Introduction to biogeomorphic responses to wildfire in fluvial ecosystems

Wildfire biogeomorphology comprises an integration of biological and geomorphic disciplines, which are fundamental to understanding fluvial system processes and their interactions with ecology in the context of wildfire (Fig. 1). Over geologic time, wildfire as an Earth-system process has occurred coincident with the appearance and development of biota and functional heterogeneous ecosystems (Archibald et al., 2018; Bowman et al., 2009; Krawchuk et al., 2009; Pausas and Keeley, 2009; Pausas and Ribeiro, 2017; McLauchlan et al., 2020). Recent research suggests that the spatial and temporal heterogeneity of wildfire patterns and their effects across a landscape—or pyrodiversity—are central in ecological contexts such as biodiversity, food webs, and feedbacks between ecological processes and fire regimes (Bowman et al., 2016; Kelly et al., 2020; Jones and Tingley, 2022). Wildfire also plays a beneficial role in terrestrial vertebrate richness (Moritz et al., 2023). Therefore, recognizing the attributes of post-wildfire geomorphic processes with respect to biological processes and functions, and looking beyond their significance as geologic hazards, is essential for a more complete understanding of the range of wildfire effects, and to better anticipate and manage them.

Wildfire biogeomorphology responds to human activities and anthropogenic change (Fig. 1). Anthropogenic activity has profoundly altered wildfire characteristics on Earth’s landscapes. Over the past millennia, for example, humans have alternately increased or decreased the incidence of wildfire. Many fire-prone ecosystems adapted to the practices of Indigenous people who intentionally burned landscapes to enhance resources (Trauernicht et al., 2015). Over this period, natural and human-managed wildfires enhanced biodiversity and ecosystem function (Bowman et al., 2009; Archibald et al., 2018). In contrast, landscape alteration (Andela et al., 2017) and human suppression of wildfire in recent centuries contributed to the buildup of fuels and the incidence of larger, higher severity wildfires, which can increase recovery time and reduce ecosystem resiliency following a wildfire (Kelly et al., 2020). Ignitions in rapidly expanding wildland-urban interface zones have further increased wildfire frequency (Radeloff et al., 2018).

At the same time that anthropogenic activities have modified fire regimes, human-induced climate change has also modified wildfire characteristics (Westerling et al., 2006; Fig. 1). Climate change since ~1980 has increased the frequency and magnitude of fire weather favorable for the ignition and spread of wildfire globally, such that many landscapes burn more frequently (Jones et al., 2022). Global warming and modified fire regimes have altered geomorphic processes and their interactions with ecosystems. Continuing changes in hillslope stability, sediment yields, and fluvial morphology are possible outcomes if more extreme rainfall events occur following wildfires in the future (East and Sankey, 2020). Human alterations to global climate, land uses, vegetation composition and structure, and fire suppression are transforming wildfire characteristics and patterns that influence ecosystem biodiversity (He et al., 2019; Kelly et al., 2020).

In this book, we approach the interactions between wildfire characteristics and geomorphic and biological processes from the point of view of Earth system scientists with intent to highlight and expand interdisciplinary studies of wildfire biogeomorphology. Biogeomorphic responses to wildfire may differ greatly because of variations in interrelated characteristics of fluvial systems (climate, basin area, topography, geology, vegetation, soils, aquatic organisms, anthropogenic activities) and wildfire characteristics (extent, severity, intensity, frequency, duration, timing, patchiness, rate of spread), as affected by thresholds, feedbacks, and the temporal and spatial scales considered (Florsheim and Chin, 2022). The numerous permutations of how these factors converge lead to different post-wildfire biogeomorphic responses among fluvial systems in different regions, and even within the same fluvial system. Nonetheless, exploring possible biogeomorphic interactions can advance our understanding of disturbance and recovery following wildfires.

Wildfire profoundly alters hydrologic and geomorphic processes and modifies physical habitat in fluvial ecosystems. As wildfires become more frequent, larger, and more severe as a result of the warming climate and prior fire suppression policies, burns may become more destructive to functional habitats within fluvial ecosystems (Kelly et al., 2020). Therefore, in addition to increased hazards to human life and property, future wildfires pose risks to ecosystems. Whereas the effect of loss of vegetation on post-wildfire hydrology and sediment supply is better understood (Shakesby and Doerr, 2006; Moody et al., 2013, Florsheim and Chin, 2022), less attention has been directed toward addressing the effects of these changes to physical habitats for biota on hillslopes, channels, and floodplains—or toward the responses of biota on these landscapes. Similarly, little attention has been given to understanding the interrelations or feedbacks between post-wildfire changes to erosion and sedimentation dynamics, and alterations to instream and riparian systems influenced by climate and anthropogenic changes. To address this gap, we present this GSA Special Paper focusing on biogeomorphic responses to wildfire in fluvial systems as a step toward advancing interdisciplinary wildfire science.

