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Dynamics of large woody debris in small streams disturbed by the 2001 Dogrib fire in the Alberta foothills
Forest Ecology and Management 256 (2008) 1751–1759
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Dynamics of large woody debris in small streams disturbed by the 2001 Dogrib fire in the Alberta foothills Trevor A. Jones, Lori D. Daniels * Department of Geography, University of British Columbia, 1984 West Mall, Vancouver, BC V6T 1Z2, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 August 2007 Received in revised form 30 November 2007 Accepted 28 February 2008
We investigated the temporal dynamics of large woody debris (LWD) in five headwater streams before and after the 2001 Dogrib fire in the foothills of Alberta. The density of LWD varied from 5 to 41 logs per 50 m of stream reach and accounted for 19.4 5.1 m3 ha1 (mean standard error) of wood in the riparian zones and 114.1 30.1 m3 ha1 of wood in the bankfull margins of the stream channel. Individual logs averaged 18.9 1.15 cm in diameter, 5.5 0.7 m in length, and 0.2 0.02 m3 in volume. Logs became significantly shorter in decay classes II–IV. Bridges were longer than partial bridges, which were longer than loose and buried LWD. Individual log volume was greatest for bridges, but not significantly different among other position classes. Bridges and loose LWD contributed little to stream morphology and function; however, 55% of partial bridges and all buried logs contributed to sediment storage, channel armouring, or riffles and pools in the stream channel. Using dendroecological methods, we estimated the year of death of 108 of 115 spruce logs. LWD resulted from tree deaths that occurred between 1874 and 2001, so that time since death ranged from 5 to 132 years. Time since death increased from decay class II to III to IV and bridges were younger than LWD in all other position classes. Due to high rates of recruitment after fire, 16.5% LWD recruited between 2001 and 2006, most of which were bridges or partial bridges in decay class II. We anticipate a delay of 30–45 years before newly recruited logs contribute significantly to stream morphology and function. Depletion rates of LWD were exponential, such that 50% of LWD would be lost to decay, erosion or downstream transport within 30 years of tree death and <12% of LWD would persist more than 100 years. Since recruitment of new LWD in post-fire lodgepole pine stands is delayed by ca. 40 years while trees establish and stands develop, we anticipate periods of ca. 70 years between stand-replacing fires and recruitment of new, functional LWD into stream channels. During this time, fire-killed snags are an important source of LWD to small streams. For headwater streams in environments susceptible to floods and erosion we recommend that buffer zones comprised of snags to be established after fires. The goal of these post-fire buffers is to ensure a supply of LWD into streams for years to decades after a standreplacing fire. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Keywords: Dendroecology Logs Decay class Position class Natural disturbance Stream morphology
1. Introduction Woody debris in upland and riparian forests, including large woody debris (LWD) defined as downed logs that intersect the bankfull margins of a stream, has been linked to many important ecological functions (Harmon et al., 1986). For example, LWD provides stream channels with structural complexity and stability (Montgomery et al., 1995; Rot et al., 2000) by contributing to the storage of sediment (May and Gresswell, 2003), creating diverse habitats in the form of step pools, plunge pools, riffles and logjams (Bilby and Ward, 1989; Montgomery et al., 1995) and by
* Corresponding author. Tel.: +1 604 822 3442; fax: +1 604 822 6150. E-mail address: [email protected] (L.D. Daniels).
dissipating stream energy during periods of high water flow (Richmond and Fausch, 1995). LWD also affects stream dynamics in both in space and time (Naiman et al., 1999; Hyatt and Naiman, 2001; Hassan et al., 2005), which is largely manifested through bank erosion, downstream sediment movement, and the formation and loss of structural elements such as pools and riffles (May and Gresswell, 2003). Many studies of LWD have focussed on the quantity and function of LWD (Hassan et al., 2005), while others have attempted to quantify how management may affect LWD quantity and composition (Bilby and Ward, 1991; McHenry et al., 1998; Gomi et al., 2001; Dahlstrom et al., 2005). Disturbance type, size and frequency plays and important role in LWD dynamics irrespective of forest type. Geomorphic disturbances such as seasonal floods, bank erosion and channel migration contribute LWD into streams
0378-1127/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.02.048
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regardless of age or stage of development of the riparian forest (Naiman and Decamps, 1997). Coarse-scale, episodic disturbances of riparian forests, such as harvesting (Chen et al., 2005), large floods (Acker et al., 2003), fire (Hedman et al., 1996), ice storms (Kraft et al., 2002) and insect outbreaks, are the major contributors of LWD into streams. Following stand-replacing disturbances, the contribution of LWD will vary with stand development processes. Young, uniform-aged forests have lower rates of woody debris recruitment and contribute smaller diameter woody debris, while older forests, with diverse diameter sizes and age structures, generate a greater range of LWD sizes as well as a greater abundance and volume of wood (Harmon et al., 1986; Spies, 1998). In absence of coarse-scale disturbances in old forests, frequent, fine-scale disturbances results in chronic input of LWD due to the combined influences of tree age, wind, insects and pathogens. As a result, older, more structurally complex forests have greater amounts of LWD than younger, less complex forests (Naiman et al., 1999; Acker et al., 2003; Benda and Sias, 2003). Understanding the links between disturbance, vegetation dynamics and structural complexity of forests has become increasingly important as many management agencies have adopted the principals of natural range of variation and emulation of natural disturbance as key objectives for forest management (Christensen et al., 1996; Landres et al., 1999). For example, the recent recognition that natural disturbances are an integral component of riparian forest dynamics has initiated discussions about whether riparian zones should be actively managed or protected as reserves and buffer zones (Beschta et al., 2004). Consequently, studies have examined the effects of management on LWD volume through time (Gomi et al., 2001). These studies have documented discrepancies between timber management and natural processes such as standreplacing fires. For example, Chen et al. (2005) found that fire resulted in a net increase in terrestrial woody debris biomass following a disturbance when compared to clear-cut harvesting or non-harvested old forests. Bragg (2000) provided a long-term perspective when he determined that the differences between clear cuts and natural disturbances such as fire last more than 120 years. Interestingly, these studies have not considered how LWD changes in function or how variation across streams may influence stream structure and function over time. Although time since tree death may be correlated with tree decay rate, it is the rate at which LWD are integrated into functional positions in streams that is most important for our understanding stream structural dynamics through time (Powell, 2006). To better understand how natural disturbances, such as fire, influence LWD function, it is critical to determine how and when LWD recruits into streams and how long LWD takes to transition into different position classes and functional roles in streams. Unfortunately, specific long-term studies of in-stream wood dynamics have not been established for the sole purpose of understanding wood function through time. However, the development of dendrochronological techniques to
determine the time since death of woody debris has dramatically increased our ability to study the dynamics of dead wood. To address the temporal dynamics of LWD in streams, we have designed a study to investigate the following questions: (1) What are the short-term impacts of fire on LWD abundance and volume in small streams? (2) What is the expected time period before LWD begins to contribute to the structure and function of streams? (3) At what rate is LWD depleted from a stream? Through insight gained from this study, we aim to inform management decisions following fires that have consequences for riparian zones and the temporal dynamics of LWD in small streams. 2. Methods 2.1. Study area We conducted this research in the Upper Foothills Natural Subregion of the Rocky Mountains in west-central Alberta. The upper foothills are characterized by steeply rolling terrain with maximum and minimum elevations of 1931 and 1430 m above sea level (m.a.s.l.), respectively, and an average slope of 218. Upland forests within this subregion are characterized by closedcanopy coniferous forests, most commonly including lodgepole pine (Pinus contorta Dougl. Ex Loud), as well as white spruce (Picea glauca (Moench) Voss) and black spruce (Picea mariana (Mill.) B.S.P.). Frequent, stand-replacing fires of an historical average fire size of 300 ha, with a mean return interval of 100 years, have created a landscape mosaic of mixed stands composed of varying amounts of lodgepole pine, white spruce and black spruce (Beckingham et al., 1996). We assessed LWD in five headwater streams affected by the Dogrib fire, which burned 9214 ha of forest near Sundre, Alberta in September 2001 (Fig. 1). According to Alberta vegetation inventories, the riparian forest composition surrounding the five stream reaches was dominated by black and white spruce with minor amounts of lodgepole pine. Based on post-fire assessments, it was apparent that forest composition in the valleys constraining the riparian zones was a mixture of black and white spruce and lodgepole pine had grown in the upland forests surrounding the riparian zones. The fire was a fast-burning crown fire, resulting in nearly 100% mortality of mature trees in the affected riparian area (E. Wassink, Sundre Forest Products, personal communication). Residual standing dead trees were blackened, however, bark, coarse branches, most of the sapwood and all of the heartwood were still intact on most trees. The burned area included part of the R11 Forest Management Unit and the Forest Management Agreement (FMA) area managed by Sundre Forest Products, a division of West Fraser Mills Ltd. Large portions of the FMA were subsequently salvage
Fig. 1. Location of the Dogrib fire and the 5 stream reaches in the Sundre Forest Products Ltd. forest management area in Alberta, Canada.
