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  • 1.
    Andersson, E.
    et al.
    Umeå universitet.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Temporal variation in the drift of plant litter and propagules in a small boreal river2002In: Freshwater Biology, ISSN 0046-5070, E-ISSN 1365-2427, Vol. 47, no 9, p. 1674-1684Article in journal (Refereed)
    Abstract [en]

    1. Rivers are linear ecosystems across landscapes with an effective transport of organisms, sediment and organic matter. Dispersal is studied mostly during single events and for single species, and there is little knowledge on how the drift of plant litter and propagules varies within and between years for entire communities. 2. We used floating traps for collecting waterborne plant litter and propagules in a small boreal river over 2 years. We installed the traps at four different locations along the river, and emptied them at least once a week during the ice-free season. We analysed propagule content by sorting and identifying species and through germination tests on bare soil. 3. In total, we recorded at least 54 taxa in the samples, and the highest density recorded in one sample was 5000 propagules per 100 g litter (dry weight). Large temporal variations in litter and propagule transport were revealed, both within and between years. 4. The longitudinal pattern was consistent between years, with an increasing mass of litter and number of propagule taxa downstream. The results highlight the importance of the temporal and longitudinal dimensions in river management.

  • 2. Andersson, Elisabeth
    et al.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Johansson, M. E.
    Effects of river fragmentation on plant dispersal and riparian flora2000In: Rivers Research and Applications: an international journal devoted to river research and management, ISSN 1535-1459, E-ISSN 1535-1467, Vol. 16, no 1, p. 83-89Article in journal (Refereed)
    Abstract [en]

    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.

  • 3. Andersson, Elisabeth
    et al.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Johansson, M. E.
    Plant dispersal in boreal rivers and its relation to the diversity of riparian flora2000In: Journal of Biogeography, ISSN 0305-0270, E-ISSN 1365-2699, Vol. 27, no 5, p. 1095-1106Article in journal (Refereed)
    Abstract [en]

    Aim The paper has four major objectives: (1) to determine whether diaspore mimics accurately represent dispersal dynamics of real diaspores in a free-flowing river; (2) to estimate distance travelled and reasons for stranding of floating diaspores along a free-flowing river; (3) to test if species composition and seedling recruitment vary with the ability of the riverbank to trap waterborne drift; and (4) to compare diaspore dispersal in a free-flowing river with that in a regulated river where current velocity has been reduced. Location The field work was conducted in two 7th-order boreal rivers in northern Sweden, the free-flowing Vindel River and the regulated Ume River. Methods We performed a series of dispersal experiments. We tested the usefulness wooden cubes of diaspore mimics for performing dispersal experiments by releasing cubes and achenes of Helianthus annuus and compare their dispersal patterns in the free-flowing Vindel River. We used the cubes to identify 50-m long sections along the river with different trapping capacity, i.e. the number of stranded diaspore mimics within a 50-m section. We then related the number of stranded diaspore mimics to the vascular plant flora, the proportions of species with long or short floating times (i.e. more than or less than 2 days, respectively), the number of seedlings, and to environmental variables within the sections. We also released wooden cubes in a run-of-river impoundment to determine the dispersal capacity of diaspores in a regulated river. Results The cubes were useful as diaspore mimics. They dispersed similarly to achenes of H. annuus. The stranding pattern of diaspore mimics was significantly associated with water current. Species richness of vascular plants per 50-m section increased with the number of stranded mimics. Seedling recruitment, and the proportions of species with short-floating and long-floating diaspores, did not vary with the number of stranded mimics. The ability of a river to transport diaspores downstream was strongly reduced by impoundment. Main conclusions We conclude that patterns of species richness of riparian vegetation is in part determined by the ability of the riverbank to trap waterborne diaspores, but differences in floating ability among species did not affect the species composition along free-flowing rivers.

  • 4. Barsoum, N
    et al.
    Nilsson, Christer
    Holm, Svante
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Lakes as potential barriers to gene flow in the hydrochorus plant species Angelica archangelica L. subsp. archangelicaManuscript (Other academic)
  • 5.
    Bång, Åsa
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Holm, Svante
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    Control of plant species diversity in riparian corridors: Konferens OIKOS 20022002Conference paper (Other academic)
    Abstract [en]

    previous study has shown that plant communities differ floristically between impoundments but remain similar within impoundments in a regulated river. The same pattern did not occur among plant species in a free-flowing river which instead showed a continuous change in species composition from the headwaters to the coast. This strongly suggests that the dispersal of plants is obstructed when dams are built on rivers. The present project will specifically study whether dams also affect the genetic diversity among and between populations of plant species. Filipendula ulmaria will be used as a first test species. A second question is whether tributaries have any impact on the distribution of genetic diversity of plant species within impoundments. A third question deals with the impact of seed and pollen dispersal on the genetic diversity among and between plant species populations. The project combines field sampling, field experiments and laboratory methods such as isoenzymes, microsatellites and other DNA-methods.

