Riparian forest structure and succession in second-growth stands of the central Cascade Mountains, Washington, USA

Riparian forest structure and succession in second-growth stands of the central Cascade Mountains, Washington, USA

Forest Ecology and Management 257 (2009) 1375–1385 Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.els...

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Forest Ecology and Management 257 (2009) 1375–1385

Contents lists available at ScienceDirect

Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco

Riparian forest structure and succession in second-growth stands of the central Cascade Mountains, Washington, USA Lauren A. Villarin a,*, David M. Chapin b, John E. Jones IIIc a

College of Forest Resources, University of Washington, Seattle, WA 98105, United States Seattle Public Utilities, Watershed Services, 19901 Cedar Falls Road S.E., North Bend, WA 98045, United States c Weyerhaeuser Co. Statistics, Math and Operations Research, P.O. Box 9777, Federal Way, WA 98063, United States b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 17 June 2008 Received in revised form 1 December 2008 Accepted 2 December 2008

We examined the relationship between landform types and riparian forest structure and succession in second-growth stands along mid order streams in the Cascade Mountains, Washington, USA. We sampled tree, sapling, seedling, and shrub characteristics across a range of fluvial geomorphic surfaces, which were classified into four landform classes, including low floodplain, high floodplain, terrace and hillslope. Landform classification was based on topographic characteristics, position relative to the stream channel, and estimated flood frequency. Statistical analyses using generalized estimating equations (GEE) showed that landform exerted a strong influence on the distribution and abundance of conifer and deciduous species and of different tree life stages. The floodplain landforms were characterized by initial disturbance from timber harvest, and ongoing fluvial disturbance, which favored the establishment of deciduous communities dominated by red alder (Alnus rubra) and maintenance of early successional riparian stands. In contrast, the terrace and hillslope landforms were also subject to timber harvest as the stand initiating agent but were unaffected by fluvial disturbance. However, based on differences in species distribution, we infer that forest structure on these two landforms differed from one another as a result of differences in soil moisture levels. Terraces and hillslopes were found to have high conifer tree abundance, but frequency of younger conifers was higher on hillslopes. Deciduous tree reproduction was very low on terraces and hillslopes. Our results also suggest that conifer recruitment in these second-growth riparian forests may be more successful on soil substrates than on coarse woody debris. We propose that the interplay between the disturbance regime (including type, frequency and intensity) and soil moisture conditions played an important role in influencing the course of riparian succession, present stand structure, and future successional trajectories and these were the primary mechanisms driving vegetation differences among landforms. ß 2008 Elsevier B.V. All rights reserved.

Keywords: Second growth Riparian Forest structure Forest succession Tree regeneration Disturbance mechanisms Fluvial processes Landform Restoration Generalized estimation equation (GEE)

1. Introduction Riparian vegetation develops on different geomorphic surfaces that are characterized by a range of disturbance types, frequencies, and intensities. It is well known that fluvial disturbance plays a major role world-wide in structuring riparian vegetation (Agee, 1988; Harris, 1987, 1988; Bendix, 1994; Baker and Walford, 1995; Hupp and Osterkamp, 1985; Hupp, 1986; Tabacchi et al., 1998; Harris, 1999; Gurnell et al., 2001; Naiman et al., 2005; Shin and Nakamura, 2005; Nakamura et al., 2007). In addition to ongoing fluvial disturbance, many riparian forests are

* Corresponding author. Present address: Weyerhaeuser Co. Forestry Research, P.O. Box 9777, Mail Stop WTC 1B10, Federal Way, WA 98063-9777, USA. Tel.: +1 253 924 4989; fax: +1 253 924 6736 fax. E-mail address: [email protected] (L.A. Villarin). 0378-1127/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2008.12.007

subject to anthropogenic disturbance. In the Pacific coastal ecoregion of northern California, Oregon, Washington, and British Columbia (hereafter referred to as the Pacific Northwest, or PNW), the primary source of human disturbance in riparian areas has been timber harvest. Development of these secondgrowth riparian forests has been influenced by both the effects of human activities and natural disturbance processes occurring across a range of landforms. Most riparian vegetation studies in the PNW have focused on naturally regenerated stands (mature or old growth) following fire and possibly ongoing fluvial disturbance (Fonda, 1974; Hawk and Zobel, 1974; Minore and Weatherly, 1994; Pabst and Spies, 1999; Nierenberg and Hibbs, 2000; Hibbs and Bower, 2001; Wimberly and Spies, 2001; Barker et al., 2002; Rot et al., 2000; Acker et al., 2003). Early research was conducted by Hawk and Zobel (1974) on the relationship between forest succession and alluvial landforms in the Cascade Mountains of Oregon along the

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McKenzie River. They classified vegetation occurring on terraces, floodplains, and glacial outwash plains to assess the spatial distribution of successional stages and trajectories. Eight distinct plant communities were identified across these landforms, and variation in edaphic factors and moisture availability was found to exert a major influence on plant species composition, forest structure and development. Fonda (1974) studied long-term forest succession in relation to floodplain terrace formations in the Hoh River valley, Washington. He found a pattern of early seral red alder (Alnus rubra) stands in the floodway zone and more developed series dominated by Sitka spruce (Picea sitchensis) and western hemlock (Tsuga heterophylla) communities on progressively older terraces. A number of studies conducted in naturally regenerating stands have shown a strong relationship between conifer to deciduous ratio with distance and elevation above the stream channel (Pabst and Spies, 1999; Nierenberg and Hibbs, 2000; Rot et al., 2000; Wimberly and Spies, 2001; Barker et al., 2002; Acker et al., 2003). In these studies, deciduous taxa, especially red alder, were typically found to be more abundant closer to the channel and at lower elevations, while conifers were more abundant farther away from the channel and at higher elevations. The gradient from deciduous to conifer with elevation and distance from the channel was usually attributed to variation in the susceptibility to fluvial disturbances, however, none of these studies delineated geomorphic surfaces based on quantitative hydrologic criteria. Rather, the relationship between fluvial disturbance and riparian forest structure has been inferred from gradients of distance and elevation from the channel. In addition, relatively few studies have examined riparian forest successional pathways in second-growth riparian stands in the PNW (Hibbs and Giordano, 1996; Hibbs and Bower, 2001; Barker et al., 2002). Following timber harvest, riparian areas often become dominated by red alder, particularly where wetter soil conditions or exposed mineral soil favors alder establishment and growth (Hibbs, 1987; Volk et al., 2003). Salmonberry (Rubus spectabilis) often occupies these sites as well, and some studies have suggested that these sites may ultimately support a shrub-dominated community as salmonberry often precludes the establishment of conifers and the red alder senesces (Henderson, 1978; Pabst and Spies, 1999; Volk et al., 2003). The inhibition of riparian conifer growth by deciduous vegetation is often perceived as detrimental to salmonid fish habitat because it reduces future recruitment of more effective conifer large woody debris (LWD). Although red alder is often a major component of riparian forests following timber harvest, the relative importance of natural fluvial disturbance versus disturbance from timber harvest in causing this pattern has not been demonstrated. In this study we examined structure and development of riparian forests in the central Cascade Mountains of Washington state that have undergone historic timber harvest and are subject to ongoing fluvial disturbance. We were interested in how riparian vegetation patterns varied across different geomorphic surface types and what role fluvial disturbance played in riparian forest development following timber harvest. The hypothesis we tested was that fluvial disturbance significantly affected riparian forest succession following timber harvest, which results in different patterns on floodplain versus non-floodplain geomorphic surfaces. There were two primary objectives of this investigation: (1) to examine riparian forest structure and succession in relation to fluvial and upland landforms and (2) to evaluate patterns of structure and succession in second-growth riparian forests we observed with those examined elsewhere in unmanaged forests.