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Table 1 Description of decay and position classes used to categorize large woody debris (LWD) Variable class
Description
Position classes Bridge Partial bridge Loose Buried
Log spans channel, touching both banks and resting on the floodplain Spanned log has broken in one or more places within the stream channel Log is no longer associated with the floodplain; fully associated with the streambed; log is fully submerged during bankfull Log has become incorporated into the streambed or the sides of the streambank; sediment is stored upstream of the log which is at least partly buried
Decay classes I II III IV
Wood has >75% bark still intact, bark adheres tightly; branches have fine (third order) branchlets; sapwood is sound, and log retains structural integrity Wood has 25–75% bark intact which, in places, is loosely attached to the bole; first order branches have a solid connection to the bole; wood is solid with evidence of decay on some outer sections of sapwood only Wood has 0–25% bark present, adhering loosely to the sapwood; first order branches and branch nubs are present and sit loosely in the bole; along some parts of the bole, wood shows significant signs of decay to depths of 5–10 cm Bark is no longer attached; branch nubs only are present; along some parts of the bole, wood is soft, crumbly or fibrous, and decay can penetrate nearly through the sapwood
logged, although riparian buffers of standing dead trees were retained along most first and second order streams and all higher order streams. McCleary (2005) classified stream channel structure, assessed sediment storage, and documented LWD recruitment and storage at stream and watershed spatial scales for the part of the Sundre Forest Products FMA that burned during the Dogrib fire. For our analysis of large woody debris, we selected five headwater streams based on the following criteria: (1) Headwater channels had an upstream drainage area <200 ha. (2) The entire reach had burned during the Dogrib fire resulting in 100% canopy tree mortality in the riparian forest surrounding the stream. (3) The forest surrounding the stream had not been harvested before the fire and was not salvaged after the fire or influenced by roads. (4) No evidence of recent or historic landslides. Streams had limited ability to transport wood due to the fact that they were narrow and shallow compared to the length and diameter of the wood within them. Using Alberta Vegetation Inventory data used by Sundre Forest Products for timber management, we confirmed that the riparian zones had mature spruce-dominated forests before the fire. Stand origins for the forests surrounding each stream were greater than 100 y.b.p. and exhibited uneven-sized diameter distributions, based on pre-fire vegetation inventories and post-fire appraisals of residual standing and recently downed woody debris (Andison and McCleary, unpublished data). The riparian zone was 40 m wide, 20 m on either side of the stream, which was the area from which mature trees had fallen and contributed to the in-stream LWD. Streams chosen for study had an average slope of 8.2 1.88 (mean standard error) and were loosely constrained by shallow hill slopes (14.5 4.88). 2.2. LWD abundance and size At each stream, we surveyed a 45–100 m reach starting at a randomly selected location that was downstream from McCleary (2005) permanent research plots. Target reach length for the study was 100 m; however, reach length varied depending on the availability of suitable stream area downstream of permanent plots and also by availability of non-salvaged forest surrounding the streams. Consequently three of the reaches were truncated at 50 m while another was truncated at 45 due to a road crossing. Stream width at bankfull was measured every 5 m along the length of each sampled stream reach.
We sampled all LWD, defined as logs that intersected the stream channel and exceeded 0.08 m in diameter and 1 m in length. We confirmed that each log was spruce based on intact bark and wood anatomy (Hoadley, 1990); pine and other species were not present. Each LWD was assigned a decay class and position class (Table 1). Decay classes categorize logs based on structural characteristics and wood integrity (Maser et al., 1979) and position classes categorize LWD location and function relative to the stream channel (Richmond and Fausch, 1995; Berg et al., 1998; Hauer et al., 1999). To assist with our field-based estimates for decay class, we documented several physical attributes commonly associated with stages of decay. These included the presence of needles, the branch order, the percentage of bark remaining, and the percent cover of vegetation and soil on the upper surface of the LWD. We classified the size of the branches attached to each log, as follows: 0 = no branches, 1 = branch stubs, 2 = first order, large branches, 3 = second order large and small branches, and 4 = third order, fine twigs. For each LWD, we used a six-class system to independently estimate the percentage of (a) visible bark remaining on the stem, (b) upper surface of the log covered by vegetation and (c) upper surface of the log covered by soil, as follows: 0 = none, 1 = 1–25%, 2 = 26–50%, 3 = 51–75%, 4 = 76–99%, and 5 = 100%. We classified the effects of fire using a three-class system of the amount of charred bark and sapwood of each LWD, as follows: 0 = no char, 1 = charred bark but outermost ring present in at least one part of the circumference, and 2 = charred bark and sapwood resulting in significant ring loss. We also classified each piece of LWD according to one or more function classes. These included: creating pools and riffles, retaining sediment, armouring banks, creating debris jams or no observable function. Due to small sample size in some function classes and to aid in analysis, LWD was subsequently assigned to one of two broader function classes: 0 = LWD has no observable effect on stream geomorphology or 1 = LWD has an effect on stream geomorphology. For each LWD, length (m) and diameter (cm) were determined based on the condition of the log and its position relative to the stream channel. For logs with a visible root plates and intact tops, we measured diameter at breast height (dbh) and total length to calculate volume (Vlog) using species-specific standard allometric equations (Honer, 1967). For segments of logs without an identifiable root plate, we measured length and diameter at both ends of the log. The volume of these logs (Vlog) was calculated using the equation for a conical paraboloid (Fraver et al., 2007). If decay along the log prevented an accurate measure of diameter at the ends, then the diameter in the middle was recorded and volume (Vlog) was calculated using Huber’s formula, which provides consistent volume estimates for woody debris with broken tops and bottoms or in advanced stages of decay (Figueiredo et al., 2000; Fraver et al., 2007).
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For LWD that crossed one or both banks of the stream channel, we measured the length within the bankfull boundaries (in-stream length) and diameter at the midpoint within these boundaries and calculated the in-stream volume (Vst) using Huber’s formula (Figueiredo et al., 2000; Fraver et al., 2007). LWD that was embedded in the stream bed was partially excavated to measure length and diameter and to obtain a sample for dendrochronological analyses. We used a general linear model (GLM, SAS Institute Inc., Cary, NC) to determine if individual LWD diameter, length, volume, in-stream length and in-stream volume varied significantly among position, decay or function classes. The models included a stream reach factor to account for differences in measured variables due to variance among the streams. All five dependent variables were logarithmically transformed to meet the assumptions of normality and equal variance. A Tukey–Kramer post hoc test was used to differentiate among classes. In the figures, the transformed variables are presented in the original units. At the spatial scale of the stream reach, we calculated (a) total volume of LWD in each riparian zone by summing Vlog in each stream reach and (b) total in-stream volume of LWD by summing Vst in each stream reach. To allow direct comparison among reaches, the total volumes were expressed in m3 ha1 of riparian and stream channel area, respectively. 2.3. LWD year of death and time since death We stratified LWD into trees that were most likely killed by fire in 2001 and logs that died before the fire. We assumed that logs in decay class (DC) II that were bridges or partial bridges most likely had been killed by fire. Secondary evidence of fire-killed trees was the presence of charcoal on the stem and intact bark, but no soil or vegetation had accumulated on the logs. We tested this assumption by estimating year of death of each log using dendrochronological methods. Cross-sectional disks were sampled from all LWD in DC II–IV; there were no LWD in DC I. Disks from LWD in DC II were air dried and then sanded following standard dendrochronological protocols (Stokes and Smiley, 1968). For wood samples that were in more advanced stages of decay (DC III and IV), the samples were frozen immediately after collection and then planed and sanded while frozen with progressively smoother sandpaper, up to 600 grit. After sanding, these disks were air dried and some were resanded by hand to ensure that ring boundaries were clearly visible. The ring-width series from the pith to the outermost ring of each wood disk was measured to the nearest 0.001 mm using a Velmex bench interfaced with a computer. Ring widths were crossdated against the regional, species-specific standard chronologies for black and white spruce (Powell, 2006), using skeleton plots and the programs COFECHA (Grissino-Mayer, 2001) and TSAPWin (RINNTECH, Engineering and Distribution, Heidelberg, Germany). Before assigning a calendar year to the outermost ring to estimate the year of death, we evaluated the quality of the bark and sapwood of each disk. When rings are lost from a sample due to erosion or decay, the crossdating results do not accurately depict year of death. For eroded or decayed samples, year of death is under-estimated and the number of years since death is over-estimated (Daniels et al., 1997). The magnitude of the error increases with advanced decay (Guyette and Cole, 1999) but has not been quantified in our study area. Therefore, subsequent analyses were conducted with the understanding that the year of death of 34 LWD were under-estimated. Time since death of all LWD was calculated as 2006 minus year of death.
