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Sediment deposition in streams adjacent to upland clearcuts and partially harvested riparian buffers in boreal forest catchments
Forest Ecology and Management 258 (2009) 1578–1585
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Sediment deposition in streams adjacent to upland clearcuts and partially harvested riparian buffers in boreal forest catchments David Kreutzweiser a,b,*, Scott Capell a, Kevin Good a, Stephen Holmes a a b
Canadian Forest Service, Natural Resources Canada, 1219 Queen St. East, Sault Ste Marie, Ontario, Canada, P6A 2E5 Cooperative Freshwater Ecology Unit, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 2C6
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 May 2009 Received in revised form 26 June 2009 Accepted 2 July 2009
The rates of fine sediment deposition were compared among three logged and three reference stream reaches 2–3 years before and 3–4 years after logging to assess the environmental impacts of partial harvesting as a novel riparian management strategy for boreal forest streams. The partial-harvest logging resulted in 10, 21 and 28% average basal area removal from riparian buffers at the three logged sites, adjacent to upland clearcut areas. No significant differences from pre-logging or reference-site sedimentation patterns were detected for two of the three logged sites. At the site with the most intense riparian logging (WR2), significant increases of 3–5 times higher than pre-logging or reference levels were detected in fine inorganic sediment (250–1000 mm) load and accumulation in the first year after logging, but no significant change was detected in fine organic sediments or very fine sediments (0.5– 250 mm). The increased inorganic sediment deposition at WR2 was temporary with no significant differences from reference or pre-logging levels detectable by summer of the second post-logging year. Logging impacts on fine sedimentation in streams appeared to have been mitigated by careful logging practices including winter harvesting in riparian areas to reduce ground disturbance, and a tendency to avoid immediate (within 3–5 m) stream-side areas. Where it is feasible and advisable to conduct partial harvesting in riparian buffers of boreal forest streams, the logging can be conducted without posing significant risk of increased sediment inputs to streams when careful logging practices are followed. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Sedimentation Streams Partial harvest Logging Riparian buffers
1. Introduction Fine sediment inputs are among the most widely implicated impacts of watershed logging on forest streams (Waters, 1995; Wood and Armitage, 1999; Croke and Hairsine, 2006). Excessive fine sediment deposition on streambeds can have detrimental effects by interfering with hydrologic exchange processes and degrading streambed quality (Scha¨lchli, 1992; Brunke and Gonser, 1997), altering invertebrate abundance or community composition (Angradi, 1999; Zweig and Rabeni, 2001; Kreutzweiser et al., 2005a), and reducing fish egg and fry survival by impinging on spawning and incubation substrates (Scrivener and Brownlee, 1989; Sutherland et al., 2002). Forest management guidelines and regulations in North America have attempted to mitigate logging impacts on fine sediment inputs by implementing various best management practices (Fortino et al., 2004; Vowell and Frydenborg, 2004; McCord et al., 2007). One of the more common best
* Corresponding author at: Canadian Forest Service, Natural Resources Canada, 1219 Queen St. East, Sault Ste Marie, Ontario, Canada, P6A 2E5. Tel.: +1 705 541 5648; fax: +1 705 541 5700. E-mail address: [email protected] (D. Kreutzweiser).
management practices for forestry is the application of riparian (shoreline) buffers or restricted-harvest reserves to minimize machine-generated ground disturbance and channeled flowpaths near streams and shorelines (Lee et al., 2004). In general, these forested riparian buffers are effective at mitigating many logging impacts on aquatic systems (Norris, 1993; Barling and Moore, 1994; Castelle et al., 1994; Ward and Jackson, 2004). However, it has recently been noted that the systematic application of no-harvest riparian buffers along streams and around other water bodies in the boreal forests of Canada is resulting in unnatural, linear patterns of older growth forest stands across the landscape resulting in ‘‘ribbons’’ along streams and ‘‘donuts’’ around lakes (Buttle, 2002; Steedman et al., 2004). These riparian buffers are often protected from natural fire and insect disturbances for several decades because the adjacent upland areas contain young, regenerating stands that are less prone to these types of disturbance. Consequently, there may be a loss of productive shoreline habitat for biota that require natural stand-replacing disturbances to flourish, and the resulting landscape patterns could be contrary to the objectives of forest ecosystem sustainability and natural-disturbance emulation. This has led to the recognition that forest researchers, managers and policy-makers must consider disturbance management, natural
0378-1127/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2009.07.005
D. Kreutzweiser et al. / Forest Ecology and Management 258 (2009) 1578–1585
landscape patterns, and alternative riparian buffer configurations as they move toward more landscape-based forest management frameworks (Scrimgeour et al., 2000; Smith et al., 2003; Kardynal et al., 2009). One alternative riparian management strategy being considered for creating more natural shoreline habitat patterns in the boreal forest is partial-harvest or variable-retention logging in riparian areas. A large-scale field study is underway at the White River Riparian Harvesting Impacts Project (WRRHIP) to determine the operational feasibility, ecological benefits, and environmental impacts of partial-harvest logging in riparian buffers of boreal forest streams. The intent of the WRRHIP is to create riparian forest gaps by partial harvesting that are sufficient to regenerate early-successional vegetation in an effort to 1) increase riparian habitat complexity, 2) sustain the riparian ecological corridor function by retaining 50% or more of the original stand, 3) simulate shoreline fire disturbance to the extent that fires tend to burn patchily as they approach water bodies (Nitschke, 2005), 4) allow timber harvest in riparian areas to partially offset declining wood supplies and (or) to reallocate standing timber elsewhere on the landscape, and 5) to continue to provide protection for streams. As part of that study, we measured fine sediment deposition in streambed sediment traps before and after the logging. Impacts on stream temperature patterns are reported elsewhere (Kreutzweiser et al., 2009). 2. Methods 2.1. Study site and experimental logging The WRRHIP is located near White River, Ontario in central Canada, 75 km inland from the northeastern shore of Lake Superior (Fig. 1). The forest type in the study area consists of boreal mixed woods including white spruce (Picea glauca), black spruce (Picea mariana), balsam fir (Abies balsamea), trembling aspen (Populus tremuloides), and white birch (Betula papyrifera). The predominant soils are thin, bouldery glacial tills, largely humo-ferric podzols, and, to some extent, brunisols over Precambrian bedrock with frequent rocky outcrops. The landscape is uninhabited, accessible only by logging and mining roads, and has a rolling-to-broken topography with multiple lakes and drainage systems. The total relief over the study area is 160 m. A meteorological station located
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Table 1 Total precipitation (mm) at a weather monitoring station located 60 km west of the study area during the sediment sampler deployment periods of each year. Period
2002
2003
2004
2005
2006
2007
Jun–Jul Aug–Sep
172 133
113 192
81 244
90 141
113 82
94 109
Total
305
305
325
231
195
203
100 km to the southeast of the study area (Wawa, Ontario, 478580 N, 848470 W) receives on average about 1000 mm of precipitation per year (1/3 of that as snowfall), with an average annual air temperature of 2 8C and daily mean temperatures ranging from 15 to 15 8C (data averaged 1971–2000). The area is typically ice and snow covered from November to May. Seasonal precipitation data from the periods in which the sediment samplers were deployed were obtained from a weather monitoring station in Pukaskwa National Park located 60 km to the west of the study area (Table 1) (http://climate.weatheroffice.ec.gc.ca/climatedata/ dailydata_e.html). Five study blocks, each on a separate watershed, were located within a 120 km2 area, and each block contained a stream study reach (Fig. 1). Two of the study sites (WR3, WR4) had not been previously logged, and served as reference sites. Three sites (WR1, WR2, WR6) contained harvest blocks in which the upland areas were clearcut by conventional feller-buncher and grapple skidders, and in which the riparian buffers (30–100 m wide) were partially harvested with the same operational equipment. All logging in the watersheds was conducted in late winter to minimize ground disturbance. The WR6 site was harvested in the winter (March) of 2004 and the WR1 and WR2 sites were harvested in the winter (February) of 2005. The partial-harvest logging prescription in the riparian buffers was ‘‘up to 50% removal of merchantable trees as evenly distributed across species and size classes as possible, in accessible portions of the riparian buffers’’, and was quantified as basal area (BA) removal. While the prescription allowed up to 50% removal at any one area, feller-buncher access trails were clearcut up to 10 m wide, and the surrounding area within reach of the buncher was then partially harvested. This resulted in a patchy distribution of gaps of varying sizes, and the overall riparian disturbance was assessed by post-logging measurements of residual trees and
Fig. 1. The WRRHIP study sites on the Boreal Shield near White River, Ontario. The solid black line is a logging road, and the study blocks containing the stream study reaches are shown in dark shading. The study blocks lie between 488210 05.7500 N, 858200 59.3700 W and 488130 47.5900 N, 858220 03.2200 W.
