Spatial and temporal patterns of plant communities near small

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Spatial and temporal patterns of plant communities near small mountain streams in managed forests

Can. J. For. Res. Downloaded from www.nrcresearchpress.com by CSP Staff on 01/27/12 For personal use only.

Lana E. D’Souza, Laura J. Six, Jonathan D. Bakker, and Robert E. Bilby

Abstract: Riparian plant communities along small streams occupy a small proportion of the total landscape but can provide disproportionally large ecological, social, and economic benefits. We examined plant communities at 25 study sites along small fish-bearing streams in temperate managed forests of the Pacific Northwest spatially as a function of distance from stream and temporally by assessing a chronosequence of stand ages: young (31–51 years), mature (52–70 years), and old (>100 years). We identified three distinct vegetation communities based on species cover and richness in shrub and herb layers: riparian (0–9 m), transitional (10–29 m), and upslope (30–80 m); 12 species were indicators of these vegetation communities. For tree species, basal area increased with stand age. Shrub species cover and richness were greatest in old stands, but herb species richness was highest in young stands. Composition varied with stand age; 15 species were indicators of these differences in composition. These results, together with information on successional and wetland status, suggest that plant communities on small fish-bearing streams reflect geomorphic and fluvial settings but also follow successional patterns found in natural forests. These stands will become some of the primary unharvested, older forests within the managed forest landscape and provide insights for effective riparian management on sites impacted by historical management practices prior to the regulations requiring riparian buffers. Résumé : Les communautés végétales riveraines le long des petits cours d’eau occupent une faible proportion du paysage total mais elles peuvent procurer des bénéfices sociaux, écologiques et économiques grandement disproportionnés. Nous avons étudié des communautés végétales dans 25 sites d’étude le long de petits cours d’eau contenant des poissons dans des forêts tempérées aménagées du Pacific Northwest, spatialement en fonction de la distance du cours d’eau et temporellement en évaluant une chronoséquence d’âges de peuplement : jeunes (31–51 ans), mature (52–70 ans) et vieux (>100 ans). Nous avons identifié trois communautés végétales distinctes sur la base de la couverture et de la richesse des espèces dans l’étage arbustif et herbacé : riveraine (0–9 m), transitionnelle (10–29 m) et pente ascendante (30–80 m); 12 espèces étaient indicatrices de ces communautés végétales. Chez les espèces arborescentes, la surface terrière augmentait avec l’âge du peuplement. La couverture et la richesse des espèces arbustives étaient la plus élevées dans les vieux peuplements mais la richesse des espèces herbacées était la plus élevée dans les jeunes peuplements. La composition variait selon l’âge du peuplement; 15 espèces étaient indicatrices de ces différences de composition. Ces résultats, avec l’information sur le statut successionnel et de terre humide, indiquent que les communautés végétales le long des petits cours d’eau qui contiennent des poissons reflètent des cadres géomorphologiques et fluviaux mais qu’elles suivent également des patrons successionnels qu’on retrouve dans les forêts naturelles. Ces peuplements deviendront certaines des vieilles forêts primaires non récoltées dans le paysage forestier aménagé et fourniront des connaissances pour un aménagement riverain efficace sur les sites exposés à d’anciennes pratiques d’aménagement, antérieures à la réglementation qui exige des zones tampons riveraines. [Traduit par la Rédaction]

Introduction Riparian areas serve as crucial interfaces between aquatic and terrestrial ecosystems. The ecological, social, and economic values of riparian areas have been increasingly recognized in the last few decades (Naiman et al. 2005), leading to the adoption of regulations that guide forestry practices along streams (Lee et al. 2004). In forested systems, riparian buffers are commonly retained along streams that have prescribed buffer widths that include influential areas for

maintaining ecological processes, preserving environmental characteristics, and protecting biota from upland management activities (Lee et al. 2004). Buffer width requirements vary regionally but generally increase with stream size and with the presence of fish (Lee et al. 2004). When protecting drainage networks, small streams are particularly significant, as they constitute a substantial proportion of the stream length within watersheds. The steeper topography along many small streams restricts flooding impacts to a relatively narrow zone that limits the spatial extent

Received 4 February 2011. Accepted 26 October 2011. Published at www.nrcresearchpress.com/cjfr on xx January 2012. L.E. D’Souza. Weyerhaeuser Global Timberlands Technology, P.O. Box 3777, MS 1B10, Federal Way, WA 98063, USA. L.J. Six and R.E. Bilby. Weyerhaeuser Global Timberlands Technology, P.O. Box 3777, MS 1A5, Federal Way, WA 98063, USA. J.D. Bakker. School of Environmental and Forest Sciences, University of Washington, Box 354115, Seattle, WA 98195, USA. Corresponding author: Lana E. D’Souza (e-mail: [email protected]). Can. J. For. Res. 42: 260–271 (2012)

doi:10.1139/X11-171

Published by NRC Research Press


D’Souza et al.

