Differences in Riparian Flora Between Riverbanks and River Lakeshores Explained by Dispersal Traits

Differences in Riparian Flora between Riverbanks and River Lakeshores Explained byDispersal TraitsAuthor(s): Christer Nilsson, Elisabet Andersson, David M. Merritt, Mats E. JohanssonSource: Ecology, Vol. 83, No. 10 (Oct., 2002), pp. 2878-2887Published by: Ecological Society of AmericaStable URL: http://www.jstor.org/stable/3072023Accessed: 11/03/2009 16:08

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Ecology, 83(10), 2002, pp. 2878-2887 ( 2002 by the Ecological Society of America

DIFFERENCES IN RIPARIAN FLORA BETWEEN RIVERBANKS AND RIVER LAKESHORES EXPLAINED BY DISPERSAL TRAITS

CHRISTER NILSSON,12,3 ELISABET ANDERSSON,I DAVID M. MERRITT,' AND MATS E. JOHANSSON'

'Landscape Ecology Group, Department of Ecology and Environmental Science, Umea University, SE-901 87 Umea, Sweden

2Department of Natural and Environmental Sciences, Mid Sweden University, SE-851 70 Sundsvall, Sweden

Abstract. Rivers and river lakes, i.e., lakes that are part of river systems, provide distinctive geomorphic and hydrologic conditions for riparian plants. This variation between lotic and lentic water bodies results in various environments for establishment and growth of plants, but also presents a range of conditions under which plant propagules are transported and deposited along riverbanks and lakeshores. Propagules may be differentially deposited in specific types of fluvial settings depending on their buoyancy. Among riverbanks and lakeshores, we predicted that lakeshores would capture the highest proportion of long-floating seeds, because short floaters will sink before reaching the shoreline. We also predicted that turbulent reaches would receive the highest proportion of short-floating seeds, because these are the sections where buoyancy is least important. Tranquil reaches of rivers were predicted to be intermediate between turbulent reaches and lakes with respect to their efficiency in capturing of long- and short-floating seeds. We tested whether these differences were mirrored in the floras of the different reach types, using reaches of a range of current velocities in 67 sites along a free-flowing river system in northern Sweden. We also related floristic differences to environmental conditions in each of the three reach types (turbulent sections, tranquil sections, and river lakes).

The proportions of species with long-floating propagules, herbs, and aquatic species were higher along river lakeshores and tranquil reaches than along turbulent reaches, and the opposite was true for species with short-floating propagules, dwarf shrubs, graminoids, and terrestrial species. The patterns remained when the largest species groups (herbs and graminoids) were tested for floating ability, i.e., the highest proportions of long-floating herbs and graminoids were found in lakes, and the lowest in turbulent reaches; tranquil reaches were intermediate or similar to lakes. Exposure to waves and currents and peat cover explained most of the variation in proportions of species with different floating abilities. We suggest that reach type is a good indicator for predicting the composition of riparian vegetation, especially the proportionate representation of species with various dispersal traits. These results suggest that there is a functional relationship between dispersal traits, channel characteristics, and plant community composition.

Key words: boreal rivers; hydrochory; lakeshore; long-floating seed; plant dispersal; riparian flora; riverbank; short-floating seed; species composition; Sweden.

INTRODUCTION

Most free-flowing rivers in boreal regions are complex and dynamic systems, characterized by turbulent and tranquil reaches, as well as river lakes. Variation in landscape topography and in the energy from water currents and waves among these reach types create different types of fluvial environments (Gregory et al. 1991, Malanson 1993, Ward 1997). Turbulent reaches (waterfalls, rapids, and runs) have coarse bed and bank material, such as gravel, cobbles, boulders, and bedrock. Sedimentation is sparse, but occurs during floods in areas of reduced flow velocity and where riparian vegetation and other protruding objects trap waterborne particles (Nepf 1999, Wohl and Cenderelli 2000). Tran-

Manuscript received 12 July 2001; revised 14 February 2002; accepted 4 March 2002.

3 Address correspondence to this author at Umea Univer- sity. E-mail: [email protected]

quil reaches exhibit weaker currents, which facilitate the deposition of small particles, such as sand, silt, clay, and fine-particulate organic matter resulting in floodplain development. Likewise, lakes are effective sediment traps, but lakeshores may be strongly eroded by waves and ice resulting in coarse shoreline material (Nilsson et al. 1993, Scrimgeour et al. 1996). These differences in habitat yield distinct plant and animal communities along morphologically different types of river reaches.

Macrophyte and bryophyte communities differ between turbulent and tranquil aquatic habitats (Vitt and Glime 1984, Englund et al. 1997), as well as algal (Poff et al. 1990, Reynolds 1995), fish (Bisson et al. 1988, Kinsolving and Bain 1993), and invertebrate communities (Stanford and Ward 1983, Quinn and Hickey 1994). Whereas physical habitat certainly exerts a strong influence on the composition of plant communities along rivers, the relationships between seed dis-

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October 2002 DISPERSAL DISTINGUISHES RIPARIAN FLORA 2879

TABLE 1. Conceptual differences between the margins of different reach types in freshwater lotic and lentic ecosystems.

