Mats E. Johansson

Do rivers function as corridors for plant dispersal

. We evaluated the importance of dispersal for species frequencies and distribution by comparing dispersal properties of vascular plant species with their frequencies along river banks. We assumed that species with long-floating seeds would be more frequent than species with short-floating seeds. We compiled data on frequencies of vascular plants and their dispersal properties from ten rivers in northern Sweden and compared these with boreal forests and grasslands in the same region. In all rivers, but in none of the reference areas, there was a positive relationship between floating capacity and frequency of species. A comparison of floating capacity between species with and without certain dispersal devices showed that seeds of vegetatively dispersed species had higher floating capacities than other seeds. For other dispersal categories (animal and wind dispersal), floating time did not differ from contrast groups. The results indicate that water dispersal has a certain role in structuring the riparian flora, and provide a basis for explaining species distribution patterns from dispersal characteristics. They also suggest that continuous river corridors are important for maintaining regional biodiversity.

Hydrochory, population dynamics and distribution of the clonal aquatic plant Ranunculus lingua

The dispersal and dynamics of the clonal aquatic plant Ranunculus Lingua were studied in a marginal river population in northern Sweden, where this species is dispersed exclusively by vegetative diaspores (i.e. floating rhizome fragments). The study focused on the extent and range of dispersal, relationships between dispersal and distribution of established stands of R. lingua, and between dispersal and environmental characteristics of the river (i.e. «river status»), the establishment success of vegetative diaspores, and the relative changes in ramet numbers between years. Between 0-8 and 2-9% of the total population of ramets was estimated to disperse each year. Most rhizomes were dispersed within 1 -5 km in a leptokurtic pattern. A river lake functioned as a dispersal barrier, preventing dispersal distances greater than 3 km (...)

Effects of river fragmentation on plant dispersal and riparian flora

We evaluated the effects of river fragmentation by dams on hydrochory (i.e. plant dispersal by water) and on plant distribution by comparing two adjacent rivers in northern Sweden, one free-flowing and the other regulated. We collected stranded drift material from both rivers in order to quantify the drift material and its species content. We also estimated the floristic continuity along the two rivers by comparing the drift flora with the riparian flora further upstream. The drift amount deposited on the riverbank, its species richness and its contribution to the species pool were higher in the free-flowing than in the regulated river. The floristic continuity was also higher in the free-flowing than in the regulated river. Copyright (C) 2000 John Wiley & Sons, Ltd.

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

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 trans- ported and deposited along riverbanks and lakeshores. Propagules may be differentially deposited in specific types of fluvial settings depending on their buoyancy. Among river- banks 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.

Intercontinental similarities in riparian-plant diversity and sensitivity to river regulation.

We asked whether assemblages of species with separate evolutionary his- tories differed in their response to similar human interventions. We assessed this by com- paring the response of riparian plant communities to river regulation on two different continents. We compared free-flowing and regulated rivers between boreal parts of North America (Alberta and British Columbia) and Europe (Sweden), using a standardized sam- pling protocol and the same field staff on both continents. Although the two regions shared few species, both riparian plant-species diversity along free-flowing rivers and the response to different kinds of flow regulation were similar between the continents. The number of riparian-plant species and their amount of cover differed among types of water-level regime, but the continental affiliation of a river-margin site did not statistically explain any of the variation. Within continents, the local flora of the regulated river-margin sites was largely similar in species composition to the free-flowing ones, but the sites along storage reservoirs were more species-poor. The similarity in the response to regulation between the continents suggests that general guidelines for rehabilitation of degraded boreal rivers are a realistic goal. The number of species and genera, plant cover, and species numbers in most trait groups (classified according to growth form and life span) were similar between free-flowing river margins in Europe and North America. Moreover, the regional native species pools of northern Sweden and Alberta were similar in size and composition of species groups, despite the fact that only 27% of the species in Alberta were found in northern Sweden. This is presumably because the floras share a common Tertiary origin and because the regions have had largely similar late-Tertiary and Quaternary histories. The most pronounced difference between the continents was that we found no exotic species on the 183 Swedish river- margin sites, whereas 9% of the species found in all 24 North American plots taken together were exotics. All North American exotics found have occurred in Europe since prehistoric times, and the difference in exotic richness most likely reflects a difference in the number of species humans have transferred from one continent to another, rather than a difference in invasibility between the regions.

Small rivers behave like large rivers: effects of postglacial history on plant species richness along riverbanks

Similarity in longitudinal patterns of vascular plant species richness along riverbanks was tested by com- paring two small and two large, unimpounded rivers empty- ing into the Gulf of Bothnia in northern Sweden. Simply put, we asked whether small rivers behave like pieces or like miniatures of large rivers. All rivers exhibited a peak in species richness at the point where the rivers begin to cut down into sediments deposited during a higher coastal stage 9200 BP. This result supports the hypothesis that plant communities along riverbanks, irrespective of size, are given a similar shape by the postglacial history of the land- scape. The mechanisms producing such similarity may relate to intermediate disturbance, maximum heterogeneity, or immigration history.

