae (primarily A. banksi and Z. cinctipes)and species of the winter stonefly genus Capnia.Nemourids were common across all 3 canyonsprior to fire and flood disturbance. Pteronarcellabadia was common in Frijoles Canyon and inGuaje Canyon prior to the Cerro Grande Fire.Interestingly, P. badia was absent from Capulin,even prior to the fire, despite the fact that thisstonefly is present in most canyons of the pla -teau. This absence is difficult to explain, becausenearby canyons with P. badia populations aresimilar to Capulin in both aquatic and riparianhabitat features. Capniidae nymphs were col-

lected throughout the study in low numbers inall 3 canyons, probably due to the coarse meshsize used for sampling. Therefore, temporaltrends for this family were included in the analy-sis for total stonefly detritivore densities butwere not specifically analyzed.

The total number of stonefly detritivores inthe 3 canyons varied considerably across bothyears and seasons where higher densities wereobserved in summer and fall sampling events(Fig. 3). Mean densities (standard deviation)were similar in the 2 prefire years for most ofthe study reaches (Frijoles, 20.3 [34.5]; Capulin,21.1 [17.9]; lower Guaje, 21.7 [17.9]; and upperGuaje, 11.9 [6.5]). No shredder stoneflies werecollected in Capulin canyon 2 weeks after thefirst postfire (100-year) flood in July 1996, andthey remained absent at study sites until fall1997. Thereafter and throughout the studyperiod, densities remained lower than meanprefire levels, and also lower than densities inunburned Frijoles Canyon. In Guaje Canyon,we were able to compare direct effects ofwildfire on detritivorous stonefly densities tothe impacts of the first 100-year flood eventafter the fire. Densities increased almost 4-fold immediately following the fire in June2000 (Fig. 3). We observed that all inverte-brates, including shredder stoneflies, were con-gregated in the leaf litter. Detritus may haveserved as a refuge from the ash covering thestream bottom.

While the fire itself did not significantlyimpact detritivorous stoneflies in Guaje Canyon,densities were reduced to zero at both lower andupper reaches 3 weeks after the first postfireflood in September 2000 (Fig. 3). Stream reachesat both elevations were flooded, despite differ -ences in fire effects on riparian vegetation,because severely burned upper hillslopes of thewatershed created intense runoff throughephem eral tributaries and overland flow (Beesonet al. 2001). However, lower-elevation sitesexperienced cumulative hydrologic effects fromupstream, which resulted in more-severe floodmagnitudes. Organisms were likely flusheddownstream during this 100-year flood event.Lower Guaje reaches, where stream morphol-ogy and bed substrate were more significantlyaltered by flooding (e.g., deep incision, shiftinggravel bed; Fig. 2), showed minimal recolo -nization of shredder stoneflies until 2002. Bycontrast, densities recovered a year earlier inthe upper Guaje reaches (Fig. 3), where the

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bed sub strate stabilized more rapidly after thefirst postfire flood season (N. Vieira, personalobservation).

Both A. banksi and P. badia were commonin Guaje Canyon, a situation that allowed us tocompare recovery trajectories of these 2 spe -cies. Recovery of shredder stoneflies in lowerGuaje Can yon was largely driven by recolo-nization success of A. banksi (Fig. 4). Popula-tions of P. badia in lower Guaje did not returnto prefire densities during the study. By con-trast, A. banksi recovered to prefire densities byMay 2002, only 2 years postfire. Similar inter-species differences were observed in upperGuaje, where A. banksi recolonized to prefiredensities earlier in spring 2001 (Fig. 4). Ptero -narcella badia recolonized upper sites by summer2001 but at lower densities than prefire densi-

ties. By 2002, P. badia densities were similarto prefire levels in the upper Guaje reach.

