Instream Wood Research Articles

What are the Conditions of Riparian Ecosystems? Identifying Impaired Floodplain Ecosystems across the Western U.S. Using the Riparian Condition Assessment (RCA) Tool.

Environmental stressors associated with human land and water-use activities have degraded many riparian ecosystems across the western United States. These stressors include (i) the widespread expansion of invasive plant species that displace native vegetation and exacerbate streamflow and sediment regime alteration; (ii) agricultural and urban development in valley bottoms that decouple streams and rivers from their floodplains and reduce instream wood recruitment and retention; and (iii) flow modification that reduces water quantity and quality, degrading aquatic habitats. Here we apply a novel drainage network model to assess the impacts of multiple stressors on reach-scale riparian condition across two large U.S. regions. In this application, we performed a riparian condition assessment evaluating three dominant stressors: (1) riparian vegetation departure from historical condition; (2) land-use intensity within valley bottoms; and (3) floodplain fragmentation caused by infrastructure within valley bottoms, combining these stressors in a fuzzy inference system. We used freely available, geospatial data to estimate reach-scale (500 m) riparian condition for 52,800 km of perennial streams and rivers, 25,600 km in Utah, and 27,200 km in 12 watersheds of the interior Columbia River Basin (CRB). Model outputs showed that riparian condition has been at least moderately impaired across ≈70% of the streams and rivers in Utah and ≈49% in the CRB. We found 84% agreement (Cohen's ĸ = 0.79) between modeled reaches and field plots, indicating that modeled riparian condition reasonably approximates on-the-ground conditions. Our approach to assessing riparian condition can be used to prioritize watershed-scale floodplain conservation and restoration by providing network-scale data on the extent and severity of riparian degradation. The approach that we applied here is flexible and can be expanded to run with additional riparian stressor data and/or finer resolution input data.

Where Does Wood Most Effectively Enhance Storage? Network‐Scale Distribution of Sediment and Organic Matter Stored by Instream Wood

AbstractWe used 48 reach‐scale measurements of large wood and wood‐associated sediment and coarse particulate organic matter (CPOM) storage within an 80 km2 catchment to examine spatial patterns of storage relative to stream order. Wood, sediment, and CPOM are not distributed uniformly across the drainage basin. Third‐ and fourth‐order streams (23% of total stream length) disproportionately store wood and coarse and fine sediments: 55% of total wood volume, 78% of coarse sediment, and 49% of fine sediment, respectively. Fourth‐order streams store ~0.8 m3 of coarse sediment and 0.2 m3 of fine sediment per cubic meter of wood. CPOM storage is highest in first‐order streams (60% of storage in 47% of total network stream length). First‐order streams can store up to 0.3 m3 of CPOM for each cubic meter of wood. Logjams in third‐ and fourth‐order reaches are primary sediment storage agents, whereas roots in small streams may be more important for storage of CPOM. We propose the large wood particulate storage index to quantify average volume of sediment or CPOM stored by a cubic meter of wood.

Relative performance of three stream bed stability indices as indicators of stream health.

Bed stability is an important stream habitat attribute because it affects geomorphology and biotic communities. Natural resource managers desire indices of bed stability that can be used under a wide range of geomorphic conditions, are biologically meaningful, and are easily incorporated into sampling protocols. To eliminate potential bias due to presence of instream wood and increase precision of stability values, we modified a stream bed instability index (ISI) to include measurements of bankfull depth (d bf) and median particle diameter (D 50) only in riffles and increased the pebble count to decrease variability (i.e., increase precision) in D 50 . The new riffle-based instability index (RISI) was compared to two established indices: ISI and the riffle stability index (RSI). RISI and ISI were strongly associated with each other but neither was closely associated with RSI. RISI and ISI were closely associated with both a diatom- and two macrovertebrate-based stream health indices, but RSI was only weakly associated with the macroinvertebrate indices. Unexpectedly, precision of D 50 did not differ between RISI and ISI. Results suggest that RISI is a viable alternative to both ISI and RSI for evaluating bed stability in multiple stream types. With few data requirements and a simple protocol, RISI may also better conform to riffle-based sampling methods used by some water quality practitioners.

