Large Woody Debris Loading Research Articles

Quantifying the role of woody debris in providing bioenergetically favorable habitat for juvenile salmon

The habitat complexity of a riverine ecosystem influences the bioenergetics of drift feeding fish. We coupled hydrodynamic and bioenergetic models to assess the influence of habitat complexity generated by large woody debris (LWD) on the growth potential of juvenile Chinook salmon (Oncorhynchus tshawytscha) in a river that lacked large wood. Simulations indicated how LWD diversified the flow field, creating pronounced velocity gradients, which enhanced fish feeding and resting activities at the sub-meter scale. Fluid drag created by individual wood structures increased under higher wood loading amounts, leading to a 5–19% reduction in the reach-averaged velocity. The reach-scale growth potential was asymptotically related to wood loading, suggesting that the river became saturated with LWD and additional loading would produce minimal benefit for the configurations we simulated. In the scenario we analyzed for illustration, LWD additions could quadruple the potential growth area available before that limit was reached for the configurations selected for demonstration. Wood depletion in the world's rivers has been documented extensively, leading to widespread attempts by river managers to reverse this trend by adding wood to simplified aquatic habitats. However, systematic prediction of the effects of wood on fish growth has not been previously accomplished. We offer a quantitative approach for assessing the influence of wood on habitat potential for fish growth at the microhabitat and reach-scales.

Simple measures of channel habitat complexity predict transient hydraulic storage in streams

Stream thalweg depth profiles (along path of greatest channel depth) and woody debris tallies have recently become components of routine field procedures for quantifying physical habitat in national stream monitoring efforts. Mean residual depth, standard deviation of thalweg depth, and large woody debris (LWD) volumes are potential metrics of habitat complexity calculated from these survey data. We used 42 intensive dye-transit studies to demonstrate the relevance of these easily measured channel habitat complexity metrics to transient hydraulic (“dead zone”) storage, a channel process important for biotic habitat as well as retention and “spiraling” of dissolved and particulate nutrients. We examined transient storage and channel morphology in small gravel and cobble-bedded upland streams (wetted width 2–5 m; slopes 2.6–8.3%) representing a wide range of flow stages, LWD loading, and channel complexity, including measurements before and after LWD was added to enhance fish habitat. While transient storage volume fraction decreased as flow stage increased in simple channels, those with complex morphology and well-developed riparian vegetation maintained high transient storage fractions even during storm flows. LWD additions increased transient storage and channel complexity over the 2 years of post-treatment measurements. We predict with considerable precision two different formulations of transient hydraulic storage fraction using single-variable linear regressions on residual depth (R 2 = 0.61–0.89), thalweg depth variance (R 2 = 0.64–0.91), or large woody debris volume (R 2 = 0.48–0.74). Demonstration of these likely causal associations contributes to understanding the process of transient storage and redefines the use of thalweg profile metrics as a new approach to quantifying morphologic and hydraulic complexity in streams.

Simulating the Effects of Forest Management on Large Woody Debris in Streams in Northern Idaho

Abstract Existing models for simulating large woody debris (LWD) loads of forest streams were adapted for forest conditions in northern Idaho. Effects of riparian management prescriptions implemented for streams within a habitat conservation planning area for bull trout and other sensitive species were evaluated based on riparian and instream LWD conditions observed along 58 randomly selected stream segments. A wood budgeting system presented by Welty et al. (2002. Riparian aquatic interaction simulator (RAIS): A model of riparian forest dynamics for the generation of large woody debris and shade. For. Ecol. Manage. 162:299–318) was employed through use of observed starting instream LWD loads and generalized depletion rates. LWD recruitment estimates were based on locally relevant growth and yield simulators, taper equations, and adjustments for tree fall directional bias. LWD loading, expressed as the number of qualifying pieces per 1,000 ft of stream, was examined under two scenarios: a no-harvest scenario and a harvest scenario. Results indicated no significant difference in the frequency distribution of simulated LWD loading between the no-harvest and harvest scenarios over a 100-year prediction period. Examination of our assumptions indicated that LWD loading was likely underestimated and less variable than would be expected. However, these assumptions had equal effects on each scenario, enabling us to confidently interpret the effects of timber harvest. The nature and extent of riparian forest harvesting evaluated in this simulation is similar to levels being considered elsewhere in the region. Therefore, simulation techniques demonstrated here could be applied elsewhere in the region for evaluating the potential effects of riparian management on fisheries resources.

