1. Background
⌅Fire is a natural element of the ecosystems that, except the polar zones, shaped global biomes at different intensities (e.g., Holz et al., 2012Holz, A., Kitzberger, T., Paritsis, J. & Veblen, T.T., 2012. Ecological and climatic controls of modern wildfire activity patterns across southwestern South America. Ecosphere, 3: 1-25. https://doi.org/10.1890/ES12-00234.1 ; Muñoz-Rojas et al., 2021Muñoz-Rojas, M., Machado de Lima, N., Chamizo, S. & Bowker, M.A. 2021. Restoring post-fire ecosystems with biocrusts: Living, photosynthetic soil surfaces. Current Opinion in Environmental Science & Health, 23: 100273. https://doi.org/10.1016/j.coesh.2021.100273 ). In recent decades, as a consequence of land use (e.g., land abandonment, industrial plantations, urban sprawl) and climate change (e.g., drought), there has been an increase high the severity of fires. Such fires can have a long-term effect on different ecosystem components such as air, water, biodiversity or soils (Pereira et al., 2021Pereira, P., Bogunovic, I., Zhao, W. & Barcelo, D., 2021. Short-term effect of wildfires and prescribed fires on ecosystem services. Current Opinion in Environmental Science & Health, 22: 100266. https://doi.org/10.1016/j.coesh.2021.100266 ). Land abandonment promoted vegetation encroachment and the increase of biomass in several parts of the world (e.g., Mantero et al., 2020Mantero, G., Morresi, D., Marzano, R., Motta, R., Mladenoff. D.J. & Garbarino, M., 2020. The influence of land abandonment on forest disturbance regimes: a global review. Landscape Ecology, 35: 2723-2744. https://doi.org/10.1007/s10980-020-01147-w ; Khorchani et al., 2021Khorchani, M., Nadal-Romero, E., Lasanta, T. & Tague, C., 2021. Effects of vegetation succession and shrub clearing after land abandonment on the hydrological dynamics in the Central Spanish Pyrenees. Catena, 204: 105374. https://doi.org/10.1016/j.catena.2021.105374.). This rewilding trend increases the ecosystem services supplied by the abandoned areas, such as carbon sequestration, air quality regulation, erosion regulation, flood regulation, water purification and recreation (Pereira et al., 2022Pereira, P., Inacio, M., Kalinauskas, M., Bogdzevič, K., Bogunovic, I. & Zhao, W., 2022. Land-use changes and Ecosystem Services. In: P. Pereira, E. Gomes, J. Rocha (eds), Mapping and forecast land use/cover changes. The Present and Future of Planning. Amsterdam, Netherlands: Elsevier. 1-27 pp. https://doi.org/10.1016/B978-0-323-90947-1.00007-7 ). Nevertheless, this fuel accumulation is also responsible for the increasing wildfire risk (Zazali et al., 2019Zazali, H.H., Towers, I.N. & Sharples, J.J., 2019. A critical review of fuel accumulation models used in Australian fire management. International Journal of Wildland Fire, 30: 42-56. https://doi.org/10.1071/WF20031 ; Davies et al., 2022Davies, K.W., Wollstein, K., Dragt, B. & O’Connor, C., 2022. Grazing management to reduce wildfire risk in invasive annual grass prone sagebrush communities. Rangelands, 44: 194-199. https://doi.org/10.1016/j.rala.2022.02.001 ). Also, the proliferation of highly flammable monoculture plantations (e.g., pines or eucalyptus trees), generally without management, is increasing the vulnerability of these environments drastically to severe wildfires (e.g., Pereira et al., 2018aPereira, P., Francos, M., Brevik, E.C., Ubeda, X. & Bogunovic, I., 2018a. Post-fire soil management. Current Opinion in Environmental Science & Health, 5: 26-32. https://doi.org/10.1016/j.coesh.2018.04.002 ). Urban sprawl and the negligence of people are increasing the risk of wildfire. The sprawl of new houses and people with careless habits (e.g., vegetation and fire management) into wildland areas is increasing the risk and vulnerability of these communities to wildfire effects. With such vulnerability due to human landscape management and behaviour, the occurrence of wildfires during drought periods is expected to increase (Bento-Gonçalves & Vieira, 2020Bento-Gonçalves, A. & Vieira, A., 2020. Wildfires in the wildland-urban interface: Key concepts and evaluation methodologies. Science of The Total Environment, 707: 135592. https://doi.org/10.1016/j.scitotenv.2019.135592 ; Gonzalez-Mathiesen et al., 2021Gonzalez-Mathiesen, C., Ruane, S. & March, A., 2021. Integrating wildfire risk management and spatial planning - A historical review of two Australian planning systems. International Journal of Disaster Risk Reduction, 53: 101984. https://doi.org/10.1016/j.ijdrr.2020.101984 ).
