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The hidden life of plants: fine root dynamics in northern ecosystems
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Climate Impacts Research Centre ; Arcum)
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Fine roots constitute a large part of the primary production in northern (arctic and boreal) ecosystems, and are key players in ecosystem fluxes of water, nutrients and carbon. Data on root dynamics are generally rare, especially so in northern ecosystems. However, those ecosystems undergo the most rapid climatic changes on the planet and a profound understanding of form, function and dynamics of roots in such ecosystems is essential.

This thesis aimed to advance our knowledge about fine root dynamics in northern ecosystems, with a focus on fine root phenology in natural plant communities and how climate change might alter it. Factors considered included thickness and duration of snow cover, thawing of permafrost, as well as natural gradients in temperature. Experiments and observational studies were located around Abisko (68°21' N, 18°45' E), and in a boreal forest close to Vindeln (64°14'N, 19°46'E), northern Sweden. Root responses included root growth, total root length, and root litter input, always involving seasonal changes therein, measured with minirhizotrons. Root biomass was also determined with destructive soil sampling. Additionally, aboveground response parameters, such as phenology and growth, and environmental parameters, such as air and soil temperatures, were assessed.

This thesis reveals that aboveground patterns or responses cannot be directly translated belowground and urges a decoupling of above- and belowground phenology in terrestrial biosphere models. Specifically, root growth occurred outside of the photosynthetically active period of tundra plants. Moreover, patterns observed in arctic and boreal ecosystems diverged from those of temperate systems, and models including root parameters may thus need specific parameterization for northern ecosystems. In addition, this thesis showed that plant communities differ in root properties, and that changes in plant community compositions can thus induce changes in root dynamics and functioning. This underlines the importance of a thorough understanding of root dynamics in different plant community types in order to understand and predict how changes in plant communities in response to climate change will translate into root dynamics. Overall, this thesis describes root dynamics in response to a variety of factors, because a deeper knowledge about root dynamics will enable a better understanding of ecosystem processes, as well as improve model prediction of how northern ecosystems will respond to climate change.

Place, publisher, year, edition, pages
Umeå: Umeå Universitet , 2016. , 24 p.
Keyword [en]
Arctic, belowground, boreal, climate change, fine roots, heath, meadow, minirhizotron, permafrost, phenology, plant community, root biomass, root growth, root litter, root production, subarctic, tundra
National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-124757ISBN: 978-91-7601-533-9OAI: oai:DiVA.org:umu-124757DiVA: diva2:954761
Public defence
2016-09-16, Björken, Sveriges Lantbruksuniversitet, Umeå, 09:00 (English)
Opponent
Supervisors
Available from: 2016-08-26 Created: 2016-08-23 Last updated: 2016-09-23Bibliographically approved
List of papers
1. The hidden season: growing season is 50% longer below than above ground along an arctic elevation gradient
Open this publication in new window or tab >>The hidden season: growing season is 50% longer below than above ground along an arctic elevation gradient
2016 (English)In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 209, no 3, 978-986 p.Article in journal (Refereed) Published
Abstract [en]

There is compelling evidence from experiments and observations that climate warming prolongs the growing season in arctic regions. Until now, the start, peak, and end of the growing season, which are used to model influences of vegetation on biogeochemical cycles, were commonly quantified using above-ground phenological data. Yet, over 80% of the plant biomass in arctic regions can be below ground, and the timing of root growth affects biogeochemical processes by influencing plant water and nutrient uptake, soil carbon input and microbial activity. We measured timing of above- and below-ground production in three plant communities along an arctic elevation gradient over two growing seasons. Below-ground production peaked later in the season and was more temporally uniform than above-ground production. Most importantly, the growing season continued c. 50% longer below than above ground. Our results strongly suggest that traditional above-ground estimates of phenology in arctic regions, including remotely sensed information, are not as complete a representation of whole-plant production intensity or duration, as studies that include root phenology. We therefore argue for explicit consideration of root phenology in studies of carbon and nutrient cycling, in terrestrial biosphere models, and scenarios of how arctic ecosystems will respond to climate warming.

Keyword
below ground, belowground, below-ground, fine roots, phenology, root growth, root production, sub-Arctic
National Category
Ecology Climate Research Physical Geography
Identifiers
urn:nbn:se:umu:diva-120667 (URN)10.1111/nph.13655 (DOI)000373378000013 ()26390239 (PubMedID)
Available from: 2016-07-28 Created: 2016-05-18 Last updated: 2016-09-23Bibliographically approved
2. Root phenology unresponsive to earlier snowmelt despite advanced aboveground phenology in two subarctic plant communities
Open this publication in new window or tab >>Root phenology unresponsive to earlier snowmelt despite advanced aboveground phenology in two subarctic plant communities
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Earlier snowmelt at high latitudes advances aboveground plant phenology, thereby affecting water, nutrient and carbon cycles. Despite the key role of fine roots in these ecosystem processes, phenological responses to earlier snowmelt have never been assessed belowground. We experimentally advanced snowmelt in two contrasting plant community types (heath and meadow) in northern Sweden and measured above- and belowground phenology (leaf-out, flowering and fine root growth). We expected earlier snowmelt to advance both above- and belowground phenology, and shrub-dominated heath to be more responsive than meadow. Snow melted on average nine days earlier in the manipulated plots than in controls, and soil temperatures were on average 0.9 °C higher during the snowmelt period of three weeks. This resulted in small advances in aboveground phenology, but contrary to our expectations, root phenology was unresponsive, with root growth generally starting before leaf-out. Both plant community types responded similarly to the snowmelt treatment, despite strong differences in dominating plant functional types, and root properties, such as root length and turnover. The lack of a response in root phenology, despite warmer soil temperatures and aboveground phenological advances, adds evidence that aboveground plant responses might not be directly translated to belowground plant responses, and that our understanding of factors driving belowground phenology is still limited, although of major importance for water, nutrient and carbon cycling.