2.4. LWD residence and depletion We quantified LWD residence times and rates of depletion using two different approaches. First, an ANOVA (GLM, SAS Institute Inc.) was used to test for differences in time since death of LWD among position and decay classes. Time-since-death estimates were transformed using a natural logarithm to meet the assumptions of normality and equal variance. For significant variables, a Tukey–Kramer post hoc test was used to differentiate among classes. In the figures, we presented the transformed variables in the original units. Second, for the 89 crossdated LWD with year of death before the fire in 2001, we assessed rates of depletion using time since death estimates. To construct the standardized cumulative distributions, we sorted the LWD by time since death, summed the number of logs beginning with the greatest time since death, and divided the cumulative values by the total number of samples (Hyatt and Naiman, 2001; Guyette et al., 2002). We plotted the cumulative, relative distribution against time since death and fit a negative exponential model using SAS (NLIN, SAS Institute Inc.): Y ¼ expat
(1)
where Y was the predicted proportion of LWD pieces in the channel at time, t, which corresponds with the time since death of the LWD in 2006, and a was a constant representing the rate of change of the negative exponential function. Because we were assessing a static distribution, the net rate of change in abundance of LWD represents the combined effects of recruitment, storage and depletion due to decay, erosion and downstream transport over time. The period of analysis was determined by the year of death of the oldest datable LWD that we sampled in the streams. 3. Results 3.1. LWD year of death Of the 115 pieces of LWD sampled, 108 were sound enough to estimate outermost ring dates and years of death by crossdating, but seven samples were decayed so that annual rings could not be distinguished. Year of death of LWD ranged from 1874 to 2001, corresponding to time since death of 132 to 5 years (37.7 3.1 years, mean standard error) (Fig. 2). Of the 115 LWD in the 5 reaches, 19 logs had accumulated in the 5 years since fire, compared to 96 logs that had been recruited and retained in the riparian zones and streams before the fire in 2001
Fig. 2. Time since death of large wood debris (LWD) in five headwater streams. LWD is classified by origin relative to the Dogrib fire in 2001.
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Table 2 Abundance, dimensions and volume of large woody debris (LWD) in the five study streams Stream number
Length of sampled reach (m)
Stream channel width (m)
Number of LWD All (n)
Post-fire (n)
Diameter (cm)
Length (m)
Volume (m )
Riparian (m3 ha1)
In-stream (m3 ha1)
1 2 3 4 5
100 50 50 50 45
1.7 2.4 1.6 5.1 4.9
23 27 5 45 15
7 3 0 8 1
17.9 18.5 22.4 15.5 20.1
8.0 4.6 5.3 5.3 4.1
0.29 0.18 0.21 0.16 0.18
16.7 23.8 5.3 36.0 15.0
77.0 231.4 82.6 110.3 69.2
23.0 (6.7)
3.8 (1.6)
18.9 (1.2)
19.4 (5.1)
114.1 (30.1)
Mean (S.E.)
59.0 (10.3)
(0.2) (0.5) (0.2) (0.7) (1.2)
3.1 (0.8)
Individual LWD
(1.2) (1.2) (2.9) (0.8) (1.1)
Total volume
(1.3) (0.6) (2.4) (0.7) (0.8)
5.5 (0.7)
3
(0.11) (0.07) (0.09) (0.04) (0.06)
0.20 (0.02)
Means are followed by standard errors in parentheses.
(Fig. 2, Table 2). The latter category included the seven logs that could not be dated due to advanced decay. The field-based assessments of decay and position class correctly identified 13 of 19 LWD that resulted from fire. Of those logs, none were in DC I, due to the fact that all needles and fine branches and some of the bark and outer rings had been burned during the fire; 12 were bridges in DC II, and one was a partial bridge in DC II. As well, four bridges and partial bridges were in DC III and two loose LWD were in DC II and III. The outermost ring of 14 LWD was 2001, confirming these trees died in the year of fire. For five LWD, the outermost rings were between 1995 and 1998; however, these trees likely died in 2001 given substantial charring that indicated loss of some rings during the fire. Furthermore, these logs were intact, bridged the stream channel, and had not accumulated soil or vegetation. The remaining LWD died before the fire in 2001. They included logs with no evidence of ring loss and years of death prior to 2001 and logs with some ring erosion and years of death prior to 1992. Five logs that died prior to fire were bridges or partial bridges in DC II, which met our field-based criteria for identifying LWD potentially caused by the fire. However, the outermost rings on these LWD ranged from 1992 to 1958 and soil and/or vegetation had accumulated on three of them, supporting our interpretation that they died before the fire in 2001.
loose (3.0 0.4 m) and buried LWD (2.8 0.4 m), but loose and buried did not differ significantly. LWD in DC II (10.0 1.0 m) were longer than LWD in DC III (4.0 0.4 m) and IV (3.9 0.6 m), which did not differ significantly. Changes in LWD length contributed to significant differences in the volume of individual LWD among position classes (F = 9.47, p < 0.0001) and decay classes (F = 5.2, p = 0.008) (Fig. 3). Bridges (0.41 0.10 m3) had significantly greater volume than partial bridges (0.14 0.04 m3), loose (0.07 0.02 m3) and buried (0.13 0.05 m3) LWD. Similarly, individual in-stream volume varied significantly among position classes (F = 2.31, p = 0.081), where bridges (0.12 0.04 m3) tended to have greater volume than partial bridges (0.07 0.01 m3), loose (0.05 0.02 m3) and buried (0.07 0.02 m3) LWD. 3.3. LWD function The function or influence of LWD on stream morphology varied among position classes (Fig. 4). Only two of 28 (7.1%) bridges and four of 21 (19.0%) loose LWD influenced sediment storage and structure of the stream channel. In contrast, over half of partial bridges (21 of 38, 55.3%) and all buried LWD (n = 21) influenced sediment in the stream channel and banks or contributed to steppools and riffles in the stream. The average time since death of functional LWD was 50.2 5.2 years, although some LWD influenced stream morphology within 5 years of death.
3.2. LWD abundance and size 3.4. LWD dynamics The abundance of LWD was highly variable among the five stream reaches (Table 2). There were between five and 41 LWD per 50 m of stream reach, with an average 21.0 7.0 LWD. The total volume of LWD was 19.4 5.1 m3 ha1, while the total in-stream volume of LWD was 114.1 30.1 m3 ha1 of stream channel area (Table 2). LWD volumes were highly variable among reaches and ranged between 5.3 and 23.8 m3 ha1 for total volume and between 69.2 and 231.4 m3 ha1 for total in-stream volume (Table 2). The post-fire cohort of LWD contributed 6.2 3.0 m3 ha1 to total volume and 15.3 6.1 m3 ha1 directly into the stream channels, while LWD that died prior to 2001 contributed 14.4 4.1 m3 ha1 to total volume and 101.9 31.3 m3 ha1 into the stream channels. Individual logs were, on average, 18.9 1.15 cm in diameter and 5.5 0.7 m long (Table 2). The mean volume of individual logs was 0.2 0.02 m3 and ranged between 0.003 and 2.09 m3 (Table 2). For 49% (56 of 115) of LWD, the entire log was within the bankfull limits of the stream channel. For the remaining 59 logs, 5–90% (36 2.8) of the total length was directly associated with the stream channel. The average in-stream volume of individual logs was 0.08 0.01 m3 and ranged between 0.006 and 1.12 m3. The length of LWD differed significantly among position (F = 20.3, p < 0.0001) and decay (F = 23.4, p < 0.0001) classes (Fig. 3). Bridges (10.53 1.1 m) were significantly longer than partial bridges (5.0 0.5 m), which were significantly longer than
Time since death of LWD increased with progressive position and decay classes (Fig. 3). The time since death of bridges was less than all other position classes (Fig. 3, F = 8.88, p < 0.0001). Due to large variance within position classes, differences in mean time since death among partial bridges, loose and buried LWD were not significant. Time since death of LWD was least in DC II and increased significantly in DC III and IV (Fig. 3, F = 29.9, p < 0.0001). The absolute number of LWD was greater after the fire than before the fire, largely due to differences in recruitment rates. In the 6 years after the fire (2001–2006), 19 LWD recruited into the streams at a rate of 0.54 LWD per year per 50 m reach. In contrast, during the 6 years immediately before the fire (1995–2000), the recruitment rate was 0.37 LWD per year per 50 m reach. During these two time intervals, we assumed that depletion due to decay, erosion and downstream transport was negligible for three reasons. (1) More than 12 years is needed for LWD to completely decay or erode. (2) Most LWD that died between 1995 and 2006 were bridges (56%) and partial bridges (32%), which are less vulnerable to downstream transport than loose (6%) and buried (6%) LWD. (3) The study streams were small and had limited capacity to move large intact logs. For the LWD that died before the fire in 2001, the negative exponential shape of the LWD depletion curve indicates that rate of
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Fig. 3. Comparison of LWD length (top) and volume (middle) and time since death (bottom) in different position (left column) and decay (right column) classes (mean + S.E.). In each graph, different superscript letters indicate significant differences among means at a = 0.05.