  • 6.
    Bång, Åsa
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Jägbrant, Veronica
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    Holm, Svante
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Differences in spatial genetic structure of riparian Filipendula ulmaria between an within river catchments in northern SwedenManuscript (preprint) (Other academic)
  • 7.
    Bång, Åsa
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    Holm, Svante
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    The potential role of tributaries as seed sources to an impoundment in northern Sweden: a field experiment with seed mimics2007In: Rivers Research and Applications: an international journal devoted to river research and management, ISSN 1535-1459, E-ISSN 1535-1467, Vol. 23, no 10, p. 1049-1057Article in journal (Refereed)
    Abstract [en]

    Fragmentation and flow regulation of rivers by large dams are known to obstruct the longitudinal dispersal of waterborne plant propagules between impoundments, and to affect plant community composition. However, even several decades after a dam has been built, impoundments may still have a relatively species-rich riparian flora. We hypothesized that free-flowing tributaries act as the major gene pools for such impoundments, thus alleviating the fragmenting effect large dams have on the main channel. The importance of tributaries as seed sources was tested by releasing wooden seed mimics in three different-sized (0.22-6.93 m3 s-1) tributaries of an impoundment in the Ume River in northern Sweden. In each tributary seed mimics were released, during the spring flood peak, from three points approximately 1, 2 and 3 km upstream the outlet in the impoundment. The importance of a tributary as a seed source increased with tributary size. Of the 9000 released seed mimics 1.5 % reached the impoundment; 1.2 % of the 9000 originated from the largest tributary and 0.3 % from the middle-sized one. The smallest tributary retained all its mimics.

  • 8.
    Bång, Åsa
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    Holm, Svante
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    What is the role of tributaries as seed sources for a run-of-river impoundment: a field experiment: Konferens SISORL 20042004Conference paper (Other academic)
    Abstract [en]

    The reservoirs in impounded rivers are blocked by dams, implying that plant seeds, especially waterborne ones, have trouble dispersing between reservoirs. We examined whether the tributaries have an important role in the dispersal of seeds to a specific reservoir. We did this by releasing small, coloured wooden cubes in three tributaries of the Stensele reservoir in northern Sweden, one small, one middle-sized and one large-sized. This work was carried out in spring during major flood. At the day of cube release, we counted the number of cubes that dispersed from the tributaries out into the reservoir. In summer, we collected the cubes that had stranded on the banks of the tributaries to learn how far they had spread within each stream. Our examinations showed that a big stream disperses most seeds the longest distance. The smaller the stream, the shorter the distance the seeds were spread, and the fewer the seeds that reached the reservoir. We concluded that large tributaries have an important role in dispersing plants to reservoirs of impounded rivers.

  • 9.
    Dahlström, Niklas
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Jönsson, Karin
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    Long-term dynamics of large woody debris in a managed boreal forest stream.2005In: Forest Ecology and Management, ISSN 0378-1127, E-ISSN 1872-7042, Vol. 210, no 1-3, p. 363-373Article in journal (Refereed)
    Abstract [en]

    Little is known about how past forest management in Sweden influenced the quantity and quality of large woody debris (LWD) in streams. The present study provides information of the long-term dynamics of LWD in a reach of a boreal stream intersecting a managed forest. Dendrochronological methods were used to reconstruct mortality years of the pieces of LWD and the general history of fire and cuttings of the surrounding riparian forest. Today, spruce dominates among the living trees, whereas the LWD is dominated by birch in the forest and by pine in the stream. Fire frequency prior to active fire suppression was similar to values reported from boreal forests. Pine trees were more abundant in the riparian forest before selective logging operations and active fire suppression began in the 1800s. Many of the pieces of LWD found in the stream today died more than 200 years ago and derived from a cohort of pines that generated in the early 1600s. Pine LWD in stream channels is highly resistant to decomposition and can reside for more than 300 years. A substantial amount of the LWD found today in managed forest streams in boreal Sweden most likely derives from the time before extensive human influence and is likely to decrease further in the future. Management of riparian forests to ascertain future supply of long-lived LWD in streams should target to increase the proportion of pine trees.