2. Materials and methods 2.1. Study area This study was conducted within the eastern portion of the Cedar River Municipal Watershed (CRMW) along the Cedar River and two of its tributaries, Boulder Creek and the Rex River. The study area is located about 50 km east of Seattle, WA, USA on the west side of the Cascade Mountains (lat 478 210 , long 1218 360 ). The area experiences a maritime climate characterized by dry summers and wet winters. Precipitation ranges from 150 to 300 cm annually with much of the precipitation at the higher elevations in the watershed occurring as snow (Franklin and Dyrness, 1973). Geologic characteristics include bedrock formations of volcanic and volcaniclastic rocks of the Huckleberry Mountain Formation, with areas of intrusive igneous rock (Frizzell et al., 1984). The geomorphology and topography of the study area is a result of alpine glaciation that ended about 20,000 years ago (Hirsch, 1975), with a landscape of both glacial U-shaped and stream incised V-shaped valleys. Valley bottom soils are derived from river alluvium and glacial till and outwash. The study area is predominantly within the western hemlock (T. heterophylla) forest zone grading into lower portions of the Pacific silver fir (Abies amabilis) zone (Franklin and Dyrness, 1973). The primary conifer species are Douglas-fir, western hemlock, western redcedar (Thuja plicata), Pacific silver fir (A. amabilis), grand fir (Abies grandis), noble fir (Abies procera) and Sitka spruce (P. sitchensis). Deciduous trees such as red alder (A. rubra), bigleaf maple (Acer macrophyllum) and black cottonwood (Populus balsamifera spp. trichocarpa) are found primarily in riparian areas. Common shrub species found in riparian areas are salmonberry, vine maple (Acer circinatum), red elderberry (Sambucus racemosa), Sitka willow (Salix sitchensis), swordfern (Polystichum munitum), Oregon grape (Mahonia nervosa), and huckleberry (Vaccinium spp.). The CRMW is owned and managed by the City of Seattle for water supply and a small amount of hydropower generation. Extensive timber harvest occurred within the CRMW beginning in the late 19th century and continuing to 1995. Currently, late-seral and old growth forest occur on about 5600 ha of the CRMW (approximately 15% of forested lands within the watershed). The stands we studied represent a mix of stand types from deciduous to conifer-dominated and were located in riparian areas along midorder, free flowing streams, with time since stand initiating disturbance (major fluvial disturbance or timber harvest) between 25 and 79 years prior to our sampling in 2003 (Table 1). The second-growth stands were largely clear-cut to the channel edge, although in one plot (Plot 14) there were a few legacy trees that evidently escaped harvest. In addition, there were two floodplain plots that were not harvested, but were subject to major fluvial disturbance. Some floodplain plots had a much younger stand age than would be expected from their time since harvest, indicating stand replacing fluvial disturbance since they were harvested. Time since harvest was determined from historic harvest maps maintained by Seattle Public Utilities, and stand age was determined using tree-ring counts from increment cores. 2.2. Site selection Riparian plots for this study were located in stands harvested between 1924 and 1969. These plots were located along third to fourth order streams upstream of Chester Morse Lake at elevations ranging from 475 to 800 m above sea level. Plots were selected randomly along low gradient (<4% gradient), unconfined to moderately confined reaches of the mainstem and North and South forks Cedar River; Rex River; and Boulder Creek. All of these reaches have unregulated flow. The number of plots per reach was

L.A. Villarin et al. / Forest Ecology and Management 257 (2009) 1375–1385 Table 1 Time since harvest or fluvial disturbance and stand age of sampling plots, Cedar River Municipal Watershed. Plot#

Year harvested

Years since harvest

Stand age

1 2 3 4 5 6 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 26 29 30 31

1929 1929 1927 1927 1930 1930 1928 1928 1928 1928 1924 1924 1926 1926 1929 1929 1937 1937 1938 1938 1969 1969 No harvestc No harvestc 1949 1949

74 74 76 76 73 73 75 75 75 75 79 79 77 77 74 74 66 66 65 65 34 34 – – 54 54

65 63 58 36 61 64 59 65 65 65 62 96b 25 65 68 25 57 50 63 54 33 29 76 32 48 45

Floodplain plota

Yes

Yes Yes Yes

Yes Yes Yes

Time since harvest data were obtained from timber harvest maps and stand age determined from increment cores. a Plots which were entirely in low to high floodplain. b Plot 13 had legacy trees that were established prior to harvest. c Plots 26 and 29 were not harvested but were in recently disturbed floodplain areas.