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Table 2 Catchment and study reach characteristics of logged and reference watersheds. Harvest blocks in logged watersheds were clearcut in upland portions and partially harvested (up to 50% BA removal intensity) in riparian buffers. Average BA harvested in riparian buffers is the average BA harvested from the total commercial tree BA available on both sides of the study reaches. Characteristic
WR1
WR2
WR3
WR4
WR6
Treatment Watershed area (ha) Harvest block area (ha) Area of watershed harvested (%) Riparian buffer area (ha) Av. BA harvested in riparian buffers (m2/ha) Av. BA harvested in riparian buffers (%) Study stream order Stream reach length in harvest block (m) Stream reach av. bankfull width (m) Stream reach av. gradient (%)
logged 817 77 9 4.5 6.6 21 3 600 5.1 2.1
logged 90 48 53 5.9 8.9 28 1 840 2.6 3.6
reference 83 0 0 no buffer 0 0 1 n/a 3.3 2.4
reference 480 0 0 no buffer 0 0 3 n/a 6.4 1.7
logged 85 75 88 6.0 3.2 10 1 550 3.9 0.8
stumps to provide a measure of average BA removal of available timber in each of the riparian buffers. Further details of the stream reaches and their catchments are provided in Table 2. 2.2. Sediment deposition measurements Fine sediment deposition on streambeds was measured with two types of sediment traps that were deployed and retrieved twice through the field seasons of 2002–2007 (deployed late May and retrieved late July, re-deployed late July and retrieved early October). This provided summer and fall sediment deposition measurements in two pre-logging and four post-logging years for WR6, and three pre- and post-logging years for WR1 and WR2. Sediment traps were also placed in the two reference streams (WR3 and WR4). Ten replicates of each trap type were placed in each study reach for both sampling periods of each year. The traps were placed on the streambeds in thalweg positions but in depositional spots (avoiding riffles and direct turbulence) at approximately 10-m intervals through the downstream sections of each study reach. This ensured that all traps at logged sites were adjacent to the riparian logging and at the downstream ends of the harvest blocks. One trap type was intended to measure the instantaneous fine sediment load on the streambed at the time of collection. It consisted of a 17 cm 17 cm 5 cm deep plastic box, open on top and filled with washed stones of a consistent size (1.5–2.5 cm diameter), with a 1.5-cm mesh screen to retain the stones. The open top was intended to allow sediment particles to deposit and re-suspend by flushing and transport with varying stream discharge over the sampling periods. When the boxes were retrieved, plastic lids were placed on top to retain the contents, and they were transferred to a storage container, preserved in 5% formalin, and stored for laboratory processing. The second trap was intended to measure the total fine sediment accumulation over the sample period. It consisted of a 2.5-cm diameter 11-cm long plastic tube, open on one end and placed upright on the streambed. Each tube was attached to a brick to hold it upright and secure, and placed beside or near the sediment box. The small tube diameter relative to its depth was intended to capture and retain sediment particles thus measuring total fine sediment accumulation. When the tubes were retrieved, a plastic lid was attached to the top to retain the sediments and the contents were preserved as described for the sediment boxes. While our specific sediment trap designs have not necessarily been used previously, the concept of collecting fine sediment particles in traps on streambeds to determine sedimentation rates is widely used (e.g., Petticrew et al., 2007). Collected sediments were separated into two size classes: fine sediments (FS) of 250–1000 mm, and very fine sediments (VFS) of 0.5–250 mm. For both types of traps, the contents were washed through 1-mm and 250-mm sieves and into a collection bucket.
Materials on the 1-mm sieve were discarded. Materials retained on the 250-mm sieve were dried at 60 8C for two days, weighed, combusted at 500 8C for 2 h, cooled and re-weighed to provide ashfree dry weights (organic fraction) and ash weights (inorganic fraction) of FS. Materials washed through the 250-mm sieve were suspended in the water of the collection bucket by a stir-bar on a magnetic stir-plate set at a consistent speed. A number of 25-mL aliquots were withdrawn by pipette at a consistent depth and placed into a suction-filter apparatus with a pre-ashed, 0.5-mm glass fiber filter (Whatman 934-AH1). Aliquots were added until the filter would not pass further water. The total subsample was calculated from the number of aliquots filtered and the volume of water collected. A minimum of 50 mL was filtered for each sample, using and combining two filters if necessary. Filters and contents were dried, weighed, combusted and re-weighed as described for FS to provide organic and inorganic fractions of VFS. 2.3. Data analysis The overall approach to data analysis was to examine trends over time in sedimentation rates among logged and reference sites, and to determine if post-logging trends at logged sites deviated significantly from post-logging trends at reference sites or from pre-logging trends. The trends over time in fine sediment deposition were compared graphically. These trends were examined for differences between logged and reference sites by repeated-measures analysis of variance (RM-ANOVA) to account for correlation in time, using a general linear model in SigmaStat 3.5 (Systat Software Inc., San Jose, CA). Because the sediment traps were deployed in the same spots in each study reach at each sample time, the individual traps at each study reach were considered site replicates (subjects) and sample time was the repeated factor. The primary test for detecting potential impacts was the site time interaction (McDonald et al., 2000). When significant (p < 0.05), indicating that trends over time differed among sites, Holm-Sidak pairwise comparisons (to control the error rate for multiple comparisons) were made between each logged site and both reference sites at each sample time. Postlogging sample times were examined first. If post-logging sedimentation levels at a logged site were not significantly higher than at least one reference site at the same sample time, then no logging impacts were inferred. If the sedimentation at a logged site was significantly higher than both reference sites at the same sample time, and if there were no significant differences between that site and both references at any time during the pre-logging period, then a logging impact on sedimentation was inferred. If significant pre-logging differences also occurred between the logged site and at least one reference site, then the post-logging increase was compared separately by t-test with the average sedimentation at both reference sites over the pre-logging period
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(suggested by McDonald et al., 2000). If significant, a logging impact at that sample time was inferred. We recognize that the individual traps at each site were not treatment replicates, but site replicates. The three logged sites were not considered replicates of logging treatment because the extent of watershed logging and the intensity of riparian logging varied among the three sites (Table 2). Rather, the focus was on trends over time at each site and separate comparisons between each logged site and both references sites. These were site comparisons from which we inferred impacts or no impacts under an assumption that trends at a logged site will not deviate significantly from trends at the reference sites in the absence of a logging disturbance. Although the study followed a BACI design, a classic BACI analysis (e.g., Stewart-Oaten et al., 1986) was not applied because it averages over sample times within the before and after periods, and consequently may not detect short-term fluctuations in response patterns. Sediment deposition data from both types of traps were expressed as g/m2 and log transformed to improve normality and homogeneity of variances prior to analysis. Graphical presentation of trends was made with non-transformed sedimentation data. Missing data arose from toppled or otherwise disturbed sediment traps. If a trap appeared to have been influenced by a local disturbance (e.g., trap tipped or tilted, evidence of someone stepping in or near a trap, upturned rootwad immediately upstream from the trap), the data were excluded if the sedimentation value was more than two standard deviations removed from the mean of 10 replicates. This process often removed one or two traps from each set of 10, rarely three, and never more than four. The general linear model for the RM-ANOVA accounted for missing values by estimating least square means in the F-statistic computations when necessary.
Fig. 2. Mean (SE) fine sediment (FS, 250–1000 mm particle sizes) load and accumulation in streambed traps of logged (suffix L) and reference (suffix R) study reaches. Samples were collected twice each year. Logging occurred at WR6 prior to the 2004 samples and at WR1 and WR2 prior to 2005.
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3. Results 3.1. Overall trends During the pre-logging period, the instantaneous sediment load in traps and the accumulation in tubes were variable among all sites and over time (Figs. 2 and 3). Among sites at a given sample time, there were as much as 5–11-fold differences in mean fine sediment (FS, 250–1000 mm) load or accumulation, while between sample times at a given site in the pre-logging period there were up to 8–22-fold differences. Very fine sediments (VFS, 0.5–250 mm) contributed 51–92% (mean 76%) of total sediment load in traps, and 29–82% with a mean of 57% of total sediment accumulation in tubes. There was less variability in the deposition and accumulation of VFS than FS among sites and over time. VFS load or accumulation was at most 4–6-fold different among sites at a given time and 4–7-fold different among times at a site. The two reference sites (WR3 and WR4) tended to have the greatest differences, with low and relatively consistent sedimentation at WR3 and higher and more variable sediment loading at WR4, thus representing a range of natural sediment deposition in these streams (Figs. 2 and 3). The organic fraction in FS load over the pre-logging period ranged from 4 to 49% among sites with a mean of 24% and a coefficient of variation (CV) of 51% (Table 3). The organic fraction of FS accumulation was higher and less variable with a range of 13– 61%, mean of 39%, and CV of 30%. Organic matter was more consistent in VFS load with 18–50% organic, mean of 39% and a CV of 19%, and slightly higher in accumulation tubes with a range of 28–56%, a mean of 46% and a CV of 14%.
Fig. 3. Mean (SE) very fine sediment (VFS, 0.5–250 mm particle sizes) load and accumulation in streambed traps of logged (suffix L) and reference (suffix R) study reaches. Samples were collected twice each year. Logging occurred at WR6 prior to the 2004 samples and at WR1 and WR2 prior to 2005.