depletion decreases with time since death (Fig. 5, F = 53332.4, p < 0.001 and MSE = 0.0005). The depletion curves also provided insight into the residence times of LWD in the study streams. If no additional LWD was recruited into the streams and rates of depletion follows the same exponential rate depicted in Fig. 5, then 50% of the LWD would be depleted in ca. 30 years and <12% would persist >100 years (Fig. 5). 4. Discussion The Dogrib fire in western Alberta resulted in a measurable increase in the abundance of LWD in headwater streams. Using dendroecological dating methods, we determined that 16.5% of the LWD in our study streams were killed and recruited into the stream as a result of the fire in 2001. The average annual rate of recruitment of LWD increased 48% between the 6 years immediately before and after the fire. Although this is a significant increase in the abundance of LWD over a short period, a large number of standing dead trees that were killed by the fire have yet to fall (McCleary, 2005; Jones, unpublished data). Given the fall
rates we observed between 2001 and 2006, we anticipate the volume of LWD will continue to increase significantly during the next several years to decades. Consequently, the rate of LWD recruitment will be sustained at a greater rate than expected under the influence of fine-scale gap-phase dynamics in mature and old spruce-dominated riparian forests also located in the foothills of Alberta (Powell, 2006). Despite the increase in abundance of LWD, there is little evidence that the post-fire cohort of LWD had an immediate influence on stream morphology. Of the 19 LWD in the post-fire cohort, 13 were bridges suspended above the stream channel, four were partial bridges with limited contact with the stream channel, and two were loose logs lying directly on the streambed. At the time of sampling, none of the LWD in the post-fire cohort had contributed to step or plunge pools morphology and only three LWD, two partial bridges and one loose log, had retained sediment. Based on the time since death of partial bridges, loose and buried LWD currently in the stream channels (Fig. 3), we anticipate a delay of 30–45 years before the newly recruited logs have a significant impact on stream morphology. These findings are
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transition among position classes and contribute to stream function. At the same time, the abundance of pre-fire LWD will deplete due to in situ decay and erosion as well as downstream transport of logs during peak flows. Assuming that the depletion rates of the post-fire cohort will be similar to the logs that died during the 124 years before the Dogrib fire, we anticipate only 9–24% of LWD that we observed in 2006 will remain in the streams in 2071, 70 years after the fire. Similarly, by 2086, 85 years after the fire, only 5–16% of LWD will remain. These losses will be mitigated only by the recruitment of the fire-killed snags adjacent to the stream, input from upstream and upslope locations (Benda et al., 2005), or by exposure of buried LWD during channel migration (Guyette et al., 2002). 5. Management implications
Fig. 4. Comparison of LWD time since death by function class and position class. Functional LWD (n = 48) influences stream morphology; non-functional LWD (n = 60) has no observable impact on sediment or hydrology. Different superscript letters indicate significant differences among means at a = 0.05. Numbers above superscript letters are sample size for each function and position class.
Fig. 5. Depletion curve for LWD based on all time since death estimates. The predicted curve was derived using the exponential function in Eq. (1). The vertical dotted line represents the Dogrib fire in 2001.
consistent with previous studies that documented delays of several decades before LWD influences stream geomorphology (Powell, 2006). In our study area, the burned snags in the riparian zones represent an important source of LWD for the stream morphology and function. Depending on rates of fall, LWD recruitment into the riparian zone may continue for several decades. Once all snags have fallen, no new sources of LWD will be available until the riparian forest is either has matured sufficiently to generate logs through self-thinning or through other mortality events (Oliver and Larson, 1996; Franklin et al., 2002). Based on the regenerating trees we observed in the surrounding riparian area, the future riparian forests surrounding our study streams will be initially dominated by lodgepole pine. In riparian pine forests of the Alberta foothills, new LWD is typically generated ca. 40 years after stand-replacing fires (Powell, 2006). Since it takes 30–45 years for LWD entering the stream to affect stream morphology, we anticipate a period of ca. 70– 85 years before new LWD is generated and becomes functional in the stream. During this lag, only the post-fire cohort of LWD will
Knowledge of time since death and rates of depletion of LWD indicate that the abundance of woody debris in headwater streams is highly variable through time and significantly influenced by disturbances and subsequent dynamics of riparian forests. Understanding the decadal-scale dynamics of LWD in relation to disturbance can contribute to sustainable forest management. For example, our results corroborate studies that indicate human impacts, such as salvaging timber from burned riparian forests, can have long-term consequences for stream geomorphology and function (Meleason et al., 2003). If trees in burned riparian forests are not retained, then the cohort of post-fire LWD and snags cannot recruit into the stream. In this scenario, the LWD that recruited before the fire will deplete due to decay, erosion and transport, but will not be replaced for prolonged periods (ca. 70 years). As the absolute abundance of LWD is reduced, structural complexity of the stream channel may decline. This assertion is supported by evidence from steam surveys of harvested landscapes in which the removal of riparian biomass resulted in significant reductions in LWD volume and step and plunge pool area, while channel width, stream velocity and sediment transport increased (Bilby and Ward, 1991; Fausch and Northcote, 1992; Ralph et al., 1994; Gomi et al., 2001). Such changes can have serious consequences for streambed stability, water quality, invertebrate and vertebrate habitats, and biodiversity (Wipfli et al., 2007). Current operational guidelines in our study area in Alberta require that a buffer of at least 10 m be retained around small permanent streams (Alberta Sustainable Resource Development, 2006). Independent of these buffers, an additional 25% of standing dead wood must be retained when salvaging burned forests (Alberta Sustainable Resource Development, 2007). To reduce risk of post-fire degradation due to erosion and to ensure long-term recruitment of LWD following fire, we recommend retaining snags immediately adjacent to riparian buffer zones in burned areas. This would create relatively large, contiguous areas with intact woody debris while meeting both operational requirements. In contrast to permanent streams, treed buffers are not required around intermittent or ephemeral streams and salvage can, and does, occur in the these riparian areas in Alberta. However, recent research has demonstrated the negative impacts of forest operations on headwater streams and the downstream effects on water quality and biodiversity (Wipfli et al., 2007). Therefore, we recommend protection of riparian forests and post-fire woody debris associated with headwater streams. For example, snags in riparian zones surrounding small streams should be left intact under the following conditions: (a) the landscape is subject to seasonal floods and erosion; (b) the riparian forest directly provides habitat for threatened, rare or endangered species; (c) the riparian forests is located upstream from habitat of threatened, rare or endangered species, particularly in landscapes that are susceptible to flooding and erosion.
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In essence, we are advocating the retention of post-fire buffer zones comprised of snags surrounding headwater streams with the goal of mitigating the inevitable depletion of the LWD. The dimensions of post-fire buffer zones should vary among forest types since LWD recruits into a stream from different distances depending on the species and size of trees and the geomorphology of the stream and terrain surrounding the riparian zone. In our study area, LWD was recruited from riparian zones that extended 20 m to either side of the stream channel. Therefore, effective post-fire buffer zones around permanent streams could be designed and retained within the context of existing operational guidelines. However, additional protection of intermittent or ephemeral headwater streams following fire is needed in the foothills of Alberta. Acknowledgements This project was funded by the Alberta Conservation Association and an NSERC Collaborative Research and Development with industrial partners Hinton Wood Products a Division of West Fraser Mills Ltd., Alberta Newsprint Company, and the Foothills Model Forest Natural Disturbance Program. Thanks to D. Andison and R. McCleary of Foothills Model Forest, R. Bonar and C. Spytz of Hinton Wood Products a Division of West Fraser Mills Ltd. for technical support; to our field and lab assistants for their hard work and dedication.