Instream large wood loads across bioclimatic regions

Instream large wood in forested rivers creates diverse physical and ecological effects and is a vital component of river process and form. The majority of research on wood in rivers has been conducted within a limited geographic range, raising questions about the applicability of resulting insights. We analyze data from 438 river segments in old-growth or unmanaged forests and 250 river segments in lightly managed forests representing diverse environments. We evaluate whether drainage area, channel width, and channel slope influence instream wood load in unmanaged forests across bioclimatic regions. Here we show that, without accounting for variations across regions, these numeric variables do not correlate significantly with wood loads. When accounting for the influence of all available potential influences on wood load in unmanaged forest rivers, bioclimatic region, drainage area, and channel width are the dominant predictors of wood load, and the relationship between wood load and channel width differs between regions. Combining data across bioclimatic regions, unmanaged rivers have significantly greater wood loads than lightly managed forests. Splitting data by bioclimatic region, unmanaged rivers in the northern dry conifer and northern wet conifer regions have significantly greater wood loads than lightly managed forests, but wood loads in the northern wet deciduous region do not differ between unmanaged and lightly managed forests. Our findings suggest that (i) bioclimatic region is a critical factor in predicting and understanding wood dynamics in rivers, (ii) even historic or relatively light levels of timber harvest and wood removal can create persistent differences in wood loads, and (iii) substantial variation in wood load among river segments within a bioclimatic region suggests that riparian forest and river management should focus on processes that maintain wood loads capable of creating desired physical and ecological effects rather than specified volumes of wood.

Properties of small instream wood as a logjam clogging agent: Implications for clogging dynamics based on wood density, water content, and depositional environment

In cooperation with large instream wood (LW) within logjams, small instream wood (SW) can control downstream flux of sediment and particulate organic matter and can play an important role for stream ecosystems. However, information regarding the density and moisture content of SW—which affects wood transport, wood decay, and mass loading—is limited. Here we investigated the SW properties, i.e., density under field conditions (in situ density), basic density, volumetric water content, and depositional environment of SW sampled from five logjams and their backwater areas in two headwater streams (second- and third-order streams) surrounded by mixed broadleaf-conifer forests in western Japan. The in situ density ranged from 0.49 to 1.25gcm−3, and pieces with densities >1.0gcm−3 accounted for 45% of all samples. Additionally, the in situ density of SW closely related to the volumetric water content (r2=0.76) rather than the basic density as an index of solidity or decay condition of wood. The SW that was partially submerged in water had a higher volumetric water content than SW exposed to air. These results indicate that a nonfloating transport cannot be ignored as an important mechanism for SW movement and that in situ density depends not on the solidity of the wood but on water sorption by SW. However, waterlogged SW should be well decayed because it has a lower basic density than air-exposed and sediment-buried SW. We conclude that the moisture conditions of the depositional environment can affect subsequent transport and decay processes of SW. Moreover, most waterlogged and sediment-buried SW, because of its high in situ density (>1.0gcm−3), may contribute to clogging between the channel bed and LW that initiate a logjam during future movements.

Riparian and geomorphic controls on thermal habitat dynamics of pools in a temperate headwater stream

AbstractStream pools serve as habitat for diverse communities composed of microbes, macroinvertebrates, and fish. Every pool presents a combination of geomorphological features, and each pool habitat hosts a unique community. These communities are tied to the physical characteristics of their habitat, especially water temperature. The effects of geomorphology and hydraulic variables on thermal habitat dynamics in pools are not well understood. In this study, the thermal regime in important stream pool habitats was investigated in relation to their riparian and geomorphological features. Our results indicate that instream wood, groundwater resurgence, and depth can locally modify the pool thermal habitat and increase thermal buffering capacity during heat‐stress events. Our findings highlight the importance of riparian vegetation and in‐channel hydrogeomorphic features in controlling pool thermal habitats.