  • 10.
    Dahlström, Niklas
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Influence of Woody Debris on Channel Structure in Old Growth and Managed Forest Streams in Central Sweden2004In: Environmental Management, ISSN 0364-152X, E-ISSN 1432-1009, Vol. 33, no 3, p. 376-384Article in journal (Refereed)
    Abstract [en]

    Anecdotal information suggests that woody debris have had an important channel-forming role in Swedish streams and rivers, but there are few data to support this view. We identified 10 streams within near-natural and 10 streams within managed forest landscapes in central Sweden, and quantified their channel characteristics and content of woody debris. All pieces of woody debris greater than 0.5 m in length and greater than 0.05 m in base diameter were included. The near-natural forests were situated in reserves protected from forest cutting, whereas the managed forests had previously faced intensive logging in the area adjacent to the stream, The two sets of streams did not differ in general abiotic characteristics such as width, slope, or boulder cover, but the number of wood pieces was twice as high and the wood volume almost four times as high in the near-natural streams. This difference resulted in a higher frequency of debris dams in the near-natural streams. Although the total pool area did not differ between the two sets of streams, the wood-formed pools were larger and deeper, and potentially ecologically more important than other pools. In contrast to what has been believed so far, woody debris can be a channel-forming agent also in steeper streams with boulder beds. In a step-wise multiple regression analysis, pool area was positively and most strongly related to the quantity of woody debris, whereas channel gradient and wood volume were negatively related. The frequency of debris dams increased with the number of pieces of woody debris, but was not affected by other variables. The management implications of this study are that the wood quantity in streams in managed forests would need to be increased if management of streams will target more pristine conditions.

  • 11.
    Dahlström, Niklas
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    The dynamics of coarse woody debris in boreal Swedish forests are similar between stream channels and adjacent riparian forests2006In: Canadian Journal of Forest Research, ISSN 0045-5067, E-ISSN 1208-6037, Vol. 36, no 5, p. 1139-1148Article in journal (Refereed)
    Abstract [en]

    Although numerous studies have focused on the dynamics of coarse woody debris (CWD) in boreal Fennoscandian forests, information on CWD in streams remains limited. To achieve a better understanding of CWD dynamics in streams we compared amounts and characteristics of CWD between streams and adjacent riparian forests in old-growth and managed forest sites, respectively. We also identified distances to the sources of CWD and evaluated these in relation to the lateral zonation of riparian trees. CWD volumes found in the stream channels were related to, but exceeded, the volumes found in the adjacent forest. In-channel volumes separated by species were better correlated with terrestrial volumes of CWD than with volumes of living trees. Tree species appeared to be zoned across the riparian zone, with slightly higher abundances of deciduous trees and lower abundances of Scots pine trees close to the stream. Similar to upland forests, riparian forests were dominated by coniferous tree species, mainly Norway spruce (Picea abies (L.) Karst.). These findings suggest large similarities in CWD input between streams and riparian forests and substantially slower decomposition rates in stream channels compared with those in riparian forest. The results provide an improved basis for creating reliable models of CWD supply and maintenance in streams based on knowledge of forest development and CWD dynamics in the terrestrial environment. Site productivity could potentially be used to predict CWD volumes in stream channels under pristine conditions.

  • 12. Dynesius, Mats
    et al.
    Jansson, Roland
    Johansson, Mats E
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Intercontinental similarities in riparian-plant diversity and sensitivity to river regulation.2004In: Ecological Applications, ISSN 1051-0761, E-ISSN 1939-5582, Vol. 14, no 1, p. 173-191Article in journal (Refereed)
    Abstract [en]

    We asked whether assemblages of species with separate evolutionary histories differed in their response to similar human interventions. We assessed this by comparing 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 Sampling 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.

  • 13. Hylander, K
    et al.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Gothner, T
    Effects of buffer-strip retention and clearcutting on land snails in boreal riparian forests2004In: Conservation Biology, ISSN 0888-8892, E-ISSN 1523-1739, Vol. 18, no 4, p. 1052-1062Article in journal (Refereed)
    Abstract [en]

    We investigated the short-term effects of forest clearcutting on land snails (terrestrial gastropods) in 15 forest stands along small streams in Sweden. Two different silvicultural treatments were applied at each site: clearcutting across the stream channel and buffer strips 10 m wide on each side of the stream. Additionally, we studied 10 reference sites in unlogged riparian forests along similar-sized streams. All sites were studied before logging and then 2.5 years after logging. After clearcutting the number of individuals in a 0.5-m(2) sample from each site decreased on average from 107 to 87, and the mean number of species per sample decreased from 9.9 to 7.7. Most species were negatively affected, but there were also clear differences in sensitivity. There were correlations between species survival and ground moisture. At the wettest clearcut sites with an almost intact bryophyte cover, the land snails were unaffected by clearcutting. This result suggests that wet or moist forest floors can serve as refugia even at very small spatial scales (e.g., shallow hollows, crevices). If this is an important mechanism, the spatial distribution of small habitats could be important for the long-term survival of the snail fauna or other small, dispersal-limited organisms at clearcut sites. In the buffer strips, the number of individuals decreased but not the number of species, indicating that buffer-strip retention is a good practice for protecting land snails in riparian forests. The varying effectiveness of the buffer strip could partly be explained by the proportion of the remaining basal area, emphasizing that buffer strips could be even more effective if efforts are made to avoid heavy damage by windthrows.

