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Mosses as mediators of climate change: implications for tree seedling establishment in the tundra
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Alpine and arctic tree line expansion depends on the establishment of tree seedlings above the current tree line, which is expected to occur with climate warming. However, tree lines often fail to respond to higher temperatures. Other environmental factors are therefore likely important for tree seedling establishment. Above the tree line, establishing seedlings encounter existing vegetation such as bryophytes, which often dominate in arctic and alpine tundra. Bryophytes modify their environment in various ways and may mediate climate change effects on establishing tree seedlings, and with that tree line expansion. The aim of this thesis was to understand if and how the environment, in particular bryophytes, mediates the impact of climate change on tree seedling establishment at the alpine and arctic tree line. This was explored by reviewing literature on tree seedling establishment at alpine and arctic tree lines globally. In addition, tree seedling survival and growth of Betula pubescens and Pinus sylvestris were assessed experimentally. Here, individuals were planted into mono-specific mats of different bryophytes species and exposed to warming and different precipitation regimes. The literature review revealed that besides from temperature, tree seedling establishment is affected by a wide range of abiotic and biotic factors including water, snow, nutrients, light, disturbance and surrounding vegetation. Furthermore the review revealed that for example vegetation can change tree seedling responses to climate change. The experiments showed that especially tree seedling survival was adversely affected by the presence of bryophytes and that the impacts of bryophytes were larger than those of the climate treatments. Seedling growth, on the other hand, was not hampered by the presence of bryophytes, which is in line with earlier findings that seedling survival, growth and seed germination do not respond similarly to changes in environmental conditions. Moreover, we found several indications that vegetation above the tree line, including bryophytes, mediated tree seedling responses to warming and precipitation or snow cover. This thesis shows that temperature alone should not be used to predict future tree seedling establishment above the alpine and arctic tree line and that extrapolations from climate envelope models could strongly over or under estimate tree line responses to warming. This underlines the value of multi-factorial studies for understanding the interplay between warming and other environmental factors and their effects on tree seedling establishment across current tree lines.

Place, publisher, year, edition, pages
Umeå: Umeå University , 2017. , 26 p.
Keyword [en]
arctic, alpine, B. pubescens, bryophytes, competition, facilitation, mosses, P. sylvestris, precipitation changes, seedlings, traits, tree line, warming
National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-130942ISBN: 978-91-7601-654-1 (print)OAI: oai:DiVA.org:umu-130942DiVA: diva2:1070385
Public defence
2017-02-24, Björken, SLU, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2017-02-03 Created: 2017-02-01 Last updated: 2017-02-03Bibliographically approved
List of papers
1. The struggle for life at the alpine tree line - Environmental factors acting on tree seedling establishment
Open this publication in new window or tab >>The struggle for life at the alpine tree line - Environmental factors acting on tree seedling establishment
(English)Manuscript (preprint) (Other academic)
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-130937 (URN)
Available from: 2017-02-01 Created: 2017-02-01 Last updated: 2017-02-01
2. Bryophyte traits explain climate-warming effects on tree seedling establishment
Open this publication in new window or tab >>Bryophyte traits explain climate-warming effects on tree seedling establishment
2017 (English)In: Journal of Ecology, ISSN 0022-0477, E-ISSN 1365-2745, Vol. 105, no 2, 496-506 p.Article in journal (Refereed) Published
Abstract [en]

Above the alpine tree line, bryophytes cover much of the tundra soil surface in dense, often monospecific carpets. Therefore, when climate warming enables tree seedling establishment above the tree line, interaction with the bryophyte layer is inevitable. Bryophytes are known to modify their environment in various ways. However, little is known about to which extent and by which mechanisms bryophytes affect the response of tree seedlings to climate warming.

We aimed to assess and understand the importance of bryophyte species identity and traits for tree seedling performance at tree line temperatures and their response to warmer conditions. Seedlings of two common, tree line-forming tree species (Betula pubescens and Pinus sylvestris) were planted into intact cushions of eight common tundra bryophyte species and bryophyte-free soil and grown for 18 weeks at current (7·0 °C) and near-future (30–50 years; 9·2 °C) tree line average growing-season temperatures. Seedling performance (biomass increase and N-uptake) was measured and related to bryophyte species identity and traits indicative of their impact on the environment.

Tree seedlings performed equally well or better in the presence of bryophytes than in bryophyte-free soil, which contrasts to their usually negative effects in milder climates. In addition, seedling performance and their response to higher temperatures depended on bryophyte species and seedlings of both species grew largest in the pan-boreal and subarctic bryophyte Hylocomium splendens. However, B. pubescens seedlings showed much stronger responses to higher temperatures when grown in bryophytes than in bryophyte-free soil, while the opposite was true for P. sylvestris seedlings. For B. pubescens, but not for P. sylvestris, available organic nitrogen of the bryophyte species was the trait that best predicted seedling responses to higher temperatures, likely because these seedlings had increased N-demands.

Synthesis. Climatically driven changes in bryophyte species distribution may not only have knock-on effects on vascular plant establishment, but temperature effects on seedling performance are themselves moderated by bryophytes in a species-specific way. Bryophyte traits can serve as a useful tool for understanding and predicting these complex interactions.

National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-130935 (URN)10.1111/1365-2745.12688 (DOI)000394587000019 ()
Available from: 2017-02-01 Created: 2017-02-01 Last updated: 2017-05-10Bibliographically approved
3. The impact of bryophytes in mediating tree seedling responses to winter stress: implications for the stress gradient hypothesis
Open this publication in new window or tab >>The impact of bryophytes in mediating tree seedling responses to winter stress: implications for the stress gradient hypothesis
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-130939 (URN)
Available from: 2017-02-01 Created: 2017-02-01 Last updated: 2017-02-01
4. Mosses mediate effects of warmer and wetter conditions on tree seedlings at the alpine tree line
Open this publication in new window or tab >>Mosses mediate effects of warmer and wetter conditions on tree seedlings at the alpine tree line
Show others...
(English)Manuscript (preprint) (Other academic)
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-130940 (URN)
Available from: 2017-02-01 Created: 2017-02-01 Last updated: 2017-02-01

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