(width) of fluvial landforms and of the riparian plant community (Richardson and Danehy 2007). This pattern has been described for boreal (Renöfålt et al. 2005), hemiboreal (Hagan et al. 2006), temperate (Pabst and Spies 1998), mixed hardwood–conifer (Goebel et al. 2006), and tropical (Drucker et al. 2008) forests. Plant communities along small streams may serve as indicators for riparian areas and aid in determining proper buffer width (Hagan et al. 2006). Plant communities along the smallest fish-bearing streams have received less research attention than those along lowerordered streams (Hagan et al. 2006) or large rivers (Naiman et al. 2005). Vegetation near small streams is often ordered along a complex environmental gradient from active fluvial streamside areas to hillslopes (Pabst and Spies 1998). Herb, shrub, and tree layers can respond at different spatial scales to the environmental gradients caused by the stream (Lyon and Sagers 2003). For example, Hagan et al. (2006) found distinct herbaceous vegetation in a narrow band near the channel but found no differences in trees or shrubs. Species distributions along small streams may also relate to their functional strategies and attributes (e.g., successional status, shade tolerance, and wetland indicator status) (Grime 2001). Prior to the adoption of regulations guiding forestry practices along streams, logging often occurred down to the streambank. The temporal effects of this historical management practice on the successional trajectory of riparian vegetation are not well understood and baseline data may yield important information for future buffer planning. Most successional studies on harvesting disturbance are limited either to the short-term effects on upslope plant community (Schoonmaker and McKee 1988; Hunt et al. 2003) or to riparian overstory contributions of large woody debris in streams (Bilby and Ward 1991; Dahlström et al. 2005). Upslope studies have found that vegetation displays considerable temporal variation, as the time since last disturbance influences forest stand density, structure, canopy closure, and forest floor organic matter and species richness (Halpern and Spies 1995; Jules et al. 2008), especially during early succession prior to canopy closure (Halpern 1988; Schoonmaker and McKee 1988). However, developmental processes and vegetation in riparian plant communities may differ from those found in upslope areas given their unique hydrologic conditions and fluvial disturbances, and it is unknown if successional and spatial patterns act independently or interact. The purpose of this study was to examine spatial and temporal variation in plant communities along small fish-bearing streams in western Washington. We focused on areas within managed forests that were logged to the streambank to provide insight for current and future riparian zone management in these forests. We hypothesized that distinct plant communities could be identified based on distance from stream and stand age. We expected these communities to be most strongly evident in the herb layer, as this layer is more species rich and thus includes more opportunities for specialization. Furthermore, we hypothesized that the species closely associated with these communities would have unique combinations of attributes such as wetland indicator status and successional status. Finally, we expected that riparian plant communities would demonstrate fewer changes over time than upslope plant communities due to hydrologic disturb-

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ance found in near-stream settings and that these communities would again be most evident in the herb layer.

Methods Study sites This study was conducted on managed forest lands in the headwaters of the Skookumchuck and Newaukum rivers with a combined watershed areas covering >97 000 ha in the western Cascade Mountains, Washington, USA (122°W, 46° N). The regional climate is maritime with wet, mild winters and dry, cool summers. Mean monthly temperature ranges from 4 °C in January to 18 °C in July and mean annual precipitation is 140 cm, occurring mainly as rain at low elevations and snow at higher elevations (Franklin and Dyrness 1988). Geology consists of tertiary volcanic and sedimentary rock with andesitic or basaltic soils (Evans 1987; Pringle 1990). Topography is steep with deeply dissected terrain and incised stream valleys. The study area is within the western hemlock forest zone (Franklin and Dyrness 1988), which is typically dominated by large, long-lived conifer trees, including Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco), western hemlock (Tsuga heterophylla (Raf.) Sarg.), and western redcedar (Thuja plicata Donn ex D. Don). Our 25 sites were adjacent to small fish-bearing streams and were characterized by perennial flow, narrow widths, steep slopes, and relatively small watersheds (Table 1). Most sites were located along third-order streams (21); two were along second-order and two were along fourth-order streams. Each study site was associated with a stream and the adjacent managed forest stand (termed a “stand”). Stands dominated by trees 100 years). For young and mature stands, we randomly selected 11 young and 10 mature stands. Only four old stands met our selection criteria; all were sampled. The ideal chronosequence would have been composed of all stand age classes from previously logged stands, but this was not feasible given the reality of the landscape and the timing of historical management activities. For example, this landscape does not contain clearcuts >100 years old; the old stands that we sampled had regenerated after wildfire. Data collection To capture vegetation characteristics within each stand, we established an 80 m × 20 m (0.16 ha) plot to enable uniform Published by NRC Research Press

