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Breaking the ice: effects of ice formation and winter floods on vegetation along streams
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Landscape Ecology Group ; Arcum)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)Alternative title
Klimatförändringar och isbildning i vattendrag : effekter på biologisk mångfald (Swedish)
Abstract [en]

Streams in cold regions are characterized by unique hydrological processes that control flow regime and water levels. One of the most important processes is the formation, growth and melting of different types of ice in and around the stream channel during winter. River ice controls major hydrologic events such as winter floods with magnitudes and frequencies often greater than those created by open-water conditions. While river management in northern countries has already recognized high risk of ice damages, the focus of the risk assessment has been mostly aimed towards the local economy; the ecological role of river ice has been less acknowledged. Along rivers in boreal Sweden, riparian vegetation has developed specific zonation with height and age of the plants increasing the further away they are from the stream channel. On lower levels the vegetation is often comprised of short-lived plants, such as annuals and biennials whereas more permanent woody vegetation is found at higher levels. This zonation has most often been explained by the resilience of different growth forms to the inundation regimes, such as the spring flood in northern systems. Within this framework, I investigated which factors drive the ice formation and how ice and ice-induced floods affect riparian and in-stream vegetation. A 3-year survey was conducted of ice formation and vegetation along 25 stream reaches and a set of experiments were used to evaluate ice as a disturbance agent. Reaches far away from lake outlets which had a low input of groundwater and a high velocity and stream power were most prone to form anchor ice, but many other factors also influenced ice formation. Streams with anchor ice experienced more frequent flooding of the riparian vegetation during winter. Our findings suggests that ice and winter floods favour diversity and create habitat heterogeneity for riparian species. On a community level, woody plants such as evergreen dwarf shrubs are eliminated when flooded during winter, opening up patches for other species to colonize, creating a dynamic riparian understory community. Significant changes in river ice conditions could develop with projected changes in climate which would have important geomorphologic, ecological and socio-economic impacts. One implication of climate change could be less ice disturbance and consequently a riparian vegetation in cold regions that slowly changes from forb to dwarf-shrub dominated with a subsequent decrease in species richness. Changes in species diversity and abundance of groups of species related to changes in ice formation could potentially cascade into riparian and in-stream processes such as nutrient cycling, litter decomposition and organism dispersal.

Place, publisher, year, edition, pages
Umeå: Umeå universitet , 2015. , 25 p.
Keyword [en]
anchor ice, climate change, in-stream, riparian, river ice, streams, vegetation
National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-99008ISBN: 978-91-7601-182-9 (print)OAI: oai:DiVA.org:umu-99008DiVA: diva2:785120
Public defence
2015-02-27, Björken, Sveriges lantbruksuniversitet, Skogsmarksgränd 901 83, Umeå, 09:30 (English)
Opponent
Supervisors
Funder
Swedish Research Council Formas
Available from: 2015-02-06 Created: 2015-02-02 Last updated: 2017-04-03Bibliographically approved
List of papers
1. Hydrological and thermal controls of ice formation in 25 boreal stream reaches
Open this publication in new window or tab >>Hydrological and thermal controls of ice formation in 25 boreal stream reaches
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The Northern Hemisphere has a high density of fluvial freshwater ecosystems, many of which become ice-covered during winter. The formation and extent of ice have both ecological and socio-economic implications. For example, ice can cause freezing of riparian vegetation and fish eggs as well as influence hydropower production; however, when, where and why ice develops in small streams is poorly described. Data from 25 stream reaches were used to study the factors controlling ice formation during two consecutive winters. We addressed where in the catchment surface or anchor ice is most likely to develop, how stream morphology influences ice formation, and how climate influences ice processes. Reaches far away from lake outlets were most prone to form anchor ice, but many other factors also influenced ice formation. We found that anchor-ice was most common where water temperature and groundwater input were low and stream power high. The in-stream substrate was also important for the formation of anchor ice as well as the current velocity, which created turbulence and super-cooled conditions if high enough. We demonstrated that ice formation in the studied streams was complex, involving many variables, thus we constructed a conceptual model describing the likelihood of various ice types to develop, based on our large dataset. To our knowledge, this model is the first to describe the complexity of ice formation in steep boreal streams. As such it will be useful for practitioners and scientists working in small rivers in the Northern Hemisphere.