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Daniels, L.D., Dobry, J., Klinka, K., Feller, M.C., 1997. Determining year of death of logs and snags of Thuja plicata in southwestern coastal British Columbia. Canadian Journal of Forest Research 27, 1132–1141. Fausch, K.D., Northcote, T.G., 1992. Large woody debris and salmonid habitat in a small coastal British-Columbia stream. Canadian Journal of Fisheries and Aquatic Sciences 49, 682–693. Figueiredo, A., Machado, S.A., Carneiro, M.R.A., 2000. Testing accuracy of log volume calculation procedures against water displacement techniques (xylometer). Canadian Journal of Forest Research 30, 990–997. Franklin, J.F., Spies, T.A., Van Pelt, R., Carey, A.B., Thornburgh, D.A., Rae Berg, D., Lindenmayer, D.B., Harmon, M.E., Keeton, W.S., Shaw, D.C., Bible, K., Chen, J., 2002. Disturbances and structural development of natural forest ecosystems with silvicultural implications, using Douglas-fir forests as an example. Forest Ecology and Management 155, 399–423. 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Identifying Wood: Accurate Results with Simple Tools. Taunton Press, Newton, CT. Honer, T.G., 1967. Standard Volume Tables and Merchantable Conversion Factors for the Commercial Tree Species of Central and Eastern Canada. Canada Department of Forestry and Rural Development, Forestry Branch, Forest Research and Services Institute. Hyatt, T.L., Naiman, R.J., 2001. The residence time of large woody debris in the Queets River, Washington, USA. Ecological Applications 11, 191–202. Kraft, C.E., Schneider, R.L., Warren, D.R., 2002. Ice storm impacts on woody debris and debris dam formation in northeastern US streams. Canadian Journal of Fisheries and Aquatic Sciences 59, 1677–1684. Landres, P.B., Morgan, P., Swanson, F.J., 1999. Overview of the use of natural variability concepts in managing ecological systems. Ecological Applications 9, 1179–1188. Maser, C., Anderson, R.G., Cromack, K.J., Williams, J.T., Martin, R.E., 1979. Dead and down woody material. In: Thomas, J.W. (Ed.), Wildlife Habitats in Managed Forests—The Blue Mountains of Oregon and Washington. USDA Forest Service, Agriculture Handbook 553, Washington, DC, pp. 78–95. May, C.L., Gresswell, R.E., 2003. Processes and rates of sediment and wood accumulation in headwater streams of the Oregon coast range, USA. Earth Surface Processes and Landforms 28, 409–424. McCleary R., 2005. Structure and function of small foothills streams and riparian area following fire. Technical Report—Fish and Watershed Program Foothills Model Forest, Hinton, AB, 97 pp. McHenry, M.L., Shott, E., Conrad, R.H., Grette, G.B., 1998. Changes in the quantity and characteristics of large woody debris in streams of the Olympic: Peninsula, Washington, USA (1982–1993). Canadian Journal of Fisheries and Aquatic Sciences 55, 1395–1407. Meleason, M.A., Gregory, S.V., Bolte, J.P., 2003. Implications of riparian management strategies on wood in streams of the Pacific Northwest. Ecological Applications 13, 1212–1221. Montgomery, D.R., Buffington, J.M., Smith, R.D., Schmidt, K.M., Pess, G., 1995. Pool spacing in forest channels. Water Resources Research 31, 1097– 1105. Naiman, R.J., Decamps, H., 1997. The ecology of interfaces: riparian zones. Annual Review of Ecology and Systematics 28, 621–658. Naiman, R.J., Elliott, S.R., Helfield, J.M., O’Keefe, T.C., 1999. Biophysical interactions and the structure and dynamics of riverine ecosystems: the importance of biotic feedbacks. Hydrobiologia 410, 79–86. Oliver, C.D., Larson, B.C., 1996. Forest Stand Dynamics. Wiley, New York. Powell, S.R., 2006. A wood inventory and dendroecological analysis of large woody debris in small streams in the Foothills Model Forest, Hinton, Alberta. M.Sc. Thesis, University of British Columbia, Vancouver, BC, 97 pp.
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Dynamics of large woody debris in small streams disturbed by the 2001 Dogrib fire in the Alberta foothills Trevor A. Jones, Lori D. Daniels * Department of Geography, University of British Columbia, 1984 West Mall, Vancouver, BC V6T 1Z2, Canada
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 August 2007 Received in revised form 30 November 2007 Accepted 28 February 2008
We investigated the temporal dynamics of large woody debris (LWD) in five headwater streams before and after the 2001 Dogrib fire in the foothills of Alberta. The density of LWD varied from 5 to 41 logs per 50 m of stream reach and accounted for 19.4 5.1 m3 ha1 (mean standard error) of wood in the riparian zones and 114.1 30.1 m3 ha1 of wood in the bankfull margins of the stream channel. Individual logs averaged 18.9 1.15 cm in diameter, 5.5 0.7 m in length, and 0.2 0.02 m3 in volume. Logs became significantly shorter in decay classes II–IV. Bridges were longer than partial bridges, which were longer than loose and buried LWD. Individual log volume was greatest for bridges, but not significantly different among other position classes. Bridges and loose LWD contributed little to stream morphology and function; however, 55% of partial bridges and all buried logs contributed to sediment storage, channel armouring, or riffles and pools in the stream channel. Using dendroecological methods, we estimated the year of death of 108 of 115 spruce logs. LWD resulted from tree deaths that occurred between 1874 and 2001, so that time since death ranged from 5 to 132 years. Time since death increased from decay class II to III to IV and bridges were younger than LWD in all other position classes. Due to high rates of recruitment after fire, 16.5% LWD recruited between 2001 and 2006, most of which were bridges or partial bridges in decay class II. We anticipate a delay of 30–45 years before newly recruited logs contribute significantly to stream morphology and function. Depletion rates of LWD were exponential, such that 50% of LWD would be lost to decay, erosion or downstream transport within 30 years of tree death and <12% of LWD would persist more than 100 years. Since recruitment of new LWD in post-fire lodgepole pine stands is delayed by ca. 40 years while trees establish and stands develop, we anticipate periods of ca. 70 years between stand-replacing fires and recruitment of new, functional LWD into stream channels. During this time, fire-killed snags are an important source of LWD to small streams. For headwater streams in environments susceptible to floods and erosion we recommend that buffer zones comprised of snags to be established after fires. The goal of these post-fire buffers is to ensure a supply of LWD into streams for years to decades after a standreplacing fire. Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved.
Keywords: Dendroecology Logs Decay class Position class Natural disturbance Stream morphology
1. Introduction Woody debris in upland and riparian forests, including large woody debris (LWD) defined as downed logs that intersect the bankfull margins of a stream, has been linked to many important ecological functions (Harmon et al., 1986). For example, LWD provides stream channels with structural complexity and stability (Montgomery et al., 1995; Rot et al., 2000) by contributing to the storage of sediment (May and Gresswell, 2003), creating diverse habitats in the form of step pools, plunge pools, riffles and logjams (Bilby and Ward, 1989; Montgomery et al., 1995) and by
* Corresponding author. Tel.: +1 604 822 3442; fax: +1 604 822 6150. E-mail address: [email protected] (L.D. Daniels).
dissipating stream energy during periods of high water flow (Richmond and Fausch, 1995). LWD also affects stream dynamics in both in space and time (Naiman et al., 1999; Hyatt and Naiman, 2001; Hassan et al., 2005), which is largely manifested through bank erosion, downstream sediment movement, and the formation and loss of structural elements such as pools and riffles (May and Gresswell, 2003). Many studies of LWD have focussed on the quantity and function of LWD (Hassan et al., 2005), while others have attempted to quantify how management may affect LWD quantity and composition (Bilby and Ward, 1991; McHenry et al., 1998; Gomi et al., 2001; Dahlstrom et al., 2005). Disturbance type, size and frequency plays and important role in LWD dynamics irrespective of forest type. Geomorphic disturbances such as seasonal floods, bank erosion and channel migration contribute LWD into streams
0378-1127/$ – see front matter . Crown Copyright ß 2008 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.02.048
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regardless of age or stage of development of the riparian forest (Naiman and Decamps, 1997). Coarse-scale, episodic disturbances of riparian forests, such as harvesting (Chen et al., 2005), large floods (Acker et al., 2003), fire (Hedman et al., 1996), ice storms (Kraft et al., 2002) and insect outbreaks, are the major contributors of LWD into streams. Following stand-replacing disturbances, the contribution of LWD will vary with stand development processes. Young, uniform-aged forests have lower rates of woody debris recruitment and contribute smaller diameter woody debris, while older forests, with diverse diameter sizes and age structures, generate a greater range of LWD sizes as well as a greater abundance and volume of wood (Harmon et al., 1986; Spies, 1998). In absence of coarse-scale disturbances in old forests, frequent, fine-scale disturbances results in chronic input of LWD due to the combined influences of tree age, wind, insects and pathogens. As a result, older, more structurally complex forests have greater amounts of LWD than younger, less complex forests (Naiman et al., 1999; Acker et al., 2003; Benda and Sias, 2003). Understanding the links between disturbance, vegetation dynamics and structural complexity of forests has become increasingly important as many management agencies have adopted the principals of natural range of variation and emulation of natural disturbance as key objectives for forest management (Christensen et al., 1996; Landres et al., 1999). For example, the recent recognition that natural disturbances are an integral component of riparian forest dynamics has initiated discussions about whether riparian zones should be actively managed or protected as reserves and buffer zones (Beschta et al., 2004). Consequently, studies have examined the effects of management on LWD volume through time (Gomi et al., 2001). These studies have documented discrepancies between timber management and natural processes such as standreplacing fires. For example, Chen et al. (2005) found that fire resulted in a net increase in terrestrial woody debris biomass following a disturbance when compared to clear-cut harvesting or non-harvested old forests. Bragg (2000) provided a long-term perspective when he determined that the differences between clear cuts and natural disturbances such as fire last more than 120 years. Interestingly, these studies have not considered how LWD changes in function or how variation across streams may influence stream structure and function over time. Although time since tree death may be correlated with tree decay rate, it is the rate at which LWD are integrated into functional positions in streams that is most important for our understanding stream structural dynamics through time (Powell, 2006). To better understand how natural disturbances, such as fire, influence LWD function, it is critical to determine how and when LWD recruits into streams and how long LWD takes to transition into different position classes and functional roles in streams. Unfortunately, specific long-term studies of in-stream wood dynamics have not been established for the sole purpose of understanding wood function through time. However, the development of dendrochronological techniques to
determine the time since death of woody debris has dramatically increased our ability to study the dynamics of dead wood. To address the temporal dynamics of LWD in streams, we have designed a study to investigate the following questions: (1) What are the short-term impacts of fire on LWD abundance and volume in small streams? (2) What is the expected time period before LWD begins to contribute to the structure and function of streams? (3) At what rate is LWD depleted from a stream? Through insight gained from this study, we aim to inform management decisions following fires that have consequences for riparian zones and the temporal dynamics of LWD in small streams. 2. Methods 2.1. Study area We conducted this research in the Upper Foothills Natural Subregion of the Rocky Mountains in west-central Alberta. The upper foothills are characterized by steeply rolling terrain with maximum and minimum elevations of 1931 and 1430 m above sea level (m.a.s.l.), respectively, and an average slope of 218. Upland forests within this subregion are characterized by closedcanopy coniferous forests, most commonly including lodgepole pine (Pinus contorta Dougl. Ex Loud), as well as white spruce (Picea glauca (Moench) Voss) and black spruce (Picea mariana (Mill.) B.S.P.). Frequent, stand-replacing fires of an historical average fire size of 300 ha, with a mean return interval of 100 years, have created a landscape mosaic of mixed stands composed of varying amounts of lodgepole pine, white spruce and black spruce (Beckingham et al., 1996). We assessed LWD in five headwater streams affected by the Dogrib fire, which burned 9214 ha of forest near Sundre, Alberta in September 2001 (Fig. 1). According to Alberta vegetation inventories, the riparian forest composition surrounding the five stream reaches was dominated by black and white spruce with minor amounts of lodgepole pine. Based on post-fire assessments, it was apparent that forest composition in the valleys constraining the riparian zones was a mixture of black and white spruce and lodgepole pine had grown in the upland forests surrounding the riparian zones. The fire was a fast-burning crown fire, resulting in nearly 100% mortality of mature trees in the affected riparian area (E. Wassink, Sundre Forest Products, personal communication). Residual standing dead trees were blackened, however, bark, coarse branches, most of the sapwood and all of the heartwood were still intact on most trees. The burned area included part of the R11 Forest Management Unit and the Forest Management Agreement (FMA) area managed by Sundre Forest Products, a division of West Fraser Mills Ltd. Large portions of the FMA were subsequently salvage
Fig. 1. Location of the Dogrib fire and the 5 stream reaches in the Sundre Forest Products Ltd. forest management area in Alberta, Canada.