Processes on riverbanks and lakeshores Turbulent reaches Tranquil reaches Lakes

Disturbance by currents high low-intermediate low Disturbance by waves low-intermediate low intermediate-high Propagule erosion high low-intermediate low Propagule deposition low-high intermediate intermediate-high Mean residence time of short intermediate long

propagule in reach before deposition

Predicted types of propa- all species regardless of mainly long-floating mainly long-floatingt gules deposited floating ability

Predicted proportions of highest proportion of short- intermediate highest proportion of long- short- and long-floating floating species floating species species in standing vegetation

Notes: Based on the hydrologic conditions, the banks of turbulent reaches of rivers are predicted to trap plant propagules with a range of floating abilities, whereas lakeshores will exhibit a dominance of long-floating propagules. We assume that the differences in seed deposition will be expressed in the standing vegetation. This leads to the prediction that, among reach types, turbulent reaches will have the highest proportion of short-floating species and lakes the highest proportion of long- floating species in the standing vegetation. Tranquil reaches are predicted to be intermediate in both cases.

t May vary as a function of discharge. T Short-floating seeds sink before deposition.

persal by water and the fluvial environment may also have a measurable influence on plant community composition.

The erosion-sedimentation cycle in boreal lotic systems may best be described as a source-sink process, in which fine material is transported from higher velocity turbulent reaches to lower velocity tranquil reaches. In this way, turbulent reaches may supply organic and mineral material to the stream channel, whereas tranquil reaches may serve as sinks for the deposition of this material. Geomorphologists recog- nize the strong relationships between channel form and the source-sink dynamics of mineral sediment move-

0 150 t

N km

Arctic Circle

2

C, 5 6 Rivers 6

1) Giertsbicken <A, 7 g2) Garghn -~~~~~~~~ ~~~3) Bjurbdicken

4) Sikfin 5) Aman

Gulf of 6) Hjuksin Bothinia 7) Krycklan

FIG. 1. The location of the study rivers (1-7) in northern Sweden.

ment in stream channels (Madej and Ozaki 1996, Thompson et al. 1999), and these dynamics are certain to be important in the dispersal dynamics of plant propagules along streams as well.

In Table 1 we present the basic hydrologic differences between reach types, and we predict that turbulent reaches generally receive the highest proportion of short-floating propagules, whereas lakes receive the highest proportion of long-floating propagules. Tran- quil reaches are predicted to receive relatively intermediate proportions of short- and long-floating propagules compared to turbulent reaches and lakeshores, as neither should be favored for stranding. In this paper we test whether these predicted differences in propagule stranding are expressed in the composition of standing vegetation.

METHODS

To cover the entire biogeographic range from the mountains to the coast, without encountering a gradient in stream size, we used seven tributaries of the Vindel River system, northern Sweden. In order from the most upstream, the tributaries were as follows: Gierts (local name Giertsbacken), Garg (Garga'n), Bjur (Bjurback- en), Sik (Sika'n), Aman, Hjuks (Hjuksaon), and Krycklan rivers (Fig. 1). The mean annual discharge at their con- fluences with the Vindel River ranges from 0.7 m3/s (Krycklan River) to 8.0 m3/s (Garg River; Table 2). Spring floods resulting from snowmelt are usually short and intense, and discharge is sensitive to rainstorms. Two other small rivers, of stream orders 4 and S (Strah- ler 1957), close to the Vindel River, have coefficients of variation of annual flows among years of 21.5% and 26.3 % (Nilsson et al. 199 lb), suggesting environmental predictability among years. The small rivers in the mountain region are dominated by morainic substrates,

2880 CHRISTER NILSSON ET AL. Ecology, Vol. 83, No. 10

TABLE 2. Hydrologic data on the rivers.

Mean Stream Channel Catch- annual order at length ment area discharge

River mouth (km) Slope (%) (kM2) (m3/s)

Gierts 5 68 0.88 486 7.7 Garg 4 77 0.23 852 8.0 Bjur 3 55 0.16 150 1.4 Sik 3 30 0.12 227 1.5 Aman 5 75 0.16 658 5.0 Hjuks 4 41 0.29 400 3.1 Krycklan 4 23 0.65 121 0.7

the small inland rivers by peat and morainic substrates, and those in the coastal region by fine sediments. River lakes are present in all tributaries. The largest of these is Giertsjaure, which has a surface area of 693 ha and is 9.3 km long. The smallest lakes are on the Gierts, Krycklan, and Bjur rivers (all unnamed lakes). They are 1.2 ha (length, L = 0.22 km), 0.3 ha (L = 0.05 km), and 1.8 ha (L = 0.27 km), respectively. The riparian vegetation may be distinctly vertically zoned, grading from forest communities furthest from the channel to shrub vegetation to herbaceous communities nearer the stream channel. The annual growing season ranges from < 140 d close the mountains to nearly 170 d in the more coastal areas.