  • 14. Hylander, Kristoffer
    et al.
    Jonsson, Bengt Gunnar
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Evaluating Buffer Strips Along Boreal Streams Using Bryophytes as indicators2002In: Ecological Applications, ISSN 1051-0761, Vol. 12, no 3, p. 797-806Article in journal (Refereed)
  • 15. Hylander, Kristoffer
    et al.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Dynesius, Mats
    Jonsson, Bengt-Gunnar
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Substrate form determines the fate of bryophytes in clear-cuts and buffer strips along small boreal streams2005In: Ecological Applications, ISSN 1051-0761, Vol. 15, no 2, p. 674-688Article in journal (Refereed)
    Abstract [en]

    Studies on the effectiveness of forest buffer strips left along streams after logging have long overlooked the biota of the buffers themselves, despite their high species richness. We investigated mosses and liverworts (bryophytes), abundant and species-rich groups in boreal forests, before and 2.5 years after logging along 15 small streams. In each Site, two 1000-m(2) (20 X 50 m) plots along the stream were inventoried; one plot in an area subjected to clear-cutting, and another in a buffer strip (10 m wide on each side of the stream). Ten plots along small streams in forest reserves in the same region were used as references. We found that less than half as many bryophyte species per plot disappeared after logging in the buffer strips compared to the clearcuts. The changes in bryophyte cover and in species composition were smaller in the buffer strips, and the species that were negatively affected in the clearcuts were less affected in the buffer strips. However, there was a significant change in species composition in the buffer strips compared to the references. Substrate form. and taxonomic group were important factors in understanding this turnover. Many species growing on substrates with a convex form (e.g., logs, tree bases, and mesic ground) decreased or disappeared, while species on concave substrates were rather unaffected. This held for both mosses and liverworts, although liverworts were generally more sensitive than mosses. The difference in response of assemblages on convex vs. concave substrates makes changes in microclimate due to logging a likely explanation. The species in most need of protection (i.e., the red-listed species) were among the ones with strongest declines in the 20 m wide buffer strips. In order to function optimally for bryophyte conservation, forests along small streams need to be protected from high wind-throw frequency and strong edge effects. Increasing the width of buffer strips at sites with known or potential values (e.g., large amounts of woody debris or boulders) should be considered a better strategy than using narrow buffer strips with a fixed width.

  • 16. Hylander, Kristoffer
    et al.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Jonsson, Bengt-Gunnar
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Göthner, Tove
    Differences in habitat quality explain nestedness in a land snail meta-community2005In: Oikos, ISSN 0030-1299, Vol. 108, no 2, p. 351-361Article in journal (Refereed)
    Abstract [en]

    We set up two alternative hypotheses on how environmental variables could foster nestedness; one of "nested habitats" and another of "nested habitat quality". The former hypothesis refers to situations where the nestedness of species depends on a nestedness of discrete habitats. The latter considers situations where all species in an assemblage increase in abundance along the same environmental gradient, but differ in specialisation or tolerance. We tested whether litter-dwelling land snails (terrestrial gastropods) in boreal riparian forest exhibited a nested community structure, whether such a pattern was related to differences in environmental variables among sites, and which of the two hypotheses that best could account for the found pattern. We sampled litter from 100 m(2) plots in 29 mature riparian forest sites along small streams in the boreal zone of Sweden. The number of snail species varied between 3 and 14 per site. Ranking the species-by-site matrix by PCA scores of the first ordination axis revealed a similarly significant nested pattern as when the matrix was sorted by number of species, showing that the species composition in this meta-community can be properly described as nested. Several environmental variables, most notably pH index, were correlated with the first PCA axis. All but two species had positive eigenvectors in the PCA ordination and the abundance increased considerably along the gradient for most of the species implying that the hypothesis of "nested habitats" was rejected in favour of the "nested habitat quality" hypothesis. Analyses of nestedness have seldom been performed on equal sized plots, and our study shows the importance of understanding that variation in environmental variables among sites can result in nested communities. The conservation implications are different depending on which of our two hypotheses is supported; a conservation focus on species "hotspots" is more appropriate if the communities are nested because of "nested habitat quality".