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Table 1 Description of decay and position classes used to categorize large woody debris (LWD) Variable class
Description
Position classes Bridge Partial bridge Loose Buried
Log spans channel, touching both banks and resting on the floodplain Spanned log has broken in one or more places within the stream channel Log is no longer associated with the floodplain; fully associated with the streambed; log is fully submerged during bankfull Log has become incorporated into the streambed or the sides of the streambank; sediment is stored upstream of the log which is at least partly buried
Decay classes I II III IV
Wood has >75% bark still intact, bark adheres tightly; branches have fine (third order) branchlets; sapwood is sound, and log retains structural integrity Wood has 25–75% bark intact which, in places, is loosely attached to the bole; first order branches have a solid connection to the bole; wood is solid with evidence of decay on some outer sections of sapwood only Wood has 0–25% bark present, adhering loosely to the sapwood; first order branches and branch nubs are present and sit loosely in the bole; along some parts of the bole, wood shows significant signs of decay to depths of 5–10 cm Bark is no longer attached; branch nubs only are present; along some parts of the bole, wood is soft, crumbly or fibrous, and decay can penetrate nearly through the sapwood
logged, although riparian buffers of standing dead trees were retained along most first and second order streams and all higher order streams. McCleary (2005) classified stream channel structure, assessed sediment storage, and documented LWD recruitment and storage at stream and watershed spatial scales for the part of the Sundre Forest Products FMA that burned during the Dogrib fire. For our analysis of large woody debris, we selected five headwater streams based on the following criteria: (1) Headwater channels had an upstream drainage area <200 ha. (2) The entire reach had burned during the Dogrib fire resulting in 100% canopy tree mortality in the riparian forest surrounding the stream. (3) The forest surrounding the stream had not been harvested before the fire and was not salvaged after the fire or influenced by roads. (4) No evidence of recent or historic landslides. Streams had limited ability to transport wood due to the fact that they were narrow and shallow compared to the length and diameter of the wood within them. Using Alberta Vegetation Inventory data used by Sundre Forest Products for timber management, we confirmed that the riparian zones had mature spruce-dominated forests before the fire. Stand origins for the forests surrounding each stream were greater than 100 y.b.p. and exhibited uneven-sized diameter distributions, based on pre-fire vegetation inventories and post-fire appraisals of residual standing and recently downed woody debris (Andison and McCleary, unpublished data). The riparian zone was 40 m wide, 20 m on either side of the stream, which was the area from which mature trees had fallen and contributed to the in-stream LWD. Streams chosen for study had an average slope of 8.2 1.88 (mean standard error) and were loosely constrained by shallow hill slopes (14.5 4.88). 2.2. LWD abundance and size At each stream, we surveyed a 45–100 m reach starting at a randomly selected location that was downstream from McCleary (2005) permanent research plots. Target reach length for the study was 100 m; however, reach length varied depending on the availability of suitable stream area downstream of permanent plots and also by availability of non-salvaged forest surrounding the streams. Consequently three of the reaches were truncated at 50 m while another was truncated at 45 due to a road crossing. Stream width at bankfull was measured every 5 m along the length of each sampled stream reach.
We sampled all LWD, defined as logs that intersected the stream channel and exceeded 0.08 m in diameter and 1 m in length. We confirmed that each log was spruce based on intact bark and wood anatomy (Hoadley, 1990); pine and other species were not present. Each LWD was assigned a decay class and position class (Table 1). Decay classes categorize logs based on structural characteristics and wood integrity (Maser et al., 1979) and position classes categorize LWD location and function relative to the stream channel (Richmond and Fausch, 1995; Berg et al., 1998; Hauer et al., 1999). To assist with our field-based estimates for decay class, we documented several physical attributes commonly associated with stages of decay. These included the presence of needles, the branch order, the percentage of bark remaining, and the percent cover of vegetation and soil on the upper surface of the LWD. We classified the size of the branches attached to each log, as follows: 0 = no branches, 1 = branch stubs, 2 = first order, large branches, 3 = second order large and small branches, and 4 = third order, fine twigs. For each LWD, we used a six-class system to independently estimate the percentage of (a) visible bark remaining on the stem, (b) upper surface of the log covered by vegetation and (c) upper surface of the log covered by soil, as follows: 0 = none, 1 = 1–25%, 2 = 26–50%, 3 = 51–75%, 4 = 76–99%, and 5 = 100%. We classified the effects of fire using a three-class system of the amount of charred bark and sapwood of each LWD, as follows: 0 = no char, 1 = charred bark but outermost ring present in at least one part of the circumference, and 2 = charred bark and sapwood resulting in significant ring loss. We also classified each piece of LWD according to one or more function classes. These included: creating pools and riffles, retaining sediment, armouring banks, creating debris jams or no observable function. Due to small sample size in some function classes and to aid in analysis, LWD was subsequently assigned to one of two broader function classes: 0 = LWD has no observable effect on stream geomorphology or 1 = LWD has an effect on stream geomorphology. For each LWD, length (m) and diameter (cm) were determined based on the condition of the log and its position relative to the stream channel. For logs with a visible root plates and intact tops, we measured diameter at breast height (dbh) and total length to calculate volume (Vlog) using species-specific standard allometric equations (Honer, 1967). For segments of logs without an identifiable root plate, we measured length and diameter at both ends of the log. The volume of these logs (Vlog) was calculated using the equation for a conical paraboloid (Fraver et al., 2007). If decay along the log prevented an accurate measure of diameter at the ends, then the diameter in the middle was recorded and volume (Vlog) was calculated using Huber’s formula, which provides consistent volume estimates for woody debris with broken tops and bottoms or in advanced stages of decay (Figueiredo et al., 2000; Fraver et al., 2007).