The wildfire’s impact depends on the severity. Usually, low and medium-severity wildfires do not have long-term impacts on the ecosystems, contrary to high-severity wildfires (e.g., Blandon et al., 2014Blandon, K.D., Emelko, M.B., Silins, U. & Stone, M., 2014. Wildfire and the Future of Water Supply. Environmental Science and Technology, 48: 8936-8943. https://doi.org/10.1021/es500130g ; Dove et al., 2020Dove, N.C., Safford, H.D., Bohlman, G.N., Estes, B.L. & Hart, S.C., 2020. High-severity wildfire leads to multi-decadal impacts on soil biogeochemistry in mixed-conifer forests. Ecological Applications, 30: e02072. https://doi.org/10.1002/eap.2072 ). Nevertheless, this dynamic does not depend only on wildfire severity but what happens after. Vegetation recuperation and the hydrological response also depend on soil properties (e.g., vulnerability to erosion, water repellency), fire history (e.g., recurrence), ecosystem affected (e.g., more or less resilient) and post-fire meteorological conditions (e.g., heavy rainfall). Fire severity and the factors mentioned will determine the impacts of wildfire (e.g., Pereira et al., 2018aPereira, P., Francos, M., Brevik, E.C., Ubeda, X. & Bogunovic, I., 2018a. Post-fire soil management. Current Opinion in Environmental Science & Health, 5: 26-32. https://doi.org/10.1016/j.coesh.2018.04.002 ; Etchells et al., 2020Etchells, H., O’Donnell, E.J., Lachlan McCaw, W. & Grierson, P.F., 2020. Fire severity impacts on tree mortality and post-fire recruitment in tall eucalypt forests of southwest Australia. Forest Ecology and Management, 459: 117850. https://doi.org/10.1016/j.foreco.2019.117850 ). In the short and long term, it is well known that wildfires negatively affect several ecosystem services (e.g., carbon sequestration, air quality regulation, erosion regulation, flood regulation, water purification, pollination, food, fibre, cultural heritage, recreation). Flood regulation is one of the most critical ecosystem services affected by wildfires, especially in the immediate period (Pereira et al., 2021Pereira, P., Bogunovic, I., Zhao, W. & Barcelo, D., 2021. Short-term effect of wildfires and prescribed fires on ecosystem services. Current Opinion in Environmental Science & Health, 22: 100266. https://doi.org/10.1016/j.coesh.2021.100266 ). Vegetation removal, root system destruction, bare soil and water repellency increase the risk of floods. This will increase the development of flash floods depending on the amount of precipitation intensity, topography and the capacity of the soil to retain water (e.g., Mueller et al., 2018Mueller, J.M., Lima, R.E., Springer, A.E. & Schiefer, E., 2018. Using Matching Methods to Estimate Impacts of Wildfire and Postwildfire Flooding on House Prices. Water Resources Research, 54: 6189-6201. https://doi.org/10.1029/2017WR022195 ; Ebel & Moody, 2020Ebel, B.A. & Moody, J.A., 2020. Parameter estimation for multiple post-wildfire hydrologic models. Hydrological Processes, 34: 4049-4066. https://doi.org/10.1002/hyp.13865 ). In case of high hydrological response, water bodies quality is also usually affected (e.g., Hohner et al., 2019Hohner, A.K., Rhoades, C.C., Wilkerson, P. & Rosario-Ortiz, F.L., 2019. Wildfires Alter Forest Watersheds and Threaten Drinking Water Quality. Accounts of Chemical Research, 52: 1234-1244. https://doi.org/10.1021/acs.accounts.8b00670 ; Emerton et al., 2020Emerton, C.A., Cooke, C.A., Hustins, S., Silins, U., Emelko, M., Lewis, T., Kruk, M.K., Taube, N., Zhu, D., Jackson, B., Stone, M., Kerr, J.G. & Orwin, J.F., 2020. Severe western Canadian wildfire affects water quality even at large basin scales. Water Research, 183: 116071. https://doi.org/10.1016/j.watres.2020.116071 ). Although there are previous works focused on wildfire impacts on