We divided the entire channels of each of the seven tributaries into 10 sections, and randomly located a 200 m long study site in the middle of each section. In the Gierts River only seven of the 10 sections were sampled, because the three most upstream sections were located above the tree line. Sites were classified as lake, tranquil, or turbulent, based upon a qualitative visual assessment. The criteria for classifying each segment included the following: (1) the relative current velocity through the segment (not measured), (2) the presence or absence of standing waves, hydraulic jumps, and other hydraulic features indicative of a high-energy fluvial environment, and (3) the width of the channel through the segment. Flow in turbulent reaches is typ- ically swift, and high-energy hydraulic features are often visible. Turbulent sections are also narrower than those classified as tranquil reaches or lakes. Following field classification, four sections in the Garg, one in the Bjur, and one in the Sik rivers were excluded from the analysis because they included both turbulent and tranquil river stretches within the 200-m section. Of the 61 study sites used in the analyses, 25 represent river-margins along turbulent sections, 12 tranquil sections, and 24 lake sections. Every stream included rep- resentatives of each reach type, but numbers varied between streams (Table 3).

To establish an objective physical basis for the field classifications, representative sections of each category were selected from the Aman, Hjuks, and Krycklan rivers and revisited in December 2000. At each of three lake sections, six tranquil sections, and five turbulent

sections, water surface slope was measured over the -200-m study reach using a surveyor's rod and level. Water surface slope provides a reliable estimate of the energy gradient or friction slope of flow, which in con- junction with water depth and channel boundary roughness governs current velocity in open channels (Mun- son et al. 1999). Energy slope is also a key determinant of shear stress and stream power exerted by the flow on the channel boundary (Munson et al. 1999). A water surface slope of 0.002 is considered the threshold between gradual and steep-gradient mountain stream channels (Wohl 2000). In addition to water surface slope, a 200-m reach-averaged channel width was measured for each of the fourteen sections from 1:50000- scale topographic maps.

Discriminant function analysis was used to derive a discriminant criterion for assigning membership of each of the sections into one of the three classification groups based upon measurements of (1) water surface slope, and (2) reach-averaged channel width. Data were log10-transformed prior to the analysis to comply with the assumptions of multivariate normality and equal covariance matrices between classes in linear discriminant analysis. Wilks' lambda from multivariate AN- OVA was used to assess the significance of differences between the groups. Cross-validation error rate was used to determine the rate at which observations were classified into the correct section type from a function developed in the absence of the observation.

Discrimination was significant between the groups (Wilks' lambda: F420 = 28.8, P < 0.0001), indicating that lakes, tranquil sections, and turbulent sections are clearly distinguishable from one another based upon the two measured channel attributes. In total, 93% of observations were correctly classified by the discriminant function. One observation (a turbulent reach on the Krycklan River) was misclassified in the analysis (cross-validation error rate, 0.067). Lakes had negli- gible water surface slopes (-0.0001) and widths ranging 650-950 m, tranquil reaches had water surface slopes ranging 0.0001-0.0022 and channel widths ranging 25-50 m, and turbulent reaches had slopes ranging 0.004-0.0328 and widths ranging 5-25 m. The misclassified section would be considered transitional between tranquil and turbulent reaches (water surface

TABLE 3. Number of samples in each reach category.

River Turbulent Tranquil Lake Total

Gierts 3 2 2 7 Garg 3 2 1 6t Bjur 4 2 3 9t Sik 3 1 5 9t Aman 2 2 6 10 Hjuks 2 2 6 10 Krycklan 8 1 1 10

t Reaches intermediate in character were omitted before analysis.


slope in the range 0.0023-0.0039). Such transitional reaches were excluded from the vegetation analysis.

Environment

At each 200 m long study site we measured the width and height of the riverbank or lakeshore. The width was measured at five locations along the 200-m section, 50 m apart, as the distance between summer low-water level and spring high-water level. The height was measured as the vertical distance between these two levels, using a rod and level. For each site we determined the number of substrate types and estimated the percentage cover of each type using Wentworth's grain size scale (boulders, >256 mm; cobbles, >64 mm; pebbles, >16 mm; gravel, >2 mm; sand, >0.6 mm; fine sediment, > 0.002mm; and clay <0.002 mm; Chorley et al. 1984), supplemented by peat and bedrock. Percentage cover was estimated for the entire 200-m section independently by two persons, final cover values were reached by consensus. We calculated substrate fineness by weighing log2-transformed values of mean particle size by percentage composition of the substrates recorded on the study site (Wright et al. 1984). Fetch (i.e., the width of the river or lake, perpendicular to the bank or shoreline) was measured on 1:50 000-scale topographic maps. Exposure was determined by esti- mating the level of wave and flow action by ranking reach margin types as follows: (1) lakes with sheltered shores; (2) tranquil reaches; (3) runs, and lakes with exposed shores; and (4) rapids and turbulent reaches (Nilsson et al. 199 lb). As stated, six transitional reaches that included both tranquil and turbulent reaches were excluded, i.e., each of the remaining sites had the same exposure level throughout the 200-m section.