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For LWD that crossed one or both banks of the stream channel, we measured the length within the bankfull boundaries (in-stream length) and diameter at the midpoint within these boundaries and calculated the in-stream volume (Vst) using Huber’s formula (Figueiredo et al., 2000; Fraver et al., 2007). LWD that was embedded in the stream bed was partially excavated to measure length and diameter and to obtain a sample for dendrochronological analyses. We used a general linear model (GLM, SAS Institute Inc., Cary, NC) to determine if individual LWD diameter, length, volume, in-stream length and in-stream volume varied significantly among position, decay or function classes. The models included a stream reach factor to account for differences in measured variables due to variance among the streams. All five dependent variables were logarithmically transformed to meet the assumptions of normality and equal variance. A Tukey–Kramer post hoc test was used to differentiate among classes. In the figures, the transformed variables are presented in the original units. At the spatial scale of the stream reach, we calculated (a) total volume of LWD in each riparian zone by summing Vlog in each stream reach and (b) total in-stream volume of LWD by summing Vst in each stream reach. To allow direct comparison among reaches, the total volumes were expressed in m3 ha1 of riparian and stream channel area, respectively. 2.3. LWD year of death and time since death We stratified LWD into trees that were most likely killed by fire in 2001 and logs that died before the fire. We assumed that logs in decay class (DC) II that were bridges or partial bridges most likely had been killed by fire. Secondary evidence of fire-killed trees was the presence of charcoal on the stem and intact bark, but no soil or vegetation had accumulated on the logs. We tested this assumption by estimating year of death of each log using dendrochronological methods. Cross-sectional disks were sampled from all LWD in DC II–IV; there were no LWD in DC I. Disks from LWD in DC II were air dried and then sanded following standard dendrochronological protocols (Stokes and Smiley, 1968). For wood samples that were in more advanced stages of decay (DC III and IV), the samples were frozen immediately after collection and then planed and sanded while frozen with progressively smoother sandpaper, up to 600 grit. After sanding, these disks were air dried and some were resanded by hand to ensure that ring boundaries were clearly visible. The ring-width series from the pith to the outermost ring of each wood disk was measured to the nearest 0.001 mm using a Velmex bench interfaced with a computer. Ring widths were crossdated against the regional, species-specific standard chronologies for black and white spruce (Powell, 2006), using skeleton plots and the programs COFECHA (Grissino-Mayer, 2001) and TSAPWin (RINNTECH, Engineering and Distribution, Heidelberg, Germany). Before assigning a calendar year to the outermost ring to estimate the year of death, we evaluated the quality of the bark and sapwood of each disk. When rings are lost from a sample due to erosion or decay, the crossdating results do not accurately depict year of death. For eroded or decayed samples, year of death is under-estimated and the number of years since death is over-estimated (Daniels et al., 1997). The magnitude of the error increases with advanced decay (Guyette and Cole, 1999) but has not been quantified in our study area. Therefore, subsequent analyses were conducted with the understanding that the year of death of 34 LWD were under-estimated. Time since death of all LWD was calculated as 2006 minus year of death.
2.4. LWD residence and depletion We quantified LWD residence times and rates of depletion using two different approaches. First, an ANOVA (GLM, SAS Institute Inc.) was used to test for differences in time since death of LWD among position and decay classes. Time-since-death estimates were transformed using a natural logarithm to meet the assumptions of normality and equal variance. For significant variables, a Tukey–Kramer post hoc test was used to differentiate among classes. In the figures, we presented the transformed variables in the original units. Second, for the 89 crossdated LWD with year of death before the fire in 2001, we assessed rates of depletion using time since death estimates. To construct the standardized cumulative distributions, we sorted the LWD by time since death, summed the number of logs beginning with the greatest time since death, and divided the cumulative values by the total number of samples (Hyatt and Naiman, 2001; Guyette et al., 2002). We plotted the cumulative, relative distribution against time since death and fit a negative exponential model using SAS (NLIN, SAS Institute Inc.): Y ¼ expat
(1)
where Y was the predicted proportion of LWD pieces in the channel at time, t, which corresponds with the time since death of the LWD in 2006, and a was a constant representing the rate of change of the negative exponential function. Because we were assessing a static distribution, the net rate of change in abundance of LWD represents the combined effects of recruitment, storage and depletion due to decay, erosion and downstream transport over time. The period of analysis was determined by the year of death of the oldest datable LWD that we sampled in the streams. 3. Results 3.1. LWD year of death Of the 115 pieces of LWD sampled, 108 were sound enough to estimate outermost ring dates and years of death by crossdating, but seven samples were decayed so that annual rings could not be distinguished. Year of death of LWD ranged from 1874 to 2001, corresponding to time since death of 132 to 5 years (37.7 3.1 years, mean standard error) (Fig. 2). Of the 115 LWD in the 5 reaches, 19 logs had accumulated in the 5 years since fire, compared to 96 logs that had been recruited and retained in the riparian zones and streams before the fire in 2001
Fig. 2. Time since death of large wood debris (LWD) in five headwater streams. LWD is classified by origin relative to the Dogrib fire in 2001.
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Table 2 Abundance, dimensions and volume of large woody debris (LWD) in the five study streams Stream number
Length of sampled reach (m)
Stream channel width (m)
Number of LWD All (n)
Post-fire (n)
Diameter (cm)
Length (m)
Volume (m )
Riparian (m3 ha1)
In-stream (m3 ha1)
1 2 3 4 5
100 50 50 50 45
1.7 2.4 1.6 5.1 4.9
23 27 5 45 15
7 3 0 8 1
17.9 18.5 22.4 15.5 20.1
8.0 4.6 5.3 5.3 4.1
0.29 0.18 0.21 0.16 0.18
16.7 23.8 5.3 36.0 15.0
77.0 231.4 82.6 110.3 69.2
23.0 (6.7)
3.8 (1.6)
18.9 (1.2)
19.4 (5.1)
114.1 (30.1)
Mean (S.E.)
59.0 (10.3)
(0.2) (0.5) (0.2) (0.7) (1.2)
3.1 (0.8)
Individual LWD
(1.2) (1.2) (2.9) (0.8) (1.1)
Total volume
(1.3) (0.6) (2.4) (0.7) (0.8)
5.5 (0.7)
3
(0.11) (0.07) (0.09) (0.04) (0.06)
0.20 (0.02)
Means are followed by standard errors in parentheses.
(Fig. 2, Table 2). The latter category included the seven logs that could not be dated due to advanced decay. The field-based assessments of decay and position class correctly identified 13 of 19 LWD that resulted from fire. Of those logs, none were in DC I, due to the fact that all needles and fine branches and some of the bark and outer rings had been burned during the fire; 12 were bridges in DC II, and one was a partial bridge in DC II. As well, four bridges and partial bridges were in DC III and two loose LWD were in DC II and III. The outermost ring of 14 LWD was 2001, confirming these trees died in the year of fire. For five LWD, the outermost rings were between 1995 and 1998; however, these trees likely died in 2001 given substantial charring that indicated loss of some rings during the fire. Furthermore, these logs were intact, bridged the stream channel, and had not accumulated soil or vegetation. The remaining LWD died before the fire in 2001. They included logs with no evidence of ring loss and years of death prior to 2001 and logs with some ring erosion and years of death prior to 1992. Five logs that died prior to fire were bridges or partial bridges in DC II, which met our field-based criteria for identifying LWD potentially caused by the fire. However, the outermost rings on these LWD ranged from 1992 to 1958 and soil and/or vegetation had accumulated on three of them, supporting our interpretation that they died before the fire in 2001.
loose (3.0 0.4 m) and buried LWD (2.8 0.4 m), but loose and buried did not differ significantly. LWD in DC II (10.0 1.0 m) were longer than LWD in DC III (4.0 0.4 m) and IV (3.9 0.6 m), which did not differ significantly. Changes in LWD length contributed to significant differences in the volume of individual LWD among position classes (F = 9.47, p < 0.0001) and decay classes (F = 5.2, p = 0.008) (Fig. 3). Bridges (0.41 0.10 m3) had significantly greater volume than partial bridges (0.14 0.04 m3), loose (0.07 0.02 m3) and buried (0.13 0.05 m3) LWD. Similarly, individual in-stream volume varied significantly among position classes (F = 2.31, p = 0.081), where bridges (0.12 0.04 m3) tended to have greater volume than partial bridges (0.07 0.01 m3), loose (0.05 0.02 m3) and buried (0.07 0.02 m3) LWD. 3.3. LWD function The function or influence of LWD on stream morphology varied among position classes (Fig. 4). Only two of 28 (7.1%) bridges and four of 21 (19.0%) loose LWD influenced sediment storage and structure of the stream channel. In contrast, over half of partial bridges (21 of 38, 55.3%) and all buried LWD (n = 21) influenced sediment in the stream channel and banks or contributed to steppools and riffles in the stream. The average time since death of functional LWD was 50.2 5.2 years, although some LWD influenced stream morphology within 5 years of death.
3.2. LWD abundance and size 3.4. LWD dynamics The abundance of LWD was highly variable among the five stream reaches (Table 2). There were between five and 41 LWD per 50 m of stream reach, with an average 21.0 7.0 LWD. The total volume of LWD was 19.4 5.1 m3 ha1, while the total in-stream volume of LWD was 114.1 30.1 m3 ha1 of stream channel area (Table 2). LWD volumes were highly variable among reaches and ranged between 5.3 and 23.8 m3 ha1 for total volume and between 69.2 and 231.4 m3 ha1 for total in-stream volume (Table 2). The post-fire cohort of LWD contributed 6.2 3.0 m3 ha1 to total volume and 15.3 6.1 m3 ha1 directly into the stream channels, while LWD that died prior to 2001 contributed 14.4 4.1 m3 ha1 to total volume and 101.9 31.3 m3 ha1 into the stream channels. Individual logs were, on average, 18.9 1.15 cm in diameter and 5.5 0.7 m long (Table 2). The mean volume of individual logs was 0.2 0.02 m3 and ranged between 0.003 and 2.09 m3 (Table 2). For 49% (56 of 115) of LWD, the entire log was within the bankfull limits of the stream channel. For the remaining 59 logs, 5–90% (36 2.8) of the total length was directly associated with the stream channel. The average in-stream volume of individual logs was 0.08 0.01 m3 and ranged between 0.006 and 1.12 m3. The length of LWD differed significantly among position (F = 20.3, p < 0.0001) and decay (F = 23.4, p < 0.0001) classes (Fig. 3). Bridges (10.53 1.1 m) were significantly longer than partial bridges (5.0 0.5 m), which were significantly longer than
Time since death of LWD increased with progressive position and decay classes (Fig. 3). The time since death of bridges was less than all other position classes (Fig. 3, F = 8.88, p < 0.0001). Due to large variance within position classes, differences in mean time since death among partial bridges, loose and buried LWD were not significant. Time since death of LWD was least in DC II and increased significantly in DC III and IV (Fig. 3, F = 29.9, p < 0.0001). The absolute number of LWD was greater after the fire than before the fire, largely due to differences in recruitment rates. In the 6 years after the fire (2001–2006), 19 LWD recruited into the streams at a rate of 0.54 LWD per year per 50 m reach. In contrast, during the 6 years immediately before the fire (1995–2000), the recruitment rate was 0.37 LWD per year per 50 m reach. During these two time intervals, we assumed that depletion due to decay, erosion and downstream transport was negligible for three reasons. (1) More than 12 years is needed for LWD to completely decay or erode. (2) Most LWD that died between 1995 and 2006 were bridges (56%) and partial bridges (32%), which are less vulnerable to downstream transport than loose (6%) and buried (6%) LWD. (3) The study streams were small and had limited capacity to move large intact logs. For the LWD that died before the fire in 2001, the negative exponential shape of the LWD depletion curve indicates that rate of
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Fig. 3. Comparison of LWD length (top) and volume (middle) and time since death (bottom) in different position (left column) and decay (right column) classes (mean + S.E.). In each graph, different superscript letters indicate significant differences among means at a = 0.05.