  • 17. Jansson, R.
    et al.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Dynesius, M.
    Andersson, E.
    Effects of river regulation on river-margin vegetation: A comparison of eight boreal rivers2000In: Ecological Applications, ISSN 1051-0761, E-ISSN 1939-5582, Vol. 10, no 1, p. 203-224Article in journal (Refereed)
    Abstract [en]

    Regulation and fragmentation by dams belong to the most widespread deliberate impacts of humans on the world's rivers, especially in the Northern Hemisphere. We evaluated the effects of hydroelectric development by comparing the flora of vascular plants in 200-m-long reaches of river margin distributed along eight entire rivers in northern Sweden. Four of these rivers were free-flowing, and four were strongly regulated for hydroelectric purposes. First, we compared species diversity per site between entire free-flowing and regulated rivers. To reduce the effects of natural, between-river variation, we compared adjacent rivers. One regulated river had lower plant species richness and cover than two adjacent free-flowing ones, whereas two other parallel rivers, one regulated and another free-flowing, did not differ significantly. Second, river-margin vegetation responded differently to different types of regulated water-level regimes. Both along run-of-river impoundments, with small but daily water-level fluctuations, and along storage reservoirs, with large fluctuations between low water levels in spring and high levels in late summer and fall, the number of species and their cover per site were lower than along the free-flowing rivers. Regulated but unimpounded reaches were most similar to free-flowing rivers, having lower plant cover per site, but similar numbers of species. For reaches with reduced discharge, evidence was mixed; some variables were lower compared to free-flowing rivers whereas others were not. However, for the last two types of regulation, statistical power was low due to small sample sizes. Third, we classified all plant species according to their dispersal mechanisms and tested whether they respond differently to different types of regulated water-level regimes. Three out of four types of regulation had higher proportions of wind-dispersed species, and two out of four had lower proportions of species without specific mechanisms for dispersal, compared to free-flowing rivers, suggesting that dispersal ability is critical for persistence following regulation. Run-of-river impoundments had higher proportions of long-floating species and species with mechanisms for vegetative dispersal, suggesting that water dispersal may still be important despite fragmentation by dams. Fourth, plant species richness and cover varied with both local factors, such as water-level regime, and regional factors, such as length of the growing season. Presence of clay and silt in the river-margin soil, preregulation position of the contemporary river margin, non-reservoir sites, low altitudes, and long growing seasons were associated with high plant species richness and cover.

  • 18.
    Jansson, R.
    et al.
    Umeå University.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences. Umeå University.
    Renöfält, Birgitta
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences. Umeå University.
    Fragmentation of riparian floras in rivers with multiple dams2000In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 81, no 4, p. 899-903Article in journal (Refereed)
    Abstract [en]

    Rivers are increasingly fragmented by dams, resulting in disruption of natural dispersal pathways and subsequent changes of riverine communities. We assessed the effect of dams as barriers to plant dispersal along rivers by comparing the flora of vascular plants between pairs of run-of-river impoundments in northern Sweden. Adjacent impoundments in similar environmental settings develop different riparian floras because species with poor floating capacity become unevenly distributed among impoundments. Such discontinuities were not found along a free-flowing river, suggesting effective dispersal of riparian plants in the absence of dams. Given that dams regulate most of the world's rivers, floristic disruptions of riparian corridors may be a global phenomenon. The extensive fragmentation of other ecosystems may have caused similar obstructions to organism dispersal, with subsequent changes in species composition.

  • 19. Johansson, M. E.
    et al.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Responses of riparian plants to flooding in free-flowing and regulated boreal rivers: an experimental study2002In: Journal of Applied Ecology, ISSN 0021-8901, E-ISSN 1365-2664, Vol. 39, no 6, p. 971-986Article in journal (Refereed)
    Abstract [en]

    1 The long history of river regulation has resulted in extensively changed ecosystem structures and processes in rivers and their associated environments. This fact, together with changing climatic and hydrological conditions, has increased the need to recover the natural functions of rivers. To develop guidelines for river restoration, comparative ecological experiments at contrasting water-level regimes are needed. We compared growth and survival of transplanted individuals of four riparian plant species (Betula pubescens, Carex acuta, Filipendula ulmaria and Leontodon autumnalis) over 2 years on four free-flowing and four regulated riverbank sites in northern Sweden. The species were chosen as representatives of dominating life-forms and species traits on different elevations of the riverbanks.

