FIRE ECOLOGY

Fire effects on aquatic ecosystems: an assessment of the current state of the science

Rebecca J. Bixby1,7, Scott D. Cooper2,8, Robert E. Gresswell3,9, Lee E. Brown4,10, Clifford N. Dahm5,11, and Kathleen A. Dwire6,12

1Department of Biology and Museum of Southwestern Biology, University of New Mexico, Albuquerque, New Mexico 87131 USA 2Department of Ecology, Evolution, and Marine Biology and Marine Science Institute, University of California, Santa Barbara, California 93106 USA 3US Geological Survey, Northern Rocky Mountain Science Center, Bozeman, Montana 59715 USA 4School of Geography and water@leeds, University of Leeds, Leeds LS2 9JT UK 5Department of Biology, University of New Mexico, Albuquerque, New Mexico 87131 USA 6US Department of Agriculture Forest Service, Rocky Mountain Research Station, Fort Collins, Colorado 80526 USA

Abstract: Fire is a prevalent feature of many landscapes and has numerous and complex effects on geological, hydrological, ecological, and economic systems. In some regions, the frequency and intensity of wildfire have increased in recent years and are projected to escalate with predicted climatic and landuse changes. In addition, prescribed burns continue to be used in many parts of the world to clear vegetation for development projects, encourage desired vegetation, and reduce fuel loads. Given the prevalence of fire on the landscape, authors of papers in this special series examine the complexities of fire as a disturbance shaping freshwater ecosystems and highlight the state of the science. These papers cover key aspects of fire effects that range from vegetation loss and recovery in watersheds to effects on hydrology and water quality with consequences for communities (from algae to fish), food webs, and ecosystem processes (e.g., organic matter subsidies, nutrient cycling) across a range of scales. The results presented in this special series of articles expand our knowledge of fire effects in different biomes, water bodies, and geographic regions, encompassing aquatic population, community, and eco- system responses. In this overview, we summarize each paper and emphasize its contributions to knowledge on fire ecology and freshwater ecosystems. This overview concludes with a list of 7 research foci that are needed to further our knowledge of fire effects on aquatic ecosystems, including research on: 1) additional biomes and geographic regions; 2) additional habitats, including wetlands and lacustrine ecosystems; 3) different fire sever- ities, sizes, and spatial configurations; and 4) additional response variables (e.g., ecosystem processes) 5) over long (>5 y) time scales 6) with more rigorous study designs and data analyses, and 7) consideration of the effects of fire management practices and policies on aquatic ecosystems. Key words: wildfire, aquatic ecosystems, streams, rivers, wetlands, ecosystem, biota, prescribed burns

Fires are natural disturbances and agents of landscape and destructive of natural vegetation, property, and life. change that have a diversity of effects across a variety of However, from an ecological perspective, fires have struc- spatial scales. Perceptions of the consequences of fire are tured many ecosystems with resilient successional trajec- closely tied to human values (Langston 1995). For exam- tories (Pyne et al. 1996, Gresswell 1999, Bowman et al. ple, the use of fire distinguishes humans from other ani- 2009). Fire management and policy tend to be focused on mal species, enhances food nutritional value, and promotes protecting human property and life and on protecting or the expansion of valued plant and animal resources. Fire salvaging the economic value of terrestrial resources, such also was an integral driver of the invention and adoption of as timber, but fire also affects freshwater resources, hab- tools, other technological innovations, and, ultimately, the itats, and biodiversity. industrialization and urbanization of human societies, cre- Given the critical importance of water resources to ating the modern world we know today (Pyne 2012). In con- human populations and natural communities globally, a trast, humans generally view uncontrolled fire as harmful thorough understanding of the effects of fire on water

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DOI: 10.1086/684073. Received 15 August 2015; Accepted 1 September 2015; Published online 22 September 2015. Freshwater Science. 2015. 