The scope of this GSA Special Paper encompasses quantitative field investigations, analyses using remote sensing, and synthetic reviews to explore linkages among flows, sediment dynamics, morphology and habitat, and biota. The goal is to provide a new perspective relevant to interdisciplinary science practiced internationally. Individual chapters report recent research and provide reviews addressing post-wildfire changes to fluvial morphodynamics, interactions between large wood contributed by wildfire and sediment transport processes in riparian zones, survival of biota such as beavers in refugia enhanced by various geomorphic processes, and interactions among post-wildfire sediment dynamics, morphology, and instream biota such as steelhead trout and aquatic invertebrates. The chapters in this book advance our understanding of biogeomorphic responses following wildfire, as described in the following chapter summaries.

McMahon et al. investigated the effects of vegetation recovery on stream responses in the first decade following six wildfires in the southern California Transverse Ranges. Using remote sensing, they found that hydrology, drought, and fire severity affected vegetation recovery in upland and riparian zones. These landscape-scale basin processes increased sediment deposition and reduced riffle-pool habitat in the first few years following the fires. Loss of riparian tree canopy from wildfire increased light levels to the water surface and led to increases in algal cover and densities of algivorous invertebrates and amphibians. Particulate organic matter and leaf-shredder invertebrates also decreased following loss of riparian canopy. Riparian vegetation recovery and biogeomorphic changes such as associated amphibian densities, for example, still responded to fire patterns a decade after the fires.

Analyzing data from field investigations and remote sensing imagery, Chin et al. illustrated changes in channel morphology, grain size, and the reflectance of vegetation ten years after the 2012 Waldo Canyon Fire, Colorado. As vegetation cover increased during the first decade following the fire, decreasing post-wildfire sediment supply and runoff from hillslopes, fluvial processes began to simultaneously reestablish step-pool bedform morphology and coarsen bed material. Fallen dead trees and cut logs from post-wildfire management activities introduced large wood to the study stream channels after the initial postfire years. The formation of frequent log jams and movement of this large wood introduced transient erosion and deposition and greater variability in instream channel morphology and grain size distributions. The spatial and temporal variability in erosion and deposition caused by instream log jams can play an important role in the long-term adjustment and ultimate recovery of the fluvial system after wildfire.

Weirich et al. reviewed factors that influence the behavior of fire on hillslopes, focusing on the effect of soil hydrophobicity on hydrologic processes. A hydrophobicity layer or patch is formed when organic material within soil vaporizes and condenses a waxy substance on soil grains during a wildfire, resulting in a water repellant layer. This hydrophobic layer minimizes rainfall infiltration and enhances post-wildfire runoff. In turn, hydrophobicity leads to accelerated erosion of soil exposed by fire, often contributing to a near-term sediment pulse disturbance. New methods in identifying the formation of hydrophobic soils on hillslopes may help in understanding hydrologic, biologic, and geomorphic responses that affect post-wildfire recovery rates and patterns.

Florsheim and Chin compared results of geomorphic field data collected after storms following two wildfires three decades apart in the same steep fluvial system in the southern California Transverse Ranges. These post-wildfire data were also compared to data collected before and after a storm that occurred after recovery from the first fire. Step-pool morphology was present in the study reach before both fires. Sedimentation after both wildfires profoundly altered streambed elevation, step height and spacing, pool depth and spacing, grain size distribution, and transport capacity—the physical elements that characterize or form habitat-supporting endemic biota. During the first storm following both fires, substantial channel sedimentation raised bed elevation and buried step-pool morphology and substantially decreased grain size. During subsequent storms, incision of the channel fill toward its pre-fire elevation initiated recovery. Temporal differences in trajectories for disturbance and recovery of physical habitat in steep streams following wildfire and subsequent storm flows depend on climate variability inherent in the semi-arid Mediterranean-type region.

O’Dowd et al. examined changes in channel morphology with benthic macroinvertebrate recovery in the decade following the 2012 Waldo Canyon Fire. Their statistical analyses quantified relationships showing that wildfire severity is a significant factor in timeline and extent of recovery. Invertebrate metrics in moderate/high-severity burned catchments reflected degraded conditions following wildfire, but these communities recovered within two years post-fire. In contrast, catchments burned at low severity and unburned sites did not exhibit degraded conditions post-fire and contained similar invertebrate metrics throughout the study. The relatively high ecological quality in unburned and low-severity burned reaches compared to moderate/high-severity burned reaches suggests that management strategies that reduce wildfire severity can protect ecosystems from short-term negative impacts of wildfire.