    • 2 In Betula and Filipendula, mean proportional growth rates were significantly higher at free-flowing sites than at regulated sites, whereas no consistent differences between free-flowing and regulated sites were found in Carex and Leontodon. Differences among species were generally in accordance with natural distribution patterns along riverbank elevation gradients and with experimental evidence on flooding tolerance, although plants of all species survived and even showed positive growth rates on elevations below their natural range of occurrence.
    • 3 Partial least squares regression was used to relate plant performance (growth and survival) to duration, frequency and timing of flooding at the different sites. Flood duration and frequency typically reduced performance in all species and during all time periods, although to various degrees. Flood events early in the experiment determined the outcome to a high degree at all sites. Variables indicating a regulated regime were mostly negatively related to plant performance, whereas free-flowing regime variables were positively related to plant performance.
    • 4 We used two of the regression models generated from our data with an acceptably high predictive power to simulate a hypothetical re-regulation scenario in run-of-river impoundments. With an overall reduction in flooding duration and frequency of 50–75%, plant performance of Filipendula at low riverbank elevations showed predicted increases of about 20–30%, levelling off to zero at the highest elevations. Reductions in summer floods represented about one-third to half of this increase.
    • 5 We conclude that for a range of species individual plant performance is clearly reduced on banks of impoundments and storage reservoirs due to changes in the water-level regime. Furthermore, our model simulation suggests that rather substantial reductions of flood duration and frequency are needed to improve plant performance on riverbanks upstream from dams in impounded rivers. River restoration principles should, however, be based on a combination of experimental data on plant performance of individual species and observed long-term changes in plant communities of regulated rivers. Consequently, successful re-regulation schemes in boreal rivers should include both reductions of summer and winter floods as well as re-introduced spring floods.
  • 20.
    Jonsson, Karin
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences, Engineering and Mathematics.
    Nilsson, Christer
    Scots Pine (pinus sylvestris L.) on Shingle Fields: A Dendrochronologic Reconstruction of Early Summer Precipitation in Mideast Sweden2009In: Journal of Climate, ISSN 0894-8755, E-ISSN 1520-0442, Vol. 22, no 17, p. 4710-4722Article in journal (Refereed)
    Abstract [en]

    Scots pine (Pinus sylvestris L.) trees growing on shingle fields offer a unique possibility to reconstruct precipitation and study climate variability in the fairly humid eastern part of central Sweden. Tree-ring characteristics were compared with monthly (1890-2001) and daily (1961-2001) climate data from an adjacent meteorological station. Chronologies for latewood (LW), earlywood (EW), and tree-ring widths (RW) were constructed from 73 living and dead trees. Correlation analyses show that tree growth is most sensitive to early summer precipitation. EW shows the strongest correlation with precipitation in May and June while LW is best correlated with June and July precipitation. A reconstruction model for May-June precipitation was calculated using principal component analysis (PCA) regression (regular regression) including EW, LW, and RW for present and previous years. The model explained 46% of the variation in May-June precipitation and allowed a reconstruction back to 1560. Information about wet and dry years was collected from historical documents and was used to validate the result. Periods with precipitation above and below the mean show agreement with previous reconstructions of spring precipitation from tree rings in Finland and of spring floods from estuary sediments in the region. Analyses of correlations between meteorological stations and reconstructed precipitation show that the model is valid for the coastal part of central Sweden. The authors conclude that Scots pine trees on shingle fields are well suited for precipitation reconstruction, and the separate analyses of LW and EW improve the reconstruction.

  • 21. Malm Renöfält, Birgitta
    et al.
    Jansson, Roland
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Spatial patterns of plant invasiveness in a riparian corridor2005In: Landscape Ecology, ISSN 0921-2973, E-ISSN 1572-9761, Vol. 20, no 2, p. 165-176Article in journal (Refereed)
    Abstract [en]

    Analysis of landscape-scale patterns of plant invasiveness can assist in interpreting spatial patterns of plant species richness. We investigated downstream variation in plant invasiveness in the riparian corridor of the free-flowing Vindel River in northern Sweden by introducing seeds of an alien species, Helianthus annuus, in 0.25 m(2) plots of natural vegetation from mountain headwaters to the coast and found a significant downstream pattern with middle reaches having the highest invasiveness. We related invasiveness to species richness, both on a reach scale (200-m long stretches of riverbank encompassing the experimental plots) and on the scale of experimental plots. We found no significant correlation between plant invasiveness and species richness, neither at the reach nor at the plot scale. The number of available soil substrates shows a significant positive quadratic relationship with location along the river and substrate fineness shows a near significant negative quadratic relationship with location along the river, with middle reaches having coarser substrates. Several studies have shown that plant species richness in riparian corridors often exhibits a quadratic pattern with highest species richness in the middle reaches of a river, similar to the pattern we found for invasiveness. Although species richness per se might not be a primary factor for invasibility, the same habitat conditions as those supporting plant species richness, can help in explaining large-scale patterns of plant invasion in riparian zones.