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Can. J. For. Res. Vol. 42, 2012 Table 1. Mean values (±SE) of study site characteristics by stand age class.


Stand age class Young Mature Old

No. of sites 11 10 4

Age (years) 48.8 (±2.0) 59.1 (±1.9) 143.0 (±3.2)

Slope (%) 38.3 (±3.5) 42.9 (±3.3) 72.5 (±5.5)

Site elevation (m) 370.1 (±23.6) 361.4 (±25.2) 464.8 (±37.3)

and dispersed sampling. Each plot was located at least 30 m from stand edges to limit influence from adjacent stands. Three parallel transects were established, running perpendicular to the stream channel and 10 m apart (Fig. 1). Transects originated at the stream edge and extended 80 m upslope (slope distance). Vegetation and physical features along each transect were sampled using a series of subplots at 10 m intervals from the channel, termed distance from stream zones (DFS zones). Riparian landform type, slope, and height above the stream were recorded in each DFS zone along each transect. Riparian landform type was based on the classification scheme of Rot et al. (2000): floodplain (3 m above the channel), or slope (>20% slope). We used the term “transition slope” to identify areas that transitioned from another landform type to a slope within a DFS zone (Pabst and Spies 1998). Physical features of these riparian landforms are compared in Table 2. Canopy cover was measured in each cardinal direction every 10 m DFS zone in the herb layer subplots along each transect using a spherical densiometer (Lemmon 1956); percent canopy cover was calculated as a mean of these values. Transects were also divided into 10 m long sections for shrub sampling, and the area between transects was divided into 10 m × 10 m subplots for overstory sampling (Fig. 1). Tree and shrub layer sampling occurred over the 10 m extent of each DFS zone and the DFS zones for these variables were, therefore, referred to by the midpoint of distance of each zone (5, 15, 25, 35, 45, 55, 65, and 75 m). Herb sampling plots were 1 m2 and were established every 10 m along each transect; DFS zones for herbs are referred to by the distance at the beginning of the subplot (0, 10, 20, 30, 40, 50, 60, 70, and 80 m). Vegetation data were collected to evaluate gradients with distance from the channel. Overstory trees and saplings were sampled in one randomly selected 10 m × 10 m plot per DFS zone for a total of eight sampled plots per stand. The species identity and diameter at breast height (DBH) (breast height = 1.3 m) were tallied for each tree (≥5 cm DBH) and sapling (DBH < 5 cm and height > 5 m). Percent cover and species identity of the shrub layer were sampled using the line-intercept method (Canfield 1941) along the three 10 m transect segments per DFS zone for a total of 24 transects per stand. Shrubs were defined as shrub (woody) species >1 m tall and tree species 1–5 m tall with DBH < 5 cm. For each transect, the shrub species encountered were recorded as well as the length of transect intersected by the shrub. Percent cover of and species identity of the herb layer were sampled in 1 m × 1 m subplots every 10 m along each transect for a total of 27 subplots per stand. The herbaceous layer was defined as herbaceous vascular plants and ferns of any height as well as shrub or tree species 1% and to the nearest 0.25% if 75%) of the basal area and tree density. These two species accounted for similar amounts of the stand basal area (43% and 40%, respectively), but T. heterophylla accounted for more of the tree density (52%) than P. menziesii (25%). Tree community composition was similar across DFS zones (p = 0.172). Tree richness averaged 1.8 species per plot and did not differ among DFS zones (Fig. 2c). In total, 106 vascular plant species were identified in this study. The herb layer comprised 95% of all species encountered; the shrub layer contained only 23% of species. Although there was considerable overlap between these two layers, five species were unique to the shrub layer: Oemleria cerasiformis, Rosa gymnocarpa, Rosa nutkana, Salix sitchensis, and Taxus brevifolia. Seven tree species were also present in both the shrub and herb layers. Common species (those that occurred in >5% of all sample units) were mostly native (97%), facultative (56%), shade tolerant (78%), and perennial forbs or woody species (76%) Table 3). Moss cover was highest in the two subplots closest to the stream (Fig. 3a), while herbaceous cover was significantly higher only at the stream (Fig. 3b). Herb richness was highest at the stream edge, declined to 40 m, and remained consistent thereafter (Fig. 3c). Shrub cover was greatest at the stream,