Burned Detritus Experiments

Cottonwood leaf packs lost a higher percent-age of their original mass compared to pineneedle leaf packs after both 27 days and 50 daysof conditioning (Table 1). Unburned cottonwoodleaves showed the highest loss of 48.8% (k = 0.01per day), while burned pine showed the lowestloss of 24.4% (k = 0.005 per day). For all treat-ments, the majority of detrital loss occurred inthe first few weeks and was likely due to leach-ing and mechanical breakdown. Total percentloss was similar between 27-day and 50-daysamples within treatment types, and there wereno significant block effects due to location inthe stream (dfmodel = 95, Pmodel = 0.7988). A

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Fig. 3. Natural log of mean density (individuals per 0.093 m2; error bars represent standard error; n = 6–9) for total stone-fly detritivores in Pajarito Plateau streams (near Los Alamos, NM) before and after (A) the 1996 Dome Fire and (B) the 2000Cerro Grande Fire. Marks on the x-axis correspond to winter, spring, summer, and fall of each year. Arrows indicate the dateof fires in Capulin and Guaje canyons, while blocks indicate summers with severe 100-year floods (thick block), moderate10-year floods, and minor 2–5-year floods (thin block). Frijoles Canyon did not experience unusual flooding during the studyperiod from 1994 to 2002.

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collapsed 2-way ANOVA model, with 27-dayand 50-day samples combined, revealed thatburn treatment, detritus type, and the burn ×detritus type interaction contributed to dif-ferences in decomposition (dfmodel = 95; allP < 0.0001). Total percent loss was highest forunburned leaves, followed by burned leaves,and then burned and unburned needles (Fig.5). In contrast to differences in leaf pack break-down rates, P. badia colonization was similar

across the 4 treatments (Fig. 5). There was nosignificant difference in colonization between27-day and 50-day treatments, or betweendetritus types or burn treatments (ANOVA:log-transformed data, dfmodel = 95, Pmodel =0.6911).

Although burned leaves and needles werenotably different in texture and color than un -burned organic material, nutritional analysisindicated a similar chemical makeup between

2011] FIRE EFFECTS ON STONEFLY DETRITIVORES 265

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Fig. 4. Natural log of mean density (individuals per 0.093 m2; error bars represent standard error; n = 6–9) for 2 shredderstonefly species, Amphinemura banksi and Pteronarcella badia, in Pajarito Plateau streams (near Los Alamos, NM) beforeand after the 2000 Cerro Grande Fire. Graphs show the impacts in (A) lower Guaje Canyon and (B) upper Guaje Canyon.Arrows indicate the fire. See Fig. 3 for postfire flooding events.

TABLE 1. Mean percent loss of dry leaf pack biomass consisting of burned and unburned ponderosa pine needles anddeciduous cottonwood leaves. Leafpacks were conditioned for 27 days or 50 days in the Cache la Poudre River, Colorado.Initial dry mass of each leaf pack was 8 g (n = 12).

Burned Unburned_________________________ ________________________Detritus type Days Mean % loss SD Mean % loss SD

Cottonwood leaves 27 38.57 5.62 47.60 1.8950 34.37 2.73 48.83 2.91

Ponderosa pine needles 27 24.39 8.66 24.58 8.5650 25.38 14.68 25.03 3.18

A

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burned and unburned treatments within eachdetritus type (Table 2). However, there weregeneral differences between needles and leaves.Burned and unburned needles were higher inlipids and carbon but lower in nitrogen contentcompared to leaves. Leaves and needles also dif-fer in lignin and tannins; however, these pa -rameters were not measured.

Microbial respiration differed substantiallyamong treatments. In both respiration trials,the first measure of dissolved oxygen levels inrange-finding beakers indicated that enoughtime had passed to detect differences in oxygenlevels between treatments. Therefore, oxygenlevels were measured in all beakers (range-find-ing and treatment beakers) at the same time.This allowed for range-finding beakers to beadded as additional replicates for the unburnedleaf and burned pine treatments (n = 5 forthese 2 treatments). Microbial respiration washighest on unburned leaves, followed by burnedleaves, and lowest on unburned and burned nee-dles (Fig. 5). There was no significant differencebe tween trials (log-transformed data, dfmodel =31, Pmodel = 0.3584) in the global model. There-fore, we ran a collapsed ANOVA model withboth trials combined. Burn treatments and detri-tus types were different (P < 0.0001 for both

