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

Forest Ecology and Management 229 (2006) 50–62 www.elsevier.com/locate/foreco

A watershed scale assessment of in-stream large woody debris patterns in the southern interior of British Columbia Xiaoyong Chen a,*, Xiaohua Wei a, Rob Scherer a,b, Chad Luider a, Wayne Darlington a a

Department of Earth and Environmental Science, University of British Columbia (Okanagan), 3333 University Way, Kelowna, BC, Canada V1V 1V7 b Forest Research Extension Partnership (FORREX), C/O Department of Earth and Environmental Science, University of British Columbia (Okanagan), 3333 University Way, Kelowna, BC, Canada V1V 1V7 Received 6 May 2005; received in revised form 20 March 2006; accepted 21 March 2006

Abstract 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 m2 of the stream area) decreased with an increase in bankfull width. LWD volume ranged from 0.78 to 1.58 m3/100 m2 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 m2 (range 265– 651 kg/100 m2) in stream size I, increased to 491 kg/100 m2 (range 81–1254 kg/100 m2) in stream size II, and slightly decreased to 465 kg/100 m2 (range 247–938 kg/100 m2) in stream size III and further decreased to 250 kg/100 m2 (range 88–533 kg/100 m2) 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. # 2006 Elsevier B.V. All rights reserved. Keywords: Woody debris; Spatial distribution; Stream order; Channel network; Forest watershed

1. Introduction The role of large woody debris (LWD) in forested stream ecosystems has been widely demonstrated. LWD creates and alters channel morphological structures (Keller and Swanson, 1979; Harmon et al., 1986; Montgomery et al., 1995) and forms a variety of habitats such as pools, riffles and waterfalls, and cover for fish and other aquatic biota (Bilby, 1981; Lisle, 1986; Elliott, 1986; Freedman et al., 1996). In addition, the pieces also regulate sediment, organic matter and nutrient concentrations (Bilby and Likens, 1980; Marston, 1982) and influence carbon cycles and sequestration (Guyette et al., 2002; Chen et al., 2005). Consequently, maintaining, restoring, and controlling an

* Corresponding author. Tel.: +1 250 807 8779; fax: +1 250 807 8005. E-mail addresses: [email protected], [email protected] (X. Chen). 0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2006.03.010

appropriate level of LWD within stream systems, in terms of quality and quantity both temporally and spatially, is an important consideration in the management of sustainable forest ecosystems. For example, riparian forest buffers and reserves have been used in North America as a best forest management practice to produce sufficient woody debris to maintain the structure, function, and biological diversity of aquatic ecosystems. Adding LWD pieces in small low-gradient streams to increase in-stream cover and number of pools, and to restore its functions have proven to be an effective approach (Montgomery et al., 1995). In any case, more information regarding the amount, distribution, dynamics and function of LWD through channel networks is required to develop sustainable management strategies at the watershed scale. However, such information is lacking for many forested stream environments. Many studies have been conducted to examine in-stream LWD characteristic and function in forested streams, particu-

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larly in Pacific Northwest. However, the majority of these studies have been at the reach scale and very few targeted watershed scales. In BC, there has also been growing interest in the studies on LWD and aquatic habitat, particularly in coastal BC (Rosenfeld et al., 2002; Heinzelmann, 2002; Hogan, 1987; Hogan and Church, 1989). None of these, however, address relationships between LWD, channel morphology, and aquatic habitat at watershed scales. Spatial scale issues are important for understanding and managing forest ecosystems (Wei et al., 2000, 2003). Scales are also important for in-stream LWD dynamics because of its large variability in space and time (Bisson et al., 1987; Naiman et al., 2002). Information at the channel reach scale is useful, but may be insufficient for understanding sources of its spatial variability (Naiman et al., 1992). Integration of the reach-scale data with information from other spatial scales may be the only viable approach to study LWD processes and dynamics (Bauer and Ralph, 1999; Wei, 2003). In order to address spatial scale issues, Bauer and Ralph (1999) suggested that a landscape context is required for evaluation of aquatic habitat indicators, and a suitable stream network classification system would also be a useful tool for delineating geomorphic features and habitat variables across variable spatial scales. Habitat variables are generally measured at the habitat unit scale (e.g. pool, riffle, or glide), they should also be assessed at the stream reach scale of various stream orders. These units can then be scaled up to address questions at the sub-watershed or watershed level (Bauer and Ralph, 1999). The amount and characteristics of LWD in stream systems depend on local climate conditions, physical features of stream channels, composition and structure of riparian forests, as well as human activities. For example, windthrow has been identified as a dominate factor in affecting the input of LWD in small or medium sized stream channels (Keller and Swanson, 1979). In general, headwater streams or small channels tend to contain abundant woody debris, while LWD becomes less abundant in a downstream direction or in higher order channels (Bilby and Likens, 1980; Bilby, 1981; Benke and Wallace, 1990). Marcus et al. (2002) also reported that the amount of woody debris per kilometer was highest in second-order streams, widely variable in third- and fourth-order streams, and relatively low in sixthorder streams. In addition, average piece size and frequency of large accumulations of LWD increase with increasing channel size through the network (Bilby and Ward, 1991). In the Pacific Northwest of the US, LWD amounts were relatively high in channels flowing through forests dominated by mature conifer stands (McHenry et al., 1998). Human activities have influenced the structure and function of in-stream LWD through stream cleaning to improve navigation, to facilitate fish migration, to maintain channel capacity to transmit water, and to avoid flooding and erosion (Maser and Sedell, 1994). In addition, clearcut harvesting of the streamside forests directly influences LWD by affecting loading dynamics (Diez et al., 2001; Gomi et al., 2001; Chen et al., 2005). As a part of the comprehensive research projects for building LWD budgets at spatial and temporal scales in southern interior of British Columbia, this study is intended to quantify the structural and functional characteristics of LWD at the