Capelli reviewed the influence of wildfire on recovery strategies for the endangered southern California steelhead. The historical range of steelhead, an anadromous fish, extended more than 400 km north of the U.S.-Mexico border including streams originating in the southern California Transverse and Peninsular Ranges. This fish is thus adapted to the dynamic southern California landscapes that experience episodic wildfires, droughts, floods, and erosion and sedimentation disturbances. Wildfire, in particular, is a disturbance that may enhance essential physical components of habitat required by steelhead. Post-wildfire geomorphic processes, for example, may add spawning gravels or large boulders associated with deep pools where steelhead can survive California’s long dry season. Future recovery and persistence of steelhead requires strategies of population redundancy and spatial separation as human activity continues to alter landscapes, climate changes, and associated geomorphic processes locally alter steelhead habitat.

In Fairfax et al., investigations using remote sensing analyses identified the role of beavers in buffering the effect of three megafires in the Rocky Mountain region of the United States. Landscapes modified by beaver dams, for example, reduced the severity of the Cameron Peak and East Troublesome Fires in Colorado and the Mullen Fire in Wyoming. Beaver dams enhanced “riverscapes” by promoting wetness in floodplains, wetlands, meadows, side channels, and the hyporheic zone. Thus, by offering refugia during wildfires, beaver dams may be important process-based strategies for restoration of functional river landscapes.

Wohl et al. reviewed the beneficial influence of large wood and beaver dams on spatial heterogeneity, an attribute that improves the resistance and resilience of various components of a river network to geomorphic disturbances caused by wildfire. In contrast, sensitive systems can undergo persistent change. Spatial scale and the specific process or landform under consideration strongly influence whether a system is resilient resistant, or sensitive. Because of the potential for biogeomorphic feedbacks to either attenuate or exacerbate postfire inputs to rivers, conservation of reach-scale spatial heterogeneity and the biota such as beavers that enhance biogeomorphic feedbacks can increase river network resilience to wildfire.

Wildfire biogeomorphology is an emerging integrative science fundamental to understanding post-wildfire processes in fluvial ecosystems. After wildfires, fluvial ecosystems may exhibit a range of spatial changes within a single burned basin, within the extent of the entire burned area consisting of numerous basins, or downstream of the burned headwater areas. Temporally, these changes may be transformative over weeks, months, years, decades, centuries or longer, or transitory over a single storm season. The chapters within this book illustrate how geomorphic processes alter landforms and microhabitats that may act as a sediment disturbance to biota and ecosystem functions.

This volume addresses several key points. First, geomorphic processes markedly affect biological responses beyond direct impacts of fire to vegetation and fauna. Namely, wildfire changes soil hydrology and hillslope erosion processes, leading to sediment pulses that change the physical elements of fluvial habitat by transforming bedforms and channel patterns. In particular, post-wildfire sedimentation fills pools and covers coarse-grained clasts in channels that provide diverse habitat essential for fish, herpetofauna, and invertebrates. Such sedimentation episodes may be transitory or long-lasting, depending on variations in post-wildfire climate conditions.

Second, biological processes also alter geomorphic responses. Examples include the influence of instream large wood and beavers on postfire adjustments and recovery. Burned trees, for example, may fall into channels introducing large wood that enhances morphologic heterogeneity. Beavers modify riverscapes in ways that enhance resilience to wildfire by increasing moisture in riparian zones. As fluvial systems recover from post-wildfire sediment disturbances, feedback between post-wildfire geomorphic processes that affect biological processes, and biological processes that affect geomorphic processes, increase biodiversity and sustainability. Together, dynamic post-wildfire biogeomorphic processes and interactions have led to resilient and functional ecosystems in fire-prone areas over the past millennium.

Findings from the authors suggest that strategies that advance interdisciplinary wildfire science and promote resilience to dynamic geomorphic processes could enhance effective management of wildfire responses. Such strategies are consistent with new paradigms in wildfire management, such as adaptive resilience, intended to reduce societal vulnerability as future fire regimes and ecosystems change (Schoennagel et al., 2017). They are especially important in the face of climate changes as human development increases in fire-prone areas (Moritz and Knowles, 2016).

The chapters in this book advance understanding of wildfire biogeomorphology needed to accommodate future climate variability and change for sustainable fire-prone fluvial ecosystems. New directions, utilizing integrated approaches such as encompassed by wildfire biogeomorphology, are necessary to adapt management strategies toward a sustainable future. A new wildfire management approach would recognize feedbacks between biological and geomorphic processes, that multiple disturbance and recovery trajectories are possible depending on post-wildfire climate characteristics, and that the variable and dynamic post-wildfire geomorphic responses support heterogeneity, biodiversity, and resilience. A new approach would also recognize the necessity for multi-scale investigations from field to remote sensing studies, and from individual storm to multi-decadal research. As such, this new approach would recognize the significance of wildfire biogeomorphology in Earth-system science that is needed for effective future interdisciplinary investigations to enhance landscape conservation, restoration, and wildfire management.

We appreciate all of the authors and reviewers who contributed to this Special Paper volume and their expertise and diligence. We thank GSA Books Science Editor Christian Koeberl and all of the GSA Publications staff for their professionalism and abundant assistance.