34(4):1340–1350. © 2015 by The Society for Freshwater Science. Volume 34 December 2015 | 1341 resources is increasingly important for guiding fire man- Most previous work on the effects of fire on fresh- agement practices and policy decisions. Some short-term water ecosystems has concentrated on wildfire effects on effects of fire on freshwater ecosystems can be similar to the physical, chemical, and biological characteristics of the effects of landuse changes (e.g., agricultural and ur- forested, montane streams and wetlands in the western ban development and logging), but fire is a pulsed dis- USA (Gresswell 1999, Pilliod et al. 2003, Rieman et al. turbance, and the duration of its effects on freshwater 2003). Authors of articles in this special series expand on ecosystems depends on terrestrial ecosystem recovery. In these topics by considering fire effects on a variety of or- contrast, landuse changes constitute a press disturbance ganisms (ranging from algae and riparian vegetation to with more permanent effects (Allan 2004, Wootton 2012, spiders and fish) and processes (including microclimate, Verkaik et al. 2013). The purpose of this special series of hydrology, and biogeochemistry; nutrient inputs, uptake, articles is to illustrate the importance and complexities and limitation; and subsidies between terrestrial–aquatic of fire as a prime driver of change in the physical, chemi- habitats and tributary–mainstem systems). These organ- cal, and biological characteristics of freshwater habitats ismal and process studies were done across a wide array of in different geographic regions and biomes (Fig. 1). Given geographic areas (North America, Europe, Australia, Asia), the projected effects of climate change on fire frequency biomes (boreal forest, Mediterranean shrublands, tropi- and intensity (Knowles et al. 2006, Seager et al. 2007, Pausas cal savanna, temperate, tropical, and semitropical wetlands and Fernández-Muñoz 2011, Westerling et al. 2011), we and forests), and habitats (rivers, riparian zones, lakes, argue that our focus on the effects of fire on freshwater wetlands). Previous work has focused on the effects of fire ecosystems is timely. on state variables, but a number of authors in this series

Figure 1. Path diagram showing probable cause–effect relationships leading from fire to stream communities. Lines without arrows indicate factors that are associated with each other, unidirectional arrows point from driver to response variables, and double- headed arrows indicate consumer–resource interactions where consumers depress and benefit from the consumption of their resources. Temp = temperature, DOC = dissolved organic C, POC = particulate organic C. 1342 | Fire effects on aquatic ecosystems R. J. Bixby et al. concentrated on effects of fire on ecosystem processes or cificeffects of riparian or wetland burning on freshwater rate variables, including nutrient uptake (Diemer et al. ecosystems, including organic matter loading, biogeochem- 2015), nutrient limitation (Klose et al. 2015), leaf decom- ical cycles, light and temperature levels, and, ultimately, the position (Rodríguez-Lozano et al. 2015), subsidies from aquatic biota, have rarely been delineated (Cooper et al. river tributaries to river main stems (Harris et al. 2015), 2015). and subsidies from streams to riparian zones (Jackson and Douglas et al. (2015) examined the effects of intensely Sullivan 2015). managed fires on the composition and structure of ripar- This special series was developed in conjunction with ian vegetation in Australia’s savannas. They compared ri- a special symposium held at the Joint Aquatic Sciences parian vegetation characteristics in burned and unburned Meeting in Portland, Oregon, in May 2014. The articles watersheds in an experiment conducted in whole water- collectively emphasize the pervasive influence of fire on sheds. Vegetation was sampled 1 y after 3 y of sequential the structure and function of aquatic ecosystems through- annual burning. The application of prescribed burning sig- out the world and underscore the importance of consider- nificantly reduced woody species richness, total species ing effects of fire on freshwater systems when furthering abundance, total basal area, the abundance of small trees, our knowledge of drivers of ecosystem change and when canopy cover, and the richness and cover of vines, but guiding or developing effective natural resource manage- increased grass cover. Results of this study identified ri- ment practices and policies. parian plant species that appeared to be adapted to dif- We built on a list of research needs identified by Ver- ferent fire frequencies and showed that riparian areas are kaik et al. (2013) to evaluate how the papers in the present considerably more sensitive to fire than the surrounding special series address some of the knowledge gaps in the savanna. literature on fire effects on aquatic ecosystems. We focus The floodplain shifting habitat mosaic concept pro- on key aspects of fire effects on riparian and wetland veg- poses that habitat patch dynamics are driven by flood etation, microclimate and hydrology, water quality, or- pulses that alter the geomorphology of channels, banks, ganic matter subsidies, and stream biota. We conclude with and floodplains, thereby creating new habitats and chang- a list of the most critical research needs. The research ing existing habitats (Stanford et al. 2005). Kleindl et al. advances reported in this special series can provide a foun- (2015) extended the shifting habitat mosaic concept to dation and springboard for future research leading to the examine the effects of multiple, different disturbances, in- formulation of effective fire management practices and cluding floods and fire, on the composition of vegetation policies that better sustain freshwater resources, habitats, along the riparian corridor of the Flathead River (British and biodiversity. Columbia and Montana). They applied a combination of path and