ecosystem services (e.g., Sil et al., 2019Sil, A., Fernandes, P.M., Rodrigues, A.P., Alonso, J.M., Honrado, J.P., Pereira, A. & Azevedo, J.C., 2019. Farmland abandonment decreases the fire regulation capacity and the fire protection ecosystem service in mountain landscapes. Ecosystem Services, 36: 100908. https://doi.org/10.1016/j.ecoser.2019.100908 ; Robinne et al., 2020Robinne, F.N., Hallema, D.W., Bladon, K.D. & Buttle, J.M., 2020. Wildfire impacts on hydrologic ecosystem services in North American high-latitude forests: A scoping review. Journal of Hydrology, 581: 124360. https://doi.org/10.1016/j.jhydrol.2019.124360 ; Pereira et al. 2021Pereira, P., Bogunovic, I., Zhao, W. & Barcelo, D., 2021. Short-term effect of wildfires and prescribed fires on ecosystem services. Current Opinion in Environmental Science & Health, 22: 100266. https://doi.org/10.1016/j.coesh.2021.100266 ; Taboada et al., 2021Taboada, A., Garcia-Llamas, P., Fernández-Guisuraga, J.M. & Calvo, L., 2021. Wildfires impact on ecosystem service delivery in fire-prone maritime pine-dominated forests. Ecosystem Services, 50: 101334. https://doi.org/10.1016/j.ecoser.2021.101334 ), a synthesis focused on the effects on flood regulation and water quality is lacking. The objective of this perspective paper is to make a synthesis of the effects of wildfires on flood regulation and water purification and propose several restoration strategies.
2. Flood regulation
⌅In the immediate period after a wildfire, vegetation removal (e.g., soil cover, interception) reduces soil protection against rainfall kinetic energy, increasing sediment detachment, erosion and overland flow (Shakesby, 2011Shakesby, R.A., 2011. Post-wildfire soil erosion in the Mediterranean: Review and future research directions. Earth-Science Reviews, 105: 71-100. https://doi.org/10.1016/j.earscirev.2011.01.001 ; Cole et al., 2020Cole, R.P., Blandon, K.D., Wagenbrenner, J.W. & Coe, D.R., 2020. Hillslope sediment production after wildfire and post-fire forest management in northern California. Hydrological Processes, 34: 5242-5259. https://doi.org/10.1002/hyp.13932 ). Also, the high temperatures on the soil surface can create a hydrophobic layer and reduce water infiltration (e.g., Doerr et al., 2006Doerr, S.H., Shakesby, R.A., Blake, W.H., Chafer, C.J., Humphreys, G.S. & Wallbrink, P.J., 2006. Effects of Differing Wildfire Severities on Soil Wettability and Implications for Hydrological Response. Journal of Hydrology, 319: 295-311. https://doi.org/10.1016/j.jhydrol.2005.06.038 ). Ash may reduce or increase water retention, depending on the temperature of combustion. Black ash can be highly repellent and increase the hydrological response. On the other hand, white ash has a higher capacity to retain water during the first rainfalls. Nevertheless, white ash can produce an impermeable layer after drying due to carbonate particle crystallization. This also depends on the type of forest affected and the species composition. Some are more flammable than others, affecting ash properties (Pereira et al., 2018bPereira, P., Brevik, E.C., Bogunovic, I. & Estebaranz, F., 2018b. Ash and soils. A close relationship in fire affected areas. In: P. Pereira, J. Mataix-Solera, X. Ubeda, G. Rein & A. Cerda (eds), Fire impacts on soils. State of the art and methods used. Sydney, Australia: CSIRO. 