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watershed scale. The specific objectives of this project are as follows: (1) to determine the spatial distribution and variation of LWD characteristics (size, amount, volume, mass, orientation, position) within different sized streams of the forested watersheds; (2) to examine the relationships between LWD characteristics and stream features through the channel networks; (3) to estimate the total density, volume and mass of LWD at the watershed scale using information derived from field surveys and a geographical information system (GIS). 2. Study area The study area, contained three major watersheds: the Wilkinson Creek watershed (675 km2), the West Kettle River watershed (1032 km2), and the Nicola River watershed (1162 km2), located in the south central interior of British Columbia near the city of Kelowna (438100 N, 798550 W), Canada (Fig. 1; Appendix A). The study area lies physiographically within the Interior Plateau with elevations ranged from 700 to 1750 m, with 75% of the total area having gradients less than 15%. In the terrestrial ecosystem, the study area is dominated by the Engelmann Spruce–Subalpine Fir (ESSF) and the Montane Spruce (MS) biogeoclimateic zones (Meidinger and Pojar, 1991). Overall, the study area has a cool and continental climate characterized by long cold winters and moderately short warm summers. Mean annual temperatures range from
2 to 4.7 8C. Mean monthly temperatures are below 0 8C for 5 months of the year, and above 10 8C for 2–4 months of the year. Annual precipitation is highly variable in the study area, and ranges from 400 to 500 mm in the drier portions to 2200 mm in the wetter areas. Most (50–70%) of the precipitation falls as snow (Meidinger and Pojar, 1991) and is stored in the seasonal snowpack. Peak stream discharges occur primarily in the spring between April and June, mostly from melting snowpacks situated above 1200 m. The hydrologic regime is characterized by the spring freshet, with a general streamflow recession through summer and autumn and an extended baseflow period through winter (Moore and Scott, 2005). The soil in the study area can be characterized as podzolic with Humo-Ferric Podzols developed from moderately to well-drained parent materials. Humus forms are generally Mors (Hemimors, Hemihumimors, and Humimors), ranging from 3 to 10 cm in thickness. Dominant tree species within the area are Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), hybird white spruce (P. glauca and P. engelmannii) and lodgepole pine (Pinus contorta). Other minor tree species include douglas-fir (Pseudotsuga menziesii), western hemlock (Tsuga heterophylla), and western red cedar (Thuja plicata). The special features of the study watersheds were the young and maturing stands of lodgepole pine that had formed following wildfire. In wet environments, maturing stands contained mixtures of lodgepole pine and hybird white spruce. Subalpine fir and hybird white spruce were the dominant, shade-tolerant, climax trees. Forests in this region experienced frequent standreplacing wildfire events. Stand replacing forest fires have been reported to occur at mean return intervals of 125–150

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Fig. 1. Geographic location of study sites in southern interior of British Columbia, Canada.

years (B.C. Ministry of Forests, 1995). In the past 50 years, clear-cut harvesting was conducted using conventional logging systems in these forested watersheds. All harvested sites were reforested 1–3 years after harvesting with lodgepole pine being the dominant species. These natural and human disturbances resulted in a forest landscape characterized by a mosaic of single cohort (even-aged) stands (B.C. Ministry of Forests, 1995). 3. Method 3.1. Experimental design The spatial distribution of LWD characteristics (frequency, density, volume, biomass, orientation, submersion, and decay state) at the watershed scale was described by quantifying woody debris within various stream orders in the drainage network of three forested watersheds. Watershed features and the distribution of stream orders were derived from spatial data using a GIS (Arc/Info) (ESRI Canada, Kelowna, BC) that included the existing DEM (1:20,000), the (Terrain Resources

Information Mapping) TRIM streams data sets, and the watershed atlas. Information regarding LWD characteristics for each of the stream orders was obtained through in-stream surveys. LWD and stream morphology investigations were carried out during the months of July to October in 2003 and 2004. A total of thirty five sites with stream orders of first to fifth were sampled. For the purpose of this study, all the study sites were selected based on the following criteria: (1) the streams were located in areas of intact mature riparian forests (>80 years); (2) the stream side forests were not disturbed by human activities, such as harvesting, road building; (3) the streams were not salvaged. Therefore, the results from this study provide a baseline of LWD characteristics in intact mature riparian forests in the southern interior of British Columbia. The stream network of each watershed was divided into four stream size categories based on stream order, as determined from a 1:20,000 map, and channel bankfull width: (1) stream size I: stream order 1 with bankfull width less than 3 m; (2) stream size II: stream orders 1–3 with bankfull width 3–5 m; (3) stream size III: stream orders 3–4 with bankfull width

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Table 1 Channel characteristics of the study watersheds by stream size categories Stream sizes

Order

Width

Depth

Area

Gradient

D

L-slope

R-slope

I II III IV

1 2–3 3–4 4–5

2.2  0.4 4.0  0.6 6.1  0.5 10.0  2.8

0.4  0.1 0.6  0.1 0.7  0.1 0.8  0.1

333.8  49.3 588.6  90.1 905.7  76.1 1497.8  405.4

4.9  3.0 4.7  3.1 3.2  1.4 3.3  1.1

12.9  4.2 10.0  3.6 9.6  3.7 16.6  6.5

17.2  8.0 15.9  9.0 13.5  3.5 11.0  5.5

19.2  9.9 14.6  7.3 16.5  5.1 16.2  10.3

Order: natural stream order; width: mean stream channel bankfull width (m); depth: mean stream channel bankfull depth (cm); area: streambed area (m2); gradient: mean stream gradient (%); D: the b-axis diameter of the largest stone moved by flowing water (cm); L-slope: mean left stream bank slop (8); R-slope: mean right stream bank slop (8). Data are mean  S.D.