graphical analysis to a 22-y data set to examine relationships among hydrology, fire, land use, geomorphic RIPARIAN AND WETLAND VEGETATION position, and floodplain habitat patch dynamics. Three When terrestrial vegetation is consumed by fire, nu- factors (fire, stream power, and geomorphic position) col- trients are mobilized, runoff and erosion increases, and lectively explained much of the variation in floodplain veg- soils may be altered. Habitat changes occur that favor etation patch composition across study reaches. Wildfire some species and impede others. The literature on the had the strongest total effect. Long-term investigation of responses and recovery of many upland vegetation types disturbance and recovery pathways in a floodplain allowed to both wildfire and prescribed fire is extensive. Because the authors to expand the shifting habitat mosaic concept of differences in the microclimate, foliar moisture, struc- from one in which landscape dynamics are driven only by ture, composition, and life histories of riparian/wetland major hydrologic events to one that incorporates the in- and upland plant species, these plant communities can re- fluences of other riverscape and landscape disturbances, spond very differently to fire (Dwire and Kauffman 2003, particularly fire. Van de Water and North 2011). Although the role of fire has been studied in some wetland vegetation types, such as those occurring in the Everglades (Richardson 2010), existing knowledge on fire effects is limited for many wet- MICROCLIMATE AND HYDROLOGY land plant communities. Recent studies have provided data Fire effects on terrestrial and wetland vegetation and on riparian vegetation responses to fire (Pettit and Naiman on soils influence aquatic ecosystems by altering micro- 2007, Halofsky and Hibbs 2009, Jackson et al. 2012, Ver- climatic regimes, increasing runoff and river discharge, and kaik et al. 2013), as well the influence of riparian conditions enhancing erosion and sediment inputs, transport, and de- on fire behavior (Halofsky and Hibbs 2008), but informa- position (Gresswell 1999, Benda et al. 2003, Coombs and tion is lacking for many riparian plant species and com- Melack 2013). As a consequence, fire effects on aquatic eco- munities. Watershed-wide effects of fire on sediment and systems are the compounded effects of 2 scales of distur- nutrient inputs have been studied extensively, but the spe- bances: seasonal or interannual increases in runoff and Volume 34 December 2015 | 1343 erosion associated with storms or snowmelt superimposed wildfires. Chemical data sets with high temporal and spa- on longer-term consequences of fire disturbance. tial resolution document hydrochemical responses to fire Fire also can affect the physical characteristics of eco- and subsequent floods and debris flows that are often non- tones, including transitions from riparian and wetland areas linear and rapid (Krause et al. 2015). For example, water- to uplands. Watts and Kobziar (2015) compared air tem- quality data analyzed from a network of sondes in the Rio perature, relative humidity (RH), and vapor pressure defi- Grande watershed, New Mexico (USA), showed dramatic cit (VPD) in patches of pond cypress (cypress domes) and decreases in dissolved O2 and pH as debris pulses moved adjacent grasslands in southern Florida, USA, 2 y after wild- downstream into a large river system after a large wildfire fire. Increasing differences in air temperature, RH, and in headwater areas (Dahm et al. 2015). VPD were observed with distance from the dome centers Reale et al. (2015) demonstrated the value of collecting into savanna habitats, but microclimates were either simi- high-resolution, continuous data from networks of water- lar or, in some cases cooler or more humid, in burned than quality sensors and streamflow gages to assess initial and in unburned domes. Watts and Kobziar (2015) attributed long-term effects of wildfire on the water quality of 2nd- this response to vigorous vegetative regrowth following fire. and 4th-order streams in the Jemez Mountains and in Their study increases our understanding of interactions be- the Rio Grande, New Mexico. Precipitation did not dif- tween cypress domes and ecotonal microclimates, thereby fer before and after the fire, but episodic postfire storms increasing the ability of resource managers to maintain these resulted in significantly elevated turbidity and specific unique plant communities under predicted scenarios of conductance (SC) (linked to soil, sediment, rock and ash greater variability in climate and fire regimes. debris, and solutes entrained from burned watershed Given that the ecological effects of smoldering fires areas). Dissolved O2 concentrations were variable in a are largely unknown, Watts et al. (2015) developed the 2nd-order stream and more muted downstream in a 4th- first conceptual model of smoldering fires in wetlands in order river. An additional study of 4 sites over 4 mo en- Florida. They focused on relationships among fire, wet- compassing the wildfire also showed stronger fire effects land hydrology, and C dynamics. Their model underscores on turbidity and SC in 1st- and 2nd-order streams than the complex and integrated