Breakdown of instream wood in low order forested streams of the Southern Chilean mountain ranges

We analyzed instream wood depletion or breakdown in terms of wood breakage (defined as the reduction in length) and decay (reduction in diameter) on channel segments of three low-order mountain streams located in southern Chile. We used a unique database, composed of 1049 individually tagged logs, which were measured and their position was georeferenced between 2005 and 2014, and re-measured and re-georeferenced after the rainy winter period of 2015. Results showed that median breakdown at the end of the survey ranged between 5 and 27% and between 10 and 25% for median decay, with highest values of 83 and 72% respectively. While median annual breakage ranged between less than 1 and 4 %·year−1 and median annual decay between 1 and 4 %·year−1. According to our results, breakdown of instream wood is influenced by many factors, the most significant being initial log size (both length and diameter), their orientation, residence time and fluvial transport (i.e. displacement length). Wood density and initial tree height at the time of recruitment were also analyzed, aiming to better understand breakage and decay processes. Results showed that in-channel logs were, as expected, less dense than living trees, and very decayed logs significant loose density compared to fresh logs. The differences between initial log size and potential minimum tree height at the time of recruitment ranged between 29 and 64%. We believe that our findings illuminates on wood depletion processes on low order streams in the Chilean mountain ranges, and can be easily extrapolated to other streams with similar conditions; however, the natural heterogeneity among river environments and hydrological conditions during the survey period may influence breakage and decay processes. Therefore, we encourage other researchers to provide data on breakdown of instream wood from different regions.

Stress responses of juvenile brown trout under winter conditions in a laboratory stream

Winter can be a challenging period for fish in northern temperate rivers and streams, particularly in those that are channelized, structurally simple or regulated by, for instance, hydropower. In these systems, dynamic sub-surface ice formation commonly occurs and stable periods with ice cover may be short. Under these adverse conditions, access to shelters has been shown to be an important factor that influences overwinter survival, and exclusion from shelters by anchor ice may cause stress. Here, stress responses of juvenile brown trout under simulated winter conditions in an artificial stream were studied. Trout were subjected to three treatments in which the trout (1) were excluded from an instream wood shelter, simulating the effects of anchor ice, (2) had access to the shelter or (3) had surface ice cover in addition to the shelter. There was a positive correlation between ventilation frequency and plasma cortisol concentration. Trout without access to shelter had 30% higher ventilation frequency than trout with instream shelter and surface ice, but no differences in cortisol concentration or stress colour were found between the treatments. River regulation that reduces surface ice and increases anchor ice formation may lead to increased stress and consequently reduce overwinter survival rates.

Large Wood and Instream Habitat for Juvenile Coho Salmon and Larval Lampreys in a Pacific Northwest Stream

AbstractThe influences of large wood on Pacific salmon are well‐studied, but such studies on nonsalmonid species such as lampreys are uncommon. To address this need, we evaluated the potential effects of large wood on larval lampreys (Pacific Lamprey Entosphenus tridentatus and potentially Western Brook Lamprey Lampetra richardsoni) as well as on juvenile Coho Salmon Oncorhynchus kisutch in a small, coastal, Oregon stream. Our objectives were to (1) identify instream habitat characteristics associated with the presence of larval lampreys and abundance of juvenile Coho Salmon and (2) evaluate how these characteristics were associated with instream wood. To address habitat use, we quantified presence of larval lampreys in 92 pools and abundance of juvenile Coho Salmon in 44 pools during summer low flows. We focused on a study reach where large wood was introduced into the stream between 2008 and 2009. Results indicated that presence of larval lampreys was significantly associated with the availability of fine sediment and deeper substrate. The abundance of juvenile Coho Salmon (fish per pool) was strongly associated with pool surface area and to a weaker extent with the proportion of cobble and boulder substrates in pools. Pools with wood, regardless of whether they were formed by wood, had significantly greater coverage of fine sediment, deeper substrate, and greater pool surface area. Taken together, these results suggest that instream wood can provide habitat associated with the presence of larval lampreys and greater abundance of juvenile Coho Salmon.Received September 11, 2016; accepted March 25, 2017 Published online May 31, 2017