266 WESTERN NORTH AMERICAN NATURALIST [Volume 71

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Fig. 5. Mean (and 95% confidence intervals) of (A) total percent loss of dry mass of, and (B) number of Pteronarcella badiacolonizing on, artificially constructed leaf packs containing burned and unburned ponderosa pine needles and cottonwoodleaves (n = 24). Mean (and 95% confidence intervals) for (C) oxygen use (% change from time zero of mg O2 ⋅ L–1 per g) ofmicrobes (n = 6–10) and (D) growth (% increase in length mm) of P. badia on each detritus treatment (n = 10) are also pre-sented. Letters represent results of Tukey’s multiple comparisons tests; different letters represent statistical differences(α = 0.05).

TABLE 2. Nutritional analysis of burned and unburnedponderosa pine needles and cottonwood leaves conditionedfor 50 days in the Cache la Poudre River, Colorado. Percentsolvent extract represents both nutritive and nonnutritivelipids.

Detritus Burn % Solvent % %type treatment extract Carbon Nitrogen

Leaves Unburned 3.8 42.8 2.0Burned 4.1 48.4 1.9

Needles Unburned 12.1 51.6 1.2Burned 10.6 55.7 1.4

A B

C D

factors; dfmodel = 31), but there was no sig -nificant interaction (dfmodel = 31, P = 0.5813).

Pteronarcella badia Growth Experiment

In the microcosm growth experiment, only9 of the 120 P. badia died from unknown causes,and these individuals were excluded from analy -sis. Pteronarcella badia grew an average of 1.4mm in length (approximately 20% of their initiallength) over 12 days on all treatments (Fig. 5).Relative growth rates were similar across detri-tus type and burn treatments, and there were nosignificant block effects due to microcosm(ANOVA: dfmodel = 39, Pmodel = 0.9211).

DISCUSSION

Stonefly detritivore populations in PajaritoPlateau streams were markedly reduced inburned canyons after intense crown wildfires,but these reductions corresponded to severepostfire flooding rather than to the direct effectsof fire. Both A. banksi and P. badia populationsshowed no resistance to 100-year floods after theDome and Cerro Grande fires and showed lowresilience to moderate flooding in the year fol-lowing the fires. P. badia was notably less resil -ient than A. banksi and had not fully recoveredin severely burned reaches of Guaje Canyon 2years after the Cerro Grande Fire. While previousstudies showed that A. banksi required morethan 6 years to recover in Capulin Canyon afterthe Dome Fire (Vieira et al. 2004), this speciesshowed much higher resilience in Guaje Can -yon, especially in lightly burned upper reaches.Other studies have shown that the nemouridZapada columbiana (Mihuc and Minshall 1995,Minshall et al. 1997), and shredders in gen-eral (Minshall et. al. 2001), were fairly resilientto fire-related disturbances following the Yellow-stone fires. Another stonefly shredder, Yoraperlasp. (Peltoperlidae), recolonized quickly throughdrift in Washington streams that remainedhydrologically stable after a wildfire (Mellon etal. 2008). Our study suggests that P. badia popu-lations may be particularly sensitive to postfireflood disturbances.

Both our field surveys and our growth experi-ments indicated that reduced food quality asso-ciated with burned riparian vegetation does notexplain the strong negative response of P. badiato the Cerro Grande Fire. Stonefly shredderscolonized ash-covered detritus at high densitiesimmediately after the fire, even in severely

burned reaches. In addition, P. badia both colo -nized and grew on burned coniferous and decidu -ous detritus in our experiments. Growth onburned detritus treatments was likely due to thefact that burning did not significantly alter pro-tein content in the leaves or lipids in the nee-dles, both of which are indicators of nutrition forstream detritivores (Short et al. 1980, Cargill etal. 1985). Lower microbial colonization onburned detritus, especially on burned needles,did not negatively impact P. badia growth. Ap -parently, there were enough microorganismsconditioning the burned detritus to render it apalatable and nutritious food source.