5–7 m; (4) stream size IV: stream orders 4–5 with bankfull width greater than 7 m. The channel morphological characteristics of the four stream size categories in the study watersheds are shown in Table 1. 3.2. Determination of LWD size, volume, orientation, position and decay state LWD (is defined in the present study as wood >0.1 m in diameter and >1 m in length) data was collected in 35 study reaches, which were representatives of EESF and MS biogeoclimateic zones, totaling 150 m in length. Each reach was divided into ten 15 m sections. LWD pieces located at least partially within or above the bankfull width of the channel were assessed by decay class, orientation, position within the channel (submergence), and distance from downstream end of the study reach. The small end diameter within bankfull width and the large end diameter within bankfull width of each LWD piece were measured using calipers. The length within the bankfull width and the total length of each LWD piece were measured using tape-measures. For the pieces having a root attached, the dimensions of the root, and the diameter at the junction of the bole and the root wad were measured. The volume of each LWD piece was calculated using the following equation based on the assumption of cylindrical shapes:
V ¼ Lp

d1 þ d2 4

2 (1)

where V was the piece volume (m3), d1 and d2 the small and large end diameters (m) of each piece, and L was the length (m) within the bankfull width of each piece. Total LWD volume within bankfull width was calculated by totaling the piece volumes in each of the 150 m study reaches. Volume per unit channel area (m3/m2) was calculated by dividing the total study P reach volume by channel area (i.e. A ¼ ð15Wi Þ, where A is channel area, 15 is the section length and Wi is the bankfull width in i section). Therefore, only LWD volume and LWD length within the bankfull width were used in the following sections. Similar to Hauer et al. (1999) three decay classes were used to describe the stage of decomposition of each LWD piece: (class 1) the debris had intact bark, or at least >50% remaining, wood hard with original color, branches or twigs present; (class 2) the debris had trace of bark (<50% of the bark remaining), no twigs observed, and wood had some

surface abrasion; (class 3) the debris had dark color, no bark and twigs observed, wood soft throughout with holes and openings. Each LWD piece was grouped into four orientation categories based upon the predominant orientation of the piece to the direction of bank orientation: (1) perpendicular or close to perpendicular to banks (60–1208/240–3008); (2) parallel or close to parallel to banks (150–2108/30–3308); (3) downstream as parallel to banks with the smaller-diameter end situated downstream (30–608/210–2408 or 120–1508/ 300–3308); (4) upstream as parallel to banks with the smaller-diameter end situated upstream (30–608/210–2408 or 120–1508/300–3308). Each LWD piece was also assigned to one of four submergence categories depending on a pieces position within the channel or spanning the channel: (1) lower half of bankfull height, (2) upper half of bankfull height, (3) spanning the channel cross-section, and (4) any combination of the above three positions. 3.3. Calculation of LWD biomass LWD wood cross-sectional disks with about 1–3 cm width were cut from each study site with a chainsaw for use in determining wood density (g/cm3). Wood disks were sampled from known tree species (i.e. spruce, fir, and pine) and unknown species from three decay classes. The fresh wood disks were weighed, and the diameter and thickness of the disks were measured. Wood sub-samples were weighed and dried to determine the ratio of dry to wet mass. In some cases, the volume of the disk was determined gravimetrically by water displacement (Krankina and Harmon, 1995). After volume determination the wood samples were oven dried at 75 8C to constant mass. LWD wood density was then calculated as ratio of its dry mass to volume. The decay class, tree species, piece volume, and the corresponding density for each LWD piece were used to calculate LWD biomass. 3.4. Measurement of stream channel features The distribution and characteristics of the study reaches features (i.e. elevation, slope, stream order, forest and riparian composition, structure and age) were obtained from readily available physiographic and forest cover digital data. This spatial information was derived using GIS analysis using the following datasets: (1) 1:20,000 Terrain Resources Information Mapping (TRIM) I/II Digital Elevation Model; (2) 1:20,000 TRIM I/II Contours; (3) 1:20,000 VRI–Vegetation Resource Inventory