Tree density (trees·ha –1)

Site characteristics All stands had high canopy cover (mean of 94%); canopy cover did not differ among DFS zones (p = 0.3094) or stand age classes (p = 0.0733). Riparian landform type exhibited little variation among DFS zones, as the topography was consistent and steep (averaging 51% slope, p = 0.5630). The greatest diversity of landforms was found immediately adjacent to the stream: 60% floodplain, 32% low terrace, and 8% high terrace; however, in nearly all cases, landforms changed within the first 10 m (68%) to a slope landform and, thus, the landform type for the DFS zone was classified as “transition slope” (Table 2). Slopes were the only landform at all sites from distances 30–80 m upslope. Due to the lack of variation in canopy cover and landform, we did not statistically test their effects on vegetation variables. In addition, aspect did not affect any site variables and had no main effects on vegetation variables.

Stand basal area (m2·ha–1)


Results

Fig. 2. Effect of distance from stream on hardwood and conifer (a) tree density, (b) stand basal area, and (c) total tree species richness. Bars indicate standard errors. For a given variable, letters indicate statistically significant differences among distance from stream zones.

Tree richness (spp.·100 m–2)

indicators. Species functional characteristics were summarized for common species that occurred in >5% of all sample units.

Can. J. For. Res. Vol. 42, 2012

Distance from stream (m)

intermediate at 15 m, and lowest from 25 m upslope (Fig. 3d). Shrub richness was also highest at the stream edge, declined to 35 m, and was consistent thereafter (Fig. 3e). Of the 101 herb layer species, 66% were found at the streamside subplots (DFS zone 0 m) compared with 54% in the transitional areas (DFS zones 10, 20, and 30 m), and 36% in upslope areas (DFS zones >30 m). Shrub and herb layer community composition differed significantly among DFS zones; in both layers, three distinct vegetation communities were evident: riparian areas (0–9 m), transitional areas (10–29 m), and upslope areas (30–80 m). Twelve species were indicators of these classes (Table 4) Published by NRC Research Press

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Table 3. All common understory species (present on >5% of all sample units) in herb and shrub layers, with lifeform, origin, successional status, wetland indicator status, and shade tolerance. Species Acer circinatum Athyrium filix-femina Berberis nervosa Blechnum spicant Bromus vulgaris Cardamine angulata Circaea alpine Corydalis scouleri Dicentra formosa Dryopteris austriaca Galium triflorum Gaultheria shallon Lactuca muralis Luzula parviflora Maianthemum dilatatum Montia sibirica Oplopanax horridus Oxalis oregano Polystichum munitum Ribes bracteosum Rubus spectabilis Rubus ursinus Sambucus racemosa Thuja plicata Tiarella trifoliata Tolmiea menziesii Trillium ovatum Tsuga heterophylla Vaccinium ovalifolium Vaccinium parvifolium Vancouveria hexandra Viola sempervirens

Lifeform* W F W F G P P P P F P W A G P A W P F W W W W W P P P W W W P P

Origin† N N N N N N N N N N N N E N N N N N N N N N N N N N N N N N N N

Successional status‡ R R R R, LS R I R R I R R R I I R I R R R R R R R R R, LS R R, LS R R R R R

Wetland indicator§ FAC FAC UPL FAC UPL FACW FAC FAC FACU FAC FACU FACU NOL FAC FAC FAC FAC NOL FACU FAC FAC FACU FACU FAC FAC FAC FACU FACU UPL NOL NOL NOL

Shade tolerance∥ T T T M T N/A T N/A T N/A N/A T N/A T N/A T N/A N/A T N/A M I T T N/A N/A N/A T M T N/A N/A

