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Peatland Bryophytes in a Changing Environment: Ecophysiological Traits and Ecosystem Function
Uppsala University, Disciplinary Domain of Science and Technology, Biology, Department of Ecology and Genetics, Plant Ecology and Evolution.
2012 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Peatlands are peat forming ecosystems in which not fully decomposed plant material builds up the soil. The sequestration of carbon into peat is mainly associated with the bryophyte genus Sphagnum (peat mosses), which dominate and literally form most peatlands. The responses of Sphagnum to environmental change help us to understand peatland development and function and to predict future changes in a rapidly changing world. In this thesis, the overarching aim was to use ecophysiological traits to investigate mechanisms behind the response of Sphagnum to elevated N deposition, and, processes connected to ecosystem shift and ecosystem function of peatlands.

Regarding elevated N deposition, three experiments were performed at different scales (country-wide to greenhouse). Independent of scale and species, apical tissue N concentration increased with increasing N input until N saturation was reached. Maximum photosynthetic rate, a trait evaluating photosynthetic capacity, increased with N input and could be well predicted by tissue N concentration. Thus, the physiological responses of Sphagnum to N deposition are often positive and I found no evidence of toxic effects. Production did, however, not increase with N input, and results of the N:P ratio suggested that P limitation, and possibly other elements, might hamper growth under high N input. The effect of P limitation was, in contrast to current view, most pronounced in fast growing species indicating species specific responses to nutrient imbalance.

I explored the puzzling, but historically frequently occurring, rich fen to bog ecosystem shift; a shift from a species-rich ecosystem dominated by brown mosses, to a species-poor one with greater carbon storage that is Sphagnum-dominated. The bog-dwelling species of Sphagnum grew well, to our surprise, when in contact with rich fen water but was not a strong competitor compared to rich fen Sphagnum species. If submerged under rich fen water (high pH), the bog Sphagnum species died while rich fen species of Sphagnum were unaffected. These results show that differences in two physiological traits (growth rate and tolerance to flooding) among species, can explain when a peatland ecosystem shift might occur.

In the last study, the function of peatlands was related to trade-offs between traits and allometric scaling in Sphagnum. Results suggested that growth strategies are determined by the distribution of Sphagnum relative to the water table in order to minimize periods with suboptimal hydration. Allometric analyses stressed the importance of resource allocation among and within shoots (apical part vs. stem), although the allocation patterns in Sphagnum were not always consistent with those of vascular plants. Interestingly, data indicated a trade-off between photosynthetic rate and decomposition rate among Sphagnum species.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2012. , 39 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 889
Keyword [en]
allometric scaling, chlorophyll fluorescence, competition, decomposition, flooding, mire, N concentration, nitrogen deposition, photosynthesis, succession, stoichiometry
National Category
Ecology
Research subject
Ecological Botany
Identifiers
URN: urn:nbn:se:uu:diva-165138ISBN: 978-91-554-8249-7 (print)OAI: oai:DiVA.org:uu-165138DiVA: diva2:471974
Public defence
2012-02-17, Zootissalen, EBC, Villavägen 9, Uppsala, 10:00 (English)
Opponent
Supervisors
Available from: 2012-01-27 Created: 2012-01-03 Last updated: 2012-02-15Bibliographically approved
List of papers
1. Photosynthetic performance in Sphagnum transplanted along a latitudinal nitrogen deposition gradient
Open this publication in new window or tab >>Photosynthetic performance in Sphagnum transplanted along a latitudinal nitrogen deposition gradient
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2009 (English)In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 159, no 4, 705-715 p.Article in journal (Refereed) Published
Abstract [en]