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(Forest Cover Information); (4) 1:50,000 Watershed Atlas, Major Watershed Basins, Watershed Sub-basins, Stream Network. Channel dimensions and morphology characteristics were measured for each 150 m study reach. Bankfull width, bankfull depth, wetted width, wetted depth, stream gradient, and left and right bank slopes were measured at 11 evenly spaced intervals (15 m apart) along the 150 m study reach. Bankfull width and height were defined by a change in vegetation (e.g. no moss cover to moss-covered ground) and a topographic break from the channel bank to the forest floor. Bankfull width, bankfull depth, wetted width, and wetted depth were measured using tape-measures. The bankfull width and bankfull depth at each study site was calculated as the average of the 11 measurements. Stream gradient and left and right bank slopes were measured with a handheld clinometer. The 11 measurements were averaged to obtain the values of stream gradient and left and right bank slopes for the entire reach. 3.5. Estimate of LWD loading at the watershed scale For a watershed, the total LWD piece (LWDP) number is the sum of LWD pieces within each stream order class, P which can be expressed using the equation:(2)LWDP ¼ Di Li Bi ; i ¼ 1; 2; . . . ; nwhere, Li is the total stream length (m) in i-order stream class, which is determined by GIS, Di the mean LWD piece density (number/m2) in i-order stream class, Bi the mean stream bankfull width (m) in i-order stream class, and Di and Bi are usually obtained from field investigation:
XX m di j ; j ¼ 1; 2; . . . ; m (3) Di ¼ n
XX m bi j ; j ¼ 1; 2; . . . ; m (4) Bi ¼ n

where dij is the measurement of LWD piece density in j representative reach of i-order stream, bij the measurement of stream bankfull width in j representative reach of i-order stream, m the number of surveyed reach and n is the number of surveyed stream. Total LWD volume and biomass in the watershed is estimated using a similar approach. 3.6. Data analysis Based on site selection criteria, we randomly selected streams at each stream order for the study. Data of stream order and bankfull width were then used to set up the four stream size categories (Appendix A). Mean values of LWD density, volume, and biomass among different stream channel sizes were tested for differences by the least squares means multiple range tests. Statistical analyses were performed using the analysis of variance (ANOVA) by SAS statistical software (SAS Institute Inc., 1999). We considered test results with significance levels ( p-values) of <0.05 to be significant. Regression analysis between LWD properties and channel morphological features of study sites was performed using the least-squares method. 4. Results 4.1. Distribution of individual in-stream LWD within the different sized streams The average LWD diameter, length, volume, and biomass generally increased with increasing stream size (Fig. 2). The mean LWD diameter in stream size I (16.4 cm) was obviously lower than that in stream size III (20.6 cm) and size IV

Fig. 2. Mean diameter (A), length (B), volume (C) and biomass (D) of individual LWD within the four stream size categories in forested watersheds in southern interior of British Columbia. Error bar represents the standard deviation. Different letters above the bars indicate significant differences (ANOVA; p < 0.05) among stream size categories. Stream size I: less than 3 m; stream size II: 3–5 m; stream size III: 5–7 m; stream size IV: larger than 7 m.

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Fig. 3. Frequency of LWD by (A) diameter; (B) length and (C) volume classes within the different stream size categories in forested watersheds in southern interior of British Columbia (stream size I: less than 3 m; stream size II: 3–5 m; stream size III: 5–7 m; stream size IV: larger than 7 m).

(20.5 cm), respectively. The mean LWD length increased from 2.3 m in stream size I to 2.9 m in stream size II, 3.1 m in steam size III, and to 3.9 m in stream size IV. LWD length in stream size IV was greater than stream sizes I–III. Due to the relatively large size (both diameter and length) of the individual pieces, stream size IV had the highest mean volume (0.18 m3/piece). This difference was significantly higher than in stream size I (0.06 m3/piece, p < 0.0001) and stream size II (0.11 m3/piece, p = 0.002). Also, the differences of LWD volume between

stream size I and both sizes II and III were significant. No statistical differences of piece volume were found between stream sizes III and IV and between stream sizes II and III. As expected, the highest individual LWD biomass was found in stream size IV (59 kg/piece), which was higher than in stream size I (20 kg) and stream size II (32 kg). LWD biomass in stream sizes II and III (41 kg,) was higher than stream size I. The LWD biomass did not differ significantly between stream sizes II and III, and stream sizes III and IV ( p > 0.05). The

Fig. 4. Relationships between individual LWD volume and channel features in forested watersheds in southern interior of British Columbia. LWD volume with (A) bankfull width; (B) bankfull depth; (C) stream area.

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results indicate that LWD size and biomass were positively correlated with stream width. In the study watersheds, the proportion of the smallest pieces number in relation to the total piece number decreased as the stream size increased (Fig. 3). For example, more than 50% of the pieces were less than 15 cm in diameter, 2 m in length, and 0.05 m3 in volume in the stream size I category. The proportion of LWD in the smallest size class decreased to 40–45% in stream size II, 30–40% in stream size III, and 20–30% in stream size I. The differences between stream size I and both sizes III and IV were significant ( p < 0.05). Conversely, the frequency of LWD in the largest size class (pieces greater than 30 cm in diameter, 5 m in length and 0.2 m3 in volume) increased with an increase in stream size. Approximately 5% of LWD in the stream size I were in the largest size class, with the percentage increasing to approximately 10% in stream size II, 15% in stream size III and slightly greater than 20% in the stream size IV. Statistical differences were found between stream size I and stream sizes III and IV ( p < 0.05). Positive correlations were found between individual LWD volumes and stream bankfull width, bankfull depth, and stream area in all the study sites (Fig. 4). 4.2. LWD density, volume and biomass per unit of stream area LWD density (pieces number per 100 m2 of stream area) decreased as stream size class increased (Fig. 5A). For instance, LWD density was 19 in stream size I, decreased to 17 in stream size II, 12 in stream size III, and 4 in stream size IV. The differences between stream sizes I and IVand stream sizes II and IV were significant. No statistical differences were found among stream sizes I–III, and between stream sizes III and IV ( p > 0.05). The rank of LWD volume and biomass per 100 m2 of stream area in relation to stream size class was similar, with stream size II > stream size III > stream size I > stream size IV. On average, LWD volume was 1.16, 1.58, 1.54, and 0.78 m3/100 m2 in stream sizes I, II, III and IV, respectively. In other words, if LWD volume in stream size I was assumed as 1.00, then relative proportions in stream sizes II–IV would be 1.36, 1.33 and 0.67, respectively. Except for the significant differences in LWD volume between stream sizes II and III and stream size IV, no significant differences were found in terms of LWD volume between stream sizes I– III, and between stream sizes I and IV ( p > 0.05). LWD biomass averaged 383 kg/100 m2 (range 265–651 kg/100 m2)