Vegetation

At each site we recorded the presence of all vascular plant species and estimated the total cover of vegetation for (1) trees and shrubs, and (2) vascular plants in the understory. Vegetation cover was estimated for the entire 200-m section independently by two persons, final cover values were reached by consensus. The nomen- clature follows Krok and Almqvist (1984), except that the taxa Carex nigra and C. juncella, Hieracium spp., Hierochloe hirta and H. odorata, Rhinanthus groen- landicus and R. minor, Salix borealis, S. myrsinifolia and S. phylicifolia, Sparganium spp., Taraxacum spp., and Thalictrum flavum and T. simplex were treated as one taxon each.

To test whether functionally different types of species respond differently to the environment along lakes, tranquil sections, and turbulent sections, we classified them into functional groups: (1) morphology based on growth forms (trees and shrubs, dwarf shrubs, forbs and ferns, graminoids), and based on location of foliage (stemmed [main foliage height > 15 cm] or nonstemmed [main foliage height <15 cm; Lid 1985]); (2) location on the riverbank or lakeshore (terrestrial vs. aquatic

species, with the latter group including amphibious species; Nilsson et al. 1994); (3) life cycle (annual vs. perennial species; Lid 1985). (4) We also compared species based on dispersal traits. First, we classified them according to their seed-floating capacity in water. Duration-of-buoyancy data were available for 62% of the species found in this study (data from Romell [1938], Danvind and Nilsson [1997], Andersson et al. [2000]). Species with propagule buoyancy times ex- ceeding two days were classified as "long-floating," whereas species with propagule buoyancy times less than two days were classified as "short-floating." There is no conclusive evidence that the two-day limit is ecologically relevant, however correlation analyses on a total of 275 species from 10 boreal rivers indicated that their frequency in the riverbank flora increased for buoyancy times longer than two days (Johansson et al. 1996). Species with buoyancy data were also classified into groups based on whether the species disperse mainly by generative (seeds and fruits) or vegetative (rhizomes, root fragments, and twigs) propagules. Sec- ond, we classified species with adaptations for anemochory. These data were derived using literature (e.g., Tutin et al. 1964-1981, Hylander 1982, Muller-Schnei- der 1986) and herbarium specimens.

Data analysis

Species functional groups were treated as proportions of the total number of species at a site. Dispersal trait groups were also treated as proportions: anemochorous species as a proportion of the total number of species at a site, and species groups with long-floating, short-floating, generative, and vegetative propagules as proportions of the number of species with buoyancy data. Bank height, number of substrates, and fetch were log10-transformed to more closely comply with the assumptions of normality in regression. We then tested for environmental and floristic differences between banks along rapids, slow-flowing sections, and lakes, using one-way ANOVAs and Tukey's honestly significant difference (Tukey's hsd). In these analyses, 10 variables described the vegetation, 15 variables described the environmental conditions, and five variables described dispersal traits. To test whether tributary was a significant factor in explaining the differences among reach types, ANOVA was used. We adjusted the Type I error rate using sequential Bonferroni tests (Holm 1979) of the Dunn-Sidalk method (Sokal and Rohlf 1995) to ensure that the experiment-wise significance level always was (x = 0.05, irrespective of the number of statistical tests.

To correct for differences in total species richness due to variation in sampling area, we calculated species density by dividing the number of species for each site by the base-10 logarithm of the area sampled (Whit- taker 1972, Connor and McCoy 1979). To provide in- telligible numbers, these values were then standardized

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TABLE 4. Comparison of total species richness and of proportions (%) of species in different functional groups among turbulent reaches, tranquil reaches, and lakes (one-way ANOVA).

Turbulent Tranquil reaches reaches Lakes

Functional group (n = 25) (n = 12) (n = 24) P

Growth forms (%) Trees and shrubs 14.6 ? 0.7 15.9 + 1.2 16.9 + 0.8 0.138 Dwarf shrubs 9.6 ? 0.7a 10.7 ? 1.2b 15.5 ? 0.8c <0.001 Herbs 45.2 1.5a 37.7 + 2.1 b 32.7 ? 1.6b <0.001 Graminoids 30.6 ? 0.8a 35.7 ? 0.8b 34.9 ? 1.1b 0.001

Location of foliage (%) Stemmed 48.5 ? 1.0 47.4 ? 1.3 49.6 ? 1.0 0.412 Nonstemmed 51.5 ? 1.0 52.6 ? 1.3 50.4 ? 1.0 0.412

Life span (%) Annuals and biennials 3.1 ? 0.2 3.5 + 0.3 3.1 ? 0.3 0.601 Perennials 96.9 ? 0.2 96.5 + 0.3 96.9 ? 0.3 0.601

Location on the riverbank (%) Terrestrial 93.5 ? 0.7a 88.7 ? 0.9b 87.9 ? 0.9b <0.001

Aquatic 6.5 + 0.7a 11.3 ? 0.9b 12.1 ? 0.9b <0.001

Total species richness 78.4 + 4.63 86.3 + 7.16 79.3 + 4.36 0.594

Notes: Data are presented as means + 1 SE; except for total species richness, all are proportions. Values in a given row with different superscript letters are significantly different (P < 0.05) according to Tukey's honestly significant difference (one-way ANOVA).

by multiplying by the base-10 logarithm of the mean sample area for all rivers.