proportional to the fraction of total available stream length for sampling represented by a given reach. At each site, two plots were established on opposite banks of the stream. Because we were interested in second-growth stands, only plots with stand age <100 years were used in this analysis (26 plots). Plots were located in the field using Global Positioning System (GPS), map and compass and the corners were monumented with rebar and PVC pipe. 2.3. Plot design Plot dimensions were 20 m  45 m with the long dimension perpendicular to the stream bank. The streamside plot boundary was established at the bankfull edge. The plot centerline was recorded with GPS and installed perpendicular to the average stream bank bearing, and the plot sides were delineated using compass bearings and distance tapes to establish a rectangular plot area. Each plot was subdivided into 5 adjacent subplots from the stream edge to the upslope edge for a total of 130 subplots. The subplot nearest the stream edge was 5 m  20 m, and the other four subplots were 10 m  20 m. The streamside plot was narrower to better characterize the vegetation along the channel edge, which typically changes markedly just a few meters into the forest. All data were recorded by their subplot location and summarized using slope-corrected subplot areas. This sampling design was developed to enable analysis of changes in vegetation structure in relation to different fluvial geomorphic surfaces and associated disturbance regimes. 2.4. Trees All trees in the plot 13 cm diameter breast height (dbh) were recorded for subplot location, species, diameter at breast height (dbh), and crown class (Henderson and Lesher, 2002). Trees were

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tagged with aluminum numbered tags for future monitoring. Heights of several trees in each crown class were measured with a laser rangefinder and recorded by tree number. Typically at least 10 trees per plot were measured for height. Increment cores of a subset (n = 4–10) of those trees measured for height were taken at each plot to estimate stand age and tree age classes. Cores were subsequently mounted and counted to determine tree age. Stem density and basal area were calculated using trees >13 cm dbh and slope corrected subplot areas. Canopy closure was measured using a spherical densitometer along the plot midline at the boundary between subplots 2 and 3 and between subplots 4 and 5. Density measurements from four directions were averaged at each location. Nomenclature for trees and shrubs followed Hitchcock and Cronquist (1976). 2.5. Seedlings and saplings Young trees were defined as stems >15 cm in height and <13 cm dbh. Three different size classes were established: class (1) between 15 and 140 cm height; class (2) >140 cm height and <8 cm dbh; class (3) >140 cm height and between 8 and 13 cm dbh. Throughout this paper, we used the term seedlings to refer to size class 1 and saplings to refer to a combination of size classes 2 and 3. In addition to size class, subplot number, species, and the substrate (mineral soil, coarse wood and stumps) were recorded for all seedlings and saplings. Seedling and sapling densities were all calculated using slope-corrected subplot areas. 2.6. Shrubs Low growing shrubs (1.8 m high) and swordfern were recorded for percent cover. Cover of all low shrubs was measured by species along the centerline transect using the line-intercept method (Canfield, 1941), with shrub cover intercepting the centerline recorded to the nearest 10 cm increment. Shrub importance values were calculated as ((average relative cover + relative frequency)/2) and used to compare the relative abundance of low shrubs across landforms. 2.7. Landform surface classification Each subplot was classified into one of four geomorphic surface types, or landforms, including the low floodplain (2 year floodplain), high floodplain (2 to 100 year floodplain), terrace (>100 year floodplain and <20% slope) and hillslope (>100 year floodplain and >20% slope). Flood frequency for a given subplot was determined by relating discharge, stage, and flow recurrence interval to an elevation profile established across the stream channel and along the centerlines of the plots on each side of the stream. The elevation profile was established following standard surveying methods using a stadia rod and a hand level positioned on a monopod. The elevation of the high floodplain (i.e., floodplain inundated less frequently than a 2-year interval, but more frequently than a 50–100-year interval) was estimated as 2 the bankfull elevation, as measured from the thalweg depth to the edge of permanent vegetation (Leopold et al., 1995; Rosgen and Silvey, 1996; Rosgen, 1998), a method widely used by the U.S. Forest Service. The 2 years, or low floodplain was defined as the elevation inundated by a peak flow with a 2-year recurrence interval. The elevation, or stage, which is inundated at the 2-year peak flow was determined for each site using the software WinXSPRO (USDA Forest Service and WEST Consultants, Inc. 1998). WinXSPRO is a channel crosssection analyzer that calculates the stage of a given discharge, as well as a variety of other hydraulic variables. The ‘‘User-Supplied Manning’s n resistance method’’ was used in this analysis.

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Manning’s n was estimated for each profile using Barnes (1967) as a guide. To estimate the 2-year peak instantaneous flow, we used the method presented in USGS (2001) for estimating peak flows in ungaged areas of gaged basins (all the basins in which riparian plots were located have USGS gaging stations). This method estimates the peak flow at the ungaged location as a proportion of the peak flow at the gaged location, based on the basin area at the ungaged location relative to that of the gaged location. For the gaged locations, the 2-year flow was derived from a log Pearson Type III distribution of peak flow data from the following gages (data for these gages are readily available from the USGS website ‘‘Surface-water Daily Statistics for Washington’’): (1) Mainstem Cedar River: USGS gage 12115000, (2) North Fork Cedar River: USGS gage 12113500, (3) Boulder Creek: USGS gage 12115700, (4) Rex River: USGS gage 12115500. 2.8. Statistical analysis The primary goal of the statistical analyses was to describe the relationship between each of the response variables and landform type. Frequency graphs were created to examine differences in tree, sapling and seedling abundance among species across the four geomorphic surface types. Generalized estimation equations (GEE) (Diggle et al., 2002) were used to model the data and to test for an association between landform type and each response, as well as for pairwise differences among the landform types. The model used to describe the expected value of each response variable was:   h Eðyi j Þ ¼ m þ Si þ L j where h(E(yij)) is the expected value of the response, possibly after transformation by a link function, m is the general mean, Sj is the effect of stream i, and Li is the effect of landform type j. The 26 plots in this study were assumed to be independent, after adjusting for stream and landform effects. However, the 5 subplots within each plot form a contiguous, linear block, and are assumed to be spatially correlated. An AR(1) correlation structure was used to model the spatial correlation. Tree basal area response variables were fit with GEE using a Gaussian family after taking a log-transformation of the response. A constant of 0.5 was added to all of the basal area data to handle zero values. Stem density was taken to be normalized count data and fit using a Poisson family with a log link function and scaled variance. The shrub-cover responses were each fit with GEE on the original scale of the data using a Gaussian family. Sapling frequency responses were analyzed as binary presence/absence variables due to the high proportion of subplots with zero frequencies. This binary response was defined as 0 if no saplings were present and 1 if there was at least one sapling present. These analyses were performed with GEE using a binomial family with a logit link. All statistical analyses were conducted using Splus 7.0 (Insightful, Inc. 2005). The test for an overall association between landform-type and each response variable was performed using a Wald-type test with model-based standard errors and a Chi-squared reference distribution (Harrell, 2001; Diggle et al., 2002). A 0.05 significance level was used to test for statistical significance. In cases where the overall Wald-test was significant, all pair-wise comparisons among landform-types were tested using a Wald-type test with a general linear contrast. If the test of equal means from the first hypothesis test was not rejected, we considered individual comparisons using the Bonferonni method of adjusting the significance level (Milliken and Johnson, 1992). We examined the effects of substrate (soil and wood) on the establishment of conifer seedlings and saplings. Results for stumps