Fig. 5. LWD density (A), volume (B) and biomass (C) per unit stream area within the different stream size categories in forested watersheds in southern interior of British Columbia. Error bar represents the standard deviation. Different letters above the bars indicate significant differences (ANOVA; p < 0.05) among stream size categories. Stream I: less than 3 m; stream II: 3–5 m; stream III: 5–7 m; stream IV: larger than 7 m.

in stream size I, increased to 491 kg/100 m2 (range 81– 1254 kg/100 m2) in stream size II, slightly declined to 465 kg/ 100 m2 (range 247–938 kg/100 m2) in stream size III, and decreased to 250 kg/100 m2 (range 88–533 kg/100 m2) in stream size IV. No significant differences were found between these four stream sizes in terms of LWD biomass except between stream sizes II and III and stream size IV ( p < 0.05, Fig. 5C). Strong correlations were found between channel characteristics and LWD amount per unit stream area (Fig. 6). LWD

Fig. 6. Relationships between stream channel features and LWD density per unit stream area in forested watersheds in southern interior of British Columbia. LWD density and (A) bankfull width and (B) stream area.

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Fig. 7. Frequency of LWD number by orientation category within the different stream channel sizes in forested watersheds in southern interior of British Columbia (P: parallel; Pd: perpendicular; D: downstream; U: upstream). Stream size I: less than 3 m; stream size II: 3–5 m; stream size III: 5–7 m; stream size IV: larger than 7 m.

Fig. 9. Frequency of LWD number by decay class categories within the four stream channel sizes in forested watersheds in southern interior of British Columbia (stream size I: less than 3 m; stream size II: 3–5 m; stream size III: 5– 7 m; stream size IV: larger than 7 m).

density was inversely related to increases in channel bankfull width and stream area.

LWD pieces in intermediate and larger sized streams (stream sizes II–IV) were found to be orientated parallel to the streamflow direction, but only 26% were found in the smallest sized streams. The differences in the proportion of LWD parallel orientation between stream size I and stream sizes II–IV were significant. There were no statistic differences in LWD parallel percentage between stream sizes II–IV. The percentage of LWD in the position of perpendicular was higher in stream size I (63%) than stream size II–IV. Again, no statistical differences in terms of LWD perpendicular percentage were found among stream sizes II–IV. The proportion of LWD in both upstream and downstream categories was unexpectedly low, indicating the instability of LWD pieces in these orientations through the stream networks.

4.3. LWD orientation, submersion and decay state at different sized streams The orientation, position (submergence), and decay state of LWD for each of the study reaches are shown in Figs. 7–9. The rank of LWD number frequency in terms of orientation in the smallest sized streams (stream size I) was perpendicular > parallel > downstream > upstream. Conversely, the percentage of LWD in orientation categories in the stream sizes II–IV was similar as parallel > perpendicular > downstream > upstream (Fig. 7). Approximately 45% of total

Fig. 8. Percentage of LWD number by submersion categories within the different stream channel sizes in forested watersheds in southern interior of British Columbia. Submersion 1: lower half of bankfull height; submersion 2: upper half of bankfull height; submersion 3: above bankfull and channel cross-section; submersion 4: any combination of the above three positions. Stream size I: less than 3 m; stream size II: 3–5 m; stream size III: 5–7 m; stream size IV: larger than 7 m.

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Table 2 Estimates of total LWD number, volume and biomass of the three forested watersheds in south interior of British Columbia, Canada Stream order Wilkinson Creek 1 2 3 4 5 Total West Kettle River 1 2 3 4 5 Total Nicola River 1 2 3 4 5 Total

Stream area (ha)

LWD number (piece)

LWD volume (m3)

826 80 60 37 15

183 32 37 37 15

355126 52748 43515 15839 6509

21206 5007 5640 2885 1185

7011 1559 1705 929 382

1018

304

473737

35923

11586

1289 183 93 85 70

285 73 57 85 71

553821 120707 67676 36264 30094

33070 11457 8772 6606 5479

10934 3567 2652 2127 1765

1720

571

808561

65385

21027

1423 222 106 55 53

315 88 65 55 53

611482 146223 77297 23353 22522

36513 13879 10019 4254 4101

12072 4321 3029 1370 1321

1859

576

880876

Stream length (km)

2

LWD biomass (t)

68766 2

22094 2

The total area is—Wilkinson Creek watershed: 675 k m , West Kettle River watershed: 1032 k m , Nicola River watershed: 1162 k m .