To determine the influence of 13 environmental factors on the composition of vascular plants, based on dispersal means, functional groups, and total species richness, we performed a stepwise multiple regression analysis. The independent variables were riverbank height, substrate fineness, number of substrates, width of the river channel or lake, and percentage cover of different substrate types. Discriminant function analysis was performed in SAS/STAT version 8.0; all other tests were performed in SPSS 8.0.

RESULTS

Floristic differences

Altogether, the study sites included 269 taxa of vascular plants. The transformed (area adjusted) species richness per site did not differ significantly between the river-margin types (P = 0.594, one-way ANOVA). However, river-margin types had different proportions of life traits. Riverbanks along turbulent reaches had higher proportions of herbs and terrestrial species than did lakeshores (Table 4). The proportions of aquatic species, dwarf shrubs, and graminoids were higher on lakeshores than on banks along turbulent reaches (Table 4). Tranquil sections had higher proportions of graminoids and aquatic species, than did turbulent sections (Table 4). Tranquil sections did not differ significantly from lakeshores in these respects (P = 0.854 and 0.832, respectively, Tukey's hsd). The proportions of dwarf shrubs on tranquil sections were intermediate and significantly different from both lakeshores and turbulent sections (Table 4). The species groups based on the location of foliage and life span did not differ signif-

icantly between type of riverbank or lakeshore (Table 4).

The proportions of short-floating species were higher along turbulent reaches than along tranquil reaches or lakes, and long-floating species were more common along both lakes and tranquil reaches than along turbulent reaches (Table 5). Species capable of dispersal with waterborne vegetative propagules were more common on lakeshores than on tranquil reaches, which also had a higher proportion of vegetatively dispersed species than did turbulent reaches (Table 5). In fact, the proportion of species with waterborne vegetative propagules was strongly positively related to that of aquatics (r2 = 0.57, P < 0.001). The proportion of anemochorous species did not differ significantly between types of riverbank or lakeshore (Table 5).

When included in the analysis, tributary was a significant factor for some life traits. For example, the Sik had a higher proportion of dwarf shrubs and aquatics than did the other tributaries, the Gierts and Krycklan had higher relative numbers of herb species, and the Bjur, Sik, Aman, and Hjuks had higher proportions of species with long-floating seeds. However, in looking at the number of each reach type sampled on each stream (see Table 3), it becomes apparent that the differences may be due to differences in the actual number of lakes on the individual streams. For example, the proportion of long-floating species per tributary was strongly positively related to the number of lakes sampled (r2 = 0.69, P < 0.001).

Environmental differences

Lake shorelines had longer fetch than the two types of riverbank (Table 6). Banks along tranquil reaches


TABLE 5. Comparison of dispersal traits among turbulent reaches, tranquil reaches, and lakes (one-way ANOVA).

Propagule dispersal Turbulent reaches Tranquil reaches Lakes trait (n = 25) (n = 12) (n = 24) P

Long-floating 68.8 ? 1.08a 77.4 1 l.32b 82.3 ? 1.25c <0.001 Herbs only 61.8 ? 1.62a 72.8 ? 2.23b 80.7 ? 2.1lc <0.001 Graminoids only 69.8 ? 1.97a 77.4 ? 2.02b 80.6 ? 1.82b <0.001

Short-floating 31.2 ? 1.08a 22.6 ? 1.32b 17.7 ? 1.25c <0.001 Herbs only 38.2 ? 1.62a 27.2 ? 2.23b 19.3 ? 2.1lc <0.001 Graminoids only 30.2 ? 1.97a 22.6 ? 2.02b 19.4 ? 1.82b <0.001

Anemochorous 30.1 ? 0.64 27.8 ? 0.91 30.3 ? 0.59 0.068 Generative 83.0 ? 0.74a 79.0 ? 0.87b 75.6 ? 0.77c <0.001 Vegetative 17.0 ? 0.74a 21.0 ? 0.87b 24.4 ? 0.77c <0.001

Notes: Data are means + 2 SE of species proportions (%). Values in a given row with different superscript letters are significantly different (P < 0.05) according to Tukey's honestly significant difference (one-way ANOVA).

had longer fetch than did turbulent reaches, but the latter were subject to stronger exposure than lakes and tranquil reaches (Table 6). Exposure did not differ significantly between lakeshores and tranquil reaches (P = 0.335, Tukey's hsd). Both turbulent and tranquil reaches had a higher percentage cover of fine sediment than did lakeshores (P = 0.002 and 0.047, respectively, Tukey's hsd). Yet, tranquil reaches had finer-textured substrate than did turbulent reaches (P = 0.005, Tu- key's hsd). Bedrock was only present along turbulent reaches, so this variable was not further analyzed.