A watershed scale assessment of in-stream large woody debris patterns in the southern interior of British Columbia

In-stream large woody debris (LWD) is a structurally and functionally important component of forested stream ecosystems. To assess the role played by LWD in sustaining aquatic ecosystems at the watershed scale, the amount, distribution, dynamics and function of LWD within channel networks have to be determined. We surveyed 35 sites in first- through fifth-order streams within forested watersheds in the southern interior of British Columbia, and the spatial variation and distribution of LWD characteristics (frequency, density, volume, biomass, orientation, submersion, and decay state) were quantified based on four stream size categories. We found that the average diameter, length, volume and biomass of individual LWD pieces increased as a function of increasing bankfull width. However, LWD density (piece per 100 m 2 of the stream area) decreased with an increase in bankfull width. LWD volume ranged from 0.78 to 1.58 m 3/100 m 2 of stream area, with intermediate sized streams (sizes II and III) having the largest value and large sized streams (size IV) having the lowest values. Results showed that LWD biomass averaged 383 kg/100 m 2 (range 265–651 kg/100 m 2) in stream size I, increased to 491 kg/100 m 2 (range 81–1254 kg/100 m 2) in stream size II, and slightly decreased to 465 kg/100 m 2 (range 247–938 kg/100 m 2) in stream size III and further decreased to 250 kg/100 m 2 (range 88–533 kg/100 m 2) in stream size IV. The large majority of LWD pieces in the smallest sized streams was orientated perpendicular to streamflow and was located in spanning the channel. Conversely, most LWD pieces in intermediate sized streams were orientated parallel to the direction of flow and were situated below the bankfull height of the channel. With a difference in the orientation and position, LWD pieces within different sized streams are expected to have varying potentials to affect streamflow and channel habitats. These results highlight the need to recognize spatial variation of in-stream LWD loading and function through channel networks when maintaining suitable LWD pieces and making riparian management decisions at watershed scales.

Large Woody Debris and Land Management in California’s Hardwood-Dominated Watersheds

Although large woody debris (LWD) has been studied extensively in conifer-dominated watersheds, relatively little is known about LWD in hardwood-dominated watersheds. Field surveys of 32 hardwood-dominated stream reaches in northern coastal California revealed that levels of LWD varied with land ownership and that living trees strongly influenced debris jam formation. Almost half of the channel-spanning debris jams, which stored the most wood and were most likely to form a pool, were formed behind a key piece that was still living. These living key pieces might provide greater longevity and stability than would otherwise be expected from hardwood LWD. Compared to streams on private land, streams on public land had significantly greater LWD loading and debris-jam frequency. Land management practices that remove wood from streams might be contributing to the degradation of salmonid habitat in California's hardwood-dominated watersheds.

Influence of wildfire and harvest on biomass, carbon pool, and decomposition of large woody debris in forested streams of southern interior British Columbia

Large woody debris (LWD) is an important component in the biogeochemistry cycle of carbon and nutrients in forested stream ecosystems. In-stream LWD volume, biomass and carbon pool were investigated in 19 forested streams in the south central interior of British Columbia. The stream channels were classified into four disturbance categories based upon condition of the adjacent riparian forest. The categories are: (1) riparian forest harvested approximately10 years ago (HT10), (2) riparian forest harvested approximately 30 years ago (HT30), (3) riparian forest burned by a wildfire approximately 40 years ago (WF), and (4) undisturbed old-growth riparian forest (OF). Streams with riparian forests that were affected by wildfire or were recently harvested were observed to have significantly higher LWD volumes, biomass, and carbon pool as compared to streams flowing through old-growth riparian forests. LWD stocks averaged 376 m −3 ha −1 (volume), 112 Mg ha −1 (biomass), 52 Mg C ha −1 (carbon) in WF, 258 m −3 ha −1, 78 Mg ha −1 and 36 Mg C ha −1 in HT10, 180 m −3 ha −1, 52 Mg ha −1 and 23 Mg C ha −1 in HT30, and 114 m −3 ha −1, 37 Mg ha −1 and 17 Mg C ha −1 in OF. Volume, biomass, and carbon stock were 2.3, 2.0, 2.1, and 1.3, 1.1 and 1.1 times higher in WF and HT10 than in OF, respectively, but LWD loading did not differ significantly between HT30 and OF. Major differences were also observed in the state of decay of LWD between the four disturbance categories based upon three decomposition classes. Our study supports the conclusion that harvesting creates a short-term increase in LWD stocks. However, harvesting may greatly reduce LWD loadings over the long-term due to relatively rapid decomposition of LWD due to increased rate of decay, transport, and reduced recruitment from the adjacent riparian forest. In the study streams, the wood density of LWD ranged from 0.273 to 0.427 g cm −3 depending upon the species and decomposition level. An average decay rate constant of 0.0095 year −1 was calculated for the LWD based upon wood density. Based upon this decay rate, the time required to loss 50% ( t 0.5) and 95% ( t 0.95) of wood is 74 and 316 years, respectively.