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Table 3 Mean % organic fraction of sediments in trap loads and tube accumulations. FS is fine sediments of 250–1000 mm, VFS is very fine sediments of 0.5–250 mm. Sites with suffix L are logged sites and those with R are reference sites. Samples were collected twice each year. Logging occurred at WR6 prior to the 2004 samples and at WR1 and WR2 prior to 2005. Sample and site
Year 02
02
03
03
04
04
05
05
06
06
07
07
FS load WR1-L WR2-L WR6-L WR3-R WR4-R
4 11 17 28 27
15 9 20 30 34
36 23 23 23 39
5 16 8 28 36
13 27 46 44 49
11 18 23 31 41
31 42 31 39 41
14 5 10 37 15
17 22 26 26 36
30 32 30 43 43
22 17 39 33 28
14 17 20 33 31
FS accumulation WR1-L WR2-L WR6-L WR3-R WR4-R
13 30 36 50 48
36 34 39 51 50
49 42 47 45 50
25 33 23 47 31
36 40 46 61 55
21 23 19 45 49
46 48 47 56 52
34 10 12 52 45
37 48 38 43 50
50 39 38 53 49
23 30 37 44 48
32 22 27 44 43
VFS load WR1-L WR2-L WR6-L WR3-R WR4-R
27 35 30 39 36
18 35 36 41 40
40 35 41 41 43
25 39 33 42 42
39 48 42 50 50
32 45 43 50 49
39 47 43 49 47
35 46 34 46 45
41 45 33 42 46
44 48 35 43 45
35 44 43 45 45
33 44 43 43 46
VFS accumulation WR1-L WR2-L WR6-L WR3-R WR4-R
29 41 40 54 49
41 44 44 52 51
43 50 50 51 49
52 34 38 53 48
43 52 48 56 54
39 45 43 52 50
60 53 50 55 54
42 44 37 53 48
46 51 46 52 51
46 50 51 54 45
35 45 48 50 46
36 43 43 51 48
at reference sites (t-test, p < 0.001). At the other two logged sites and at the remaining post-logging sample times there were no significant differences from at least one of the reference sites in the inorganic FS accumulation (Holm-Sidak, p > 0.05). There were no indications of logging impacts on organic FS loads or accumulation. While there were significant differences among sites over time (site time, p < 0.001), none of the postlogging organic FS measurements at logged sites were significantly different from both reference sites (Holm-Sidak, p > 0.05), and all were within the range of pre-logging measurements (Fig. 2C and D). Organic FS did not significantly increase after logging, but the proportion of organic FS in load and accumulation samples declined at WR2, sample time 2, in comparison to pre-logging and other post-logging sample times (Table 3) because of the significant increase in inorganic FS loads and accumulation at that sample time. 3.3. Logging impacts on deposition of very fine sediment (VFS, 0.5–250 mm) No significant effects of logging on the deposition of VFS were detected. There were significant differences among sites over time (site time, p < 0.001) for all VFS measurements but none of the post-logging concentrations at logged sites were significantly higher than at both reference sites (Holm-Sidak, p > 0.05) (Fig. 3). The instantaneous VFS load measurements in traps tended to be higher in post-logging years than in pre-logging years, particularly in summer samples (Fig. 3A and C), but the post-logging patterns were similar among logged and reference sites. These differences in VFS load patterns were not observed among the VFS accumulation measurements (Fig. 3B and D).
3.2. Logging impacts on deposition of fine sediments (FS, 250–1000 mm)
4. Discussion
Trends over time in average inorganic FS loads were significantly different among sites (site time, p < 0.001). At one of the three logged sites (WR2), there was a significant increase in inorganic FS by the second sampling time after logging (Fig. 2A). The average inorganic FS load was 5 times higher than the highest reference site (WR4) at the same sample time (Holm-Sidak, p < 0.05). At pre-logging time 3, the FS load at WR2 was also significantly higher than both reference sites (Holm-Sidak, p < 0.05), but the post-logging increase was 2.7 times higher than the highest pre-logging sediment load, and was significantly higher than the average pre-logging sediment load at both reference sites (t-test, p < 0.001). The inorganic FS load at WR2 was again higher than at both reference sites at post-logging time 5 (Holm-Sidak, p < 0.05), but that increase was not outside the range of pre-logging measurements at WR2 (Fig. 2A) and was only marginally higher than the average pre-logging FS load at the reference sites (t-test, p = 0.043). The inorganic FS load at WR1 was also significantly higher than at both reference sites at post-logging time 3 (Holm-Sidak, p < 0.05). However, this was less than at least two pre-logging measurements at that site and was not significantly different from the average pre-logging inorganic FS load at the reference sites (t-test, p = 0.491). At all other post-logging sample times, there were no significant differences between logged and at least one reference site (Holm-Sidak, p > 0.05). The inorganic FS accumulation in tubes followed a similar pattern, albeit at about double the concentrations than in instantaneous load measurements (Fig. 2B), with significant differences among sites over time (site time, p < 0.001). Inorganic FS accumulation at the second post-logging sample time at WR2 was significantly higher than at both reference sites (Holm-Sidak, p < 0.05) and higher than the average pre-logging accumulation
The upland clearcuts and riparian partial-harvest logging along these boreal forest streams had negligible impact on fine sediment deposition and accumulation on streambeds. No significant post-logging changes in sedimentation rates were detected at two of three logged sites. At the site where the riparian logging was at the highest intensity (WR2, average BA removal of 28%), there was a significant but temporary increase in deposition and accumulation of inorganic FS. The increase was 5 times higher than at the reference sites and 3 times higher than the highest pre-logging sedimentation rate, and was measured only in the fall of the first post-logging year (2005). By the following spring and for the next two years, no differences from reference or pre-logging sedimentation rates were detected. No significant increases were detected among organic FS, inorganic VFS, or organic VFS samples over the 3year post-logging period. There was no obvious point source for the short-term increase in inorganic FS at WR2. Although all the sediment traps were located downstream from the logging road crossing, the sediment increases did not appear to originate from the road. The crossing consisted of a gravel road over a culvert and was constructed several years before the logging. We observed no evidence of disturbance to the road crossing during or after the logging operations, and no evidence of sediment inputs from the road to the stream. If the inorganic FS increases had originated from the road, it was expected that samplers closer to the road would contain higher amounts of inorganic FS than those further downstream. We measured the correlation between sediment concentration and the distance from the road, and found no significant correlation in load (Pearson product moment correlation r = 0.493, p = 0.147) or accumulation (r = 0.421, p = 0.259) samples.
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Given that the temporary increases in FS at WR2 were restricted to inorganic fractions, the increased sediments appeared to be from ground disturbance to mineral soil. However, ground disturbance in the riparian buffers was minimal. Plot surveys conducted by FPInnovations, FERIC Division, found no evidence of machine-generated disturbance to mineral soil in the riparian buffers of WR2, while 2.5% of the upland clearcut area had measurable mineral soil disturbance (P. Hamilton, pers. comm.). Either the inorganic sediments originated from outside the riparian areas (from clearcuts in the uplands) or they resulted from small, localized disturbances that were not detectable by the plot surveys. Croke et al. (1999) demonstrated experimentally that large sediment volumes can be produced from relatively small disturbed areas. Croke and Hairsine (2006) also point out that rainfall intensity, especially frequency and magnitude of rainfall events, are more dominant controls on sediment transport to streams than area disturbed. The increased FS at WR2 were coincident with an unusually high rainfall event in early October of 2005. Although the total precipitation in the August– September period of 2005 was not higher than in two prelogging years (Table 1), the meteorological station from which we obtained rainfall data recorded 111 mm of precipitation over a 48-h period a few days before the sediment traps were collected in October. The highest previous rainfall over a 48-h period during the study was 64 mm in 2002, and maximum precipitation over a 48-h period in early October ranged from 10 to 39 mm in the remaining years (http://climate.weatheroffice. ec.gc.ca/climatedata/dailydata_e.html). At WR2, relatively small ground disturbance in combination with a large rain event could have produced the short-term FS increases in the fall of the first post-logging year. It is also possible that the increased FS at WR2 after the large rainfall event in October 2005 was the remobilization of stored instream sediment deposits from further upstream (Croke and Hairsine, 2006). However, if remobilization of stored instream sediments was the source of the temporary increase at WR2, we would have expected a similar increase in the organic sediment fraction. Without quantification of overland sediment transport and (or) the use of conservative sediment tracers for source determination, it is not possible to unequivocally attribute the increased sediment load at WR2 to riparian or upland ground disturbance from logging activities. The differential patterns between pre- and post-logging VFS loads appeared to have resulted from drought conditions in the post-logging period. Rainfall through the sampling season of the post-logging years was 30% less than the pre-logging years (Table 1). In addition, summer stream baseflow levels were continuously recorded (every half hour) by a project partner with electronic water level-loggers at sites WR1 and WR2 during the summer and fall of 2004, 2005 and 2006 (J. McLaughlin, pers. comm.). Those data indicated that stream water levels in late summer and early fall (August–September) were 21–50% lower in 2005, and 10–40% lower in 2006 than in 2004 (Fig. 4). The average water level over that period at WR1 was 248.9 mm in 2004, and was significantly lower in 2005 at 115.2 mm (t-test, p < 0.001) and in 2006 at 124.8 mm (p < 0.001). The average water level at WR2 was 143.3 mm in 2004, and was significantly lower in 2005 at 107.6 mm (p < 0.001) and in 2006 at 110.4 mm (p < 0.001). We also observed that stream channel conditions at most sites, particularly WR1, WR3, WR4, and WR6 were much drier through the summers of 2005 and 2006 in comparison to any other year, with extended late summer periods of little or no obvious surface water flow. Lower water levels in streams during the post-logging years evidently decreased the flushing and transport of sediments out of traps by high water events, resulting in higher instantaneous load
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Fig. 4. Water levels in WR1 (A) and WR2 (B) during late summer and early fall of 2004, 2005, and 2006. Points are measurements recorded every 12 h.