and coarse woody debris (CWD) were combined into one wood substrate category. A formal test for an association between substrate type and sapling presence/absence was not possible using the same modeling assumptions for the landform analyses due to the sparcity of the dataset. Instead, we modeled sapling and seedling presence with GEE separately for both soil and wood substrates. We calculated the expected probabilities and 95% confidence intervals from the appropriate GEE model and a balanced linear combination of the model covariates. Differences in conifer seedling or sapling presence by substrate were assessed and were considered statistically significant by the degree of overlap between the confidence intervals. The less overlap the stronger the difference. In addition, because we did not standardize conifer seedling/sapling frequency by substrate area, we could not compare differences in frequency between substrates directly. However, we compared each substrate’s change in seedling to sapling frequency, which did not depend on the area of each substrate. 3. Results 3.1. Tree basal area There was a strong trend of increasing conifer basal area moving away from the channel, with the floodplains having the lowest basal area (Table 2). Conifer basal area on the two floodplain landforms was significantly lower than on the terraces and hillslopes. Differences in deciduous basal area among geomorphic surfaces were less pronounced than with the conifers. Basal area was significantly higher on the two floodplain landforms compared to that on hillslopes but did not differ significantly between either of the floodplains landforms and terraces (Table 2). Using tree basal area as a basis for determining cover type, landforms varied strongly with respect to percentage of subplots

Table 2 Average conifer and deciduous basal area (m2/ha) and stem density (#/ha) contrast estimates for all pair-wise comparisons among landform types, where LF = low floodplain, HF = high floodplain, T = terrace, and H = hillslope. Response

Tree type

Contrast

Estimate

Basal area

Conifer

HF:LF T:LF H:LF T:HF H:HF H:T HF:LF LF:T LF:H HF:T HF:H T:H

1.9 4.9 7.2 2.6 3.8 1.5 1.4 1.4 2.6 1.9 3.6 1.9

HF:LF T:LF H:LF T:HF H:HF H:T HF:LF LF:T LF:H HF:T HF:H T:H

1.5 1.8 2.4 1.2 1.6 1.3 1.3 1.7 2.6 2.2 3.5 1.5

Deciduous

Stem density

Conifer

Deciduous

Significance

0.01 0.001 0.05 0.01

0.05 0.01

0.01 0.01 0.001

Estimates represent the ratio of expected values for each pair of landform types (e.g., estimate of 5.0 means a 5 greater value of the first to the second landform type). Significant differences are noted with p-values. In cases where the overall test of association between landform type and the response was not significant, pvalues were adjusted using a Bonferonni correction prior to testing for significance.

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Table 3 Conifer and deciduous sapling presence/absence contrast estimates for all pair-wise comparisons among landform types, where LF = low floodplain, HF = high floodplain, T = terrace, and H = hillslope. Response

Tree type

Contrast

Estimate

Significance

Sapling presence

Conifer

HF:LF T:LF H:LF HF:T H:HF H:T LF:HF LF:T LF:H HF:T HF:H H:T

3.0 1.6 7.5 1.85 2.52 4.7 1.08 9.1 7.1 8.4 6.4 1.32

0.05

Deciduous

Fig. 1. Percent of subplots classified by cover type for each landform (conifer 70% conifer basal area; deciduous 70% deciduous basal area; mixed <70% both conifer and deciduous basal area; non-forested = no trees [dbh > 13 cm] present).

classified by cover type (Fig. 1). Along the gradient of increasing elevation above the channel from low floodplain to hillslope, there was a trend toward lower proportion of deciduous and higher proportion of conifer-dominated subplots. On both low and high floodplains more than half of subplots were deciduous-dominated and an additional 23% of subplots on both floodplain landforms were mixed deciduous-conifer. On hillslopes, over 80% of subplots were conifer-dominated and only 4% were deciduous-dominated. Over 50% of terrace subplots were conifer-dominated, while 25% were deciduous-dominated. 3.2. Stem density Stem density for conifers displayed a similar pattern to that of basal area (Table 2), with conifer density becoming progressively greater on higher geomorphic surfaces. The test for an association between landform type and conifer density was not significant, hence all individual tests used a Bonferonni correction. Only the contrast between the low floodplain and the hillslope was significantly different from zero at an adjusted 0.1 significance level (Table 2). The expected conifer stem density of the hillslope was nearly 2.5 times greater than the stem density of the low floodplain. Because this was not significant at the Bonferonni adjusted level of 0.0083, we used caution in interpreting this result. The Wald test for an association between landform and deciduous stem density gave very strong evidence for a significant difference among landform types (approximate p-value <0.0001). Patterns of deciduous stems were similar to that of basal area (Table 2). There was greater deciduous stem density on the two floodplains compared to the two higher landforms. The high floodplain showed the highest deciduous tree density of all the geomorphic surfaces, although the difference with the low floodplain was not significant. The density of deciduous trees was significantly greater on the high floodplain surface than on either the terrace or hillslope landforms. 3.3. Sapling presence GEE analysis of sapling density (size classes 2 and 3) with respect to landform was not very informative. Because saplings were often absent in subplots, resulting in a high number of zero values, none of the pairwise comparisons showed evidence of statistically significant differences using this approach. As an alternative, results for sapling frequency response among geomorphic surface types were analyzed as a binary variable (Table 3). A significant association between landform type and conifer

0.01

0.05 0.01 0.01 0.01 0.01

Estimates represent the ratio of the expected odds of sapling presence for each pair of landform types. Significant differences are noted with p-values.