Distribution, recruitment, and geomorphic significance of large woody debris in an alluvial forest stream: Tonghi Creek, southeastern Australia

The complex yet poorly understood interactions between riparian vegetation, large woody debris and fluvial geomorphology in an anthropogenically undisturbed reach of an alluvial, sand-bed forest stream in SE Australia have been determined. Riparian vegetation exhibits lateral and vertical zonation of understorey and overstorey species. The dominant riparian tree species, Tristaniopsis laurina (water gum), grows within the channel and on the floodplain within one channel width of the stream. Larger Eucalyptus species only grow on the highest parts of the floodplain and on a low Pleistocene river terrace. A complete large woody debris (LWD) census conducted in the 715-m-long study reach revealed that water gum comprises 17.6% of the total LWD loading, which, at 576 m 3 ha −1, is high for a stream with a catchment area of 187 km 2. Although most LWD has a small diameter (0.1–0.3 m), the greatest contribution to the total volume of LWD is by pieces with a diameter between 0.3 and 0.7 m. A high proportion of LWD (10.4%) has a blockage ratio greater than 10%. The spatial distribution of LWD is random both longitudinally and within individual meander bends. Dominant recruitment processes of LWD vary by species. T. laurina trees are recruited to the channel by minor bank erosion and senescence, while the Eucalyptus species are predominantly recruited from the highest parts of the floodplain/low-river terrace by episodic windthrow during large storms. Multiple radiocarbon dates of outer wood of immobile LWD indicate a maximum residence time of 240±40 years BP for T. laurina timber. The high loading of LWD combined with the extensive root systems of riparian vegetation stabilize Tonghi Creek. Log steps form natural wooden drop-structures with a mean height of 29 mm that were responsible for 20.5% of the total head loss under base flow conditions ( Q=0.08 m 3 s −1). Large woody debris is buried in the bed at depths of up to 2.3 m and is responsible for an estimated 49% of the 11, 600 m 3 of sand stored in the study reach. Pools are spaced at 0.8 channel widths and 82% of pools are formed by scour over, under, around, or beside LWD or by the impoundment of water upstream of debris dams. Due to the high density of hardwood timber species, debris dams, however, do not readily form in Tonghi Creek as the timber is difficult to transport and LWD usually sinks to the bed of the stream. Despite the high degree of channel stability provided by LWD, high blockage ratios in the channel result in relatively frequent overbank flows. These flows are often concentrated in chutes across the neck of meanders or multiple loops, which can develop into cutoffs and channel avulsions, respectively.

A preliminary study on in-stream large woody debrids in broadleaved and Korean pine forest in Changbai Mountain, Northeast China

This study was conducted in Erdaobaihe River passing through the broadleaved and Korean pine forest located on the north slope of Changbai Mountain. In-stream large woody debris (LWD) in two segments of the river channel was investigated with base diameter, top diameter, length, and decay class. To study relationship between in-stream LWD and adjacent riparian forest, species of each log of LWD in segment 1 was identified, and the riparian forest was examined by setting a 32m×24m quadrat consisting of twelve 8m×8m small quadrats. The results showed that, in segment 1, in-stream LWD loading was 1.733 m3/100m or 10.83 m3·hm−2, and in segment 2, it was 1.709 m3/100m or 21.36 m3·hm−2. In-stream LWD in decay class III and IV were accounted for a high proportion, which was different from that in the broadleaved and Korean pine forest, and the possible reason might be different decomposing velocities due to different decomposing conditions. Logs of LWD in stream and living trees in riparian forest declined as diameter increased, and it was in a reverse J-shaped distribution except logs of LWD in segment 1 in the first diameter class. Volumes of LWD in stream and living trees in riparian forest increased as diameter increased, and it was in a typical J-shaped distribution. Loading and species component of in-stream LWD were correlative to status of riparian forest to a certain extent, and there also existed difference. Comparing the correlation and difference was helpful to study on dynamic of the riparian forest.

Effectiveness of large woody debris in stream rehabilitation projects in urban basins

Urban stream rehabilitation projects commonly include log placement to establish the types of habitat features associated with large woody debris (LWD) in undisturbed streams. Six urban in-stream rehabilitation projects were examined in the Puget Sound Lowland of western Washington. Each project used in-stream log placement as the primary strategy for achieving project goals; none included systematic watershed-scale rehabilitation measures. The effectiveness of LWD in these projects was evaluated by characterizing physical stream conditions using common metrics, including LWD frequency and pool spacing, and by sampling benthic macroinvertebrates. In all project reaches where pre-project data existed, pool spacing narrowed after LWD installation. All project sites exhibited fewer pools for a given LWD loading, however, than has been reported for forested streams. In project reaches where the objective was to control downstream sedimentation, only limited success was observed. At none of the sites was there any detectable improvement in biological conditions due to the addition of LWD. Our results indicate that, although LWD projects can modestly improve physical habitat in a stream reach over a time scale of 2–10 years, they apparently do not achieve commensurate improvement in biological conditions.