measurements at logged and reference sites in post-logging than in pre-logging years. No such patterns were observed in the sediment accumulation traps (Fig. 3B and D) where flushing of particles did not occur under higher flow in pre-logging years. This indicates that VFS deposition and retention on streambeds can be increased during low-flow periods in these boreal forest streams. Drewson et al. (2007) reviewed sediment studies from other regions and also concluded that increased fine sediment deposition on streambeds often results from low or reduced flow because sediment particles tend to settle out under those conditions. The implications for benthic invertebrate communities of drought-induced sedimentation increases are not clear because they can be confounded by other effects of low flow on those communities (Cazaubon and Giudicelli, 1999; Boulton, 2003; Suren et al., 2003). In our study, both the inorganic and organic fractions of VFS load increased during low-flow years in contrast to the logging-induced FS increases which were restricted to inorganic fractions. The drought-induced increases in VFS loads could potentially reduce invertebrate taxa sensitive to fine sediments (Wood and Armitage, 1999) and (or) increase detritivorous taxa that prefer organic-rich fine sediments (Kreutzweiser et al., 2005b). Most previous studies reporting logging impacts on sediment inputs to streams were conducted where riparian buffers were not applied or where buffers were intact. In general, watershed logging in the absence of riparian buffers often results in large, detrimental increases in fine sediment inputs to streams. Reported increases have been variable and site specific, but were often in the 10–15-fold range, and as high as 700-fold
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(Webster et al., 1992; Binkley and Brown, 1993; Waters, 1995; Martin et al., 2000). In most cases, sediment inputs from logging operations are associated with roads, trails and stream crossings with comparatively little input from actual tree harvesting (Grayson et al., 1993; Croke et al., 1999; Croke and Mockler, 2001). Intact riparian buffers are often effective at reducing or eliminating fine sediment inputs to streams (Barling and Moore, 1994; Davies and Nelson, 1994; Hickey and Doran, 2004; Ward and Jackson, 2004; Lakel et al., 2006). Our study demonstrated that upland clearcuts and partialharvest logging in riparian buffers of boreal forest streams at up to 50% BA removal (average 10–30% removal but up to 50% at any one spot) can be conducted with minimal risk of significant and sustained increases in fine sediment inputs. Previous studies have also demonstrated that partial-harvest logging by best management practices in riparian areas of other forest types resulted in little or no sedimentation increases (Kreutzweiser and Capell, 2001; DeGroot et al., 2007; Hemstad et al., 2008). 5. Conclusions Partial-harvest logging at up to 50% BA removal in riparian buffers of boreal forest streams with upland clearcuts did not affect sediment deposition or accumulation rates at two of three logged sites. Significant increases did occur in fine inorganic sediment load and accumulation at the most intensively logged site, but the increases were temporary with no significant differences from reference or pre-logging levels by the second post-logging year. Riparian logging impacts appeared to have been mitigated by careful logging practices including winter harvesting in the study blocks, and a tendency to avoid immediate stream-side areas. While the prescription allowed for tree removal to stream sides, the areas immediately adjacent to the stream (within 3–5 m) were often too steep, or rocky, or wet for access by operational equipment. Harvesting in or near riparian areas during frost-free seasons could increase the risk of ground disturbance and transport of sediments to streams, but this was not assessed in our study. Partial-harvest logging could be considered an ecologically sound riparian management strategy if the continuing WRRHIP demonstrates ecological and operational benefits of logging in riparian buffers of boreal forest streams. Where it is feasible and advisable to conduct partial harvesting in riparian buffers of boreal forest streams, the logging can be conducted without posing significant risk of increased sediment inputs to streams when careful logging practices are followed. Acknowledgements We thank the students who assisted with field sampling and sample processing: Ted Atkinson, Kara Herridge, Elisa Muto, Chantal Nicholson, and Mandy Roberts. Domtar Inc. supported the project and conducted the experimental logging. Further support was provided by the Ontario Living Legacy Trust Grant #07-018 (SBH) and the Enhanced Forest Productivity Science Program Grant #010-2-R1 (DPK). References Angradi, T.R., 1999. Fine sediment and macroinvertebrate assemblages in Appalachian streams: a field experiment with biomonitoring applications. Journal of the North American Benthological Society 18, 49–66. Barling, R.W., Moore, I.D., 1994. Role of buffer strips in management of waterway pollution: a review. Environmental Management 18, 543–558. Binkley, D., Brown, T.C., 1993. Forest practices as nonpoint sources of pollution in North America. Water Resources Bulletin 29, 729–740. Boulton, A.J., 2003. Parallels and contrasts in the effects of drought on stream macroinvertebrate assemblages. Freshwater Biology 48, 1173–1185.
Brunke, M., Gonser, T., 1997. The ecological significance of exchange processes between rivers and groundwater. Freshwater Biology 37, 1–33. Buttle, J., 2002. Rethinking the donut: a case for hydrologically relevant buffer zones. Hydrological Prococess 16, 3093–3096. Castelle, A.J., Johnson, A.W., Conolly, C., 1994. Wetland and stream buffer size requirements: a review. Journal of Environmental Quality 23, 878–882. Cazaubon, A., Giudicelli, J., 1999. Impact of the residual flow on the physical characteristics and benthic community (algae, invertebrates) of a regulated Mediterranean river: the Durance, France. Regulated Rivers Research and Management 15, 441–461. Croke, J., Mockler, S., 2001. Gully initiation and road-to-stream linkage in a forested catchment, southeastern Australia. Earth Surface Processes and Landforms 26, 205–217. Croke, J., Hairsine, P., 2006. Sediment delivery in managed forests: a review. Environmental Reviews 14, 59–87. Croke, J., Hairsine, P., Fogarty, P., 1999. Sediment production, redistribution and storage on logged forest hillslopes in south-eastern Australia. Hydrological Processes 13, 2705–2720. Davies, P.E., Nelson, M., 1994. Relationships between riparian buffer widths and the effects of logging on stream habitat, invertebrate community composition, and fish abundance. Australian Journal of Marine and Freshwater Research 45, 1289–1305. DeGroot, J.D., Hinch, S.G., Richardson, J.S., 2007. Effects of logging second-growth forests on headwater populations of coastal cutthroat trout: a 6-year, multistream, before-and-after field experiment. Transactions of the American Fisheries Society 136, 211–226. Drewson, Z.S., James, A.B.W., Death, R.G., 2007. A review of the consequences of decreased flow for instream habitat and macroinvertebrates. Journal of the North American Benthological Society 26, 401–415. Fortino, K., Hershey, A.E., Goodman, K.J., 2004. Utility of biological monitoring for detection of timber harvest effects on streams and evaluation of Best Management Practices: a review. Journal of the North American Benthological Society 23, 634–646. Grayson, R.B., Haydon, S.R., Jayasuriya, M.D.A., Finlayson, B.L., 1993. Water quality in mountain ash forests—separating the impacts of roads from those of logging operations. Journal of Hydrology 150, 459–480. Hemstad, N.A., Merten, E.C., Newman, R.M., 2008. Effects of riparian forest thinning by two types of mechanical harvest on stream fish and habitat in northern Minnesota. Canadian Journal of Forest Research 38, 247–256. Hickey, B., Doran, B., 2004. A review of the efficiency of buffer strips for the maintenance and enhancement of riparian ecosystems. Water Quality Research Journal of Canada 39, 311–317. Kardynal, K.J., Hobson, K.A., Van Wilgenburg, S.L., Morissette, J.L., 2009. Moving riparian management guidelines towards a natural disturbance model: an example using boreal riparian and shoreline forest bird communities. Forest Ecology Management 257, 54–65. Kreutzweiser, D.P., Capell, S.S., 2001. Fine sediment deposition in streams after selective forest harvesting without riparian buffers. Canadian Journal of Forest Research 31, 2134–2142 (see also erratum in Canadian Journal of Forest Research 32, 1108). Kreutzweiser, D.P., Capell, S.S., Good, K.P., 2005a. Effects of fine sediment inputs from a logging road on stream insect communities: a large-scale experimental approach in a Canadian headwater stream. Aquatic Ecology 39, 55–66. Kreutzweiser, D.P., Capell, S.S., Good, K.P., 2005b. Macroinvertebrate community responses to selection logging in riparian and upland areas of headwater catchments in a northern hardwood forest. Journal of the North American Benthological Society 24, 208–222. Kreutzweiser, D.P., Capell, S.S., Holmes, S.B., 2009. Stream temperature responses to partial-harvest logging in riparian buffers of boreal mixedwood forest watersheds. Canadian Journal of Forest Research 39, 497–506. Lakel, W.A., Aust, W.M., Dolloff, C.A., 2006. Seeing the trees along the streamside. Journal of Soil and Water Conservation 61, 23–29. Lee, P., Smyth, C., Boutin, S., 2004. Quantitative review of riparian buffer width guidelines from Canada and the United States. Journal of Environmental Management 70, 165–180. Martin, C.W., Hornbeck, J.W., Likens, G.E., Buso, D.C., 2000. Impacts of intensive harvesting on hydrology and nutrient dynamics of northern hardwood forests. Canadian Journal of Fisheries and Aquatic Sciences 57 (Suppl. 2), 19–29. McCord, S.B., Grippo, R.S., Eagle, D.M., 2007. Effects of silviculture using best management practices on stream macroinvertebrate communities in three ecoregions of Arkansas, USA. Water, Air and Soil Pollution 184, 299–311. McDonald, T.L., Erickson, W.P., McDonald, L.L., 2000. Analysis of count data from before-after control-impact studies. Journal of Agricultural, Biological and Environmental Statistics 5, 262–279. Nitschke, C.R., 2005. Does forest harvesting emulate fire disturbance? A comparison of effects on selected attributes in coniferous-dominated headwater systems. Forest Ecology and Management 214, 305–319. Norris, V., 1993. The use of buffer zones to protect water quality: a review. Water Resource Management 7, 257–272. Petticrew, E.L., Krein, A., Walling, D.E., 2007. Evaluating fine sediment mobilization and storage in a gravel-bed river using controlled reservoir releases. Hydrological Processes 21, 198–210. Scha¨lchli, U., 1992. The clogging of coarse gravel river beds by fine sediment. Hydrobiologia 235, 189–219.