LWD position located within the bankfull channel reflected the degree LWD impacts water flow patterns and fish habitat at varying flow levels. The proportion of LWD completely submersed within the lower half of bankfull height (submersion category 1) was only 3 and 5% in stream sizes I and IV, respectively, and was lower than those in stream sizes II and III (12 and 16%, respectively, p < 0.05) (Fig. 9). This indicates that more pieces touch the streambed at low flow in the intermediate sized streams than in both small and large sized streams. No statistical differences were found in piece number in the lower position (submersion category 1) between stream sizes I and IV, and sizes II and III. In submersion category 2, the percentage of LWD was significantly lower in stream size I than stream sizes II–IV. The results indicate that (1) LWD impacts on streamflow in small sized streams were limited even in the duration of highlevel flow, and (2) LWD control of streamflow in large sized streams is enhanced during the period of high-level flow. A great majority of the LWD pieces in all stream sizes were in the advanced decay class (decay class 3) (Fig. 9). The differences in LWD decay class 3 between stream size I and both stream sizes II and III were significant. No significant differences were found in LWD decay class 1 among the four stream size categories ( p > 0.05). The significant differences in decay class 2 were found between stream size III and the stream sizes I and IV. 4.4. Estimate of LWD at watershed level Based on collected GIS data and our in-stream field surveys, LWD number, volume and biomass in the study watersheds were calculated (Table 2).

Although the total stream area within the studied watersheds accounts for a very small proportion of the total watershed area (approximately 0.5%), the accumulated LWD number, volume, and biomass were considerable in the stream networks. In general, the first stream order accounted for approximately 75% of total stream length, 50–60% of total stream area, 60% of total LWD number, and 55% of total LWD volume and biomass. The fifth-stream order accounted for less than 5% of total stream length, about 10% of total stream area with 3% total LWD number, and about 6% of total LWD volume and biomass. The intermediate stream order (the second- to fourth-order streams) accounted for the remaining percentages. 5. Discussion Patterns and forms of in-stream LWD distribution and accumulation depend on the characteristics of riparian structure, streamflow power and channel systems, as well as LWD pieces (Lienkaemper and Swanson, 1987; Bilby and Ward, 1989; Marcus et al., 2002; Abbe and Montgomery, 2003). Mean individual LWD size (diameter, length, volume, and biomass) increased with increasing stream size, but LWD density appeared to decrease with increasing stream size. The findings are consistent with the previous studies (Bilby and Ward, 1989; Montgomery et al., 1995; Beechie and Sibley, 1997; Marcus et al., 2002). Wider streams usually exhibit higher streamflow discharge and have a higher capacity to move material downstream. Conversely, smaller sized channels exhibit less streamflow volume and have a lower capacity to transport material such as LWD (Bilby and Ward, 1989; Montgomery et al., 1995; Piegay and Gurnell, 1997).

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It is also noted that the relationships of stream bankfull width and LWD length were an important factor in affecting the spatial distribution of LWD size and density within the stream network of watersheds (Piegay and Gurnell, 1997; Marcus et al., 2002). Lienkaemper and Swanson (1987) pointed out that downstream movement of woody debris is strongly related to length of individual LWD. When LWD was considerably longer than channel width it results in relatively stable wood pieces. Braudrick and Grant (2001) also showed that length and diameter of trees increased the stability of woody debris. In streams where the channel width is less than the height of streamside forests, the fallen logs often bridge across the channel and are relatively stable and are thus not easily transported downstream (Nakamura and Swanson, 1993). Abbe and Montgomery (2003) used the ratio of the log diameter to bankfull depth and the ratio of log length to bankfull width to describe the log stability thresholds for key, racked, and loose pieces. They found that both matter for explaining the stability of large key-member logs serve as the foundation for log jams. Here we use the stability index (n), defined as the ratio of mean LWD length (L) and mean channel bankfull width (B), namely n = L/B to describe the stability of LWD pieces in relation to stream size. The results showed that the stability index was 1.1, 0.7, 0.5 and 0.4 for the stream sizes I–IV, respectively. The index is a useful indicator to characterize the LWD movement in the channel network. The larger the value of n, the less likely the LWD piece will be transported downstream; therefore, a higher v indicates a higher stability. In comparison to in-stream LWD loadings reported from the coast areas of the Pacific Northwest, our data was low. For instance, mean LWD biomass per unite of channel area was about 2345–2520 kg/100 m2 in coastal British Columbia (Harmon et al., 1986; Fausch and Northcote, 1992) and 2700 kg/100 m2 in Oregon’s Cascade Mountains (Harmon et al., 1986), which were 5–10 times higher than in our study streams. LWD total volume in coastal streams of southeast Alaska ranged 3.9–7.9 m3/100 m2 with a mean of 6.1 m3/ 100 m2 (Robison and Beschta, 1990) and about 7.0 m3/100 m2 for Washington coastal streams (Grette, 1985), which was two to four times higher than in this study sites. The lower LWD biomass and volume in the study watersheds are mainly related to the relative short circle period of wildfire in this area. It was reported that the average return intervals of stand replacing forest fires ranged from 125 to 150 years in ESSF and MF biogeoclimatic zones in British Columbia (B.C. Ministry of Forests, 1995). In contrast, average recurrence intervals for stand-resetting wildfires were about 200 years in coastal forests of the Olympic Peninsula and Oregon Coast Range (Agee, 1990). The range in mean LWD diameter and LWD total volume in the southern interior streams of British Columbia reported from this study (Figs. 2 and 5) was comparable with than values for Boreal Shield streams in Ontario (14–20 cm in diameter, Kreutzweiser et al., 2005) and Boreal Cordillera streams in Yukon Territory (12–19 cm in diameter and 0.05–1.27 m3/100 m2 in volume, Mossop and Bradford, 2004).