feedbacks between burn depths in higher-order downstream sites. These results suggest and extent of smoldering fires on local and regional hydrol- that flow pathways, geomorphology, and biogeochemical ogy and predicts that increased burn depths and extended processes moderate fire effects on water quality along the hydroperiods reduce initiation and frequency of fire in these river continuum. habitats. Fires kill or damage vegetation and alter soil chemistry, Peatlands cover ∼17% of the land surface area in the thereby reducing uptake. Therefore, nutrients, such as N UK and are distributed broadly across the headwater and P, are often mobilized by fire, which results in in- areas of most major river watersheds. Brown et al. (2015) creased loading to stream and river ecosystems (Sherson synthesized current knowledge about how rivers in peat- et al. 2015). These postfire nutrient pulses, which usually lands respond to both wildfires and prescribed burns. The are associated with floods, can increase nutrient concen- hydrologic response of peatland streams to fire is complex. trations many fold. Diemer et al. (2015) extend our knowl- Peak flows are lower during many precipitation events but edge of long-term fire effects on nutrient dynamics in greater during the largest rainfall events in burned than in streams to the boreal forests of Central Siberia. Boreal for- unburned watersheds. Furthermore, concentrations of dis- est streams and their ecosystems are highly susceptible to solved organic C (DOC) in surface waters are higher in the effects of climate change, including the intensity, fre- burned than unburned watersheds. The authors present a quency, duration, and extent of forest fires. Diemer et al. conceptual model that illustrates linkages and feedbacks (2015) showed that fires in boreal forests alter stream among the hydrological, chemical, and biological proper- chemistry for many years and affect the retention and ties and processes of watersheds following fire. This model export of N and P in these stream networks. Streams in provides a framework for identifying knowledge gaps and for watersheds that had burned within the 4 to 10 y before forecasting changes in peatland streams related to the re- the study in Central Siberia had lower DOC and higher – moval of vegetation by wildfire or prescribed burning. NO3 concentrations, differing from nutrient responses to fire in boreal regions of North America. WATER QUALITY Fire effects on water quality are of particular concern ORGANIC MATTER SUBSIDIES to water resource managers because of potential effects Fires modify the inputs of dissolved and particulate (e.g., on water supply systems and aquatic communities. Ad- ash and charcoal) organic matter into streams by damag- vances in technology and instrumentation (e.g., sondes) ing or killing upland vegetation (Earl and Blinn 2003). Where allow the collection of continuous water-quality data to riparian or wetland vegetation is destroyed or damaged by monitor changes related to complex disturbances, such as fire, the canopy opens, thereby decreasing allochthonous 1344 | Fire effects on aquatic ecosystems R. J. Bixby et al. inputs and increasing light and temperature levels, which to a very limited literature on fire effects on detrital dy- promote autochthonous production, with repercussions for namics and leaf breakdown rates (Koetsier et al. 2010, aquatic communities and food webs (Beakes et al. 2014, Jackson et al. 2012). Results reported by Vaz et al. (2015) Cooper et al. 2015). In some cases, a pulse of leafy and and Rodríguez-Lozano et al. (2015) suggest that fire ef- woody debris from damaged vegetation can occur after ri- fects on detrital dynamics can be long-lived (>5 y) (also parian fires. Allochthonous inputs often decrease subse- see Robinson et al. 2005). quent to the loss of riparian vegetation, but organic inputs eventually rebound as riparian vegetation recovers (Britton 1990). Furthermore, postfire hydrological conditions can STREAM BIOTA greatly affect the biomass of organic matter on stream bot- Although immediate effects of fire on the stream biota toms when floods mobilize and transport organic matter may be muted, stream biological communities usually to downstream areas. Riparian trees damaged by fire may change radically with postfire floods, which scour stream not fall into or across streams until years after the fire, usu- substrates and remove most organisms (Gresswell 1999, ally in association with wind throw or floods (Robinson Minshall 2003). Furthermore, effects on aquatic commu- et al. 2005, Bendix and Cowell 2010). nities can be modified by pre- or postfire drought (Rugen- Harris et al. (2015) compared watersheds that were ski and Minshall 2014). The responses of different types burned and then affected by subsequent debris flows to of organisms to fire and floods or droughts are related to watersheds that had not burned or had been burned with- their life cycles, dispersal abilities, and the availability and out debris flows 4 y after a major fire. They document a distribution of refugia. Short-lived, fast-colonizing species large increase in sediment export during spring runoff in often dominate after fires and floods