D. Kreutzweiser et al. / Forest Ecology and Management 258 (2009) 1578–1585 Scrimgeour, G.J., Tonn, W.M., Paszkowski, C.A., Aku, P.M.K., 2000. Evaluating the effects of forest harvesting in littoral benthic communities within a natural disturbance-based management model. Forest Ecology and Management 126, 77–86. Scrivener, J.C., Brownlee, M.J., 1989. Effects of forest harvesting on spawning gravel and incubation survival of chum (Oncorhynchus keta) and coho salmon fry (Oncorhynchus kisutch) in Carnation Creek, British Columbia. Canadian Journal of Fisheries and Aquatic Sciences 46, 681–696. Smith, D.W., Russell, J.S., Burke, J.M., Prepas, E.E., 2003. Expanding the forest management framework in the province of Alberta to include landscapebased research. Journal of Environmental Engineering and Science 2, S15–S22. Steedman, R.J., Allan, C.J., France, R.L., Kushneriuk, R.S., 2004. Land, water, and human activity in boreal watersheds. In: Gunn, J.M., Steedman, R.J., Ryder, R.A. (Eds.), Boreal Shield Watersheds: Laketrout Ecosystems in a Changing Environment. CRC Press, Boca Raton, Florida, pp. 59–86. Stewart-Oaten, A., Murdoch, W.W., Parker, K.R., 1986. Environmental impact assessment: pseudoreplication in time? Ecology 67, 929–940. Suren, A.M., Biggs, B.J.F., Duncan, M.J., Bergey, L., 2003. Benthic community dynamics during summer low-flows in two rivers of contrasting enrichment 2. Invertebrates. New Zealand Journal of Marine and Freshwater Research 37, 71–83.
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Sutherland, A.B., Meyer, J.L., Gardiner, E.P., 2002. Effects of land cover on sediment regime and fish assemblage structure in four southern Appalachian streams. Freshwater Biology 47, 1791–1805. Vowell, J.L., Frydenborg, R.B., 2004. A biological assessment of best management practice effectiveness during intensive silvicultural and forest chemical application. Water, Air and Soil Pollution Focus 4, 297–307. Ward, J.M., Jackson, C.R., 2004. Sediment trapping within forested streamside management zones: Georgia Piedmont, USA. Journal of the American Water Resources Association 40, 1421–1431. Waters, T.F., 1995. Sediment in Streams: Sources, Biological Effects, and Control. American Fisheries Society, Bethesda, Maryland. Webster, J.R., Golladay, S.W., Benfield, E.F., Meyer, J.L., Swank, W.T., Wallace, J.B., 1992. Catchment disturbance and stream responses: an overview of stream research at Coweeta Hydrologic Laboratory. In: Boon, P.J., Calow, P., Petts, G.E. (Eds.), River Conservation and Management. John Wiley & Sons Ltd., Chichester, pp. 231–253. Wood, P.J., Armitage, P.D., 1999. Sediment deposition in a small lowland stream– management implications. Regulated Rivers Research and Management 15, 199–210. Zweig, L.D., Rabeni, C.F., 2001. Biomonitoring for deposited sediment using benthic invertebrates: a test on 4 Missouri streams. Journal of the North American Benthological Society 20, 643–657.
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Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco
Sediment deposition in streams adjacent to upland clearcuts and partially harvested riparian buffers in boreal forest catchments David Kreutzweiser a,b,*, Scott Capell a, Kevin Good a, Stephen Holmes a a b
Canadian Forest Service, Natural Resources Canada, 1219 Queen St. East, Sault Ste Marie, Ontario, Canada, P6A 2E5 Cooperative Freshwater Ecology Unit, Laurentian University, 935 Ramsey Lake Road, Sudbury, Ontario, Canada, P3E 2C6
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 May 2009 Received in revised form 26 June 2009 Accepted 2 July 2009
The rates of fine sediment deposition were compared among three logged and three reference stream reaches 2–3 years before and 3–4 years after logging to assess the environmental impacts of partial harvesting as a novel riparian management strategy for boreal forest streams. The partial-harvest logging resulted in 10, 21 and 28% average basal area removal from riparian buffers at the three logged sites, adjacent to upland clearcut areas. No significant differences from pre-logging or reference-site sedimentation patterns were detected for two of the three logged sites. At the site with the most intense riparian logging (WR2), significant increases of 3–5 times higher than pre-logging or reference levels were detected in fine inorganic sediment (250–1000 mm) load and accumulation in the first year after logging, but no significant change was detected in fine organic sediments or very fine sediments (0.5– 250 mm). The increased inorganic sediment deposition at WR2 was temporary with no significant differences from reference or pre-logging levels detectable by summer of the second post-logging year. Logging impacts on fine sedimentation in streams appeared to have been mitigated by careful logging practices including winter harvesting in riparian areas to reduce ground disturbance, and a tendency to avoid immediate (within 3–5 m) stream-side areas. Where it is feasible and advisable to conduct partial harvesting in riparian buffers of boreal forest streams, the logging can be conducted without posing significant risk of increased sediment inputs to streams when careful logging practices are followed. Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved.
Keywords: Sedimentation Streams Partial harvest Logging Riparian buffers
1. Introduction Fine sediment inputs are among the most widely implicated impacts of watershed logging on forest streams (Waters, 1995; Wood and Armitage, 1999; Croke and Hairsine, 2006). Excessive fine sediment deposition on streambeds can have detrimental effects by interfering with hydrologic exchange processes and degrading streambed quality (Scha¨lchli, 1992; Brunke and Gonser, 1997), altering invertebrate abundance or community composition (Angradi, 1999; Zweig and Rabeni, 2001; Kreutzweiser et al., 2005a), and reducing fish egg and fry survival by impinging on spawning and incubation substrates (Scrivener and Brownlee, 1989; Sutherland et al., 2002). Forest management guidelines and regulations in North America have attempted to mitigate logging impacts on fine sediment inputs by implementing various best management practices (Fortino et al., 2004; Vowell and Frydenborg, 2004; McCord et al., 2007). One of the more common best
* Corresponding author at: Canadian Forest Service, Natural Resources Canada, 1219 Queen St. East, Sault Ste Marie, Ontario, Canada, P6A 2E5. Tel.: +1 705 541 5648; fax: +1 705 541 5700. E-mail address: [email protected] (D. Kreutzweiser).