In contrast to our study, Mihuc and Minshall(1995) found that 10 of 11 insect taxa could notgrow on 100% burned detritus, and they citedlow lipid and protein content and reduced mi -crobial densities as potential explanations. Com-pared to our study, Mihuc and Minshall (1995)also found greater reductions in microbial colo-nization and alteration of nutritional content asa result of more-intense burning. Our experi-mental burn treatment was more similar to their35% natural burn. They found slightly higherbacterial colonization on this naturally burnedmaterial, and as such, 3 generalist detritivoresgrew on this treatment, including the nemouridstonefly Z. columbiana. In addition to growingon partially burned materials, Z. columbiana alsofed and grew on periphyton, demonstrating thatthis species has a flexible detritivore-herbivorefeeding strategy and is not a specialist on CPOM.Dietary flexibility was corroborated by the factthat this species showed isotope signatures forboth periphyton and detritus in the field (Mihucand Minshall 2005) and this flexibility mayexplain the high resilience of Z. columbianapopulations after wildfire (Minshall et al. 1997).Herbivory is common in other North Americannemourids (Vieira et al. 2006) and may havefacilitated A. banksi colonization success afterpostfire floods abated in burned plateau canyons.

While postfire flooding clearly impacteddetritivorous stonefly densities, we could notseparate organism loss via flooding from theindirect effects of reduced detrital retention,which can also influence shredder populations(Muotka and Laasonen 2002, Lepori et al.2005). Most aquatic organisms in Guaje andCapulin canyons, in cluding detritivorous stone-flies, were extirpated after moderate to severepostfire flooding (Vieira et al. 2004). We alsoobserved repeated flushing of detritus as a

2011] FIRE EFFECTS ON STONEFLY DETRITIVORES 267

result of these floods, with mini mal additionsof new leaf litter until riparian vegetationreestablished. Others have observed reducedor variable retention of CPOM due to hydro-logic instability after wildfires (e.g., McIntyreand Minshall 1996, Robinson et al. 2005, Arkleet al. 2010). Our findings are consistent withthose of Arkle et al. (2010), who found that post-fire hydrology interacts with riparian cover,de trital resources, and sediment loads in a com -plex fashion to influence benthic invertebraterecovery. For example, recovery trajectories topre fire levels differed widely across studyreaches. Shredder stonefly densities recoveredearliest in upper Guaje Canyon, where the ri -parian zone was minimally damaged in the fire.By contrast, densities of stonefly detritivores,and also of total invertebrate shredders (Vieiraet al. 2004), had not fully recovered in CapulinCanyon 6 years postfire, even though maturewoody riparian species had reestablished.

Recovery trajectories not only differed acrossstudy sites, but also between stonefly species.Pteronarcella badia populations demonstratedless resilience than A. banksi populations afterthe Cerro Grande Fire. Even after flood mag-nitudes dampened, P. badia had still not recov-ered in either lightly or severely burned reachesof Guaje Canyon by the end of the study. Bycontrast, A. banksi populations recovered toprefire densities in upper Guaje reaches oneyear after the fire and in the severely burnedlower reaches by spring 2002. Species traitsrelated to hydrologic disturbance may explainthese differences in recovery trajectories (Rader1997, Richards et al. 1997, Townsend et al.1997b). First, P. badia is semivoltine and thusmay require a higher degree of hydrologicaland streambed stability to complete their lifecycle. For exam ple, Feminella (1996) demon-strated that another pteronarcid, Pteronarcyssp., was restricted to streams with a high levelof flow permanence and stability, whereas uni-voltine Amphinemura sp. and Capniidae wereubiquitous even in in termittent streams in Ala -bama. Second, P. badia nymphs are larger bod-ied and probably more easily entrained byfloods compared to A. banski, which can seekrefugia in microhabitats. Finally, P. badia mayhave weaker dispersal mechanisms. For exam-ple, nymphs of this species are less prone todrift compared to the smaller-bodied nemouridsand capniids. Pteronarcella badia is a strongflyer and could have colonized via aerial dis-

persal in Guaje Canyon. However, both P. badiaand A. banksi adults tend to remain local (within1 km) after emergence (Vieira et al. 2006).