*Lifeforms: W, woody shrubs and tree seedlings; F, ferns; G, graminiods; P, perennial forbs; A, annual forbs (adapted from the PLANTS database (USDA NRCS 2010)). † Orgin status: N, native; E, exotic (USDA NRCS 2010). ‡ Successional status: R, residual species (Dyrness 1973; Halpern 1989); I, invader species (Dyrness 1973; Halpern 1989); LS, late seral species (McKenzie et al. 2000). § Wetland indicator status: FAC, facultative species; UPL, obligate upland species; FACW, facultative wetland species; FACU, facultative upland species; NOL, not on list for USDA Region 9 (USDA NRCS 2010). ∥ Shade tolerance: T, tolerant; M, intermediate; I, intolerant; N/A, not available (USDA NRCS 2010).

with some overlap between herb and shrub layers. In the herb layer, four herbs and one shrub were indicators of riparian areas; the strongest indicator, Cardamine angulata, is a facultative wetland species. In the shrub layer, three shrub species, Ribes bracteosum, Rubus spectabilis, and Oplopanax horridus, were indicators of riparian areas. The combined cover of these three species accounted for most of the shrub cover in the riparian area (60%) and less in the other communities (22% in transitional areas and 7% upslope areas). Vancouveria hexandra, a late-successional perennial forb, was the only indicator species found solely in transitional areas. Multiple species were indicators of both transitional and either riparian or upslope areas. Two species, Polystichum munitum (facultative upland) and Berberis nervosa (obligate upland), were indicators of upslope areas.

Stand age Tree density was similar across stand ages, but basal area was significantly higher in old than in young and mature stands (Table 5). Tree diameter did not vary by stand age (p = 0.4904), although young stands had fewer large trees than old stands (Table 5). Tree species composition and richness did not differ across age classes (Table 5), although the density of P. menziesii was 40% lower (85 versus 140 trees·ha–1) and that of T. plicata was 222% higher (112 versus 35 trees·ha–1) in young than in old stands. Moss and herb covers did not differ among age classes (Figs. 4a and 4b). Shrub cover was nearly double in old than in young and mature stands (Fig. 4c). Herb richness was highest in young stands, while shrub richness tended to be higher in old stands (Figs. 4d and 4e). Published by NRC Research Press

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Shrub cover (%) Shrub richness (spp.·10 m transect –1)

Herb cover (%) Herb richness (spp.·m–2)


Moss cover (%)

Fig. 3. Effect of distance from stream on (a) moss cover, (b) herb cover, (c) herb richness, (d) shrub cover, and (e) shrub richness. Bars indicate standard errors. For a given variable, letters indicate statistically significant differences among distance from stream zones.

Community composition by stand age class varied between herb and shrub layers. In the shrub layer, community composition of old stands differed from those of the mature and young stands. In the herb layer, community composition differed among all stand age classes. Fifteen species were identified as indicators of one or more of stand ages (Table 6). Young stands had three indicator species, old stands had seven indicator species, and no species were indicators solely of mature stands. Herb layer species consisted primarily of perennial residual species with few invader or annual species present. All but one species was native, the exception being Lactuca muralis, an exotic, annual, invader species that was an indicator of young and mature stands but was absent in old stands. Old stands supported some late-successional herb species (Blechnum spicant and Trillium ovatum) but differed in community composition primarily due to increased pres-

ence of shade-tolerant, residual shrubs (Acer circinatum, B. nervosa, Gaultheria shallon, Vaccinium ovalifolium, and Vaccinium parviflorum) and saplings of shade-tolerant trees.

Discussion This study examined spatial and temporal vegetation patterns along small fish-bearing streams in temperate managed forests. We found that distance from stream and stand age influenced the plant community, but acted independently. We suggest that vegetation patterns found with distance from stream are governed by the geomorphic and fluvial settings that influence the disturbance patterns and moisture availability along small montane streams, while the demonstrated temporal patterns were consistent with successional processes found with stand development. Plant attributes differed bePublished by NRC Research Press

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Table 4. Herb and shrub indicator species from riparian (R) (0–9 m), transitional (T) (10–29 m), and upslope (UP) (30– 80 m) areas.