Temporal dynamics of instream wood in headwater streams draining mixed Carpathian forests

Instream wood can reside in fluvial systems over varying periods depending on its geographical context, instream position, tree species, piece size, and fluvial environment. In this paper, we investigate the residence time of two typical species representing a majority of instream wood in steep headwaters of the Carpathians and located under mixed forest canopy. Residence times of individual logs were then confronted with other wood parameters (i.e., wood dimensions, mean annual increment rate, tree age, class of wood stabilisation and decay, geomorphic function of wood pieces, and the proportion of the log length within the active channel). Norway spruce (Picea abies (L.) Karst.) samples indicated more than two times longer mean and maximal residence times as compared to European beech (Fagus sylvatica L.) based on the successful cross-dating of 127 logs. Maximum residence time in the headwaters was 128years for P. abies and 59years for F. sylvatica. We demonstrate that log age and log diameter played an important role in the preservation of wood in the fluvial system, especially in the case of F. sylvatica instream wood. By contrast, we did not observe any significant trends between wood residence time and total wood length. Instream wood with geomorphic functions (i.e., formation of steps and jams) did not show any differences in residence time as compared to nonfunctional wood. Nevertheless, we found shorter residence times for hillslope-stabilised pieces when compared to pieces located entirely in the channel (either unattached or stabilised by other wood or bed sediments). We also observed changes of instream wood orientation with respect to wood residence time. This suggests some movement of instream wood (i.e., its turning or short-distance transport), including pieces longer than channel width in the steep headwaters studied here (1.5≤W≤3.5m), over the past few decades.

Frederick J. Swanson’s 1976–1979 papers on the effects of instream wood on fluvial processes and instream wood management

The recognition of instream wood as a key element of river ecosystems and a driver of fluvial processes is now well established, but it did not start until the second half of the twentieth century. A landmark reference work which led to the development of concepts and methods which are still employed in instream wood studies today was the work by Frederick J. Swanson and his papers published between 1976 and 1979. In this article we revisit these papers, highlighting the pioneering observations about the effects of instream wood on fluvial morphology, the description of the instream wood sources and recruitment processes and the discussion about management of wood in rivers. The instream wood research has grown dramatically since the late 1970s, however many knowledge gaps remain; this short historical review illustrates the importance of continuing and developing those research lines into the future.

Morphological response of channels to long-term human interventions in mountain basins on the example of the Moravskoslezské Beskydy Mts (Czechia)

The paper summarizes the history of human interactions with mountain streams on the example of the flysch Western Carpathians, Czechia. These are represented by indirect impacts since the 16th century, mainly corresponding to extensive changes in land use and species composition of forests, and by direct human interventions as timber floating with the removal of instream wood (since the end of the 18th and beginning of the 19th century) and check-dam constructions (since 1906). Mountain streams are very sensitive to changes in sediment supply, hydrological regime or direct interventions and produce a fast morphological response. Thus, hydromorphological assessments and management of mountain streams should take into consideration the contemporary land use at the basin scale in historical perspective, sediment connectivity and the occurrence of instream wood as important elements of stream habitat.