or droughts (Min- the burned, but not in the unburned, watersheds. Stream shall 2003, Grace 2006, Malison and Baxter 2010a). DOC concentrations were 75% greater in watersheds with Klose et al. (2015) studied the effects of wildfire and fires and debris flows than in unburned watersheds, but postfire flooding in southern California on algal abun- concentrations of chlorophyll a and the chlorophyll a ∶ or- dance, community composition, and nutrient limitation (as- ganic matter ratio were higher in unburned watersheds. sessed with nutrient-diffusing substrata) in stream reaches Macroinvertebrate export from tributary streams to the in unburned and burned catchments. They also considered main stem was dominated by r-strategist taxa (Chironomi- reaches where riparian vegetation did or did not burn. Re- dae, Baetidae, and Simuliidae) in streams draining burned sults suggest that algal responses (e.g., density, biovolume, watersheds, and export of invertebrate biomass was greater chlorophyll a, and species composition) to fire and nutri- from streams in burned watersheds with debris flows than ent enrichment are driven primarily by fire effects on ri- from streams in unburned watersheds (Harris et al. 2015). parian canopy cover and associated light and temperature Vaz et al. (2015) reviewed changes in large wood in- levels, flood disturbance intensities, and nutrient concen- puts, distributions, structural complexity, and invertebrate trations. Decreased riparian cover mediated faster algal re- associations after fires. Their review was based primarily covery postfire. The results provide insights into processes on their research in Portuguese streams, but they also that create and maintain habitat heterogeneity in riparian examined the effects of wildfire on large wood subsidies to and stream habitats. a lake in northern Minnesota, USA. Their results extend Most information on wildfire effects on stream and our knowledge of the effects of wildfire on large wood river ecosystems is derived from studies of single wildfire inputs to streams and lakes and suggest that fire may sim- events in cool headwater systems. In contrast, Whitney plify the structure of wood in streams but increase habi- et al. (2015) quantified changes in riverine habitat, ben- tat complexity in lakes. thic algal chlorophyll a concentration, and both warm- Rodríguez-Lozano et al. (2015) reported that stream and coldwater invertebrate and fish communities after macroinvertebrate functional feeding groups recovered consecutive fires that covered >100 km2 in southwestern within 1 or 2 y after wildfire but that leaf-litter inputs de- New Mexico, USA. Cumulative fire effects, fire size, and creased and leaf-litter breakdown rates increased in a post-wildfire rainfall were strongly associated with the stream draining a watershed that had burned 8 y before siltation of river beds, decreases in chlorophyll a concen- the study relative to a stream in an unburned watershed. tration, and decreases in the biomass of most insect taxa Their results suggest that microbially mediated leaf de- and 6 of 7 native fish species. Among native fish species, composition rates are enhanced by increased tempera- the Headwater Chub Gila nigra (100% loss) and Spikedace tures engendered by the opening of the riparian canopy by Meda fulgida were lost from streams in burned watersheds fire and that total (microbial + shredder) leaf breakdown for up to 2 y postfire. Fish kills are thought to have resulted + rates were increased by shredder aggregation in coarse- from hypoxia, and elevated concentrations of NH4 ,trace mesh leaf bags in the stream in the burned watershed metals, and ferrocyanides generated by wildfires. Nonnative where leaf litter inputs were low. These results contribute warmwater fish, crayfish, and amphibian larvae were less Volume 34 December 2015 | 1345 affected by fire, results suggesting that fires threaten native are present (Gresswell 1999). Sedell et al. (2015) used a taxa more strongly than invasive taxa. qualitative, heuristic model to map the predicted distri- Verkaik et al. (2015) considered how stream macro- butions of postfire debris slides in the Colorado Rocky invertebrate community responses to fire are mediated by Mountains. They compared these maps to the distribu- interactions with preceding droughts or subsequent flood tion of Colorado River Cutthroat Trout (Oncorhynchus events. This global-scale, multisite analysis included data clarkii pleuriticus) populations. The results indicated that from central Idaho, USA, northeastern Spain, and Victo- interconnected trout populations would be resilient to ria, Australia. Macroinvertebrate community responses to wildfire-induced debris flows. They also showed that trout wildfire after 9–11 mo (lower taxonomic richness, higher populations in headwater streams and lakes probably total macroinvertebrate abundance, and high percentages act as refuge populations for the recolonization of lower of Chironomidae, Simuliidae, and Baetidae) were similar stream reaches that are at much higher risk from debris across all 3 regions, but the magnitude of the response flows. differed among regions. The greatest differences between Rosenberger et