Keyword
climate change, phenology, fine roots, snowmelt, arctic, alpine, root growth, root production
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-124754 (URN)
Available from: 2016-08-23 Created: 2016-08-23 Last updated: 2016-08-25
3. Dwelling in the deep – permafrost thawing strongly increases plant root growth and root litter input
Open this publication in new window or tab >>Dwelling in the deep – permafrost thawing strongly increases plant root growth and root litter input
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Plant roots play a key role in ecosystem carbon and nutrient cycling. Climate warming induced thawing of permafrost exposes large amounts of carbon and nitrogen at greater soil depths that hitherto have been detached from plant influences. Whether plant roots can reach and interact with these carbon and nitrogen sources upon permafrost thaw remains unknown. Here, we use a long-term permafrost thaw experiment and a short-term deep fertilization experiment in northern Sweden to assess changes in vegetation composition and root dynamics (deep nitrogen uptake, root depth distribution, root growth and phenology, root mortality and litter input) related to permafrost thaw, both in active layer and in newly thawed permafrost. We show that Eriophorum vaginatum and Rubus chamaemorus, both relatively deep-rooting species, can take up nitrogen released at depth of permafrost thaw, despite the late release time in autumn when plant activity is expected to have ceased. Also, root dynamics changed drastically after a decade of experimental permafrost thaw. Total root length, root growth and root litter input all strongly increased, not only in the active layer but also in the newly thawed permafrost, and the timing of root growth was related to the seasonality of soil thaw. These responses were driven by Eriophorum vaginatum, which differed greatly in root dynamics compared to the other species and thus worked as an ecosystem engineer. This study demonstrates that soil organic matter currently locked-up at depth in permafrost is no longer detached from plant processes upon thaw. Given the pivotal role that roots have in the carbon cycle and the importance of the large carbon stocks in arctic soils, the changes observed here have the potential to feedback onto the global climate system.

Keyword
permafrost, root phenology, fine roots, minirhizotron, belowground, permafrost thaw, root biomass, root exudation, root litter
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-124753 (URN)
Available from: 2016-08-23 Created: 2016-08-23 Last updated: 2016-08-25
4. Short-term climate change manipulation effects do not scale up to long-term legacies: effects of an absent snow cover on boreal forest plants
Open this publication in new window or tab >>Short-term climate change manipulation effects do not scale up to long-term legacies: effects of an absent snow cover on boreal forest plants
2016 (English)In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745Article in journal (Refereed) Epub ahead of print
Abstract [en]

1. Despite time-lags and nonlinearity in ecological processes, the majority of our knowledge about ecosystem responses to long-term changes in climate originates from relatively short-term experiments.

2. We utilized the longest ongoing snow removal experiment in the world and an additional set of new plots at the same location in northern Sweden to simultaneously measure the effects of longterm (11 winters) and short-term (1 winter) absence of snow cover on boreal forest understorey plants, including the effects on root growth and phenology.

3. Short-term absence of snow reduced vascular plant cover in the understorey by 42%, reduced fine root biomass by 16%, reduced shoot growth by up to 53% and induced tissue damage on two common dwarf shrubs. In the long-term manipulation, more substantial effects on understorey plant cover (92% reduced) and standing fine root biomass (39% reduced) were observed, whereas other response parameters, such as tissue damage, were observed less. Fine root growth was generally reduced, and its initiation delayed by c. 3 (short-term) to 6 weeks (long-term manipulation).

4. Synthesis. We show that one extreme winter with a reduced snow cover can already induce ecologically significant alterations. We also show that long-term changes were smaller than suggested by an extrapolation of short-term manipulation results (using a constant proportional decline). In addition, some of those negative responses, such as frost damage and shoot growth, were even absolutely stronger in the short-term compared to the long-term manipulation. This suggests adaptation or survival of only those individuals that are able to cope with these extreme winter conditions, and that the short-term manipulation alone would overpredict long-term impacts. These results highlight both the ecological importance of snow cover in this boreal forest, and the value of combining short- and long-term experiments side by side in climate change research.

Keyword
minirhizotron, Norway spruce, Picea abies, plant–climate interactions, root phenology, snow removal, soil frost, understory, Vaccinium
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-124756 (URN)10.1111/1365-2745.12636 (DOI)
Available from: 2016-08-23 Created: 2016-08-23 Last updated: 2016-09-23

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