59

A large proportion of LWD pieces in small sized streams were oriented perpendicular to streamflow. Whereas the majority of the LWD were oriented parallel to streamflow in intermediate and large sized streams (Fig. 7). The change in LWD orientation from the small sized streams to intermediate and large sized streams indicates that LWD pieces reaches a stable state throughout the movement in the channel networks. Braudrick and Grant (2000) conducted flume experiments with woody pieces shorter than bankfull width and found that orientation to streamflow, presence of a rootwad, log density, and log diameter were the dominative factors in LWD transport, and they demonstrated that woody pieces stability increased if the pieces were rotated parallel to streamflow within the fluvial systems. The finding that the proportion of LWD oriented perpendicular to the channel declined with increasing stream size in this study is consistent with previous observations (Bilby and Ward, 1989; Richmond and Fausch, 1995). The highest percentage of LWD in a perpendicular orientation in stream size I reflected that most LWD that entered these small sized streams were naturally oriented perpendicular to channel, which implies that LWD did not rotate during normal flow. In fact, even in the duration of high flow, a great amount of LWD still can not get through the flow and thus can not been rotated. This is because a high percentage of LWD was found to be in the position above the bankfull and channel cross-section in small sized streams (Fig. 8). The position of LWD in streams reflects the potential influence of LWD on channel habitat, hydraulic condition, and sediment deposition (Robison and Beschta, 1990). The low percentage of LWD in the positions of submersions 1 and 2 indicates that the LWD has limited influence on streamflow and aquatic environments at both low and high flow in these small sized streams. This situation raises the question of how we can manage LWD in practice to balance the proportion of LWD in the submersion categories in order to enhance the LWD function in channel morphology and habitat in small sized streams. In contrast to small streams, most LWD in the intermediate sized streams was oriented parallel to the streamflow, indicating that the LWD has been moved and re-oriented. The high proportion of LWD located below the bankfull height of the channel (Fig. 8) revealed that LWD can enter the stream bottoms where it interacts flow and affects erosion and sediment deposition during the period of low flow. The majority of LWD was oriented parallel to the streamflow in large sized streams. A low proportion of LWD located in submersion 1 and a high percentage of LWD located in submersion 2 demonstrated that the impacts of LWD on streamflow and aquatic environments were limited at base flow time, and were greatly enhanced during the high flow periods. With the increase of stream width, the proportion of advanced decay LWD piece was increased in the study sites. It may attribute to the variation of local microclimate condition at different stream orders. The direct radiation solar energy was increased with the increasing stream sizes and the temperature would be increased. It would enhance

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microbial activities and, therefore, result in the acceleration of wood piece decomposition (Mackensen et al., 2003). The reason that advanced decay LWD amount was slightly declined in the largest sized streams might be related to the LWD size in these steams. The mean individual LWD diameter was largest in the largest sized streams. It was reported that decomposition rate was inversely related to log diameter due to lower surface:volume ratio which reduced access to decomposers and lowered rates of gas and water exchange in proportion to volume (Harmon et al., 1986; Mackensen et al., 2003). Stream networks in mountainous watersheds of southern British Columbia typically exhibit stream gradients that have a potential to result in downstream migration of LWD. This trend corresponds to LWD delivery from lower order streams (upstream) to higher order streams (downstream). The spatial heterogeneity of in-stream LWD characteristics (size, amount, volume, biomass, decomposition status, position, and orientation) at the watershed level highlights the need to recognize and account for the variation of LWD through channel network when evaluating the structural and functional role of LWD in stream ecosystems. Also, this needs to be considered when designing riparian management strategies to maintain the suitable LWD loadings in forested watersheds. Such structural variation of LWD loadings will result in different distribution patterns of carbon and other nutrient elements, and thus form a variety of riparian and aquatic habitats through the channel network. Few studies have been reported to estimate the total LWD number, volume, and biomass in stream systems at the watershed scale. Two broad approaches have been proposed to quantify the total amount of objects in the larger scale (Upton and Finglfton, 1985; Magurran, 1988). The first is to derive a mean value from a typical unit within the large area, and apply this mean to the whole area, regardless of spatial variation and distribution of the object. The second approach considers distribution features of the object over a large area. From this, the values of the whole area are aggregated from varying representative units corresponding to the distribution patterns of the object within the large scale. Our results along with other studies (Bilby and Ward, 1989; Montgomery et al., 1995; Beechie and Sibley, 1997; Marcus et al., 2002) showed that LWD number, volume, and biomass vary with the size of the stream. Therefore, the second approach is a more appropriate and an accurate method to estimate the LWD values through the stream networks within a watershed. As a first step to quantifying LWD at multiple spatial scales, the present study has accounted for stream size in addition to the spatial variability and distribution of LWD loading in order to aggregate woody debris to the whole watershed. There exists considerable room for improvement and refinement of this calculated approach. For example, besides channel bankfull width (although it is a dominant parameter), other channel morphological features, such as gradient, bank slope, the structure of streamside forests, and land-use conditions should be considered for a more detailed analysis.