management practices for forestry is the application of riparian (shoreline) buffers or restricted-harvest reserves to minimize machine-generated ground disturbance and channeled flowpaths near streams and shorelines (Lee et al., 2004). In general, these forested riparian buffers are effective at mitigating many logging impacts on aquatic systems (Norris, 1993; Barling and Moore, 1994; Castelle et al., 1994; Ward and Jackson, 2004). However, it has recently been noted that the systematic application of no-harvest riparian buffers along streams and around other water bodies in the boreal forests of Canada is resulting in unnatural, linear patterns of older growth forest stands across the landscape resulting in ‘‘ribbons’’ along streams and ‘‘donuts’’ around lakes (Buttle, 2002; Steedman et al., 2004). These riparian buffers are often protected from natural fire and insect disturbances for several decades because the adjacent upland areas contain young, regenerating stands that are less prone to these types of disturbance. Consequently, there may be a loss of productive shoreline habitat for biota that require natural stand-replacing disturbances to flourish, and the resulting landscape patterns could be contrary to the objectives of forest ecosystem sustainability and natural-disturbance emulation. This has led to the recognition that forest researchers, managers and policy-makers must consider disturbance management, natural
0378-1127/$ – see front matter . Crown Copyright ß 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2009.07.005
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landscape patterns, and alternative riparian buffer configurations as they move toward more landscape-based forest management frameworks (Scrimgeour et al., 2000; Smith et al., 2003; Kardynal et al., 2009). One alternative riparian management strategy being considered for creating more natural shoreline habitat patterns in the boreal forest is partial-harvest or variable-retention logging in riparian areas. A large-scale field study is underway at the White River Riparian Harvesting Impacts Project (WRRHIP) to determine the operational feasibility, ecological benefits, and environmental impacts of partial-harvest logging in riparian buffers of boreal forest streams. The intent of the WRRHIP is to create riparian forest gaps by partial harvesting that are sufficient to regenerate early-successional vegetation in an effort to 1) increase riparian habitat complexity, 2) sustain the riparian ecological corridor function by retaining 50% or more of the original stand, 3) simulate shoreline fire disturbance to the extent that fires tend to burn patchily as they approach water bodies (Nitschke, 2005), 4) allow timber harvest in riparian areas to partially offset declining wood supplies and (or) to reallocate standing timber elsewhere on the landscape, and 5) to continue to provide protection for streams. As part of that study, we measured fine sediment deposition in streambed sediment traps before and after the logging. Impacts on stream temperature patterns are reported elsewhere (Kreutzweiser et al., 2009). 2. Methods 2.1. Study site and experimental logging The WRRHIP is located near White River, Ontario in central Canada, 75 km inland from the northeastern shore of Lake Superior (Fig. 1). The forest type in the study area consists of boreal mixed woods including white spruce (Picea glauca), black spruce (Picea mariana), balsam fir (Abies balsamea), trembling aspen (Populus tremuloides), and white birch (Betula papyrifera). The predominant soils are thin, bouldery glacial tills, largely humo-ferric podzols, and, to some extent, brunisols over Precambrian bedrock with frequent rocky outcrops. The landscape is uninhabited, accessible only by logging and mining roads, and has a rolling-to-broken topography with multiple lakes and drainage systems. The total relief over the study area is 160 m. A meteorological station located
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Table 1 Total precipitation (mm) at a weather monitoring station located 60 km west of the study area during the sediment sampler deployment periods of each year. Period
2002
2003
2004
2005
2006
2007
Jun–Jul Aug–Sep
172 133
113 192
81 244
90 141
113 82
94 109
Total
305
305
325
231
195
203
100 km to the southeast of the study area (Wawa, Ontario, 478580 N, 848470 W) receives on average about 1000 mm of precipitation per year (1/3 of that as snowfall), with an average annual air temperature of 2 8C and daily mean temperatures ranging from 15 to 15 8C (data averaged 1971–2000). The area is typically ice and snow covered from November to May. Seasonal precipitation data from the periods in which the sediment samplers were deployed were obtained from a weather monitoring station in Pukaskwa National Park located 60 km to the west of the study area (Table 1) (http://climate.weatheroffice.ec.gc.ca/climatedata/ dailydata_e.html). Five study blocks, each on a separate watershed, were located within a 120 km2 area, and each block contained a stream study reach (Fig. 1). Two of the study sites (WR3, WR4) had not been previously logged, and served as reference sites. Three sites (WR1, WR2, WR6) contained harvest blocks in which the upland areas were clearcut by conventional feller-buncher and grapple skidders, and in which the riparian buffers (30–100 m wide) were partially harvested with the same operational equipment. All logging in the watersheds was conducted in late winter to minimize ground disturbance. The WR6 site was harvested in the winter (March) of 2004 and the WR1 and WR2 sites were harvested in the winter (February) of 2005. The partial-harvest logging prescription in the riparian buffers was ‘‘up to 50% removal of merchantable trees as evenly distributed across species and size classes as possible, in accessible portions of the riparian buffers’’, and was quantified as basal area (BA) removal. While the prescription allowed up to 50% removal at any one area, feller-buncher access trails were clearcut up to 10 m wide, and the surrounding area within reach of the buncher was then partially harvested. This resulted in a patchy distribution of gaps of varying sizes, and the overall riparian disturbance was assessed by post-logging measurements of residual trees and
Fig. 1. The WRRHIP study sites on the Boreal Shield near White River, Ontario. The solid black line is a logging road, and the study blocks containing the stream study reaches are shown in dark shading. The study blocks lie between 488210 05.7500 N, 858200 59.3700 W and 488130 47.5900 N, 858220 03.2200 W.
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Table 2 Catchment and study reach characteristics of logged and reference watersheds. Harvest blocks in logged watersheds were clearcut in upland portions and partially harvested (up to 50% BA removal intensity) in riparian buffers. Average BA harvested in riparian buffers is the average BA harvested from the total commercial tree BA available on both sides of the study reaches. Characteristic
WR1
WR2
WR3
WR4
WR6
Treatment Watershed area (ha) Harvest block area (ha) Area of watershed harvested (%) Riparian buffer area (ha) Av. BA harvested in riparian buffers (m2/ha) Av. BA harvested in riparian buffers (%) Study stream order Stream reach length in harvest block (m) Stream reach av. bankfull width (m) Stream reach av. gradient (%)
logged 817 77 9 4.5 6.6 21 3 600 5.1 2.1
logged 90 48 53 5.9 8.9 28 1 840 2.6 3.6
reference 83 0 0 no buffer 0 0 1 n/a 3.3 2.4
reference 480 0 0 no buffer 0 0 3 n/a 6.4 1.7
logged 85 75 88 6.0 3.2 10 1 550 3.9 0.8
stumps to provide a measure of average BA removal of available timber in each of the riparian buffers. Further details of the stream reaches and their catchments are provided in Table 2. 2.2. Sediment deposition measurements Fine sediment deposition on streambeds was measured with two types of sediment traps that were deployed and retrieved twice through the field seasons of 2002–2007 (deployed late May and retrieved late July, re-deployed late July and retrieved early October). This provided summer and fall sediment deposition measurements in two pre-logging and four post-logging years for WR6, and three pre- and post-logging years for WR1 and WR2. Sediment traps were also placed in the two reference streams (WR3 and WR4). Ten replicates of each trap type were placed in each study reach for both sampling periods of each year. The traps were placed on the streambeds in thalweg positions but in depositional spots (avoiding riffles and direct turbulence) at approximately 10-m intervals through the downstream sections of each study reach. This ensured that all traps at logged sites were adjacent to the riparian logging and at the downstream ends of the harvest blocks. One trap type was intended to measure the instantaneous fine sediment load on the streambed at the time of collection. It consisted of a 17 cm 17 cm 5 cm deep plastic box, open on top and filled with washed stones of a consistent size (1.5–2.5 cm diameter), with a 1.5-cm mesh screen to retain the stones. The open top was intended to allow sediment particles to deposit and re-suspend by flushing and transport with varying stream discharge over the sampling periods. When the boxes were retrieved, plastic lids were placed on top to retain the contents, and they were transferred to a storage container, preserved in 5% formalin, and stored for laboratory processing. The second trap was intended to measure the total fine sediment accumulation over the sample period. It consisted of a 2.5-cm diameter 11-cm long plastic tube, open on one end and placed upright on the streambed. Each tube was attached to a brick to hold it upright and secure, and placed beside or near the sediment box. The small tube diameter relative to its depth was intended to capture and retain sediment particles thus measuring total fine sediment accumulation. When the tubes were retrieved, a plastic lid was attached to the top to retain the sediments and the contents were preserved as described for the sediment boxes. While our specific sediment trap designs have not necessarily been used previously, the concept of collecting fine sediment particles in traps on streambeds to determine sedimentation rates is widely used (e.g., Petticrew et al., 2007). Collected sediments were separated into two size classes: fine sediments (FS) of 250–1000 mm, and very fine sediments (VFS) of 0.5–250 mm. For both types of traps, the contents were washed through 1-mm and 250-mm sieves and into a collection bucket.
Materials on the 1-mm sieve were discarded. Materials retained on the 250-mm sieve were dried at 60 8C for two days, weighed, combusted at 500 8C for 2 h, cooled and re-weighed to provide ashfree dry weights (organic fraction) and ash weights (inorganic fraction) of FS. Materials washed through the 250-mm sieve were suspended in the water of the collection bucket by a stir-bar on a magnetic stir-plate set at a consistent speed. A number of 25-mL aliquots were withdrawn by pipette at a consistent depth and placed into a suction-filter apparatus with a pre-ashed, 0.5-mm glass fiber filter (Whatman 934-AH1). Aliquots were added until the filter would not pass further water. The total subsample was calculated from the number of aliquots filtered and the volume of water collected. A minimum of 50 mL was filtered for each sample, using and combining two filters if necessary. Filters and contents were dried, weighed, combusted and re-weighed as described for FS to provide organic and inorganic fractions of VFS. 2.3. Data analysis The overall approach to data analysis was to examine trends over time in sedimentation rates among logged and reference sites, and to determine if post-logging trends at logged sites deviated significantly from post-logging trends at reference sites or from pre-logging trends. The trends over time in fine sediment deposition were compared graphically. These trends were examined for differences between logged and reference sites by repeated-measures analysis of variance (RM-ANOVA) to account for correlation in time, using a general linear model in SigmaStat 3.5 (Systat Software Inc., San Jose, CA). Because the sediment traps were deployed in the same spots in each study reach at each sample time, the individual traps at each study reach were considered site replicates (subjects) and sample time was the repeated factor. The primary test for detecting potential impacts was the site time interaction (McDonald et al., 2000). When significant (p < 0.05), indicating that trends over time differed among sites, Holm-Sidak pairwise comparisons (to control the error rate for multiple comparisons) were made between each logged site and both reference sites at each sample time. Postlogging sample times were examined first. If post-logging sedimentation levels at a logged site were not significantly higher than at least one reference site at the same sample time, then no logging impacts were inferred. If the sedimentation at a logged site was significantly higher than both reference sites at the same sample time, and if there were no significant differences between that site and both references at any time during the pre-logging period, then a logging impact on sedimentation was inferred. If significant pre-logging differences also occurred between the logged site and at least one reference site, then the post-logging increase was compared separately by t-test with the average sedimentation at both reference sites over the pre-logging period
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(suggested by McDonald et al., 2000). If significant, a logging impact at that sample time was inferred. We recognize that the individual traps at each site were not treatment replicates, but site replicates. The three logged sites were not considered replicates of logging treatment because the extent of watershed logging and the intensity of riparian logging varied among the three sites (Table 2). Rather, the focus was on trends over time at each site and separate comparisons between each logged site and both references sites. These were site comparisons from which we inferred impacts or no impacts under an assumption that trends at a logged site will not deviate significantly from trends at the reference sites in the absence of a logging disturbance. Although the study followed a BACI design, a classic BACI analysis (e.g., Stewart-Oaten et al., 1986) was not applied because it averages over sample times within the before and after periods, and consequently may not detect short-term fluctuations in response patterns. Sediment deposition data from both types of traps were expressed as g/m2 and log transformed to improve normality and homogeneity of variances prior to analysis. Graphical presentation of trends was made with non-transformed sedimentation data. Missing data arose from toppled or otherwise disturbed sediment traps. If a trap appeared to have been influenced by a local disturbance (e.g., trap tipped or tilted, evidence of someone stepping in or near a trap, upturned rootwad immediately upstream from the trap), the data were excluded if the sedimentation value was more than two standard deviations removed from the mean of 10 replicates. This process often removed one or two traps from each set of 10, rarely three, and never more than four. The general linear model for the RM-ANOVA accounted for missing values by estimating least square means in the F-statistic computations when necessary.