The percentage cover of trees and shrubs per site was significantly higher along turbulent reaches than along lakeshores (Table 6), as was the percentage cover of vascular plants in the understory (Table 6). Tranquil reaches did not differ significantly from the other two types in their percentage cover of trees and shrubs (P > 0.05, Tukey's hsd). A wide range of the variation in

the dependent variables (5-65%) was explained in regression analyses (Table 7). Exposure to waves and currents, and percentage cover of peat were best related to the variation in the proportions of species with different floating abilities, while fetch was most strongly correlated with the variation in the proportions of propagule types (generative or vegetative). The variation in total species richness per site was most related to the percentage cover of cobbles. The proportions of aquatic and terrestrial species were best explained by the fetch, the proportion of herbs by the percentage cover of peat, and the proportion of graminoids by exposure.

DISCUSSION

Dispersal traits have been shown to be important for understanding floristic patterns along rivers, both for species (Staniforth and Cavers 1976) and communities (Schneider and Sharitz 1988, Nilsson et al. 1991a). In

TABLE 6. Comparison of environmental variables among turbulent reaches, tranquil reaches, and lakes (one-way ANOVA).

Turbulent reaches Tranquil reaches Lakes Environmental variable (n = 25) (n = 12) (n = 24) P

Fetch (m)t 27.8 ? 3.56a 57.1 ? 11.6b 510 ? 69.4c <0.001 Width of riverbank or lakeshore (m) 17.4 ? 3.56ab 54.0 ? 27.2a 15.2 3.45b 0.034t Height of riverbank or lakeshore (m)t 1.23 0.1 la 1.22 ? 0.l6ab 0.90 ? 0.08b 0.024t Exposure 3.24 ? 0.16a 1.83 ? 0.llb 2.21 ? 0.17b <0.001 Substrate fineness (+) 3.72 ? 1.13a 11.3 ? 2.05b 5.65 ? 1.45ab 0.007 No. substratest 3.84 ? 0.30a 2.67 ? 0.53b 3.54 ? 0.38ab 0.039t

Substrate type (%)

Peat 29.7 ? 7.5Oa 65.1 ? ll.lb 64.5 ? 7.98b 0.004 Fine sediment 30.8 ? 7.63a 26.3 ? 10.6a 1.0 ? 0.58b 0.002 Sand 10.5 ? 3.22 3.50 ? 2.71 5.75 ? 2.58 0.278 Gravel 1.56 ? 0.71 1.33 ? 1.25 1.71 ? 0.92 0.976 Pebbles 1.00 ? 0.61 0.42 ? 0.42 1.42 ? 0.51 0.551 Cobbles 3.20 ? 1.24 2.75 ? 2.49 4.83 ? 1.73 0.667 Boulders 22.8 5.44a 1.08 ? 0.83b 20.8 ? 5.19ab 0.031t

Percentage cover (%)

Trees and shrubs 60.6 ? 3.92a 44.2 ? 5.18ab 29.1 ? 4.26b <0.001 Vascular plants in the understory 62.6 ? 3.79a 58.8 ? 4.89ab 46.2 ? 4.24b 0.014

Notes: Data are means ? 1 SE. Values in a given row with different superscript letters are significantly different (P < 0.05) according to Tukey's honestly significant difference (one-way ANOVA).

t Variables were logi0-transformed prior to analysis. t Not significant after Bonferroni correction, using a sequential procedure (Holm 1979) of the Dunn-Sidak method (Sokal

and Rohlf 1995).


TABLE 7. Results of the stepwise multiple regression analyses on proportion of species with given propagule type or in given functional group; entries are standardized P values (n = 61).

Propagule type/functional group HEIGt SUFI NOSUt FETCt EXPO PEAT BOUL COBB PEBB FISE R dJ

Transformed species richness ... ... ... - *- ... -- 0.47 *- ... 0.21

Propagule type Long-floating propagules -0.18 ... ... 0.33 -0.37 0.37 0.29 ... 0.65 Short-floating propagules 0.18 ... -0.33 0.37 -0.37 -0.29 . *... 0.65 Generative propagules * - *- -0.54 ... -0.29 ... *- 0.26 * 0.50 Vegetative propagules * - -- 0.54 - 0.29 ... *- -0.26 ... 0.50

Functional group Trees and shrubs ... ... -- 0.24 - ... 0.38 -0.42 ... 0.21 Dwarf shrubs ... *-- -0.19 0.31 -- -- *- -- -0.24 -0.52 0.51 Herbs ... 0.44 0.21 -0.37 *-- -0.60 ... ... 0.25 - 0.57 Graminoids *- ... * - -0.31 0.29 ... -- -0.24 ... 0.35 Nonstemmed ... - - -- 0.26 *- ... 0.05 Stemmed - - * - ... -0.26 * - - - 0.05 Aquatic ... * - 0.39 -0.31 ... *- ... ... 0.29 Terrestrial -- ... * -0.39 0.31 ... ... - ... ... 0.29