sapling presence was observed from this analysis using the Wald test (p < 0.05 level). The hillslope showed a greater chance of having conifer saplings present than the low floodplain or the terrace with 7.5 and over 4.5 times greater odds, respectively; however, conifer sapling presence on hillslope did not differ from the high floodplain. In addition, the odds of observing a conifer sapling on the high floodplain was approximately 3 times more likely, on average, than on the low floodplain. The Wald test for an association between landform and deciduous sapling presence was also statistically significant at a 0.05 level. The results indicated a strong trend toward deciduous presence on the floodplain geomorphic surfaces when compared to the upland. The odds of observing a deciduous sapling increased dramatically on the low floodplain when compared to the terrace and hillslope by an average of approximately 9 and 7 times, respectively. Similarly the expected odds of observing deciduous saplings on the high floodplain were nearly 8.5 and 6.5 times greater, on average, than the terrace and the hillslope. However, there were no significant differences in the pairwise comparison between the two floodplain surfaces or the two upland surfaces. 3.4. Shrub cover There was strong statistical evidence for an overall association between landform type and shrub cover. The GEE results showed a trend of decreasing low shrub cover when moving from the floodplain to the upland, although these relationships were only significant for the hillslope comparisons (Table 4). Importance values (Table 5) showed that salmonberry was the most important shrub species found on both low and high floodplains and also had high importance on terraces; however, it had low importance on hillslopes. Swordfern had the highest importance on terraces and hillslopes and was also of moderate importance on the two Table 4 Percent low shrub cover contrast estimates for all pair-wise comparisons among landform types, where LF = low floodplain, HF = high floodplain, T = terrace, and H = hillslope. Response

Contrast

Estimate

Shrub cover

LF-HF LF-T LF-H HF-T HF-H T-H

5.4 17.2 38 11.8 32.6 20.8

Significance

0.001 0.001 0.05

Estimates represent the difference in expected percent low shrub cover for each pair of landform types. Significant differences are noted with p-values.

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Table 5 Average importance value of low shrub cover (shrubs < 2 m in height) in subplots by landform type where LF = low floodplain, HF = high floodplain, T = terrace, and H = hillslope. Species

Landform

Scientific name

Common name

LF

HF

T

H

Acer circinatum Amelanchier alnifolia Berberis nervosa Gaultheria shallon Holodiscus discolor Menziesia ferruginea Oplopanax horridus Physocarpus capitatus Polystichum munitum Ribes bracteosum Ribes lacustre Rosa gymnocarpum Rubus leucodermis Rubus parviflorus Rubus spectabilis Rubus ursinus Sambucus racemosa Salix sitchensis Vaccinium parvifolium Vaccinium species

Vine maple Saskatoon, serviceberry Oregon grape Salal Oceanspray Fool’s huckelberry Devils club Pacific ninebark Swordfern Stink currant Black gooseberry Baldhip rose Blackcap Thimbleberry Salmonberry Trailing blackberry Red elderberry Sitka willow Red huckleberry Blueberry

21 0 0 0 4 0 8 0 24 21 0 2 0 20 46 21 7 10 4 0

19 1 1 1 7 2 8 1 34 8 1 0 2 24 47 14 3 5 5 2

23 2 0 0 0 8 7 0 51 6 0 0 0 4 40 6 9 0 2 7

12 2 22 0 0 0 6 0 44 4 0 0 0 0 12 13 0 0 2 4

Importance value = (average relative cover + relative frequency)/2. Only species with importance value of 1 on at least one landform are shown.

floodplain landforms. Vine maple had moderate importance on both floodplains and terraces but somewhat lower importance on hillslopes. 3.5. Sapling frequency by substrate Comparisons of wood versus soil substrate were based on conifer seedlings and saplings from all landforms combined. When comparing seedlings to saplings on each substrate, there was a higher probability of a conifer sapling occurring on soil substrate than a seedling. On CWD substrate, the probability of a sapling occurring was slightly lower than that of a seedling, but the overlapping confidence intervals indicated this difference was not significant. Differences were considered statistically significant by the degree of overlap between the confidence intervals (Fig. 2). The less overlap the stronger the difference. Fig. 3. Frequency of common (a) tree, (b) sapling, and (c) seedling species occurrence across the four landform categories (ALRU = Alnus rubra; ACMA = Acer macrophyllum; TSHE = Tsuga heterophylla; PSME = Pseudotsuga menziesii; THPL = Thuja plicata; PISI = Picea sitchensis).

3.6. Species frequency

Fig. 2. Probability estimates and 95% confidence intervals for the expected probability of conifer seedling and sapling presence as a function of size and substrate. Differences were considered statistically significant by the degree of overlap between the confidence intervals. The less overlap the stronger the difference. The confidence intervals are not symmetric since all estimation takes place on the logit-scale. Seedlings are represented as size class 1 (SC1) while saplings are represented by the sum of size classes 2 and 3 (SC23). Substrate classes are soil and wood. All estimates come from different GEE models.

The frequency of species across geomorphic surfaces was examined descriptively and found to vary markedly among geomorphic surfaces (Fig. 3). The most common deciduous tree species on floodplain surfaces was red alder, occurring in roughly 80% of subplots on the low and the high floodplains. Western hemlock and Douglas-fir were the most frequent conifers on floodplains. Except for Sitka spruce, conifer taxa generally showed an increasing trend on the gradient from low floodplain to hillslope. On terraces and hillslopes, western hemlock showed the highest frequency and Douglas-fir was also high on hillslopes. Red alder also had moderately high frequency on terraces and hillslopes, occurring in roughly 60% of terrace and hillslope subplots. Sitka spruce occurred at low frequencies (9–12%) on floodplain and terrace surfaces but was virtually absent on hillslopes. Big leaf maple had the highest frequency on terraces but occurred similarly on the other three surfaces.