al. (2015) documented that Rainbow burned and unburned watersheds in macroinvertebrate Trout (Oncorhynchus mykiss) were present throughout communities were found in Australia, where fire was ac- streams in burned watersheds a decade after fires and de- companied by ongoing drought and persistent low flows. bris flows, but that individuals in older age classes were In contrast, macroinvertebrate recovery was faster in the least abundant in streams in burned watersheds with de- cold–temperate climate of Idaho and the Mediterranean bris flows and most abundant in streams in unburned climate of northeastern Spain, where postfire floods may watersheds. Rainbow Trout from burned watersheds also have acted to re-establish or reset biotic colonization pro- were characterized by fast growth, low lipid content, and cesses. These interactions between hydrological and fire early maturity compared to those in unburned watersheds. events are likely to become more pronounced with cli- Dunham et al. (2007) reported that stream temperatures mate change. were higher in burned watersheds with debris flows than The effects of wildfire and hydrological disturbances in unburned watersheds and burned watersheds without on stream invertebrates also can affect subsidies of emerg- debris flows. Rosenberger et al. (2015) developed models ing stream insects to riparian zones, thereby altering the whose output suggested that moderate warming associated availability of food resources for riparian predators (Mali- with wildfire and channel disturbance history associated son and Baxter 2010b). Jackson and Sullivan (2015) inves- with faster individual trout growth exacerbate competition tigated the effects of fire on linked aquatic and terrestrial for food resulting in decreases in trout densities. habitats in the Mediterranean climate of California, which is characterized by high interannual variability in precipi- FUTURE RESEARCH RECOMMENDATIONS tation and frequent high-severity wildfires. They assessed The articles in this series expand our knowledge of the effects of wildfire on stream geomorphology; the den- the effects of fire on aquatic ecosystems to different geo- sity and community composition of aquatic benthic macro- graphic regions, biomes, habitats, and both rate and state invertebrates; and the densities, tissue Hg concentrations, variables. The research presented here emphasizes the im- trophic position, and food sources of riparian spiders (family portance of fire ‘type’ (wildfire vs prescribed fire), differ- Tetragnathidae) in Yosemite National Park. Although dif- ent prescribed burn approaches (e.g., large forest burns, ferences in spider responses between paired burned and strips to mitigate fire spread, patches to create mosaics), unburned study sections were not statistically significant, fire effects on riparian and wetland vegetation, and pre- modeling suggested that variability in benthic invertebrate and postfire hydrological events on riparian–stream sub- density, watershed-scale fire frequency, and precipitation sidies, stream and wetland communities, and ecosystem are important predictors of tetragnathid spider density and processes. All of these topics have implications for the trophic position. Perhaps most important, precipitation was effective management of aquatic resources. Fire effects on related to multiple spider responses, a relationship suggest- aquatic ecosystems are inherently complex. They depend ing that climate variability could have greater effects on the on the characteristics (e.g., extent, intensity, severity, tim- aquatic–terrestrial ecological linkages than the influence of ing, frequency) of fires and previous or subsequent hydro- fire alone. logical events (e.g., drought and floods) and on features of Effects of fire on physical and chemical conditions and watersheds (e.g., slopes, soils, and vegetation) and receiving on biological communities can affect populations of apex waters (e.g., lentic or lotic, discharge, geomorphology, and predators in streams, such as fish (Rieman et al. 2003, biota). Future research on the effects of fire on aquatic sys- Sestrich et al. 2011, Beakes et al. 2014). Wildfires and sub- tems requires increased focus on a wider array of combi- sequent floods can kill or remove fish in isolated, small, nations of fire, hydrology, watershed geomorphology, and headwater streams, but fish populations appear to re- aquatic conditions, and models integrating fire effects and cover quickly, provided no barriers to fish immigration natural resource management. As a consequence, we pro- 1346 | Fire effects on aquatic ecosystems R. J. Bixby et al. pose that future investigations be expanded to address 7 fire effects on the regional distributions and abun- research foci: dances of the aquatic biota. Such assessments will require a combination of extensive and intensive 1. Additional geographic areas and biomes. Fire is sampling across the landscape using a probabilistic regularly used to manage savannas and to clear sampling design. rainforests or wetlands for agricultural activities, 4. Additional response variables. Most investigators but very little information exists on the effects of have concentrated on documenting changes