Notes: Predictor variable abbreviations are as follows: HEIG, riverbank or lakeshore height; SUFI, substrate fineness; NOSU, no. substrates; FETC, fetch; EXPO, exposure; and proportion along the 200 m stretch of peat (PEAT), boulders (BOUL), cobbles (COBB), pebbles (PEBB), and fine sediments (FISE). The proportions of bedrock, gravel, and sand were not entered in the equations and therefore are not included in the table. No predictor variables were entered for the dependent variables "annuals" and "perennials" (not included in the table).

t Data have been logi0-transformed.

this study we found that dispersal characteristics of standing vegetation may be explained in part by the interaction between buoyancy characteristics of seeds and hydraulic attributes of the channel. The spatial distribution of stranded seeds along riverbanks is a function of stream hydraulics (Merritt and Wohl 2002). As hydraulic patterns of lakes are simple, relative to those in river channels, stranding patterns of waterborne seeds along lakeshores are largely governed by wind speed and direction over the lake surface. In channels with flowing water, hydraulics become much more complex and also vary as a function of channel boundary characteristics and discharge.

In areas where currents are swift, such as along turbulent reaches, the importance of seed floating ability diminishes (Danvind and Nilsson 1997). Along turbulent reaches, seed stranding is largely dependent upon hydraulic factors such as turbulent waves caused by bursting, which deliver seeds to banks, and through the trapping of seeds by emergent objects such as rocks and vegetation (Nepf 1999). Turbulent flow results in irregular mixing and high shear stress throughout the water column, keeping even waterlogged or nonbuoy- ant seeds in suspension, thereby increasing the likelihood that any seed, not only those with long floating ability, will be deposited along riverbanks. Vegetation along turbulent reaches had the lowest proportion of long-floating species and the highest proportion of short-floating species compared to other reach types, indicating that factors other than floating ability govern patterns of seed deposition in these areas.

The processes governing seed deposition in turbulent reaches are likely to differ between low and high discharges. At low discharge, when velocity through tur-

bulent reaches is lower and roughness elements such as rocks and vegetation are exposed above the water surface, these factors are likely to be very important in trapping seeds. However at higher discharges when these features are drowned out, it is more likely that surges of water and turbulent waves force seeds onto riverbanks. In addition, the strips of riparian forest flooded at higher discharges also trap seeds. In effect, turbulent reaches may be very efficient in trapping seeds, especially in outer curves and in backwater reaches (Andersson et al. 2000).

Along tranquil reaches and lakes, the proportion of long-floating species in the vegetation was found to be 12-20% higher than along turbulent reaches (Table 5). Because of the slower flow velocity through tranquil reaches and lakes, the residence time of seeds is higher than along turbulent reaches. The seeds of short-floating species have a greater likelihood of sinking, fa- voring species with long-floating ability in more tranquil hydraulic environments.

However, tranquil reaches are not necessarily always sink areas for propagules. Whereas velocity in tranquil reaches is low relative to turbulent reaches at intermediate discharges, as discharge increases, a threshold may be crossed so that velocity in tranquil reaches exceeds that of turbulent reaches (Keller 1971). "Ve- locity reversal" is due to the formation of zones of recirculation along tranquil reach margins and the con- finement of downstream flow to a strong jet in the center of the channel (Thompson et al. 1999). Velocity reversal at high flows has been recognized by geomorphologists as an important process in pool main- tenance, whereby pools fill with mineral sediment during low discharges and then are scoured at higher dis-


charges. The source-sink dynamics of seed transport and deposition are likely to be affected by these processes as well. Thus, rather than turbulent sections serving as source areas and tranquil reaches serving as sinks of propagules, the source-sink dynamics between reach types may vary as a function of discharge. Tran- quil reaches may serve as the depositories of large numbers of seeds during normal discharges, but shift to source areas for the remobilization of stored seeds at higher discharges. Such processes, suggesting long transport times for some seeds, may contribute to the fact that most seeds in the drift deposits along riverbanks may actually be dead (Nilsson and Grelsson 1990).

Delivery of waterborne seeds to lakeshores is governed entirely by wind currents, which vary tremen- dously in strength and direction through time. Seeds may be blown back and forth across a lake surface many times before finally reaching a lakeshore. The low proportion of short-floating species on lakeshores reflects the fact that most of the short-floating propagules sink before reaching the shoreline. Consequently, short-floating species are not well represented in the standing vegetation along lakeshores. Whereas lakes are very efficient seed traps for all types of seeds, the slow delivery of seeds to lakeshores serves as a filter, eliminating short floaters. These patterns are also sup- ported by the results in Jansson et al. (2000) who found higher proportions of long-floating species on the banks of run-of-river impoundments, than along free-flowing rivers. Run-of-river impoundments have hydrologic conditions that are more similar to lakes than to river reaches. Therefore, beyond the habitat differences between turbulent and tranquil reaches and lakes, there are hydraulic factors related directly to dispersal that may be a key to understanding community composition along river-margins.