  • 22. Naiman, R.J.
    et al.
    Bunn, S.E.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Petts, G.E.
    Pinay, G.
    Thompson, L.C.
    Legitimizing fluvial ecosystems as users of water: an overview2002In: Environmental Management, ISSN 0364-152X, Vol. 30, no 4, p. 455-467Article in journal (Refereed)
  • 23.
    Nilsson, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Andersson, Elisabet
    Merritt, David
    Johansson, Mats
    Differences in riparian flora between riverbanks and river lakeshores explained by dispersal traits2002In: Ecology, ISSN 0012-9658, Vol. 83, no 10, p. 2878-2887Article in journal (Refereed)
  • 24.
    Nilsson, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Berggren, K.
    Umeå University.
    Alterations of riparian ecosystems caused by river regulation2000In: BioScience, ISSN 0006-3568, E-ISSN 1525-3244, Vol. 50, no 9, p. 783-792Article in journal (Refereed)
  • 25.
    Nilsson, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Bång, Åsa
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Holm, Svante
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Control of plant species diversity in riparian corridors: Konferens ESEB 20032003Conference paper (Other academic)
    Abstract [en]

    previous study has shown that plant communities differ floristically between impoundments but remain similar within impoundments in a regulated river. The same pattern did not occur among plant species in a free-flowing river which instead showed a continuous change in species composition from the headwaters to the coast. This strongly suggests that the dispersal of plants is obstructed when dams are built on rivers. The present project will specifically study whether dams also affect the genetic diversity among and between populations of plant species. Filipendula ulmaria will be used as a first test species. A second question is whether tributaries have any impact on the distribution of genetic diversity of plant species within impoundments. A third question deals with the impact of seed and pollen dispersal on the genetic diversity among and between plant species populations. The project combines field sampling, field experiments and laboratory methods such as isoenzymes, microsatellites and other DNA-methods.

  • 26.
    Nilsson, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Pizzuto, J. E.
    Moglen, G. E.
    Palmer, M. A.
    Stanley, E. H.
    Bockstael, N. E.
    Thompson, L. C.
    Ecological Forecasting and the Urbanization of Stream Ecosystems: Challenges for Economists, Hydrologists, Geomorphologists, and Ecologists2003In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 6, no 7, p. 659-674Article in journal (Refereed)
    Abstract [en]

    The quantity and quality of freshwater resources are now being seriously threatened, partly as a result of extensive worldwide changes in land use, and scientists are often called upon by policy makers and managers to predict the ecological consequences that these alterations will have for stream ecosystems. The effects of the urbanization of stream ecosystems in the United States over the next 20 years are of particular concern. To address this issue, we present a multidisciplinary research agenda designed to improve our forecasting of the effects of land-use change on stream ecosystems. Currently, there are gaps in both our knowledge and the data that make it difficult to link the disparate models used by economists, hydrologists, geomorphologists, and ecologists. We identify a number of points that practitioners in each discipline were not comfortable compromising on-for example, by assuming an average condition for a given variable. We provide five instructive examples of the limitations to our ability to forecast the fate of stream and riverine ecosystems one drawn from each modeling step: (a) Accurate economic methods to forecast land-use changes over long periods (such as 20 years) are not available, especially not at spatially explicit scales; (b) geographic data are not always available at the appropriate resolution and are not always organized in categories that are hydrologically, ecologically, or economically meaningful; (c) the relationship between low flows and land use is sometimes hard to establish in anthropogenically affected catchments; (d) bed mobility, suspended sediment load, and channel form-all of which are important for ecological communities in streams-are difficult to predict; and (e) species distributions in rivers are not well documented, and the data that do exist are not always publicly available or have not been sampled at accurate scales, making it difficult to model ecological responses to specified levels of environmental change. Meeting these challenges will require both interdisciplinary cooperation and a reviewed commitment to intradisciplinary research in the fields of economics, geography, quantitative spatial analysis, hydrology, geomorphology, and ecology.

  • 27.
    Nilsson, Christer
    et al.
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Svedmark, M.
    Basic principles and ecological consequences of changing water regimes: riparian plant communities2002In: Environmental Management, ISSN 0364-152X, Vol. 30, no 4, p. 468-480Article in journal (Refereed)
    Abstract [en]

    Recent research has emphasized the importance of riparian ecosystems as centers of biodiversity and links between terrestrial and aquatic systems. Riparian ecosystems also belong among the environments that are most disturbed by humans and are in need of restoration to maintain biodiversity and ecological integrity. To facilitate the completion of this task, researchers have an important function to communicate their knowledge to policy-makers and managers. This article presents some fundamental qualities of riparian systems, articulated as three basic principles. The basic principles proposed are: (1) The flow regime determines the successional evolution of riparian plant communities and ecological processes. (2) The riparian corridor serves as a pathway for redistribution of organic and inorganic material that influences plant communities along rivers. (3) The riparian system is a transition zone between land and water ecosystems and is disproportionately plant species-rich when compared to surrounding ecosystems. Translating these principles into management directives requires more information about how much water a river needs and when and how, i.e., flow variables described by magnitude, frequency, timing, duration, and rate of change. It also requires information about how various groups of organisms are affected by habitat fragmentation, especially in terms of their dispersal. Finally, it requires information about how effects of hydrologic alterations vary between different types of riparian systems and with the location within the watershed.