depletion decreases with time since death (Fig. 5, F = 53332.4, p < 0.001 and MSE = 0.0005). The depletion curves also provided insight into the residence times of LWD in the study streams. If no additional LWD was recruited into the streams and rates of depletion follows the same exponential rate depicted in Fig. 5, then 50% of the LWD would be depleted in ca. 30 years and <12% would persist >100 years (Fig. 5). 4. Discussion The Dogrib fire in western Alberta resulted in a measurable increase in the abundance of LWD in headwater streams. Using dendroecological dating methods, we determined that 16.5% of the LWD in our study streams were killed and recruited into the stream as a result of the fire in 2001. The average annual rate of recruitment of LWD increased 48% between the 6 years immediately before and after the fire. Although this is a significant increase in the abundance of LWD over a short period, a large number of standing dead trees that were killed by the fire have yet to fall (McCleary, 2005; Jones, unpublished data). Given the fall
rates we observed between 2001 and 2006, we anticipate the volume of LWD will continue to increase significantly during the next several years to decades. Consequently, the rate of LWD recruitment will be sustained at a greater rate than expected under the influence of fine-scale gap-phase dynamics in mature and old spruce-dominated riparian forests also located in the foothills of Alberta (Powell, 2006). Despite the increase in abundance of LWD, there is little evidence that the post-fire cohort of LWD had an immediate influence on stream morphology. Of the 19 LWD in the post-fire cohort, 13 were bridges suspended above the stream channel, four were partial bridges with limited contact with the stream channel, and two were loose logs lying directly on the streambed. At the time of sampling, none of the LWD in the post-fire cohort had contributed to step or plunge pools morphology and only three LWD, two partial bridges and one loose log, had retained sediment. Based on the time since death of partial bridges, loose and buried LWD currently in the stream channels (Fig. 3), we anticipate a delay of 30–45 years before the newly recruited logs have a significant impact on stream morphology. These findings are
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transition among position classes and contribute to stream function. At the same time, the abundance of pre-fire LWD will deplete due to in situ decay and erosion as well as downstream transport of logs during peak flows. Assuming that the depletion rates of the post-fire cohort will be similar to the logs that died during the 124 years before the Dogrib fire, we anticipate only 9–24% of LWD that we observed in 2006 will remain in the streams in 2071, 70 years after the fire. Similarly, by 2086, 85 years after the fire, only 5–16% of LWD will remain. These losses will be mitigated only by the recruitment of the fire-killed snags adjacent to the stream, input from upstream and upslope locations (Benda et al., 2005), or by exposure of buried LWD during channel migration (Guyette et al., 2002). 5. Management implications
Fig. 4. Comparison of LWD time since death by function class and position class. Functional LWD (n = 48) influences stream morphology; non-functional LWD (n = 60) has no observable impact on sediment or hydrology. Different superscript letters indicate significant differences among means at a = 0.05. Numbers above superscript letters are sample size for each function and position class.
Fig. 5. Depletion curve for LWD based on all time since death estimates. The predicted curve was derived using the exponential function in Eq. (1). The vertical dotted line represents the Dogrib fire in 2001.
consistent with previous studies that documented delays of several decades before LWD influences stream geomorphology (Powell, 2006). In our study area, the burned snags in the riparian zones represent an important source of LWD for the stream morphology and function. Depending on rates of fall, LWD recruitment into the riparian zone may continue for several decades. Once all snags have fallen, no new sources of LWD will be available until the riparian forest is either has matured sufficiently to generate logs through self-thinning or through other mortality events (Oliver and Larson, 1996; Franklin et al., 2002). Based on the regenerating trees we observed in the surrounding riparian area, the future riparian forests surrounding our study streams will be initially dominated by lodgepole pine. In riparian pine forests of the Alberta foothills, new LWD is typically generated ca. 40 years after stand-replacing fires (Powell, 2006). Since it takes 30–45 years for LWD entering the stream to affect stream morphology, we anticipate a period of ca. 70– 85 years before new LWD is generated and becomes functional in the stream. During this lag, only the post-fire cohort of LWD will
Knowledge of time since death and rates of depletion of LWD indicate that the abundance of woody debris in headwater streams is highly variable through time and significantly influenced by disturbances and subsequent dynamics of riparian forests. Understanding the decadal-scale dynamics of LWD in relation to disturbance can contribute to sustainable forest management. For example, our results corroborate studies that indicate human impacts, such as salvaging timber from burned riparian forests, can have long-term consequences for stream geomorphology and function (Meleason et al., 2003). If trees in burned riparian forests are not retained, then the cohort of post-fire LWD and snags cannot recruit into the stream. In this scenario, the LWD that recruited before the fire will deplete due to decay, erosion and transport, but will not be replaced for prolonged periods (ca. 70 years). As the absolute abundance of LWD is reduced, structural complexity of the stream channel may decline. This assertion is supported by evidence from steam surveys of harvested landscapes in which the removal of riparian biomass resulted in significant reductions in LWD volume and step and plunge pool area, while channel width, stream velocity and sediment transport increased (Bilby and Ward, 1991; Fausch and Northcote, 1992; Ralph et al., 1994; Gomi et al., 2001). Such changes can have serious consequences for streambed stability, water quality, invertebrate and vertebrate habitats, and biodiversity (Wipfli et al., 2007). Current operational guidelines in our study area in Alberta require that a buffer of at least 10 m be retained around small permanent streams (Alberta Sustainable Resource Development, 2006). Independent of these buffers, an additional 25% of standing dead wood must be retained when salvaging burned forests (Alberta Sustainable Resource Development, 2007). To reduce risk of post-fire degradation due to erosion and to ensure long-term recruitment of LWD following fire, we recommend retaining snags immediately adjacent to riparian buffer zones in burned areas. This would create relatively large, contiguous areas with intact woody debris while meeting both operational requirements. In contrast to permanent streams, treed buffers are not required around intermittent or ephemeral streams and salvage can, and does, occur in the these riparian areas in Alberta. However, recent research has demonstrated the negative impacts of forest operations on headwater streams and the downstream effects on water quality and biodiversity (Wipfli et al., 2007). Therefore, we recommend protection of riparian forests and post-fire woody debris associated with headwater streams. For example, snags in riparian zones surrounding small streams should be left intact under the following conditions: (a) the landscape is subject to seasonal floods and erosion; (b) the riparian forest directly provides habitat for threatened, rare or endangered species; (c) the riparian forests is located upstream from habitat of threatened, rare or endangered species, particularly in landscapes that are susceptible to flooding and erosion.
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In essence, we are advocating the retention of post-fire buffer zones comprised of snags surrounding headwater streams with the goal of mitigating the inevitable depletion of the LWD. The dimensions of post-fire buffer zones should vary among forest types since LWD recruits into a stream from different distances depending on the species and size of trees and the geomorphology of the stream and terrain surrounding the riparian zone. In our study area, LWD was recruited from riparian zones that extended 20 m to either side of the stream channel. Therefore, effective post-fire buffer zones around permanent streams could be designed and retained within the context of existing operational guidelines. However, additional protection of intermittent or ephemeral headwater streams following fire is needed in the foothills of Alberta. Acknowledgements This project was funded by the Alberta Conservation Association and an NSERC Collaborative Research and Development with industrial partners Hinton Wood Products a Division of West Fraser Mills Ltd., Alberta Newsprint Company, and the Foothills Model Forest Natural Disturbance Program. Thanks to D. Andison and R. McCleary of Foothills Model Forest, R. Bonar and C. Spytz of Hinton Wood Products a Division of West Fraser Mills Ltd. for technical support; to our field and lab assistants for their hard work and dedication.
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