R versus T Species Herb layer Cardamine angulata Circaea alpina Ribes bracteosum Rubus spectabilis Tolmiea menziesii Athyrium filix-femina Oxalis oregana Vancouveria hexandra Polystichum munitum Berberis nervosa Shrub layer Ribes bracteosum Oplopanax horridus Rubus spectabilis Vaccinium ovalifolium

R versus UP

T versus UP

R

T

p

R

UP

p

T

UP

p

R R R R R R–T R–T T T–UP UP

61.0 30.8 44.1 23.5 59.1 39.6 41.2 1.7 26.3 0.0

0.4 3.2 0.5 1.3 2.4 15.8 39.6 29.3 64.4 20.0

0.001 0.018 0.001 0.013 0.001 0.066 0.386 0.033 0.006 0.04

62.3 39.6 47.7 26.2 68.0 60.9 44.5 2.4 26.7 0.0

0.1 0.0 0.0 0.4 0.0 1.7 16.0 12.8 60.4 45.3

0.001 0.001 0.001 0.001 0.001 0.001 0.004 0.430 0.005 0.004

5.1 13.5 5.6 5.9 17.9 32.7 35.9 20.7 48.8 3.3

1.0 0.1 0.0 1.6 0.0 2.3 17.6 6.2 44.9 37.7

0.214 0.005 0.035 0.379 0.001 0.001 0.023 0.036 0.564 0.003

R R–T R–T T

44.1 42.0 44.5 0.2

1.5 7.5 13.1 26.7

0.002 0.006 0.021 0.020

51.2 51.2 62.9 0.4

0.0 0.0 1.0 8.6

0.001 0.001 0.001 0.298

9.2 27.6 32.9 20.1

0.1 0.3 1.8 2.7

0.010 0.001 0.001 0.002

Note: For each comparison, the data shown are the habitat indicator value in each zone and the p value for the larger indicator value. Strong habitat indicator species (shown in bold) have indicator value > 25 and p ≤ 0.05.

Table 5. Overstory vegetation characteristics and tree sapling density (mean (±SE)) for young, mature, and old stand age classes. Stand age class Overstory vegetation characteristic Canopy cover (%) Tree density (trees·ha–1) Density by size class Small trees 50 cm DBH (%) Stand basal area (m2·ha–1) Tree diameter (DBH) Tree species richness per site Sapling density (trees·ha–1)

Young (31–51 years) 93.1 (±0.6) 468.7 (±47.8) 36.0 54.3 9.7 44.7 30.9 3.7 71.3

(±4.0) (±4.0)a (±4.0)a (±5.4)a (±1.5) (±0.3) (±27.6)

Mature (52–70 years) 93.9 (±0.5) 534.1 (±45.5) 37.0 44.7 18.3 57.7 32.9 3.4 38.4

(±3.8) (±3.8)ab (±3.8) b (±5.2)ab (±1.4) (±0.3) (±26.4)

Old (100+ years) 95.7 (±0.9) 562.5 (±75.5) 47.8 30.8 21.4 79.6 32.8 3.5

(±6.4) (±6.4)b (±6.4)b (±8.7)b (±2.4) (±0.5) *

p 0.0654 0.4188 0.0304 — — — 0.0081 0.4904 0.2784 0.3535

Note: For a given variable, letters indicate statistically significant differences among stand age classes. *Missing data.

tween herb, shrub, and tree layers. There were few differences in overstory vegetation with distance from stream or stand age. Herb and shrub layers displayed three distinct understory plant communities running perpendicular to the stream: riparian areas (0–10 m), transitional areas (10– 30 m), and upslope areas (30–80 m). Additionally, herb composition differed between all age classes with greatest species richness in young stands, while shrub cover and species richness were highest in old stands. Distance from stream We found that riparian areas on small fish-bearing streams possessed limited areas with environmental conditions suitable for floodplain plant species. Riparian landforms at our study sites were relatively homogeneous; the greatest landform diversity was observed within 10 m of the channel edge, which correspond to the narrow (10 m) riparian plant communities that we identified based on the herb and shrub layers. Vegetation along small streams is greatly influenced by underlying geomorphic processes that shape the stream

valley and landforms (Goebel et al. 2006). The incised valleys found at our study sites restrict stream movement and create fewer fluvial landforms that are located immediately adjacent to the channel. The spatial compression and restriction of low-lying landforms creates a narrow riparian zone along the channel that is a common feature in the V-shaped valleys typical along streams in mountainous terrain (Pabst and Spies 1998; Rot et al. 2000). In addition to riparian landforms, the vegetation gradient from stream edge to upslope at our study stands is likely influenced by differing environmental gradients (Pabst and Spies 1999; Lyon and Sagers 2003). We did not observe differences in site characteristics (e.g., canopy cover or slope conditions) with distance from stream, but other studies found that riparian areas along confined, small streams commonly exhibit a physical environment characterized by very steep environmental gradients in soil moisture, humidity, fluvial disturbance, and light availability (Richardson and Danehy 2007). Vegetation response to environmental gradients based on plant traits can vary by functional group (Lyon and Published by NRC Research Press