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Western hemlock seedlings occurred in greater abundance than any other species, with similar frequency across all geomorphic surfaces, ranging from 45% (low floodplains) to 55% (hillslopes) (Fig. 3). Other taxa occurred at much lower frequencies (<20% across all species and geomorphic surfaces). Red alder, Douglas-fir, and big-leaf maple emergents were all higher on floodplain surfaces than terraces or hillslopes. Alder and western hemlock were the most common sapling (>1.4 m height, <13 cm dbh) taxa on floodplains (Fig. 3). Hemlock was also the most common sapling species on terraces and hillslopes, but had much higher frequency on hillslopes (85%) compared to terraces (<30%). Frequencies of seedlings and saplings of the same species often differed substantially with respect to landform (Fig. 3). For instance, western hemlock saplings were much more frequent on hillslope subplots (85%) than were seedlings (55%), in contrast there were far fewer terrace subplots with hemlock saplings (30%) than with seedlings (50%). 4. Discussion One of the primary objectives of this study was to examine the influence of landform and disturbance (both natural and anthropogenic) on riparian forest structure and succession in secondgrowth stands. Relatively few studies have examined riparian forest successional pathways in second-growth stands in the PNW (Hibbs and Giordano, 1996; Hibbs and Bower, 2001; Barker et al., 2002). In addition, while some studies have shown a general association between riparian stand characteristics and local geomorphology, our research utilized analytical techniques not previously applied to this question, enabling us to more quantitatively describe the relationship between vegetation and landform. Our results generally agreed with previous research in the PNW indicating that fluvially disturbed valley floor landforms support dense deciduous overstories and less fluvially disturbed upland landforms support conifer canopies (Keddy and MacLellan, 1990; Pabst and Spies, 1999). Our results suggest that the type and intensity of the biological and physical processes dictating the course of riparian forest succession in the CRMW following timber harvest has varied across geomorphic types. We propose that this was a function of three primary mechanisms including disturbance type, soil moisture, and topography, which have interacted to cause the differences in forest structure and successional patterns we observed. The key agents of disturbance were timber harvest and/or fluvial processes. Second-growth stands occurring across all four geomorphic types were influenced by prior timber harvest. While floodplain landforms in the CRMW are subject to ongoing fluvial disturbance, terraces and hillslopes were affected only by timber harvest as the stand initiating disturbance and have not experienced major fluvial disturbance since then. Our second primary objective was to compare patterns of riparian forest structure in these second-growth stands to those of unmanaged stands. We found that structure in riparian stands ranging from 24 to 69 years since harvest was similar to that of unmanaged stands from other areas in the PNW, with a generally greater abundance of deciduous species near the channel and increasing abundance of conifers away from the channel. Much of our analysis of mechanisms underlying these patterns should also be relevant to unmanaged stands. 4.1. Overstory tree structure Conifer basal area increased on higher landforms moving upslope from the floodplain landforms (Table 2), a pattern consistent with several other studies from a variety of locations (Pabst and Spies, 1999; Nierenberg and Hibbs, 2000; Rot et al., 2000; Hibbs and Bower, 2001). Conifer stem density also showed

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an increasing, but non-significant trend from floodplain landforms to hillslopes (Table 2). Douglas-fir and western hemlock trees occurred in small numbers on the low floodplain and in moderate numbers on the high floodplain, consistent with a greater opportunity on the high floodplain to establish and persist under less frequent fluvial disturbance (Fig. 3). Thus, conifer trees were able to become established on disturbed floodplains, but poor representation of mature trees suggests that competition with deciduous plants and frequent disturbance kept basal area low. On the other hand, hillslopes had drier conditions without fluvial disturbance, resulting in less competition from deciduous trees and shrubs during the early stages of stand development and enabling conifers to achieve higher density and larger volumes. Deciduous trees, primarily red alder, were more abundant on floodplains than hillslopes (Table 2), with terraces being transitional, also consistent with the riparian studies cited above (Fig. 3). Fluvial disturbances were likely the primary reason why red alder dominated on the floodplain landforms. Red alder is able to establish and grow quickly on disturbed, nutrient poor and wetter soils (Burns and Honkala, 1990). However, it also had moderate frequency on hillslopes (60%). The moderate to high frequency of alder across all geomorphic surfaces is likely a product of widespread disturbance from both timber harvest and fluvial activity on floodplains and from timber harvest alone on terraces and hillslopes within the last 70 years in the study area. Because terraces and hillslopes were not subject to extensive fluvial disturbance since initiation of current stands, we hypothesize that differences in overstory characteristics on these two surfaces were likely due, either directly or indirectly, to differences in soil moisture availability as a function of topography. It is likely that terraces tend to have higher soil moisture than hillslopes due to their topographic position. Although we did not measure soil moisture, proxies were used to generate a relative indication of the moisture regime that did not rely on direct measures (Grayson and Western, 2001). Douglas-fir and Sitka spruce are two good proxies for soil moisture. Both become established in open areas following disturbance, but relative to one another Douglas-fir is favored in drier sites and Sitka spruce in wetter sites (Franklin and Dyrness, 1973; Minore, 1979). Although it is possible that differences in soil nutrients could also play a role in their distribution, the higher frequency of Douglas-fir (68% versus 32%) but lower frequency of Sitka spruce (0% versus 12%) on hillslopes compared to terraces is strongly indicative of lower moisture conditions on hillslopes. 4.2. Tree establishment Patterns of conifer regeneration varied among landform types. Conifer recruitment generally increased from the valley floor to the upland (Table 3). Differences in conifer recruitment were not likely due to light availability, as the range of canopy closure among all four landforms was fairly narrow (86–93% closure). The lower frequency of conifer saplings on the low floodplain can be explained by ongoing fluvial disturbance causing conifer mortality while favoring continued red alder establishment. The lower fluvial disturbance frequency on the high floodplain likely enabled more conifers to persist compared to the low floodplain. But the pattern of increasing conifer sapling frequency with higher elevation was disrupted with respect to the terraces, where recruitment was lower than for all the other landforms (Table 3). For instance, the frequency of hemlock saplings on the hillslope subplots were almost double that of the terrace subplots (Fig. 3). Other studies have also found lower conifer regeneration on terraces. Pabst and Spies (1999) reported a lower amount of conifer regeneration on terraces compared to either floodplains or hillslopes in riparian areas in their northern subregion of the Oregon Coast Range. Hibbs and Bower (2001) found lower than