Can rarity and diversity be predicted in vegetation along river banks

Abstract This paper examines the possibility of predicting the location of rare species and species-rich habitats along rivers. Vascular plants were recorded in 200 m-long sections of bank, 10 km apart at 149 sites along the four largest unregulated rivers in Sweden. Species-rich sites and sites with rare species were scattered along the rivers, and could not be predicted either from the downstream position or from the overall type of river habitat. The only predictable feature found was that the species richness was higher on sites with rare species than on sites without rare species. The results suggest that each river has largerly unique vegetation features, and that extensive field surveys are the only guarantee of success in the selection of river sites for the purpose of conservation.

Effects of leaf-litter accumulation on riparian plant diversity across Europe

Floods in rivers can deposit large quantities of organic debris in the riparian corridor and cause considerable change to its vegetation. We studied the importance of leaf-litter accumulation on riparian vegetation in different climates by constructing gradients ranging from complete litter removal to addition of up to 6000 g/m2 (35–40 cm deep) of leaf litter and recorded the consequent changes in vegetation. Experiments were located along rivers in northern and southern Sweden, Italy, and Romania. Plant biomass decreased with increasing litter mass but did not vary with site-dependent variables such as climate and soil. Species density for various plant life traits varied with litter mass, summer temperature, and summer precipitation. Litter mass was the most important variable for all life traits except rhizomatous and stoloniferous species, which were most dependent upon summer precipitation. These findings are useful for the prediction of responses in riparian vegetation following future climatic changes.

Responses of riparian plants to accumulation of silt and plant litter: the importance of plant traits

. A 2-yr field experiment was used to determine the response of riparian plants to accumulation of litter or silt in a river flood-plain meadow in northern Sweden. Such disturbances occur regularly in free-flowing rivers but are likely to change as a result of global changes in land use or climate. We anticipated that plants with different traits would differ in their response to litter and silt accumulation. We quantified plant response as relative change in above-ground biomass, and regressed it on either litter mass or silt depth, and on plant traits such as lateral spread, plant height, relative growth rate, seed mass and seed persistence in soil. The relative changes in riparian plant biomass following litter or silt accumulation were negatively related to litter mass and silt depth, and positively related to most examined plant traits such as seed mass, seed persistence and lateral spread. The vegetation recovery in the second season was largely determined by plant traits; litter or silt accumulation had no significant effect. Litter accumulation selected for large-seeded species, but silt accumulation selected for species with strong ability of lateral spread. Seed persistence was a useful variable in predicting species recovery from both litter and silt accumulation. Plant height was negatively related to plant recovery, but relative growth rate was not significantly related to relative change in plant biomass after silt or litter accumulation. Our results imply that plant traits are important variables to consider for predicting the responses of riparian vegetation to deposition of organic and inorganic matter.

The influence of hydrological regimes on sex ratios and spatial segregation of the sexes in two dioecious riparian shrub species in northern Sweden

River management practices have altered the hydrological regimes of many rivers and also altered the availability of regeneration niches for riparian species. We investigated the impact of changed hydrological regimes on the sex ratios and the Spatial Segregation of the Sexes (SSS) in the dioecious species Salix myrsinifolia Salisb.–phylicifolia L. and S. lapponum L. by studying the free-flowing Vindel River and the regulated Ume River in northern Sweden. We surveyed sex ratios of these species in 12 river reaches on the Vindel River and in 17 reaches on the Ume River. In addition, we surveyed the sex and location above mean river stage of 1,002 individuals across both river systems to investigate the SSS of both species. Cuttings were collected from male and female individuals of S. myrsinifolia–phylicifolia from both rivers and subjected to four different water table regimes in a greenhouse experiment to investigate growth response between the sexes. We found an M/F sex ratio in both river systems similar to the regional norm of 0.62 for S. myrsinifolia–phylicifolia and of 0.42 for S. lapponum. We found no evidence of SSS in either the free-flowing Vindel River or the regulated Ume River. In the greenhouse experiment, hydrological regime had a significant effect on shoot and root dry weight and on root length. Significantly higher shoot dry weights were found in females than in males and significantly different shoot and root dry weights were found between cuttings taken from the two rivers. We concluded that changed hydrological regimes are likely to alter dimensions of the regeneration niche and therefore to influence sex ratios and SSS at an early successional stage, making it difficult to find clear spatial patterns once these species reach maturity and can be sexed.