The results show that plant traits have some value for comparing and predicting differences in riparian vegetation. Among the two dominant plant groups, herbs dominated the turbulent reaches, and graminoids dominated the tranquil reaches and lakeshores (Table 4). There are probably several group-inherent characteristics that may explain these differences in habitat preferences. For example, the graminoids in our data set are on average taller and have a higher degree of lateral spread than the herbs (Lid 1985, Hodgson et al. 1995). One might ask whether these differences were more important than floating characteristics. In fact, mean floating time differed, but not significantly (P > 0.05), between graminoids (65 d) and herbs (49 d), and the ratio of long floaters to short floaters was significantly higher in graminoids (79.5%:20.5%) than in herbs (60.4%:39.6%; P < 0.05, x2 test). However, when tested against reach type, both graminoids and herbs had the highest ratio of long floaters to short floaters in lakes, decreasing to tranquil and further to turbulent reaches (Table 5). Thus, the pattern found for the entire

species pool remained consistent within the two largest plant groups, further supporting our original predic- tions.

The life span of plants cannot function as a grouping variable in this region, simply because there were only three percent annual and biennial species and little variation between sites. The location of foliage on the plants did not differ between riverbank and lakeshore categories, suggesting that light is not a limiting factor in any of these land-water interfaces.

The fact that aquatic plants occurred in higher proportions on banks along tranquil sections and lakes than along turbulent reaches might be caused by a combination of two factors. First, tranquil sections had finer textured substrates, which favor regeneration of wet- land plants, especially small-seeded species (Keddy and Constabel 1986). Second, banks along tranquil sections and lakes were less exposed than along turbulent reaches, where ice scouring and high flows easily can break or dislodge plants in the water (Hutchinson 1975, Haslam 1978). Thus, both establishment and survival of aquatic species are favored along tranquil reaches and lakes rather than along turbulent reaches due in large part to differences in habitat.

The proportion of species with adaptations for wind dispersal did not vary much along the river courses, implying that wind dispersal of riparian species was not important for structuring species composition. Log- ically, wind-dispersed species should be favored in open areas, such as riverbanks or lakeshores with a long fetch. The dispersal distance of wind-dispersed species increases with increasing wind velocity (Hen- sen and Muller 1997). However, because anemochorous seeds often have hairs, plumes, or wing-like struc- tures that increase the surface area of the seed, they are all able to remain on the water surface by surface tension alone. In other words, if propagules with spe- cial adaptations for wind dispersal are brought into the water, they can be transported downstream, but can also be blown directly to any type of riverbank or lakeshore. This combination of dispersal mechanisms may even out eventual differences caused solely by anemochory.

To conclude, it is possible to predict the composition of vegetation at the scale of landscapes along free- flowing small rivers, although the relationships between environmental and biotic variables are complex. We have shown that an effective variable is reach type, which may be objectively classified based upon measurements of water surface slope and channel width. Lakeshores, banks along tranquil sections, and banks along turbulent reaches, had successively higher proportions of short-floating species. This reflected the conditions for hydrochory along the different sections of a river. Wide river sections with weak currents favor species with propagules that can stay afloat long enough to become stranded, germinate, and establish, while more of the short-floating propagules sink before reaching the shore in slow-moving water. An alterna-


tive explanation would be that the differences between reach types in the proportions of short-floating and long-floating species depend on differences in sediment characteristics. The only statistically significant differences in substrate between reach types were found for the proportions of fine sediment and peat. If these differences were ecologically important, turbulent and tranquil reaches would be dominated by species pre- ferring finer sediment, and lakes would be dominated by peat-growing plants.

Recurring patterns in form and physical process along rivers have long been recognized by ecologists as factors influencing the distributions of vascular plants and plant community types. The strong associations that have been drawn between distinct plant community types and specific geomorphic features along streams are most often explained in terms of the physical attributes of the fluvial features (hydrologic characteristics, soil texture, exposure to disturbance, etc.) (Harris 1987, 1988, Hupp 1990, Kalliola et al. 1991, Stevens et al. 1995, Hupp and Osterkamp 1996). Our findings indicate that other factors, such as the interplay between duration of seed buoyancy and the hydraulic properties of specific river reaches, are in fact botanically meaningful as well. This approach may serve as a useful framework for understanding the distributions of plants and probably also other types of organisms along many different stream types.

ACKNOWLEDGMENTS

We thank Mats Dynesius, Alf Ekblad, Sven Hellqvist, and Roland Jansson for assistance in the field, and Susanne Backe for starting the life trait classification of plants. We are also grateful to Roland Jansson, Rebecca Sharitz as a journal reviewer, Scott D. Wilson, and an anonymous reviewer for com- ments that greatly improved the manuscript. Financial support was provided by the Swedish Society for the Conservation of Nature, the Kempe Foundation, the Swedish World Wide Fund for Nature, and the Swedish Natural Science Research Council.

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Differences in Riparian Flora Between Riverbanks and River Lakeshores Explained by Dispersal Traits