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expected regeneration on terraces compared with higher than expected regeneration on slopes. Because terrace and hillslope landforms were elevated above the channel and not subject to fluvial disturbance, other mechanisms were likely driving differences in conifer recruitment between the two landforms. We hypothesize that soil moisture, mediated by shrub cover, is the other major factor besides disturbance frequency affecting conifer recruitment in CRMW riparian areas. Shrub cover was significantly higher in hillslope compared to terrace subplots (Table 4). There were eight shrub species with higher importance value on terraces than on hillslope, with salmonberry showing the most difference in importance value between the two landforms (Table 5). In contrast, there were only two shrub species with higher importance value on hillslopes, with the biggest difference shown by the relatively low growing Oregon grape. We hypothesize that the reduced conifer recruitment on terraces is a function of higher shrub cover, especially that of salmonberry, which in turn is a function of higher soil moisture on terraces compared to hillslopes. Again using proxies as indicators of differences in soil moisture (Grayson and Western, 2001), salmonberry is characteristic of higher soil moisture conditions compared to Oregon grape (Franklin and Dyrness, 1973). The much higher importance of salmonberry but much lower importance of Oregon grape is strongly indicative of higher soil moisture on terraces, consistent with the relative frequency of Sitka spruce and Douglas-fir discussed above. Thus, on floodplains periodic disturbance provides opportunities for conifer recruitment despite relatively high shrub cover, while on hillslopes lower shrub cover reduces competition against young conifers. Conifer recruitment on terraces is lowest due to a lack of periodic fluvial disturbance combined with higher soil moisture, which results in higher shrub cover. The negative effect of shrub cover, particularly of salmonberry, on riparian conifer regeneration has been reported in a number of other studies (Minore and Weatherly, 1994; Pabst and Spies, 1999; Nierenberg and Hibbs, 2000; Hibbs and Bower, 2001), however, the role of soil moisture, as controlled by landform and topography, in ultimately driving suppression of conifer regeneration by shrubs is not typically acknowledged. In contrast to conifer recruitment, deciduous recruitment, particularly that of alder, can be explained primarily by disturbance frequency. We found that deciduous regeneration was similar on the two floodplain landforms but was significantly lower on the terrace and hillslope surfaces (Table 3). The higher frequency of alder saplings on floodplains than on terraces and hillslopes (Fig. 3) was consistent with the hypothesis that frequent fluvial disturbance enables the establishment of this species. The general lack of alder seedlings on the floodplain surfaces was likely related to the length of time since the most recent major flood events; 1990 and 1996 (Fig. 3). Alder trees 6–13 years in age would likely be saplings (>1.3 m high but less than 13 cm diameter), based on a regression of alder age versus diameter (dbh in cm) collected from our study plots (age = 1.65  cm dbh–4.12; R2 = 0.58, n = 57). Thus, most of the alder saplings found on the floodplain landforms sampled in this study likely established following major flood events in 1990 and 1996. These findings suggest that floodplain surfaces will continue to be dominated by deciduous trees as long as flooding occurs periodically (at least every 50–100 years). The lack of a major disturbance since logging on terraces and hillslopes has provided little opportunity for recruitment of deciduous tree species following stand initiation. 4.3. Substrate effects on regeneration Although it was not one of our primary objective, the effects of wood versus soil substrate on conifer regeneration was also

addressed by data from our study. The role of ‘‘nurse logs’’ as sites of establishment for trees, especially conifers, has been investigated in numerous studies over the past 20 years. Most of the previous work has been conducted in old-growth forests and, if in riparian areas, along larger river systems in the coastal mountains of Washington and Oregon. We are aware of only one study of substrate effects on conifer recruitment in second-growth stands in the Cascade Mountains, that of Beach and Halpern (2001). In studies conducted in old-growth Picea-Tsuga forests in Washington and Oregon, seedling survival was higher on wood substrate, compared to soil, primarily due to negative effects of a thick moss layer (Harmon and Franklin, 1989). Mckee et al. (1982) conducted a study in terrace forests along the South Fork of the Hoh River, Washington and found that downed logs were a much more favorable site than soil for conifer seedling establishment. In these highly productive coastal rainforests, deep accumulations of litter, duff and moss mats cannot only limit soil moisture availability but may serve as a mechanical barrier making it difficult for seedlings to root in organic matter (Caccia and Ballare, 1998; Beach and Halpern, 2001). In addition, herbivory may play a role. Leslie (1982) found that the amount of Roosevelt elk (Cervis canadensis var. roosevelti) browse on young trees was higher for trees rooted in soil compared to that on elevated logs (Leslie, 1982). On the floodplain of the Queets River, Washington, Fetherston et al. (1995) found higher regeneration of conifers on wood compared to soil substrate, consistent with seedling mortality from flooding being higher on soil than on logs. These and other studies provide the impression that logs on the forest floor are highly important for recruitment of conifers in a variety of setting across the PNW. The substrate effects on regeneration we observed were not entirely consistent with this view. Our results suggested that conifer recruitment may ultimately be more successful on the forest floor than on logs. Although we found a similar probability of seedling (<1.4 m height) presence on wood compared to soil, there was a significantly higher probability of saplings found on soil when compared to wood (Fig. 2). This difference suggested that as seedlings matured into saplings there was greater mortality on wood. This inference was not dependent on the relative abundance of substrate types, which we did not quantify, because we were comparing each substrate’s difference in seedling and sapling frequency—not the difference in frequency between substrates. Even if sapling frequency per unit substrate area were higher on wood than on soil, the overall sapling frequency on soil per subplot is higher, indicating that current regeneration in riparian forests in these second-growth forests is mostly occurring on soil compared to wood. Interestingly, in one of the most often cited papers supporting the notion that logs on the forest floor are important for conifer recruitment, Harmon and Franklin (1989) noted that mortality rates of conifer seedlings on logs were high due to competition among seedlings and instability of the wood substrate and that long-term recruitment of trees on the forest floor may be higher than expected based on seedling distribution. A similar pattern was described in a northeastern temperate forest by Caspersen and Saprunoff (2005), who found that CWD was the most preferred substrate for germination but soil was more favorable for growth and survival of saplings. Although, Beach and Halpern (2001) reported higher seedling recruitment on logs than on the ground in second-growth riparian stands in Washington state, their data pertained primarily to emerging seedlings, and not sapling survival. There are several possible reasons why the forest floor may be a more advantageous substrate for sapling survival. It could be a more physically stable environment than rotting logs, as bark sloughing and wood fragmentation lead to seedling mortality (Harmon et al., 1986; Harmon and Franklin, 1989). In addition,