Keyword
anchor ice, boreal, channel morphology, groundwater, ice, river ice, stream
National Category
Earth and Related Environmental Sciences Oceanography, Hydrology, Water Resources
Identifiers
urn:nbn:se:umu:diva-99003 (URN)
Funder
Swedish Research Council Formas
Available from: 2015-02-02 Created: 2015-02-02 Last updated: 2015-02-04
2. The role of ice dynamics in shaping vegetation in flowing waters
Open this publication in new window or tab >>The role of ice dynamics in shaping vegetation in flowing waters
2014 (English)In: Biological Reviews, ISSN 1464-7931, E-ISSN 1469-185X, Vol. 89, no 4, 791-804 p.Article, review/survey (Refereed) Published
Abstract [en]

Ice dynamics is an important factor affecting vegetation in high-altitude and high-latitude streams and rivers. During the last few decades, knowledge about ice in streams and rivers has increased significantly and a respectable body of literature is now available. Here we review the literature on how ice dynamics influence riparian and aquatic vegetation. Traditionally, plant ecologists have focused their studies on the summer period, largely ignoring the fact that processes during winter also impact vegetation dynamics. For example, the freeze-up period in early winter may result in extensive formation of underwater ice that can restructure the channel, obstruct flow, and cause flooding and thus formation of more ice. In midwinter, slow-flowing reaches develop a surface-ice cover that accumulates snow, protecting habitats under the ice from formation of underwater ice but also reducing underwater light, thus suppressing photosynthesis. Towards the end of winter, ice breaks up and moves downstream. During this transport, ice floes can jam up and cause floods and major erosion. The magnitudes of the floods and their erosive power mainly depend on the size of the watercourse, also resulting in different degrees of disturbance to the vegetation. Vegetation responds both physically and physiologically to ice dynamics. Physical action involves the erosive force of moving ice and damage caused by ground frost, whereas physiological effects - mostly cell damage - happen as a result of plants freezing into the ice. On a community level, large magnitudes of ice dynamics seem to favour species richness, but can be detrimental for individual plants. Human impacts, such as flow regulation, channelisation, agriculturalisation and water pollution have modified ice dynamics; further changes are expected as a result of current and predicted future climate change. Human impacts and climate change can both favour and disfavour riverine vegetation dynamics. Restoration of streams and rivers may mitigate some effects of anticipated climate change on ice and vegetation dynamics by, for example, slowing down flows and increasing water depth, thus reducing the potential for massive formation of underwater ice.

Keyword
climate change, ecological restoration, ecosystem degradation, river and stream ice, vegetation
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-96808 (URN)10.1111/brv.12077 (DOI)000343998100002 ()24393609 (PubMedID)
Available from: 2014-12-11 Created: 2014-12-03 Last updated: 2017-12-05Bibliographically approved
3. Vegetation patterns in small boreal streams relate to ice and winter floods
Open this publication in new window or tab >>Vegetation patterns in small boreal streams relate to ice and winter floods
2015 (English)In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 103, no 2, 431-440 p.Article in journal (Refereed) Published
Abstract [en]

In-stream and riparian vegetation are species rich, productive and dynamic. Their patterns insmall boreal streams are largely driven by seasonal flow regimes. Traditionally, flow-related processes during the growing season, particularly the spring flood, have been seen as the most important, whereas vegetation has been viewed as being dormant and ‘less affected’ during winter.

Riparian and in-stream vegetation were inventoried during the summers 2011–2013 in eight reaches of northern Swedish streams. Along each reach, the ice formation was surveyed during winter by visual inspections and with permanently placed cameras. We then evaluated the potential effects of ice regimes and winter flooding on riparian and in-stream vegetation during 3 years by relating the abundance of winter floods caused by anchor ice to the cover, composition and biomass of vegetation.

We found that the numbers of winter floods were higher along reaches with anchor-ice formation than in reaches without. We also found that species diversity of riparian vegetation was higher inthe reaches with anchor ice. This resulted from a lower cover of riparian dwarf shrubs and a higher cover of graminoids and forbs along reaches with anchor ice. We also found a lower cover of instream algae but a higher cover of bryophytes in anchor-ice reaches. These patterns were consistent throughout the study period although there were interannual differences in temperature, water levels and ice cover.

During our study period, we encountered an average of 20 shifts per winter between freezing and thawing, while there was an average of 10 shifts per winter during 1960–1990. This indicates a warming climate in high latitudes. Higher temperatures and more shifts between freezing and thawing may initially increase ice dynamics. However, with further increases in mean temperature, ice production should eventually decrease.