6. Conclusion Our study shows that spatial variation and distribution of instream LWD characteristics vary as a function of stream size in the forested stream ecosystems in southern interior of British Columbia. LWD density and volume are correlated to channel bankfull width, bankfull depth and stream surface area. The change of woody pieces from perpendicular to parallel orientation within the stream systems reveals that LWD trends to be stable through the channel networks. It is because LWD piece has larger surface area to the hydraulic flow direction in perpendicular than in parallel orientation within the channel. The results gained from this research have direct implications to the development of guidelines for the management of riparian forests at watershed level. Due to spatial heterogeneity of LWD at watershed level, It is important for the managers to consider the LWD abundance, position and orientation at the drainage basin scale when making development and conservation strategy. In addition, LWD characteristics at different sized streams can be used as reference in the BC southern interior region to simulate and assess natural and human disturbance impacts on material transport and stream habitats at watershed and landscape scales. In small sized streams, LWD exhibits the small size, high density, low volume and biomass per unit of stream area. The majority of LWD are perpendicular to the streamflow and spanning the channel, and thus they have no chance to interact with streamflow even during periods of high flow. LWD in intermediate sized streams exhibits high volume and biomass per unit of stream area. The largest proportion of pieces in intermediate sized streams is found below bankfull height where LWD directly impacts the aquatic environments. In large sized streams, LWD number, volume, and biomass per unit of stream area are low, but mean individual LWD size is high. A great proportion of LWD is found at the upper bankfull height zone. High variations in LWD structure and function in different sized streams reflect the spatial heterogeneity of forested watersheds. These results also highlight the need to making LWD and riparian management decisions at watershed scales in light of maintaining a variety of habitat features over a range of stream orders. Acknowledgements This research was funded through the Natural Science and Engineering Research Council (NSERC) project and British Columbia Forestry Science Program (FSP) project. We gratefully thank Julie Brown, Kristin Storrey, and Tanya Seebacher for assistance in collection of field data. Numerous individuals from Riverside Forest Products Ltd., Gorman Bros. Lumber Ltd., Weyerhaeuser (Canada) Ltd., Dobson Engineering Ltd., the Ministry of Forests and the Ministry of Sustainable Resource Management also assisted with identification of field study sites. We thank the several anonymous reviewers who provided many helpful comments on the manuscript.

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Appendix A Geomorphologic characteristics of the 35 sites in south interior of British Columbia. Sites are located on streams within three forested watersheds (see Fig. 1). Stream name

Size

Order

Elev

Grad

Width

Depth

L-slope

R-slope

Nicola Creek Bobcat Creek Municipal Creek (1) Dome (1) Sunset Main CP671 Reed Creek Cotton Creek (1) Sunset Main CP672 Dome (2) Wilkinson Creek Sterling Creek Municipal Creek (2) Terrace Creek Beak Creek 240 Creek Ellis Creek (1) Ellis Creek (2) North Ellis Creek Joe Rich Creek Lower Kettle Upper West Kettle Upper West Kettle (2) Upper West Kettle (1) Saunier Creek North Ellis Creek (2) Wilkinson Creek 3 Sterling Creek (2) Sterling Creek (1) Power Creek North Ellis Creek (1) Bald Range Creek Upper West Kettle (3) Wilkinson Creek (1) Wilkinson Creek (2) Upper West Kettle (4)

I I I I I I I I I II II II II II II II II II II II II II II II III III III III III IV IV IV IV IV IV

1 1 1 1 1 1 1 1 1 2 1 2 3 1 2 2 2 3 3 3 2 2 2 3 3 4 4 4 3 4 4 3 5 4 5

1380 1659 1587 1504 1471 1712 1451 1331 1510 1583 1321 1580 858 1354 1596 1405 1382 1540 939 1357 1418 1452 1512 1107 919 1507 1038 1005 1147 1195 738 1434 1348 1504 1414

2.2 4.2 6.8 1.1 2.5 5.4 10.3 4.2 7.8 5.4 5.1 5.7 2.8 14.3 3.7 5.3 5.5 6.3 1.8 3.3 1.7 4.0 2.2 2.6 5.2 2.7 1.3 3.1 3.9 1.8 4.6 3.1 4.3 3.7 2.1

1.6 2.0 2.0 2.1 2.1 2.4 2.4 2.5 2.8 3.2 3.4 3.4 3.4 3.5 3.5 3.8 3.8 4.1 4.2 4.2 4.5 4.8 4.9 4.9 5.6 5.7 6.2 6.4 6.8 8.0 8.1 8.1 9.4 11.2 15.1

0.4 0.4 0.4 0.5 0.4 0.5 0.3 0.4 0.6 0.5 0.4 0.6 0.4 0.5 0.4 0.7 0.6 0.7 0.5 0.5 0.6 0.6 0.6 0.7 0.7 0.7 0.6 0.6 0.7 0.7 1.0 0.9 0.7 0.9 0.8

15.1 6.5 11.6 9.3 13.8 28.3 17.5 26.5 26.2 34.1 30.7 24.2 11.0 ND 17.1 7.6 10.2 14.4 7.5 16.1 6.5 20.3 4.7 18.5 16.6 15.5 14.5 7.6 13.1 12.9 4.5 16.5 17.4 9.5 5.1

20.0 3.2 16.6 12.8 13.5 37.0 16.4 29.5 23.7 22.3 15.6 26.4 8.9 ND 25.1 7.1 16.0 15.7 4.7 22.5 7.5 16.0 8.8 7.1 23.1 16.5 17.6 8.7 16.5 13.2 6.7 22.1 12.5 8.4 34.1

Size: stream size; order: natural stream order; Elev: site elevation (m); Grad: mean stream gradient (%); width: mean stream channel bankfull width (m); depth: mean stream channel bankfull depth (cm); L-slope: mean left stream bank slope (8); R-slope: mean right stream bank slope (8). ND means no data for that variable.

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