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Sagers 2003) and can be influenced by stream size (Lamb and Mallik 2003). Low-ordered streams can have an overstory composition in the riparian area similar to upslope, while unique understory plant communities are present near stream (Acker et al. 2003; Hagan et al. 2006). On the slightly larger fish-bearing streams that we studied, overstory trees responded to environmental conditions with greater proportions of hardwoods at the streamside and understory plants had a distinct transitional community. Because of their larger maximum size, the spatial scale at which patterning is detectable for trees is greater than that for understory vegetation, demonstrating that herbs and shrubs are better indicators for the compressed environmental gradients found in smaller riparian areas along confined channels. Despite their limited size, the riparian plant communities along our study streams were more diverse and supported a

Shrub cover (%) Shrub richness (spp.·site –1)

Herb cover (%) Herb richness (spp.·m–2)


Moss cover (%)

Fig. 4. Effect of stand age class on (a) moss cover, (b) herb cover, (c) herb richness, (d) shrub cover, and (e) shrub richness. Bars indicate standard errors. For a given variable, letters indicate statistically significant differences among stand age classes.

plant community distinct from that on the transitional and upland surfaces. Similar to others, we found a pattern of decreasing abundance and species richness of understory herbs and shrubs (Pabst and Spies 1998; Dieterich et al. 2006) and moss (Stewart and Mallik 2006) with distance from stream. Additionally, upslope plant communities had less understory plant cover and few indicator species. The mechanisms responsible for maintaining the gradient in plant community characteristics along these small streams could be related to susceptibility to fluvial disturbance and moisture availability. In this study, indicator species followed a gradient of facultative wetland species in the riparian areas to facultative and obligate upland species in upslope habitats, suggesting a moisture gradient. Similar patterns in indicator species were reported by Goebel et al. (2006). Moss cover was highest closest to the stream, further suggesting a moist microclimate Published by NRC Research Press

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Table 6. Herb and shrub indicator species associated with young (Y), mature (M), and old (O) stand age classes.


Y versus M Species Herb layer Athyrium filix-femina Galium triflorum Lactuca muralis Oxalis oregana Rubus ursinus Blechnum spicant Berberis nervosa Gaultheria shallon Trillium ovatum Shrub layer Rubus spectabilis Acer circinatum Gaultheria shallon Thuja plicata Tsuga heterophylla Vaccinium ovalifolium Vaccinium parviflorum

Y versus O

M versus O

Y

M

p

Y

O

p

M

O

p

Y Y Y–M Y–M Y–O Y–O M–O O O

24.4 33.0 13.5 46.3 31.3 32.9 6.2 17.9 16.9

7.5 4.6 17.2 15.9 2.4 3.1 27.1 5.1 0.4

0.035 0.007 0.695 0.001 0.001 0.001 0.011 0.075 0.002

32.5 39.4 30.5 68.9 25.8 11.7 3.1 8.6 7.7

3.1 0.2 0.1 0.0 12.3 28.4 49.4 47.9 16.4

0.027 0.002 0.003 0.001 0.332 0.331 0.001 0.002 0.267

14.1 19.7 29.9 48.5 4.4 1.4 11.3 2.8 0.3

4.5 0.9 0.0 0.0 26.8 34.8 41.7 55.6 25.7

0.325 0.049 0.009 0.001 0.007 0.001 0.007 0.001 0.001

Y Y–O O O O O O

22.2 25.8 1.8 5.8 13.9 13.5 23.7

9.1 7.2 9.2 4.4 6.1 0.3 13.3

0.108 0.017 0.753 0.782 0.219 0.002 0.242

35.8 21.3 0.2 0.6 3.0 2.9 15.9

1.0 19.7 46.5 56.0 65.4 30.2 36.9

0.007 0.806 0.001 0.001 0.001 0.001 0.035

17.9 8.2 0.1 0.5 1.4 0.1 10.5

1.2 27.2 48.7 56.1 68.0 36.6 43.3

0.120 0.032 0.001 0.001 0.001 0.001 0.004

Note: For each comparison, the data shown are the habitat indicator value in each zone and the p value for the larger indicator value. Strong habitat indicator species (shown in bold) have indicator value > 25 and p ≤ 0.05.