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competition from a thicker moss cover found on logs may inhibit tree establishment relative to soil substrates. Six and Halpern (2008) conducted research on understory substrate effects in upland areas of the same study area where our work was performed. They found three times as many species were associated with the forest floor than with logs. The authors also found moss mats to be twice as thick on CWD then on the ground. They proposed that moss desiccation on logs could inhibit root establishment. Our results and those of other studies cited above argue for a more nuanced view of the role of coarse woody debris in conifer recruitment. In the second-growth stands we examined, the development of the layer of moss and duff on the forest floor was likely not as thick as in wet old-growth conditions such as those in Picea-Tsuga forests on the Hoh River or the Coast Range of Oregon. Although flooding undoubtedly affects seedling and sapling mortality on floodplains, our analysis was across all landforms, including those unaffected by flooding. Thus, potential mortality from flooding would play less of a role in our study sites than on the dynamic floodplain and terraces of the Queets River studied by Fetherston et al. (1995). The relative importance of downed logs in conifer regeneration is likely to be variable depending on site conditions and should be evaluated with regard to survivorship to sapling size trees as well as initial seedling establishment. 4.4. Disturbance effects on successional trajectories The interaction of different sources of disturbance across different landforms within the second-growth riparian forests of the CRMW has resulted in a range of possible successional trajectories, which we have illustrated with a simple conceptual model (Fig. 4). In this model, high to moderate frequency of flood disturbance on floodplains regularly creates new substrate available for tree establishment. Although conifers often become established on these surfaces, stands typically become dominated by red alder, which readily outcompetes conifer seedlings. The

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competitive advantage of red alder comes from its prolific seed production, fast growth relative to conifers, and ability to fix N in often thin floodplain soils (Burns and Honkala, 1990). On lower floodplains, flood disturbance resets successional development relatively frequently, and basal area remains low. On higher floodplain surfaces, flooding is less frequent and disturbance effects are also likely less severe. The lower disturbance frequency and intensity enables red alder to mature and accumulate substantial basal area. Basal area on high floodplains may subsequently increase if conifers have been successful at establishing under the alder canopy or escaped intense alder competition. When these riparian forests in the CRMW were harvested, no riparian buffers were retained and trees were often cut to the channel edge. Presence of large stumps indicated that mature conifers were a component on some high floodplain surfaces in these riparian forests prior to harvest. About 38% (Fig. 1) of the high floodplain subplots we sampled were conifer-dominated or on trajectories toward conifer dominance (i.e., currently mixed conifer-deciduous). Where deciduous trees dominate on high floodplains (55% of upper floodplain subplots), however, it may be difficult for conifers to replace them. Deciduous-dominated stands, especially those with a red alder overstory and salmonberry understory, tended to have few conifer seedlings or saplings and may persist as shrubdominated with alder senescence, unless conifers can gradually accumulate or fluvial disturbance resets succession. Persistence of a salmonberry-dominated shrub community following red alder senescence has been reported elsewhere in the PNW (Henderson, 1978; Minore and Weatherly, 1994; Hibbs and Bower, 2001). Our results suggest that this community type is most likely to develop on high floodplain landforms, where the combination of soil conditions and disturbance frequency was most conducive for this successional pathway. However, our results indicate that high floodplains can follow widely different possible successional trajectories, depending on initial site and stand conditions (e.g., soil moisture levels) and frequency and intensity of fluvial

Fig. 4. Disturbance regimes and successional trajectories characteristic of different riparian landforms in the upper CRMW, Washington.

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disturbance (e.g., infrequent but low intensity disturbance may open up sites for conifer establishment). On terraces the lack of fluvial disturbance in initiating or affecting ongoing forest development following timber harvest may result in different trajectories than occur on floodplains. In the second-growth stands we sampled, conifers or mixed coniferdeciduous stands comprise about 75% of terrace subplots. As these stands mature, and red alder senesces, most terrace surfaces will be dominated by conifers. The vast majority of hillslope surfaces in CRMW riparian areas likely developed directly into coniferdominated stands, with less than 5% of hillslope subplots classified as deciduous-dominated and less than 15% mixed conifer-deciduous. Lower conifer sapling abundance on terraces compared to hillslopes, however, suggests that forest structure later in succession may differ on these two surface types. Fewer conifer saplings on terraces (Table 3) suggests that development of more complex vertical structure with different canopy layers may take longer on terraces than on hillslopes. On deciduous-dominated terraces (25% of terrace subplots) with few conifer trees and saplings, persistent shrub communities and slow development of conifer overstory can be expected, as on high floodplains. Riparian areas on hillslopes will follow trajectories similar to those of surrounding upland forests: long-term persistence of Douglas-fir, increasing abundance of western hemlock, and variable amounts of western redcedar. Successional trajectories for riparian forests proposed here are hard to compare to those by Fonda (1974) on the Hoh River, Washington and by Hawk and Zobel (1974) in the McKenzie River valley, Oregon. The latter two studies were of long-term forest development (several centuries) following natural fluvial and fire disturbance, in contrast to early stand development following timber harvest in the CRMW. Patterns of riparian forest development in the CRMW were consistent with the successional model of Van Pelt et al. (2006) for the Queets River, Washington. However, the Queets River floodplain is much larger and, with its glacially derived high sediment supply, more dynamic than the upper Cedar River and its tributaries, most of which are only moderately confined and without anastomosing channels and frequent avulsion events. Because of the differences in scale and disturbance regime between the Queets and Cedar rivers, some aspects of the Van Pelt et al., model are not as important in the CRMW. For example, conversion of floodplains to terraces is less common in the smaller, more confined streams of the CRMW compared to the widely meandering Queets River. Also, the opportunity for abandonment of large channels and subsequent succession is restricted to only a few areas on the Cedar and Rex rivers, but represents another important successional pathway on the Queets River. The limited historical period over which commercial timber harvest has occurred in the PNW precludes making long-term predictions with any confidence of riparian second-growth forest succession, in this watershed or elsewhere. However, these results have provided some insight into differences in stand development in riparian areas as a result of different disturbance regimes and site conditions, both of which are closely related to landform. Acknowledgements The authors would like to thank Robert E. Bilby, Weyerhaeuser Company, Seattle Public Utilities and Paul E. Sampson, University of Washington for financial and technical support throughout this research endeavor. We thank our field assistant Ashley Adams and numerous other people who helped in the field. We also thank two anonymous reviewers for very helpful comments in strengthening and tightening up the manuscript.

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