39-67 pp.). Overall, the flood regulation capacity is reduced immediately after a fire and is especially damaged in areas affected by high-severity wildfires (Brogan et al., 2019Brogan, D.J., Nelson, P.A. & MacDonald, L.H., 2019. Spatial and temporal patterns of sediment storage and erosion following a wildfire and extreme flood. Earth Surface Dynamics, 7: 563-590. https://doi.org/10.5194/esurf-7-563-2019 ; Alexandra & Finlayson, 2020Alexandra, J. & Finlayson, M., 2020. Floods after bushfires: rapid responses for reducing impacts of sediment, ash, and nutrient slugs. Australasian Journal of Water Resources, 24: 9-11. https://doi.org/10.1080/13241583.2020.1717694 ) (Figure 1) or hyper-dry conditions (Moody & Ebel, 2012Moody, J.A. & Ebel, B.A., 2012. Hyper-dry conditions provide new insights into the cause of extreme floods after wildfire. Catena, 93: 58-63. https://doi.org/10.1016/j.catena.2012.01.006 ). In these circumstances, if a high rainfall intensity occurs in sloped areas, flash floods are highly likely (e.g., Liu et al., 2022Liu, T., McGuire, L.A., Oakley, N. & Cannon, F., 2022. Temporal changes in rainfall intensity-duration thresholds for post-wildfire flash floods in southern California. Natural Hazards System Sciences Journal, 22: 361-376. https://doi.org/10.5194/nhess-22-361-2022 ). Climate change will increase the probability of flash floods in wildfire-affected areas, as observed elsewhere (e.g., Touma et al., 2022Touma, D., Stevenson, S., Swain, D.L., Singh, D., Kalashnikov, D. & Huang, X., 2022. Climate change increases risk of extreme rainfall following wildfire in the western United States. Science Advances, 8: eabm0320. https://doi.org/10.1126/sciadv.abm0320 ). Previous works identified the occurrence of flash floods immediately after wildfires in Australia (Nyman et al., 2011Nyman, P., Sheridan, G.J., Smith, H., Lane, P.J.N., 2011. Evidence of debris flow occurrence after wildfire in upland catchments of south-east Australia. Geomorphology, 125: 383-401. https://doi.org/10.1016/j.geomorph.2010.10.016 ), the United States (Vieira et al., 2004Vieira, N.K.M., Clements, W.H., Guevara, L.S. & Jacobs, B.F., 2004. Resistance and resilience of stream insect communities to repeated hydrologic disturbances after a wildfire. Freshwater Biology, 49: 1243-1259. https://doi.org/10.1111/j.1365-2427.2004.01261.x ), Italy (Coscarelli et al., 2021Coscarelli, R., Aguilar, E., Petrucci, O., Vicente-Serrano, S.M. & Zimbo, F., 2021. The Potential Role of Climate Indices to Explain Floods, Mass-Movement Events and Wildfires in Southern Italy. Climate, 9: 156. https://doi.org/10.3390/cli9110156 ), Greece (Filis et al., 2020Filis, C., Spyrou, N. I., Diakakis, M., Kotroni, V., Lagouvardos, K., Papagiannaki, K. & Lekkas, E., 2020. Post-wildfire flash flooding in small mountainous catchments: post-fire effects and characteristics of the November 2019 flash flood in Kineta, Greece. EGU General Assembly 2020, Vienna. https://doi.org/10.5194/egusphere-egu2020-5501 ), Spain (Ortega-Becerril et al., 2022Ortega-Becerril, J.A., Garrote, J., Vicente, Á. & Marqués, M.J., 2022. Wildfire-Induced Changes in Flood Risk in Recreational Canyoning Areas: Lessons from the 2017 Jerte Canyons Disaster. Water, 14: 2345. https://doi.org/10.3390/w14152345 ) or Uganda (Jacobs et al., 2016Jacobs, L., Maes, J., Mertens, K., Sekajugo, J., Thiery, W., van Lipzig, N., Poesen, J., Kervyn, M. & Dewitte, O., 2016. Reconstruction of a flash flood event through a multi-hazard approach: focus on the Rwenzori Mountains, Uganda. Natural Hazards, 84: 851-876. https://doi.org/10.1007/s11069-016-2458-y ). Therefore, it is a recurrent problem in several areas of the world. The extension of wildfire impacts on flood regulation depends on the fire severity (Figure 1). In areas where wildfire severity is low, the impact is reduced, while when the wildfire severity is high, the effect on flood regulation capacity is larger (e.g., Dahm et al., 2015Dahm, C.N., Candelaria-Ley, R.I., Reale, C.S., Reale, J.K. & Van Horn, D.J., 2015. Extreme water quality degradation following a catastrophic forest fire. Freshwater Biology, 60: 2584-2599. https://doi.org/10.1111/fwb.12548 ). The timing of precipitation is key to