Our research on 2 fires of the Pajarito Plateauconfirms that ecological responses in burnedwatersheds of the western United States varydramatically across individual fires, taxonomicgroups, burned streams, and even across reacheswithin a burned stream (e.g., Minshall et al.1997, Gresswell 1999, Malison and Baxter 2010).We found notable differences in how detritivo-rous stonefly species responded to wildfiresversus postfire flooding, and also in how thesestoneflies recovered between burned canyons ofthe plateau. Differences can be evaluated incontext of the mosaic of fire disturbance and thelandscape that characterizes the plateau. Forexample, the forested headwaters of Guaje Can -yon were relatively unburned in the CerroGrande Fire, whereas the Dome Fire burnedCapulin Canyon up to the intermittent springseeps where the perennial reach begins. Thepresence of pristine, perennial headwaters inGuaje Canyon provided a nearby source ofinvertebrate colonists, especially for A. banksi,which has stronger drift behaviors than P. badia.Unburned headwaters in Guaje Canyon alsolikely provided CPOM to burned reaches below.While recolonization from upstream springshas been noted in other montane desert streamsfollowing floods (Molles 1985), invertebratecommunities in the seeps of upper CapulinCanyon are fairly depauperate (Vieira et al.2004). Therefore, postfire colonization of stone-flies in burned reaches of Capulin Canyonlikely occurred via aerial adult disper sal. Thecanyons-mesa topography of the plateau, therarity of perennial tributaries, and the inter-mittency between the perennial stream seg -ments and the Rio Grande River pose strongcolonization barriers to dispersal. The relianceon aerial versus drift dispersal, especially giventhese barriers, may explain why A. banksi re -covery was significantly delayed in CapulinCanyon compared to Guaje Canyon.

Our study suggests that headwaters of thePajarito Plateau system should be considered forfire management in unburned watersheds andprioritized for rehabilitation efforts in burnedwatersheds. Such efforts will reduce postfireflooding magnitudes, maintain refugia for aquaticorganisms, provide a colonist pool for degradedreaches, and preserve riparian resources andthe detrital subsidies they provide to lotic food

268 WESTERN NORTH AMERICAN NATURALIST [Volume 71

webs downstream and in the headwaters.Lake et al. (2007) argue that headwaters nor-mally must be functional in order to supportrestoration actions in degraded reaches down-stream, especially in those headwaters proneto hydrologic disturbance, because downstreamreaches are inherently impacted by streamstability above. Riparian fuel reduction man-agement through prescribed burns, thinning,or other management actions may provide asolution to protect riparian vegetation andstreams in headwater areas. This relatively newpractice has obvious potential benefits in pro-tecting critical habitats, but how much thesebenefits are realized is largely untested andthus unknown (Stone et al. 2010). For exam-ple, only a handful of studies exist to indicatethat prescribed burns have minimal impacts tostream communities compared to wildfires(e.g., Beche et al. 2005, Arkle and Pilliod2010). Until riparian fuel management andheadwater protection strategies gain trac -tion and can be optimized for regional land-scapes and riparian types (Pettit and Naiman2007), streams will remain at the mercy ofsevere fires and postfire flooding in westernwatersheds.

ACKNOWLEDGMENTS

Thanks go to Will Clements and Boris Kon -dratieff at Colorado State University for facili-tating this research, and to the CSU HughesScholar and Merit Work Study programs forproviding support for undergraduate studentresearch. We greatly appreciated project supportfrom Brian Jacobs, U.S. National Park Service,Bandelier National Monument; from Craig Allenand Kay Beeley, U.S. Geological Survey; andfrom the Santa Fe National Forest. Two anony-mous reviewers for the journal contributed tomanuscript improvements. Funding for thisproject was provided by the National ParkService, the U.S. Forest Service, and the U.S.Geological Survey.

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Received 1 April 2010Accepted 14 February 2011

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