Response of Bed Sediments on the Grade‐Control Structure Management of a Small Piedmont Stream

AbstractThe grain sizes of 53 small lateral and mid‐channel deposits were evaluated on the 2.7 km long channel‐reach of a headwater piedmont stream that has been affected by check‐dams and bed sills constructions (ca. 14 constructions per km), riprap bank stabilisations and the removal of instream wood. Such interventions have led to the loss of natural vertically oscillating bedforms, and pool‐riffles have been replaced by plane beds. The evaluated sediment deposits were divided based on their positions relative to grade‐control structures on downstream, intermediate and upstream types. The individual types of deposits produced different downstream trends in the grain sizes that reflected their positions with respect to the grade‐control structures (GCSs). The downstream deposits generally produced slow downstream coarsening, which may point to the increased efficiency of the energy dissipation of the GCSs under a gradual downstream decrease in the channel slopes. However, they were generally not coarser than the other two types. The upstream deposits did not indicate any notable downstream grain‐size trend. Despite the significant downstream decrease in the channel slopes and related bankfull shear stresses at the positions of the intermediate deposits and the lack of a lateral sediment supply in the studied longitudinal profile, the sediment calibre of the deposits did not produce any systematic downstream fining. Thus, the observed grain sizes most likely pointed to the selective scouring of the fine sediments, which was caused by the presence of the GCSs and their effects on the decreasing sediment connectivity. Copyright © 2016 John Wiley & Sons, Ltd.

Biogeomorphological effects of leaf accumulations in stepped-bed channels: Exploratory study, Moravskoslezské Beskydy Mountains, Czech Republic

Abstract The stepped-bed system, with a step-like longitudinal profile, is typical morphology in steep headwater streams. These systems are created by a series of coarse sediments or instream wood (steps with supercritical flows) interspaced with finer material (forming pools with subcritical flows). In the case of well-developed steps and pools, the resulting channel-reach morphology is referred to as “step-pool” morphology. In this study, we identify a previously undescribed type of step-pool formation, the “foliaged step-pool”, in the high-gradient Stoligy Stream of the Moravskoslezské Beskydy Mountains. The defining feature of this formation is the significant presence of leaves in the step structure. The geometry of the steps and pools was measured and the parameters that characterise the distribution, amount and function of leaves acting in these areas were defined. Statistical results showed differences between non-foliaged and foliaged step-pool formations, in which the latter showed a significant increase in storage level, influencing the channel’s hydrodynamics. Particle-size analyses demonstrated that foliaged step-pool formations had finer sediment in the pools, which indicates that there are differences in sediment transport processes between foliaged and non-foliaged formations. These results offer new insights into stepped-bed and step-pool morphology, providing directions for further research on small streams in deciduous forested regions.

Effects of Passive and Structural Stream Restoration Approaches on Transient Storage and Nitrate Uptake

Understanding nitrogen dynamics in headwater streams is important for ascertaining how they influence downstream nutrient loads and identifying strategies for reducing loading through stream restoration. We compare nitrate uptake associated with two restoration approaches in headwater streams, Sheep Creek and Nunn Creek, of northern Colorado, USA. Segments of Sheep Creek were exclosed (fenced off) from open rangeland cattle grazing in the 1950s, allowing riparian corridors of these segments to naturally revegetate (passive approach), while other segments have been continually grazed. In 2003, restoration structures including cross vanes, J‐hook vanes, rootwads, log vanes, and bank riprap (structural approach) were installed along portions of Nunn Creek for trout habitat enhancement and local bank stabilization. We performed detailed physical characterizations and multiple nutrient injections of Br⁻ and NO₃ ⁻ to estimate transient storage and nitrate uptake in four reaches along Sheep Creek (two reaches exclosed from grazing and two grazed reaches) and two reaches along Nunn Creek (one with restoration structures and one without structures). Parameters of transient storage and nitrate uptake were estimated with the one‐dimensional transport with inflow and storage model run through universal inverse modelling code for optimization. Responses of transient storage and nitrate uptake to restoration techniques depended upon the type and extent of restoration implemented, as well the context and physical setting of each study reach. For example, in the higher‐gradient pair of Sheep Creek reaches, the restored reach showed greater nitrate uptake, while in the lower‐gradient pair of reaches, the non‐restored reach had greater uptake. At Nunn Creek, the reach with instream wood but without restoration structures exhibited more transient storage and nitrate uptake when compared with the reach with extensive J‐hook vane structures. Copyright © 2016 John Wiley & Sons, Ltd.