producing floods after wildfires. For instance, in Mediterranean areas, fire season can last until August/September (e.g., Francos et al., 2016Francos, M., Pereira, P., Alcañiz, M., Mataix-Solera, J. & Úbeda, X., 2016. Impact of an intense rainfall event on soil properties following a wildfire in a Mediterranean environment (North-East Spain). Science of the Total Environment, 572: 1353-1362. https://doi.org/10.1016/j.scitotenv.2016.01.145 ; Turco et al., 2017Turco, M., von Hardenberg, J., AghaKouchak, A., Llasat, M.C., Provenzale, A. & Trigo, R.M., 2017. On the key role of droughts in the dynamics of summer fires in Mediterranean Europe. Scientific Reports, 7: 81. https://doi.org/10.1038/s41598-017-00116-9 ). In the last years, even in October (e.g., Portugal), wildfires with high severity occurred (Figueiredo et al., 2021Figueiredo, R., Pauperio, E. & Romao, X., 2021. Understanding the Impacts of the October 2017 Portugal Wildfires on Cultural Heritage. Heritage, 4: 2580-2598. https://doi.org/10.3390/heritage4040146 ), likely due to climate change. During summer and Autumn (October and November), isolated thunderstorms and intense rainfall periods are frequent (e.g., Roye et al., 2018Roye, D., Lorenzo, N. & Martin-Vide, J., 2018. Spatial-temporal patterns of cloud-to-ground lightning over the northwest Iberian Peninsula during the period 2010-2015. Natural Hazards, 92: 857-884. https://doi.org/10.1007/s11069-018-3228-9 ; Henin et al., 2021Henin, R., Ramos, A.M., Pinto, J.G. & Liberato, M.L., 2021. A ranking of concurrent precipitation and wind events for the Iberian Peninsula. International Journal of Climatology, 41: 1421-1437. https://doi.org/10.1002/joc.6829 ). If intense rainfall periods occur in recently burned areas, the probability of flash floods increases substantially. The increased fire season length and extreme rainfall due to climate change may create “the perfect storm” for flash flood development in wildfire-affected areas.
The burned area flood regulation capacity increases with increasing vegetation coverage (Pereira et al., 2021Pereira, P., Bogunovic, I., Zhao, W. & Barcelo, D., 2021. Short-term effect of wildfires and prescribed fires on ecosystem services. Current Opinion in Environmental Science & Health, 22: 100266. https://doi.org/10.1016/j.coesh.2021.100266 ). Several biomes (e.g., Mediterranean) are adapted to fire disturbance (e.g., Santana et al., 2018Santana, V.M., Baeza, M.J., Valdecantos, A. & Vallejo, V.R., 2018. Redirecting fire-prone Mediterranean ecosystems toward more resilient and less flammable communities. Journal of Environmental Management, 215: 108-115. https://doi.org/10.1016/j.jenvman.2018.03.063 ). Nevertheless, wildfire recurrence has detrimental impacts on soil properties (e.g., Pereira et al., 2018aPereira, P., Francos, M., Brevik, E.C., Ubeda, X. & Bogunovic, I., 2018a. Post-fire soil management. Current Opinion in Environmental Science & Health, 5: 26-32. https://doi.org/10.1016/j.coesh.2018.04.002 ) and delays vegetation recuperation (e.g., Moghli et al., 2022Moghli, A., Santana, V., Baeza, M.J., Pastor, E. & Soliveres, S., 2022. Fire Recurrence and Time Since Last Fire Interact to Determine the Supply of Multiple Ecosystem Services by Mediterranean Forests. Ecosystems, 25: 1358-1370. https://doi.org/10.1007/s10021-021-00720-x ). This may severely affect the restoration of burned areas’ flood regulation capacity, especially in a climate change context where short-term wildfire recurrence is expected to increase (Elia et al., 2020Elia, M., Giannico, V., Spano, G., Lafortezza, R. & Sanesi, G., 2020. Likelihood and frequency of recurrent fire ignitions in highly urbanized Mediterranean landscapes. International Journal of Wildland Fire, 29: 120-131. https://doi.org/10.1071/WF19070 ).