Fig. 2. Mean (SE) fine sediment (FS, 250–1000 mm particle sizes) load and accumulation in streambed traps of logged (suffix L) and reference (suffix R) study reaches. Samples were collected twice each year. Logging occurred at WR6 prior to the 2004 samples and at WR1 and WR2 prior to 2005.
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3. Results 3.1. Overall trends During the pre-logging period, the instantaneous sediment load in traps and the accumulation in tubes were variable among all sites and over time (Figs. 2 and 3). Among sites at a given sample time, there were as much as 5–11-fold differences in mean fine sediment (FS, 250–1000 mm) load or accumulation, while between sample times at a given site in the pre-logging period there were up to 8–22-fold differences. Very fine sediments (VFS, 0.5–250 mm) contributed 51–92% (mean 76%) of total sediment load in traps, and 29–82% with a mean of 57% of total sediment accumulation in tubes. There was less variability in the deposition and accumulation of VFS than FS among sites and over time. VFS load or accumulation was at most 4–6-fold different among sites at a given time and 4–7-fold different among times at a site. The two reference sites (WR3 and WR4) tended to have the greatest differences, with low and relatively consistent sedimentation at WR3 and higher and more variable sediment loading at WR4, thus representing a range of natural sediment deposition in these streams (Figs. 2 and 3). The organic fraction in FS load over the pre-logging period ranged from 4 to 49% among sites with a mean of 24% and a coefficient of variation (CV) of 51% (Table 3). The organic fraction of FS accumulation was higher and less variable with a range of 13– 61%, mean of 39%, and CV of 30%. Organic matter was more consistent in VFS load with 18–50% organic, mean of 39% and a CV of 19%, and slightly higher in accumulation tubes with a range of 28–56%, a mean of 46% and a CV of 14%.
Fig. 3. Mean (SE) very fine sediment (VFS, 0.5–250 mm particle sizes) load and accumulation in streambed traps of logged (suffix L) and reference (suffix R) study reaches. Samples were collected twice each year. Logging occurred at WR6 prior to the 2004 samples and at WR1 and WR2 prior to 2005.
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Table 3 Mean % organic fraction of sediments in trap loads and tube accumulations. FS is fine sediments of 250–1000 mm, VFS is very fine sediments of 0.5–250 mm. Sites with suffix L are logged sites and those with R are reference sites. Samples were collected twice each year. Logging occurred at WR6 prior to the 2004 samples and at WR1 and WR2 prior to 2005. Sample and site
Year 02
02
03
03
04
04
05
05
06
06
07
07
FS load WR1-L WR2-L WR6-L WR3-R WR4-R
4 11 17 28 27
15 9 20 30 34
36 23 23 23 39
5 16 8 28 36
13 27 46 44 49
11 18 23 31 41
31 42 31 39 41
14 5 10 37 15
17 22 26 26 36
30 32 30 43 43
22 17 39 33 28
14 17 20 33 31
FS accumulation WR1-L WR2-L WR6-L WR3-R WR4-R
13 30 36 50 48
36 34 39 51 50
49 42 47 45 50
25 33 23 47 31
36 40 46 61 55
21 23 19 45 49
46 48 47 56 52
34 10 12 52 45
37 48 38 43 50
50 39 38 53 49
23 30 37 44 48
32 22 27 44 43
VFS load WR1-L WR2-L WR6-L WR3-R WR4-R
27 35 30 39 36
18 35 36 41 40
40 35 41 41 43
25 39 33 42 42
39 48 42 50 50
32 45 43 50 49
39 47 43 49 47
35 46 34 46 45
41 45 33 42 46
44 48 35 43 45
35 44 43 45 45
33 44 43 43 46
VFS accumulation WR1-L WR2-L WR6-L WR3-R WR4-R
29 41 40 54 49
41 44 44 52 51
43 50 50 51 49
52 34 38 53 48
43 52 48 56 54
39 45 43 52 50
60 53 50 55 54
42 44 37 53 48
46 51 46 52 51
46 50 51 54 45
35 45 48 50 46
36 43 43 51 48
at reference sites (t-test, p < 0.001). At the other two logged sites and at the remaining post-logging sample times there were no significant differences from at least one of the reference sites in the inorganic FS accumulation (Holm-Sidak, p > 0.05). There were no indications of logging impacts on organic FS loads or accumulation. While there were significant differences among sites over time (site time, p < 0.001), none of the postlogging organic FS measurements at logged sites were significantly different from both reference sites (Holm-Sidak, p > 0.05), and all were within the range of pre-logging measurements (Fig. 2C and D). Organic FS did not significantly increase after logging, but the proportion of organic FS in load and accumulation samples declined at WR2, sample time 2, in comparison to pre-logging and other post-logging sample times (Table 3) because of the significant increase in inorganic FS loads and accumulation at that sample time. 3.3. Logging impacts on deposition of very fine sediment (VFS, 0.5–250 mm) No significant effects of logging on the deposition of VFS were detected. There were significant differences among sites over time (site time, p < 0.001) for all VFS measurements but none of the post-logging concentrations at logged sites were significantly higher than at both reference sites (Holm-Sidak, p > 0.05) (Fig. 3). The instantaneous VFS load measurements in traps tended to be higher in post-logging years than in pre-logging years, particularly in summer samples (Fig. 3A and C), but the post-logging patterns were similar among logged and reference sites. These differences in VFS load patterns were not observed among the VFS accumulation measurements (Fig. 3B and D).
3.2. Logging impacts on deposition of fine sediments (FS, 250–1000 mm)
4. Discussion
Trends over time in average inorganic FS loads were significantly different among sites (site time, p < 0.001). At one of the three logged sites (WR2), there was a significant increase in inorganic FS by the second sampling time after logging (Fig. 2A). The average inorganic FS load was 5 times higher than the highest reference site (WR4) at the same sample time (Holm-Sidak, p < 0.05). At pre-logging time 3, the FS load at WR2 was also significantly higher than both reference sites (Holm-Sidak, p < 0.05), but the post-logging increase was 2.7 times higher than the highest pre-logging sediment load, and was significantly higher than the average pre-logging sediment load at both reference sites (t-test, p < 0.001). The inorganic FS load at WR2 was again higher than at both reference sites at post-logging time 5 (Holm-Sidak, p < 0.05), but that increase was not outside the range of pre-logging measurements at WR2 (Fig. 2A) and was only marginally higher than the average pre-logging FS load at the reference sites (t-test, p = 0.043). The inorganic FS load at WR1 was also significantly higher than at both reference sites at post-logging time 3 (Holm-Sidak, p < 0.05). However, this was less than at least two pre-logging measurements at that site and was not significantly different from the average pre-logging inorganic FS load at the reference sites (t-test, p = 0.491). At all other post-logging sample times, there were no significant differences between logged and at least one reference site (Holm-Sidak, p > 0.05). The inorganic FS accumulation in tubes followed a similar pattern, albeit at about double the concentrations than in instantaneous load measurements (Fig. 2B), with significant differences among sites over time (site time, p < 0.001). Inorganic FS accumulation at the second post-logging sample time at WR2 was significantly higher than at both reference sites (Holm-Sidak, p < 0.05) and higher than the average pre-logging accumulation
The upland clearcuts and riparian partial-harvest logging along these boreal forest streams had negligible impact on fine sediment deposition and accumulation on streambeds. No significant post-logging changes in sedimentation rates were detected at two of three logged sites. At the site where the riparian logging was at the highest intensity (WR2, average BA removal of 28%), there was a significant but temporary increase in deposition and accumulation of inorganic FS. The increase was 5 times higher than at the reference sites and 3 times higher than the highest pre-logging sedimentation rate, and was measured only in the fall of the first post-logging year (2005). By the following spring and for the next two years, no differences from reference or pre-logging sedimentation rates were detected. No significant increases were detected among organic FS, inorganic VFS, or organic VFS samples over the 3year post-logging period. There was no obvious point source for the short-term increase in inorganic FS at WR2. Although all the sediment traps were located downstream from the logging road crossing, the sediment increases did not appear to originate from the road. The crossing consisted of a gravel road over a culvert and was constructed several years before the logging. We observed no evidence of disturbance to the road crossing during or after the logging operations, and no evidence of sediment inputs from the road to the stream. If the inorganic FS increases had originated from the road, it was expected that samplers closer to the road would contain higher amounts of inorganic FS than those further downstream. We measured the correlation between sediment concentration and the distance from the road, and found no significant correlation in load (Pearson product moment correlation r = 0.493, p = 0.147) or accumulation (r = 0.421, p = 0.259) samples.