Sources and interpretation of channel complexity in forested subalpine streams of the Southern Rocky Mountains

AbstractWe evaluate correlations between stream geomorphic complexity and characteristics of the adjacent riparian forest, valley geometry, and land use history in forested subalpine streams of the Colorado Front Range. Measures of geomorphic complexity focus on cross‐sectional, planform, and instream wood piece and logjam variables. We categorize adjacent riparian forests as old‐growth unmanaged forest (OU), younger unmanaged forest (YU), and younger managed forest (YM), and valley geometry as laterally confined, partly confined, or unconfined. Significant differences in geomorphic stream complexity between OU, YU, and YM result primarily from differences in wood pieces and logjams, and these differences correlate strongly with pool volume and organic matter storage. Significant differences in planform and cross‐sectional complexity correlate more strongly with valley geometry, but do not explain as much of the observed variability in complexity between streams as do the wood variables. Unconfined OU streams have the largest wood loads and the greatest complexity, whereas legacy effects of logging, tie‐drives, and channel simplification create lower complexity in YM streams, even relative to YU streams flowing through similarly aged forest. We find that management history of riparian forests exerts the strongest control on reduced functional stream channel complexity, regardless of riparian forest stand age.

Effects of riparian buffer width on wood loading in headwater streams after repeated forest thinning

Forested riparian buffer zones are used in conjunction with upland forest management, in part, to provide for the recruitment for large wood to streams. Small headwater streams account for the majority of stream networks in many forested regions. Yet, our understanding of how riparian buffer width influences wood dynamics in headwater streams is relatively less developed compared to larger fish-bearing streams. The effects of riparian buffer width on instream wood loading after thinning can be difficult to discern due to the influence of basin characteristics and reach-scale geomorphology on wood recruitment, breakage and redistribution. We assessed the relationships between instream wood loading, geomorphology and riparian buffer width in small headwater streams after upland thinning. Then we examined the distances between pieces of stream wood and their sources, or the distance from which wood volumes were recruited to these streams. Data were collected along 34 stream reaches at six different sites in a replicated field experiment, comparing three no-harvest streamside buffer treatments (∼6-m, 15-mminimum, and ∼70-m widths). At each site, second-growth forests were thinned first to 200trees per ha [tph] and ∼10years later to 85tph, alongside an unthinned reference unit (∼400tph). We measured wood loading (m3/100m) four times: (1) prior to thinning; (2) year 5 post-1st thinning; (3) immediately prior to the 2nd thinning; and (4) year 1 post-2nd thinning. The majority of wood volume was in late stages of decay, most likely biological legacies from the previous forest stand, and distributed along the streambank. Surprisingly, wood volume in early stages of decay was higher in stream reaches with a narrow 6-m buffer than in stream reaches with larger 15- and 70-m buffers and the unthinned reference units. Additionally, wood volume increased with drainage basin area. Only 45% of wood in late stages of decay could be associated with a particular source. Yet, 82% and 85% of sourced wood in early and late stages of decay, respectively, originated from within 15m of streams. Expected continue low rates will likely result in declining volumes of wood in late stages of decay. Thinning and directional felling of logs into to streams could be used to augment wood volumes in the near term, and accelerate the development of large-diameter logs for future inputs. However, the relationship between instream wood loading and basin area suggests that instream wood loading depends on management across the entire watershed.