3. Water purification
⌅Vegetation removal decrease the ecosystem capacity to purify water (Figure 1). Also, wildfire ash has many potentially toxic elements, such as metals and metalloids (Murphy et al., 2020Murphy, S.F., McCleskey, R.B., Martin, D.A., Holloway, J.M. & Writer, J.H., 2020. Wildfire-driven changes in hydrology mobilize arsenic and metals from legacy mine waste. Science of the Total Environment, 743: 140635. https://doi.org/10.1016/j.scitotenv.2020.140635 ) or Polycyclic Aromatic Hydrocarbons (PAHs) (Silva et al., 2015Silva, V., Pereira, J.L., Campos, I., Keizer, J.J., Gonçalves, F. & Abrantes, N., 2015. Toxicity assessment of aqueous extracts of ash from forest fires. Catena, 135: 401-408. https://doi.org/10.1016/j.catena.2014.06.021 ). Previous works also observed that firefight products released toxic elements into the environment (e.g., fluorosurfactants or perfluorosurfactants) (Peshoria et al., 2020Peshoria, S., Nandini, D., Tanwar, R.K. & Narang, R., 2020. Short-chain and long-chain fluorosurfactants in firefighting foam: a review. Environmental Chemistry Letters, 18: 1277-1300. https://doi.org/10.1007/s10311-020-01015-8 ). After the first rainfalls, these elements can be incorporated into the soil matrix and pollute groundwater (Fernandez-Marcos, 2022Fernandez-Marcos, M.L., 2022. Potentially Toxic Substances and Associated Risks in Soils Affected by Wildfires: A Review. Toxics, 10: 31. https://doi.org/10.3390/toxics10010031 ) or transported in overland flow and polluting rivers (Meneses et al., 2019Meneses, B.M., Reis, E., Reis, R. & Vale, M.J., 2019. Post-wildfires effects on physicochemical properties of surface water: the case study of Zêzere watershed (Portugal). Ribagua, 6: 34-48. https://doi.org/10.1080/23863781.2019.1596771 ), lakes (Pelletier et al., 2022Pelletier, N., Chetelat, J., Sinon, S. & Vermaire, J.C., 2022. Wildfires trigger multi-decadal increases in sedimentation rate and metal loading to subarctic montane lakes. Science of the Total Environment, 824: 153738. https://doi.org/10.1016/j.scitotenv.2022.153738 ) and estuaries (Barros et al., 2022Barros, T.L., Bracewell, S.A., Mayer-Pinto, M., Dafforn, K.A., Simpson, S.L., Farrell, M. & Johnston, E.L., 2022. Wildfires cause rapid changes to estuarine benthic habitat. Environmental Pollution, 308: 119571. https://doi.org/10.1016/j.envpol.2022.119571 ) (Figure 1). Numerous studies found negative impacts of toxic materials from burned areas on fauna (e.g., Gomez-Isaza et al., 2022Gomez-Isaza, D.F., Cramp, R.L. & Franklin, C.E., 2022. Fire and rain: A systematic review of the impacts of wildfire and associated runoff on aquatic fauna. Global Change Biology, 28: 2578-2595. https://doi.org/10.1111/gcb.16088 ) and flora (e.g., Thompson et al., 2019Thompson, V.F., Marshall, D.L., Reale, J.K. & Dahm, C.N., 2019. The effects of a catastrophic forest fire on the biomass of submerged stream macrophytes. Aquatic Botany, 152: 36-42. https://doi.org/10.1016/j.aquabot.2018.09.001 ). This increase in pollution affects drinkable water (e.g., Robinne et al., 2020Robinne, F.N., Hallema, D.W., Bladon, K.D. & Buttle, J.M., 2020. Wildfire impacts on hydrologic ecosystem services in North American high-latitude forests: A scoping review. Journal of Hydrology, 581: 124360. https://doi.org/10.1016/j.jhydrol.2019.124360 ), affecting the supply to large cities. Examples of this were found in urban areas located in California (Proctor et al., 2020Proctor, C.R., Lee, J., Yu, D., Shah, A.D. & Whelton, A.J., 2020. Wildfire caused widespread drinking water distribution network contamination. AWWA Water Science, 2: e1183. https://doi.org/10.1002/aws2.1183 ), Colorado (Hohner et al., 2019Hohner, A.K., Rhoades, C.C., Wilkerson, P. & Rosario-Ortiz, F.L., 2019. Wildfires Alter Forest Watersheds and Threaten Drinking Water Quality. Accounts of Chemical Research, 52: 1234-1244. https://doi.org/10.1021/acs.accounts.8b00670 ) or Canada (Emelko et al., 2016Emelko, M.B., Stone, M., Silins, D., Allin, D., Collins, A.L., Williams, C.S.H., Martens, A.M. & Bladon, K.D., 2016. Sediment-phosphorus dynamics can shift aquatic ecology and cause downstream legacy effects after wildfire in large river systems. Global Change Biology, 22: 1168-1184. https://doi.org/10.1111/gcb.13073 ). Generally, after a wildfire, there has been observed an increase in freshwater turbidity (Rust et