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Given that the temporary increases in FS at WR2 were restricted to inorganic fractions, the increased sediments appeared to be from ground disturbance to mineral soil. However, ground disturbance in the riparian buffers was minimal. Plot surveys conducted by FPInnovations, FERIC Division, found no evidence of machine-generated disturbance to mineral soil in the riparian buffers of WR2, while 2.5% of the upland clearcut area had measurable mineral soil disturbance (P. Hamilton, pers. comm.). Either the inorganic sediments originated from outside the riparian areas (from clearcuts in the uplands) or they resulted from small, localized disturbances that were not detectable by the plot surveys. Croke et al. (1999) demonstrated experimentally that large sediment volumes can be produced from relatively small disturbed areas. Croke and Hairsine (2006) also point out that rainfall intensity, especially frequency and magnitude of rainfall events, are more dominant controls on sediment transport to streams than area disturbed. The increased FS at WR2 were coincident with an unusually high rainfall event in early October of 2005. Although the total precipitation in the August– September period of 2005 was not higher than in two prelogging years (Table 1), the meteorological station from which we obtained rainfall data recorded 111 mm of precipitation over a 48-h period a few days before the sediment traps were collected in October. The highest previous rainfall over a 48-h period during the study was 64 mm in 2002, and maximum precipitation over a 48-h period in early October ranged from 10 to 39 mm in the remaining years (http://climate.weatheroffice. ec.gc.ca/climatedata/dailydata_e.html). At WR2, relatively small ground disturbance in combination with a large rain event could have produced the short-term FS increases in the fall of the first post-logging year. It is also possible that the increased FS at WR2 after the large rainfall event in October 2005 was the remobilization of stored instream sediment deposits from further upstream (Croke and Hairsine, 2006). However, if remobilization of stored instream sediments was the source of the temporary increase at WR2, we would have expected a similar increase in the organic sediment fraction. Without quantification of overland sediment transport and (or) the use of conservative sediment tracers for source determination, it is not possible to unequivocally attribute the increased sediment load at WR2 to riparian or upland ground disturbance from logging activities. The differential patterns between pre- and post-logging VFS loads appeared to have resulted from drought conditions in the post-logging period. Rainfall through the sampling season of the post-logging years was 30% less than the pre-logging years (Table 1). In addition, summer stream baseflow levels were continuously recorded (every half hour) by a project partner with electronic water level-loggers at sites WR1 and WR2 during the summer and fall of 2004, 2005 and 2006 (J. McLaughlin, pers. comm.). Those data indicated that stream water levels in late summer and early fall (August–September) were 21–50% lower in 2005, and 10–40% lower in 2006 than in 2004 (Fig. 4). The average water level over that period at WR1 was 248.9 mm in 2004, and was significantly lower in 2005 at 115.2 mm (t-test, p < 0.001) and in 2006 at 124.8 mm (p < 0.001). The average water level at WR2 was 143.3 mm in 2004, and was significantly lower in 2005 at 107.6 mm (p < 0.001) and in 2006 at 110.4 mm (p < 0.001). We also observed that stream channel conditions at most sites, particularly WR1, WR3, WR4, and WR6 were much drier through the summers of 2005 and 2006 in comparison to any other year, with extended late summer periods of little or no obvious surface water flow. Lower water levels in streams during the post-logging years evidently decreased the flushing and transport of sediments out of traps by high water events, resulting in higher instantaneous load
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Fig. 4. Water levels in WR1 (A) and WR2 (B) during late summer and early fall of 2004, 2005, and 2006. Points are measurements recorded every 12 h.
measurements at logged and reference sites in post-logging than in pre-logging years. No such patterns were observed in the sediment accumulation traps (Fig. 3B and D) where flushing of particles did not occur under higher flow in pre-logging years. This indicates that VFS deposition and retention on streambeds can be increased during low-flow periods in these boreal forest streams. Drewson et al. (2007) reviewed sediment studies from other regions and also concluded that increased fine sediment deposition on streambeds often results from low or reduced flow because sediment particles tend to settle out under those conditions. The implications for benthic invertebrate communities of drought-induced sedimentation increases are not clear because they can be confounded by other effects of low flow on those communities (Cazaubon and Giudicelli, 1999; Boulton, 2003; Suren et al., 2003). In our study, both the inorganic and organic fractions of VFS load increased during low-flow years in contrast to the logging-induced FS increases which were restricted to inorganic fractions. The drought-induced increases in VFS loads could potentially reduce invertebrate taxa sensitive to fine sediments (Wood and Armitage, 1999) and (or) increase detritivorous taxa that prefer organic-rich fine sediments (Kreutzweiser et al., 2005b). Most previous studies reporting logging impacts on sediment inputs to streams were conducted where riparian buffers were not applied or where buffers were intact. In general, watershed logging in the absence of riparian buffers often results in large, detrimental increases in fine sediment inputs to streams. Reported increases have been variable and site specific, but were often in the 10–15-fold range, and as high as 700-fold
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(Webster et al., 1992; Binkley and Brown, 1993; Waters, 1995; Martin et al., 2000). In most cases, sediment inputs from logging operations are associated with roads, trails and stream crossings with comparatively little input from actual tree harvesting (Grayson et al., 1993; Croke et al., 1999; Croke and Mockler, 2001). Intact riparian buffers are often effective at reducing or eliminating fine sediment inputs to streams (Barling and Moore, 1994; Davies and Nelson, 1994; Hickey and Doran, 2004; Ward and Jackson, 2004; Lakel et al., 2006). Our study demonstrated that upland clearcuts and partialharvest logging in riparian buffers of boreal forest streams at up to 50% BA removal (average 10–30% removal but up to 50% at any one spot) can be conducted with minimal risk of significant and sustained increases in fine sediment inputs. Previous studies have also demonstrated that partial-harvest logging by best management practices in riparian areas of other forest types resulted in little or no sedimentation increases (Kreutzweiser and Capell, 2001; DeGroot et al., 2007; Hemstad et al., 2008). 5. Conclusions Partial-harvest logging at up to 50% BA removal in riparian buffers of boreal forest streams with upland clearcuts did not affect sediment deposition or accumulation rates at two of three logged sites. Significant increases did occur in fine inorganic sediment load and accumulation at the most intensively logged site, but the increases were temporary with no significant differences from reference or pre-logging levels by the second post-logging year. Riparian logging impacts appeared to have been mitigated by careful logging practices including winter harvesting in the study blocks, and a tendency to avoid immediate stream-side areas. While the prescription allowed for tree removal to stream sides, the areas immediately adjacent to the stream (within 3–5 m) were often too steep, or rocky, or wet for access by operational equipment. Harvesting in or near riparian areas during frost-free seasons could increase the risk of ground disturbance and transport of sediments to streams, but this was not assessed in our study. Partial-harvest logging could be considered an ecologically sound riparian management strategy if the continuing WRRHIP demonstrates ecological and operational benefits of logging in riparian buffers of boreal forest streams. Where it is feasible and advisable to conduct partial harvesting in riparian buffers of boreal forest streams, the logging can be conducted without posing significant risk of increased sediment inputs to streams when careful logging practices are followed. Acknowledgements We thank the students who assisted with field sampling and sample processing: Ted Atkinson, Kara Herridge, Elisa Muto, Chantal Nicholson, and Mandy Roberts. Domtar Inc. supported the project and conducted the experimental logging. Further support was provided by the Ontario Living Legacy Trust Grant #07-018 (SBH) and the Enhanced Forest Productivity Science Program Grant #010-2-R1 (DPK). References Angradi, T.R., 1999. Fine sediment and macroinvertebrate assemblages in Appalachian streams: a field experiment with biomonitoring applications. Journal of the North American Benthological Society 18, 49–66. Barling, R.W., Moore, I.D., 1994. Role of buffer strips in management of waterway pollution: a review. Environmental Management 18, 543–558. Binkley, D., Brown, T.C., 1993. Forest practices as nonpoint sources of pollution in North America. Water Resources Bulletin 29, 729–740. Boulton, A.J., 2003. Parallels and contrasts in the effects of drought on stream macroinvertebrate assemblages. Freshwater Biology 48, 1173–1185.
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