in the fire on aquatic ecosystems in the tropics (e.g., trop- abundance and biomass of aquatic organisms, with ical South America, Africa, Asia, Australia (Mal- little attention given to more subtle or indirect bi- mer 2004, Townsend and Douglas 2004, Cochrane ological responses to fire. For example, indirect, 2010). Furthermore, the incidence of fire has in- sublethal effects of fire on fish distributions, food creased in many regions and biomes where fire availability, growth, reproductive potential, and effects have been little studied (e.g., Arctic and bo- population structure have received little attention real areas, temperate rainforests, grasslands, and (Gresswell 2004, Beakes et al. 2014). In addition, semi-arid savannas) (Jacobs et al. 2007, Betts and investigations of the indirect effects of fire on food Jones 2009, Larson et al. 2013, Veach et al. 2014). webs, including subsidies, parasites, pathogens, With the enhanced availability of data from differ- and predators, could expand our knowledge of the ent biomes and regions, it should be possible to ramifying effects of catastrophic disturbance on undertake more detailed meta-analyses of fire ef- biological interactions and community structure fects (e.g., Verkaik et al. 2015) to look for generali- (Hossack et al. 2013b, c, Cooper et al. 2015, Jack- ties in the responses of the aquatic biota and eco- son and Sullivan 2015). Authors of articles in this system processes in different types of ecosystems series have provided some data on fire effects on to fire (Brown et al. 2013). stream ecosystem processes, such as nutrient up- 2. Other aquatic habitats. Most literature on fire take and limitation and leaf inputs and decomposi- effects on aquatic systems is focused on streams, tion rates, but research on these and related topics with few data available on fire effects on lakes, (e.g., nutrient spiraling, microbial activity, primary ponds, and wetlands (Prepas et al. 2009, Kotze and secondary production, stream metabolism) are 2013, Lewis et al. 2014). Like the addition of dif- promising avenues for research on the effects of fire ferent biomes mentioned above, inclusion of other on aquatic ecosystems. aquatic habitats support generalizations (or unique 5. Longer time frames. A substantial amount of lit- characteristics) that describe fire effects on a large erature is available on short-term (<5 y) stream variety of aquatic ecosystems. responses to fire (Gresswell 1999, Verkaik et al. 3. Fires with different characteristics. To date most 2013), but the longer term effects of fire on aquatic research has been concentrated on the effects of ecosystems are largely unknown. Although some severe or large fires on stream ecosystems. Many stream variables recover quickly after fire, a lim- fires across a landscape are small and seemingly ited number of studies report long-term effects inconsequential, but these fires are underrepre- of fire on riparian vegetation, organic subsidies, sented in research programs. Most prehistorical and and aquatic communities (Robinson et al. 2005, historical fire practices appear to have involved Hossack et al. 2013a, Rugenski and Minshall frequent, small, and low-intensity fires, but current 2014, Kleindl et al. 2015, Rodríguez-Lozano et al. fire regimes have been greatly altered by human 2015). Limited results indicate some fire effects population expansion, increased ignition sources, can be long-lived, but much longer time series and, in some areas, fuel management and fire- of data are needed to evaluate the legacy effects suppression practices (Stephens et al. 2007). In- of fire. Furthermore, long-term monitoring of a creased research on the effects of fires differing in number of systems in a given area will increase severity, extent, and frequency could guide the for- the probability that at least one will burn by wild- mulation of fire management practices that better fire (Jackson and Sullivan 2015), increasing the sustain water-associated resources. Even within a strength of our inferences by incorporating both given fire perimeter, research often is focused on pre- and postfire data (Verkaik et al. 2013). the most severely and extensively burned areas, 6. More rigorous study designs and analyses. Effects and more subtle fire effects on aquatic systems are of fire on aquatic ecosystems may depend on the often ignored. Last, no landscape or regional quanti- spatial pattern of burning. Statistical inferences tative assessments have been done of fire effects on could be strengthened by greater attention to site aquatic ecosystems over a complete fire season or selection, which is often opportunistic or based across years, including no analyses of cumulative on logistical considerations. In most cases, sites are Volume 34 December 2015 | 1347