Wood density and moisture sorption and its influence on large wood mobility in rivers

Dynamics of large wood in aquatic systems significantly influence physical and ecological processes in rivers. Wood mobility is notably becoming a critical issue, not only in the context of restoration, but also in terms of flooding and hazard potential. Although the number of studies focusing on instream wood has increased substantially over the last few years, physical properties of wood have rarely been measured in aquatic systems. Instead, forest industry-based standards are often used. In this study, we quantitatively assess properties of instream wood density using decayed samples from the Rhône River stored within the Génissiat Reservoir and green samples from the Ain River floodplain (France). Using in-situ and laboratory experiments, we demonstrate how wood density varies between species, how density changes with moisture sorption and decay, and how density affects buoyancy. Results illustrate that both green (e.g., 800±170kg·m−3) and instream woods (e.g., 660±200kg·m−3) have much greater densities than standard values used in the literature (500kg·m−3). Sorption processes differ in green versus instream wood; moisture desorption of green wood is faster than absorption, whereas for instream wood, absorption is faster than desorption. These findings and the related changes in density affect wood buoyancy and mobility and therefore influence wood dynamics in rivers. Finally, two case studies illustrate how more accurate density values can be used to improve wood transport modeling and wood budget estimates based on numerical simulation and ground video-imagery-based monitoring.

Detecting Fluvial Wood in Forested Watersheds using LiDAR Data: A Methodological Assessment

It has long been known that large wood in rivers increases channel complexity and is a primary driver of geomorphic change in forested mountain streams in the Pacific Northwest. Studies analyzing the presence and distribution of fluvial wood are often limited in their spatial extents to the site or reach scales because of the intensive fieldwork required for comprehensive wood surveys. Remote sensing techniques are beginning to allow researchers to assess fluvial wood dynamics and distributions on a basin or regional scale. We used 2009 high-resolution light detection and ranging (LiDAR) point cloud data to detect and quantify wood within five forested watersheds in the Oregon Coast Range. We filtered the LiDAR data to remove the forest canopy over the stream channel and visually inventoried fluvial wood based on its distinct shape within the channels. We derived several wood and stream morphometric variables to test theories relating to wood abundance and positioning in the lower reaches of streams. We were able to detect fluvial wood with confidence; however, validation of results with ground-truth data was difficult in the study due to the dynamic and mobile nature of wood through time. We mapped a total of 163 single logs and 55 logjams within the five study watersheds. We did not find statistically significant differences between individual pieces and jam positioning in relation to slope; however, the surveyed wood was often found in areas of lower stream power. This research shows that it is possible to use height-filtered LiDAR to detect in-stream wood in densely forested watersheds and has the potential to be employed in future wood studies across broad spatial scales. Copyright © 2015 John Wiley & Sons, Ltd.

Fire meets inland water via burned wood: and then what?

Wood is a key structural element in aquatic ecosystems. Wood provides habitat complexity, alters geomorphology, retains organic and inorganic material, promotes hyporheic flow, and acts as substrate for biofilms and invertebrates. Fire is an important disturbance promoting wood recruitment into inland waters, but most studies have focused on streams in western North America. Less is known about fire-derived wood dynamics on other continents or in lake environments. Here, we review fire effects on the recruitment, distribution, and function of in-stream wood, with emphasis on a series of studies from the Euro-Mediterranean. The amount of large wood in these streams was low and is expected to decline in the future because of wildfire. Wildfire engendered inputs of wood with low structural complexity, probably reducing habitat heterogeneity for aquatic organisms. Fire also provided wood with greater diameter than wood recruited by other means, but its longevity may be shorter because burned wood was more decayed and less anchored in the channel than unburned wood. Wood delivery processes are important because macroinvertebrate colonization differed between fire-derived wood that fell directly into the river and wood conditioned first on the forest floor. We present a case study describing wood dynamics in a lake after a wildfire in northern Minnesota, USA. In this study, wildfire created an area of lake shoreline with disproportionately more wood than areas unaffected by wildfire. In contrast to streams, burned wood was more complex than unburned wood in the lake system. One of the explanations may be greater scouring, abrasive action by stream flows, which breaks down burned wood faster than in lakes. Given the expected increase in the probability and severity of fire around the world, information in our review can be used to help manage riparian zones of streams and lakes.