favorable to mosses (Hylander et al. 2002; Stewart and Mallik 2006) extended only a relatively short distance from the stream. Effects of stand age Despite differences in management history or stand initiation processes, the vegetation at our study sites displayed successional trends similar to those in found in natural forests ecosystems (Franklin et al. 2002). It is likely that the initial site conditions found at all of our sites varied considerably within and between stands due to site variation and different types and intensities of management activities and fire presence (e.g., use of broadcast burning, wildfire, etc.). While the initial effects of these activities remain unknown, their impacts on vegetation establishment and growth have likely diminished over time as the stands developed. For example, a comparison of stands that initiated after logging or natural wildfire in two age classes (25–40 and 70–100 years old) found few long-term effects on plant species diversity, composition, or productivity between stands (Reich et al. 2001). Additionally, short-term effects of silviculture practices (Bailey et al. 1998; Thomas et al. 1999) may not be present in older stands. A study of mature stands (51 years) found that thinning and fertilization had few long-term effects (27 years after treatment) on understory species richness or cover (He and Barclay 2000). Similarities in understory vegetation at sites with differing stand histories suggest that plants are quite resilient to environmental change and that successional processes are significant for predicting long-term vegetation response after short-term disturbance impacts have subsided. Early-successional patterns in managed forests are characterized by shifts in abundance of residual and invader species (Halpern 1989). Consequently, plant species diversity through stand development is highest in early succession before canopy closure, lowest in midsuccessional stands after

canopy closure, and intermediate in late succession as canopy structure diversifies (Jules et al. 2008). Most previous studies have focused on the early aspects of this developmental sequence (Halpern 1988; Schoonmaker and McKee 1988). Our midsuccessional stands were primarily composed of perennial residual species with few invader or annual species and declining species richness. This suggests a general resiliency in the plant community with a short-term presence of invader species, which decline with increased presence of shade-tolerant, residual shrubs (Schoonmaker and McKee 1988; Moola and Vasseur 2008). Unique from other studies, the observed differences were not related to overstory canopy cover, which was high in all stands, although the increased shrub layer cover likely further reduced light availability for the herbaceous layer in old stands, thus creating conditions where only the most shade-tolerant species could persist on the forest floor. Although riparian and upslope areas had distinct compositions, the absence of an age and distance from stream interaction indicates that riparian and upslope forests followed similar successional trajectories. Distance from stream patterns were consistent across varying stand ages, even though steepness and stand initiations differed; either these difference did not have a measureable impact or we were unable to detect interactions due to lack of statistical power. The similarity in forest conditions in riparian areas along small streams and upslope areas is likely a reflection of the limited occurrence of low-lying landforms along these streams and the fact that the types of disturbances most likely to impact these riparian areas occur only rarely, such as debris torrents or fire. Over time, we suspect that our stands will continue to follow the successional patterns found in natural stands and will achieve greater complexity and more open conditions as vertical and horizontal diversification occurs (Franklin et al. 2002; Jules et al. 2008). Published by NRC Research Press


270

Conclusions Cumulatively in length, small fish-bearing streams comprise a substantial portion of regulated streams requiring buffers but are less studied than headwater or larger systems. These results examine the legacy of disturbance effects by fire and historical management practices such as logging to the stream prior to the establishment of riparian buffers. Understanding the long-term response of riparian vegetation along small streams to disturbance can help ensure that the ecological properties of these areas are adequately protected. Riparian areas found on these streams support distinct and diverse riparian plant communities that are limited in size but disproportionally important for conservation. Current forest practices require riparian buffers that range in mean width from ∼15 to 29 m (Lee et al. 2004). In this study, we found that riparian plant communities extended from the stream edge out 9 m with transitional plant communities from 10 to 29 m from the stream edge. This study suggests that current buffer requirements should protect the riparian plant communities on small streams, but management activities could influence transitional communities. In the future, riparian buffers will support the majority of unharvested, older forests within managed forest landscape. Therefore, these areas are of considerable ecological significance within managed landscapes. Additional research on the effects of forest management on the vegetation of streamside areas will be required to more fully appreciate how these systems are responding to current buffer requirements.

Acknowledgments We thank Steve Duke for statistical assistance, Janette Bach for spatial analysis support, David Giblin for plant identifications, John Heffner, Storm Beech, Peter James, Selina Hunstiger, John Cousins, Roxanne Nanninga, Scott Groce, and Kevin Biner for their excellent field support, and several anonymous reviewers for their helpful input.

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