not selected probabilistically (Hankin and Reeves wildland areas (Syphard et al. 2007, McMorrow 1988, Gresswell et al. 2004), do not address issues et al. 2009). This pattern emphasizes the impor- related to spatial pattern, and do not account for tance of evaluating pre- and postfire effects of possible spatial autocorrelation (Ganio et al. 2005, roads, building construction, and landuse regula- Gresswell et al. 2006). Studies in which changes tions (e.g., zoning) on stream community struc- through time are compared within and among wa- ture and ecosystem processes at the wildland– tersheds are rare, but such studies could greatly in- developed land interface. crease the scope of our conclusions. Fire effects on aquatic ecosystems are mediated through linkages CONCLUSIONS from vegetation and soils to hydrological, geomor- fi phological, and chemical responses to biotic and eco- In many regions, res are becoming more severe and ff system process responses (e.g., Brown et al. 2015); frequent in association with e ects of global climate and fi fi ff causal pathway analysis (structural equation model- landuse changes. Both wild res and prescribed res a ect ing) may strengthen inferences regarding the mech- terrestrial and aquatic ecosystems in numerous and com- anistic routes leading from fire to stream responses plex ways. This special series expands our knowledge of fi (Grace 2006; Fig. 1). re as a primary driver of hydrological, geochemical, and 7. Fire management practices. Numerous manage- biological changes in riparian, wetland, and aquatic habi- ment practices have been used before, during, and tats. In some cases, this expansion occurred via research after fires, but studies of the effects of these prac- into unexplored habitats, biomes, and response variables. tices on freshwater ecosystems are limited despite Novel approaches, including continuous monitoring, mod- the important ecosystem services and high bio- eling, and probabilistic sampling designs, aid our abilities fi diversity provided by these critical habitats. Of to generalize and predict outcomes from re. Many of the particular interest are aquatic responses to the studies in this series also highlight the multifaceted nature fi use of fire retardant to contain fire spread, con- of aquatic ecosystem responses to re; i.e., the interaction fi struction and maintenance of in-stream structures of re with climatic variables (temperature, precipitation), (e.g., debris dams) to intercept postfire sediment which drive diverse interactions among hydrological, geo- and debris, applications that stabilize hillslopes morphological, hydrochemical, biological, and ecosystem (e.g., hydromulch, reseeding), and pre- and post- processes. Last, we recommend key research needs includ- fire vegetation removal (e.g., via prescribed burns, ing expansion to additional geographic regions, biomes, mechanical removal, salvage logging) (Karr et al. habitats, and response variables; larger spatial and tempo- fi ff 2004, Reeves et al. 2006). Most investigators have ral scales; and res with di erent characteristics. We also ff found muted and short-lived stream ecological emphasize the critical need for research on the e ects of fi responses to prescribed burns (Britton 1991a, b, re management practices and policies on aquatic ecosys- Bêche et al. 2005, Arkle and Pilliod 2010). How- tems and for consideration of aquatic ecosystems when mak- fi ever, some responses have been more substantial ing re management and policy decisions. (Brown et al. 2015, Douglas et al. 2015), and little investigation has been done of the effects of differ- ACKNOWLEDGEMENTS ent prescribed fire severities, extent, and spatial Our gratitude is extended to Editor Pamela Silver and Edi- configurations on aquatic ecosystems. The man- torial Assistant Sheila Storms for their invaluable assistance in agement of fire and fuel loads in riparian areas organizing this special issue on fire effects. Blake Hossack pro- presents especially difficult challenges (Beschta vided valuable comments. We thank Sheila Wiseman for draft- fi et al. 2004, Stone et al. 2010, McDaniel 2015), ing the gure. RJB and CND acknowledge funding through the particularly where dominated by flammable exotic New Mexico Experimental Program to Stimulate Competitive Research (National Science Foundation [NSF]). SDC was sup- taxa (e.g., Acacia [acacia], Arundo [giant reed], ported by funds from the NSF’s Rapid Response and Long- Tamarix [salt cedar]) (Lambert et al. 2010, Le Term Ecological Research programs. LEB’s contribution was fi Maitre et al. 2011, Drus et al. 2013). During re- supported via the EMBER (Effects of Moorland Burning on the fighting activities, nutrients from fire retardants Ecohydrology of River basins) project funded by the UK’s Natu- can increase stream nutrient concentrations (To- ral Environment Research Council (NE/G00224X/1). Any use of bin et al. 2015), have apparently caused fish kills trade, firm, or product names is for descriptive purposes only (NMFS 2008), and, when coupled with drought, and does not imply endorsement by the U.S. Government. have had synergistic, negative effects on organ- isms in mesocosm experiments (Martin et al. LITERATURE CITED 2014). Last, wildfires in many countries are started Allan, J. D. 2004. Landscapes and riverscapes: the influence of by humans, and the incidence of wildfire increases land use on stream ecosystems. Annual Review of Ecology, with the encroachment of human activities into Evolution, and Systematics 35:257–284. 1348 | Fire effects on aquatic ecosystems R. J. Bixby et al.

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