Synthesis. Ice and winter floods caused by anchor ice appear to be important disturbance agents that allow less competitive species to establish along small boreal streams. If ice dynamics is reduced, the composition and production of riparian and in-stream vegetation may be changed, with possible consequences for the entire stream ecosystem.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2015
Keyword
anchor ice, bryophytes, climate change, determinants of plant community diversity and structure, disturbance, ice regime, riparian, vascular plants, vegetation, winter flooding
National Category
Ecology
Research subject
Ecological Botany; Hydrology
Identifiers
urn:nbn:se:umu:diva-98969 (URN)10.1111/1365-2745.12355 (DOI)000350549000014 ()
Funder
Swedish Research Council Formas
Available from: 2015-02-02 Created: 2015-01-30 Last updated: 2017-12-05Bibliographically approved
4. Effects of ice and floods on vegetation in streams in cold regions: implications for climate change
Open this publication in new window or tab >>Effects of ice and floods on vegetation in streams in cold regions: implications for climate change
2014 (English)In: Ecology and Evolution, ISSN 2045-7758, E-ISSN 2045-7758, Vol. 4, no 21, 4173-4184 p.Article in journal (Refereed) Published
Abstract [en]

Riparian zones support some of the most dynamic and species-rich plant communities in cold regions. A common conception among plant ecologists is that flooding during the season when plants are dormant generally has little effect on the survival and production of riparian vegetation. We show that winter floods may also be of fundamental importance for the composition of riverine vegetation. We investigated the effects of ice formation on riparian and in-stream vegetation in northern Sweden using a combination of experiments and observations in 25 reaches, spanning a gradient from ice-free to ice-rich reaches. The ice-rich reaches were characterized by high production of frazil and anchor ice. In a couple of experiments, we exposed riparian vegetation to experimentally induced winter flooding, which reduced the dominant dwarf-shrub cover and led to colonization of a species-rich forb-dominated vegetation. In another experiment, natural winter floods caused by anchor-ice formation removed plant mimics both in the in-stream and in the riparian zone, further supporting the result that anchor ice maintains dynamic plant communities. With a warmer winter climate, ice-induced winter floods may first increase in frequency because of more frequent shifts between freezing and thawing during winter, but further warming and shortening of the winter might make them less common than today. If ice-induced winter floods become reduced in number because of a warming climate, an important disturbance agent for riparian and in-stream vegetation will be removed, leading to reduced species richness in streams and rivers in cold regions. Given that such regions are expected to have more plant species in the future because of immigration from the south, the distribution of species richness among habitats can be expected to show novel patterns.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2014
Keyword
anchor ice, climate change, in-stream mosses, northern Sweden, plants, riparian vegetation, streams, winter floods
National Category
Environmental Sciences Ecology
Identifiers
urn:nbn:se:umu:diva-97221 (URN)10.1002/ece3.1283 (DOI)000344752000013 ()
Available from: 2014-12-18 Created: 2014-12-12 Last updated: 2017-12-05Bibliographically approved
5. Extreme events in streams and rivers in arctic and subarctic regions in an uncertain future
Open this publication in new window or tab >>Extreme events in streams and rivers in arctic and subarctic regions in an uncertain future
2015 (English)In: Freshwater Biology, ISSN 0046-5070, E-ISSN 1365-2427, Vol. 60, no 12, 2535-2546 p.Article in journal (Refereed) Published
Abstract [en]

We review the predicted changes in extreme events following climate change in flowing waters in arctic and subarctic regions. These regions are characterised by tundra or taiga ecosystems in either erosional or depositional glacial landforms or presently glacierised areas of the Northern Hemisphere. The ecological and geomorphic effects of extreme meteorological and hydrological events, such as episodes of strongly increased precipitation, temperatures and flows, can be exacerbated by altered base conditions. For example, winter temperature variations between frost and thaw will become more frequent at many places because mean temperature during the winter is closer to 0 °C, potentially leading to changes in the production of ice and intensified disturbance of riparian and aquatic habitats during extreme floods. Additionally, thawing of permafrost and glaciers can lead to increased bank erosion because of thaw slump and glacial outburst floods. We discuss the abiotic and biotic effects of these and other extreme events, including heavy precipitation, floods, drought and extreme air or water temperatures, and summarise our findings in a model that aims to stimulate further research in this field.

Place, publisher, year, edition, pages
Wiley-Blackwell, 2015
Keyword
climate change, extreme events, floods, permafrost, rivers
National Category
Oceanography, Hydrology, Water Resources Climate Research
Identifiers
urn:nbn:se:umu:diva-99006 (URN)10.1111/fwb.12477 (DOI)000364233800007 ()
Available from: 2015-02-02 Created: 2015-02-02 Last updated: 2017-12-05Bibliographically approved

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