  • 28. Strayer, D.L.
    et al.
    Beighley, R.E.
    Thompson, L.C.
    Brooks, S.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Pinay, G.
    Naiman, R.J.
    Effects of Land Cover on Stream Ecosystems: Roles of Empirical Models and Scaling Issues2003In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629, Vol. 6, no 5, p. 407-423Article in journal (Refereed)
    Abstract [en]

    We built ernpirical models to estimate the effects of land cover on stream ecosystems in the mid-Adantic region (USA) and to evaluate the spatial scales over which such models are most effective. Predictive variables included land cover in the watershed, in the streamside corridor, and near the study site, and the number and location of dams and point sources in the watershed. Response variables were annual nitrate flux; species richness of fish, benthic macroinvertebrates, and aquatic plants; and cover of aquatic plants and riparian vegetation. All data were taken from publicly available databases, mostly over the Internet. Land cover was significantly correlated with all ecological response variables. Modeled R-2 ranged from 0.07 to 0.5, but large data sets often allowed us to estimate with acceptable precision the regression coefficients that express the change in ecological conditions associated with a unit change in land cover. Dam and pointsource variables were ineffective at predicting ecological conditions in streams and rivers, probably because of inadequacies in the data sets. The spatial perspective (whole watershed, streamside corridor, or local) most effective at predicting ecological response variables varied across response variables, apparently in concord with the mechanisms that control each of these variables. We found some evidence that predictive power fell in very small watersheds (less than 1-10 km(2)), Suggesting that the spatial arrangement of landscape patches may become critical at these small scales. Empirical models can replace, constrain, or be combined with more mechanistic models to understand the effects of land-cover change on stream ecosystems.

  • 29. Xiong, Shaojun
    et al.
    Johansson, Mats
    Hughes, Francine
    Hayes, Adrian
    Richards, Keith
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Interactive effects of soil moisture, vegetation canopy, plant litter and seed addition on plant diversity in a wetland community2003In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 91, no 6, p. 976-986Article in journal (Refereed)
    Abstract [en]

    1 We carried out a factorial experiment to examine how groundwater availability (low and high sites with intermediate or rare flooding), vegetation canopy, leaf litter and seed availability interacted to determine the species richness of a productive wet grassland community in Wicken Fen National Nature Reserve, Cambridgeshire, UK. Seeds of 18 species were added to half the plots in each of eight combinations of elevation, canopy and litter, and seedling emergence was observed for two growing seasons.2 Both individual and interactive effects on plant diversity and colonization were determined for all four examined factors. Interactive effects explained 41-63% of the total variation in both species richness and numbers of individuals growing from added seeds.3 Neither elevation nor vegetation canopy had significant individual effects on total species richness, but their interaction was significant. Litter addition limited seedling emergence at the low elevation but favoured it at the high elevation.4 The relative importance of vegetation canopy and plant litter in affecting plant community composition varied with the community parameter considered (species richness or number of seedlings), elevation and stage of vegetation development. In general, plant litter was more important in determining species richness, whereas the vegetation canopy was more important in determining seed germination and seedling emergence. Plant litter was also more important than vegetation canopy at an early stage of vegetation development and at low elevation.5 Seed availability was the most important factor in determining overall species richness in the studied community. The influence of the local seed bank was very limited. Seedling emergence and seedling species richness were generally enhanced by lower elevation and seed addition, but depressed by vegetation and litter addition.6 The complex relationships observed have considerable implications for ecological modelling and ecosystem restoration. Manipulation of one factor may produce unexpected effects on other factors, which may induce a series of consequences for the whole community. Further knowledge on how natural communities are organized and maintained is needed to guide the management of ecosystems.

  • 30.
    Xiong, Shaojun
    et al.
    Umea Univ, Landscape Ecol Grp, Dept Ecol & Environm Sci, SE-90187 Umea.
    Nilsson, Christer
    Mid Sweden University, Faculty of Science, Technology and Media, Department of Natural Sciences.
    Johansson, Mats E
    Umea Univ, Landscape Ecol Grp, Dept Ecol & Environm Sci, SE-90187 Umea.
    Jansson, Roland
    Umea Univ, Landscape Ecol Grp, Dept Ecol & Environm Sci, SE-90187 Umea.
    Responses of riparian plants to accumulation of silt and plant litter: the importance of plant traits2001In: Journal of Vegetation Science, ISSN 1100-9233, E-ISSN 1654-1103, Vol. 12, p. 481-490Article in journal (Refereed)
    Abstract [en]

    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.

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