al., 2019Rust, A.J., Randell, J., Todd, A.S. & Hogue, T.S., 2019. Wildfire impacts on water quality, macroinvertebrate, and trout populations in the Upper Rio Grande. Forest Ecology and Management, 453: 117636. https://doi.org/10.1016/j.foreco.2019.117636 ), major cations and ions, and metallic elements or PAHs (Mansilha et al., 2019Mansilha, C., Duarte, C.G., Melo, A., Ribeiro, J., Flores, D. & Espinha Marques, J., 2019. Impact of wildfire on water quality in Caramulo Mountain ridge (Central Portugal). Sustainable Water Resources Management, 5: 319-331. https://doi.org/10.1007/s40899-017-0171-y ). The factors that affect post-fire water purification are similar to the ones mentioned in flood regulation. Previous works observed that these impacts could be medium (5 years after) (Rust et al., 2018Rust, A.J., Hogue, T.S., Saxe, S. & McGray, J., 2018. Post-fire water-quality response in the western United States. International Journal of Wildland Fire, 27: 203-216. https://doi.org/10.1071/WF17115 ) or long-term (10 years after) (Yu et al., 2018).
4. Restoration measures
⌅Post-wildfire restoration needs to be considered carefully because several innervations can be more damaging to the soil than the wildfire itself. They are usually appropriate when the wildfire has high severity and there are goods to protect from floods or landslides in downstream areas (Pereira et al., 2018aPereira, P., Francos, M., Brevik, E.C., Ubeda, X. & Bogunovic, I., 2018a. Post-fire soil management. Current Opinion in Environmental Science & Health, 5: 26-32. https://doi.org/10.1016/j.coesh.2018.04.002 ). Several works identified that several post-fire managing strategies (e.g., Savage logging) induce a very high soil degradation and erosion (e.g., Francos et al., 2019Francos, M., Ubeda, X. & Pereira, P., 2019. Impact of torrential rainfall and salvage logging on post-wildfire soil properties in NE Iberian Peninsula. Catena, 177: 210-218. https://doi.org/10.1016/j.catena.2019.02.014 ; Fernandez et al., 2021Fernandez, C., Fonturbel, T. & Vega, J.A., 2021. Cumulative effects of salvage logging and slash removal on erosion, soil functioning indicators and vegetation in a severely burned area in NW Spain. Geoderma, 393: 115004. https://doi.org/10.1016/j.geoderma.2021.115004 ). If some intervention measure is planned, the best option is to do when the vegetation covers the soil. This will minimize the degradation (Pereira et al., 2018aPereira, P., Francos, M., Brevik, E.C., Ubeda, X. & Bogunovic, I., 2018a. Post-fire soil management. Current Opinion in Environmental Science & Health, 5: 26-32. https://doi.org/10.1016/j.coesh.2018.04.002 ).
When the wildfire has a low or medium severity, the best approach is not to intervene and leave the vegetation to recover naturally. In several ecosystems (e.g., the Mediterranean), vegetation can recover fast after a wildfire. However, in areas where wildfires are more recurrent, the vegetation capacity to recover is low (e.g., Smith-Ramírez et al., 2021Smith-Ramirez, C., Castillo-Mandujano, J., Becerra, P., Sandoval, N., Allende, R. & Fuentes, R., 2021. Recovery of Chilean Mediterranean vegetation after different frequencies of fires. Forest Ecology and Management, 485: 118922. https://doi.org/10.1016/j.foreco.2021.118922 ). In this case, some intervention may be needed. Nevertheless, it is crucial to analyze case by case. Several strategies are applied to restore fire-affected areas, such as mulching (e.g., hydromulching, straw, wood chips), site preparation (e.g., tree plantation, ripping and post-fire mounding), channel treatments, erosion barriers, seeding or salvage logging. Mulching and seeding techniques can serve as a nature-based solution to improve the soil condition in the immediate period post-wildfire. Erosion barriers reduce soil erosion Therefore, if some restoration practices are needed, these practices are the best for soil restoration (Pereira et al., 2018aPereira, P., Francos, M., Brevik, E.C., Ubeda, X. & Bogunovic, I., 2018a. Post-fire soil management. Current Opinion in Environmental Science & Health, 5: 26-32. https://doi.org/10.1016/j.coesh.2018.04.002 ) (Figure 1). Overall, they reduce the impact of wildfire on flood regulation and water purification since they reduce sediment transport and water transport that can trigger flash floods or water quality degradation in rivers and lakes.