The Influence of Geomorphic Factors and Geographic Region on Large Woody Debris Loading and Fish Habitat in Alaska Coastal Streams

Large woody debris (LWD) and channel data from three Alaska coastal regions with varying geomorphic channel types were analyzed to document regional variability in LWD abundance, define geomorphic characteristics affecting LWD abundance, and identify relationships between LWD abundance and the formation of pools and gravel bars in streams. Large woody debris abundance was significantly lower at the northern edge of the coastal coniferous forest than in Southeast Alaska and was significantly greater in alluvial gravel-bedded channels than in contained boulder–bedrock channels. More pools and gravel bars were formed by LWD in alluvial channels than in contained channels. Pool spacing (the number of channel widths between pools) decreased with increasing LWD abundance (pieces/km) and was significantly influenced by the interaction between LWD abundance and channel width. As channel width increased, pool spacing was more strongly influenced by changes in LWD abundance, but the relative change in pool spacing diminished with increasing LWD load. The percentage of stream area in pools was insensitive to changes in LWD abundance and was best predicted by channel type. The percentage of habitat units with gravel as the dominant substrate was positively related to LWD abundance and negatively related to stream gradient.

Density and size of juvenile salmonids in response to placement of large woody debris in western Oregon and Washington streams

Thirty streams in western Oregon and Washington were sampled to determine the responses of juvenile salmonid populations to artificial large woody debris (LWD) placement. Total pool area, pool number, LWD loading, and LWD forming pools were higher in treatment (LWD placement) than paired reference reaches during summer or winter. Juvenile coho salmon (Oncorhynchus kisutch) densities were 1.8 and 3.2 times higher in treated reaches compared with reference reaches during summer and winter, respectively. The response (treatment minus reference) of coho density to LWD placement was correlated with the number of pieces of LWD forming pools during summer and total pool area during winter. Densities of age-1+ cutthroat trout (Oncorhynchus clarki) and steelhead trout (Oncorhynchus mykiss) did not differ between treatment and reference reaches during summer but were 1.7 times higher in treatment reaches during winter. Age-1+ steelhead density response to treatment during summer was negatively correlated with increases in pool area. Trout fry densities did not differ between reaches, but the response of trout fry to treatment was negatively correlated with pool area during winter. Our research indicates that LWD placement can lead to higher densities of juvenile coho during summer and winter and cutthroat and steelhead during winter.

In-stream large woody debris loading and riparian forest seral stage associations in the southern Appalachian Mountains

Large woody debris (LWD) is an important ecological component of mountain streams. However, the relation of LWD loading and riparian forest composition is poorly understood in the southern Appalachians. In this study, 500-m reaches of 11 riparian forest–stream systems representing a 300-year sere were inventoried and measured to obtain quantitative estimates and descriptions of in-stream LWD. Loading volumes ranged from 7.1 to 31.2 m3/100 m of stream, or between 3.6 and 13.2 kg/m2. LWD loadings were highly variable during midseral stages of plant community succession, primarily because of the wide range in loading of American chestnut (Castaneadentata (Marsh.) Borkh.). Loadings increased linearly in late-successional through old-growth systems over a 165-year interval. Eastern hemlock (Tsugacanadensis (L.) Carrière) and American chestnut were the most dominant carry-over LWD species in midsuccessional stream systems. Loading of eastern hemlock LWD increased from midsuccessional through old-growth stages as the species became dominant in the riparian forest. Without carry-over debris, LWD loadings would be extremely low in midsuccessional stream systems. American chestnut was a major component of LWD in midsuccessional stream systems, despite the fact that it has been unavailable for recruitment for decades.

Pool Spacing in Forest Channels

Field surveys of stream channels in forested mountain drainage basins in southeast Alaska and Washington reveal that pool spacing depends on large woody debris (LWD) loading and channel type, slope, and width. Mean pool spacing in pool‐riffle, plane‐bed, and forced pool‐riffle channels systematically decreases from greater than 13 channel widths per pool to less than 1 channel width with increasing LWD loading, whereas pool spacing in generally steeper, step‐pool channels is independent of LWD loading. Although plane‐bed and pool‐riffle channels occur at similar low LWD loading, they exhibit typical pool spacings of greater than 9 and 2–4 channel widths, respectively. Forced pool‐riffle channels have high LWD loading, typical pool spacing of <2 channel widths, and slopes that overlap the ranges of free‐formed pool‐riffle and plane‐bed channel types. While a forced pool‐riffle morphology may mask either of these low‐LWD‐loading morphologies, channel slope provides an indicator of probable morphologic response to wood loss in forced pool‐riffle reaches. At all study sites, less than 40% of the LWD pieces force the formation of a pool. We also find that channel width strongly influences pool spacing in forest streams with similar debris loading and that reaches flowing through previously clear‐cut forests have lower LWD loading and hence fewer pools than reaches in pristine forests.