Although restoration measures need to be applied in areas affected by high-severity fires, it is important to rethink their application in the future. The use of nature-based solutions cannot be discarded. The current land abandonment trend (responsible for the biomass increase) (e.g., Piccinelli et al., 2020Piccinelli, S., Brusa, G. & Cannone, N., 2020. Climate warming accelerates forest encroachment triggered by land use change: A case study in the Italian Prealps (Triangolo Lariano, Italy). Catena, 195: 104870. https://doi.org/10.1016/j.catena.2020.104870 ) and the increase in droughts, frequency, length and severity (Chiang et al., 2021Chiang, F., Mazdiyasni, O. & Agha-Kouchak, A., 2021. Evidence of anthropogenic impacts on global drought frequency, duration, and intensity. Nature Communications, 12: 2754. https://doi.org/10.1038/s41467-021-22314-w ) will affect the wildfire regime extending the wildfire season and the vulnerability of the ecosystem. In this context, previous works showed that it is expected an increase in wildfire severity and recurrence (Halofsky et al., 2020Halofsky, J.E., Peterson, D.L. & Harvey, B.J., 2020. Changing wildfire, changing forests: the effects of climate change on fire regimes and vegetation in the Pacific Northwest, USA. Fire Ecology, 16: 1-26. https://doi.org/10.1186/s42408-019-0062-8 ; Moghli et al., 2022Moghli, A., Santana, V., Baeza, M.J., Pastor, E. & Soliveres, S., 2022. Fire Recurrence and Time Since Last Fire Interact to Determine the Supply of Multiple Ecosystem Services by Mediterranean Forests. Ecosystems, 25: 1358-1370. https://doi.org/10.1007/s10021-021-00720-x ). In this case, ecosystem restoration to mitigate the impacts of wildfire on flood regulation and water purification may be more needed since they may be more fragile and the post-wildfire recuperation more problematic. This is also essential to reduce the impacts on water availability. According to previous works, extreme wildfire risks may threaten the water supply (Robinne et al., 2021Robinne, F.N., Hallema, D.W., Bladon, K.D., Flannigan, M.D., Boisramé, G., Bréthaut, C.M., Doerr, S., Di Baldassarre, G., Gallagher, L.A., Hohner, A.K., Khan, S.J., Kinoshita, A.M., Mordecai, R., Nunes, J.P., Nyman, P., Santín, S., Sheridan, G., Stoof, C.R., Thompson, M.P., Waddington, J.M., & Wei, Y., 2021. Scientists’ warning on extreme wildfire risks to water supply. Hydrological processes, 35: e14086. https://doi.org/10.1002/hyp.14086 ).
5. Conclusions
⌅Although wildfires are a natural element of ecosystems, they may cause some negative temporal impacts, especially if they have a high severity. Wildfires decrease the ecosystem’s capacity to regulate floods and water purification in the immediate period after. Normally until the vegetation recover. The risk may increase if the wildfire reaches a high severity and occur intense precipitations. The combination of these factors may trigger the development of flash floods and water quality degradation. The risk of floods and water quality degradation depends on the capacity of the ecosystems to recover and the post-wildfire meteorological conditions. In some cases, the impacts of wildfires can be long-term. The burned ecosystems restoration may be needed if wildfires reach high severity, are recurrent, or there is a risk of flooding or landslides. Normally ecosystem restoration in low and medium-severity wildfires is not needed. However, in a global change context (e.g., land abandonment and climate change), wildfire recurrence and severity are increasing. In this context, restoration measures may be more needed to mitigate the impacts of wildfires on flood regulation and water purification