Increased N deposition in Europe has affected mire ecosystems. However, knowledge on the physiological responses is poor. We measured photosynthetic responses to increasing N deposition in two peatmoss species (Sphagnum balticum and Sphagnum fuscum) from a 3-year, north-south transplant experiment in northern Europe, covering a latitudinal N deposition gradient ranging from 0.28 g N m(-2) year(-1) in the north, to 1.49 g N m(-2) year(-1) in the south. The maximum photosynthetic rate (NPmax) increased southwards, and was mainly explained by tissue N concentration, secondly by allocation of N to the   photosynthesis, and to a lesser degree by modified photosystem II activity (variable fluorescence/maximum fluorescence yield). Although climatic factors may have contributed, these results were most likely attributable to an increase in N deposition southwards. For S. fuscum, photosynthetic rate continued to increase up to a deposition level of 1.49 g N m(-2) year(-1), but for S. balticum it seemed to level out at 1.14 g N m(-2) year(-1). The results for S. balticum suggested that transplants from different origin (with low or intermediate N   deposition) respond differently to high N deposition. This indicates that Sphagnum species may be able to adapt or physiologically adjust to high N deposition. Our results also suggest that S. balticum might be more sensitive to N deposition than S. fuscum. Surprisingly, NPmax was not (S. balticum), or only weakly (S. fuscum) correlated with biomass production, indicating that production is to a great extent is governed by factors other than the photosynthetic capacity.

Keyword
Chlorophyll fluorescence, Chlorophyll, Carbon dioxide exchange, Photosynthesis, Peatlands
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-120352 (URN)10.1007/s00442-008-1261-1 (DOI)000264103800003 ()
Available from: 2010-03-11 Created: 2010-03-11 Last updated: 2017-12-12Bibliographically approved
2. Physiological responses to nitrogen and sulphur addition and raised temperature in Sphagnum balticum
Open this publication in new window or tab >>Physiological responses to nitrogen and sulphur addition and raised temperature in Sphagnum balticum
2009 (English)In: Oecologia, ISSN 0029-8549, E-ISSN 1432-1939, Vol. 161, no 3, 481-490 p.Article in journal (Refereed) Published
Abstract [en]

Sphagnum, the main genus which forms boreal peat, is strongly affected by N and S deposition and raised temperature, but the physiological mechanisms behind the responses are largely unknown. We measured maximum photosynthetic rate (NPmax), maximum efficiency of photosystem II [variable fluorescence (F v)/maximum fluorescence yield (F m)] and concentrations of N, C, chlorophyll and carotenoids as responses to N and S addition and increased temperature in Sphagnum balticum (a widespread species in the northern peatlands) in a 12-year factorial experiment. NPmax did not differ between control (0.2 g N m−2 year−1) and high N (3.0 g N m−2 year−1), but was higher in the mid N treatment (1.5 g N m−2 year−1). N, C, carotenoids and chlorophyll concentration increased in shoot apices after N addition. F v/F m did not differ between N treatments. Increased temperature (+3.6°C) had a small negative effect on N concentration, but had no significant effect on NPmax or F v/F m. Addition of 2 g S m−2 year−1 showed a weak negative effect on NPmax and F v/F m. Our results suggest a unimodal response of NPmax to N addition and tissue N concentration in S. balticum, with an optimum N concentration for photosynthetic rate of ~13 mg N g−1. In conclusion, high S deposition may reduce photosynthetic capacity in Sphagnum, but the negative effects may be relaxed under high N availability. We suggest that previously reported negative effects on Sphagnum productivity under high N deposition are not related to negative effects on the photosynthetic apparatus, but differences in optimum N concentration among Sphagnum species may affect their competitive ability under different N deposition regimes.

Keyword
Chlorophyll fluorescence, Nutrient deposition, Peatlands, Photosynthesis, Photosynthetic nitrogen use efficiency
National Category
Biological Sciences
Identifiers
urn:nbn:se:uu:diva-120447 (URN)10.1007/s00442-009-1406-x (DOI)000269010300004 ()19593588 (PubMedID)
Available from: 2010-03-12 Created: 2010-03-12 Last updated: 2017-12-12Bibliographically approved
3. Direct physiological effects of nitrogen on Sphagnum: a greenhouse experiment
Open this publication in new window or tab >>Direct physiological effects of nitrogen on Sphagnum: a greenhouse experiment
2012 (English)In: Functional Ecology, ISSN 0269-8463, E-ISSN 1365-2435, Vol. 26, no 2, 353-364 p.Article in journal (Refereed) Published
Abstract [en]

1. Bogs are nutrient-poor peatland ecosystems that are sensitive to nitrogen (N) deposition. Production of peat mosses (i.e. the peat-forming genus Sphagnum) is known to decrease under elevated N deposition, but the causal mechanisms are poorly understood. 2. It is predicted that increased N deposition will cause changes in Sphagnum species composition, with fast-growing species benefiting from increased N availability in contrast to slow-growing species. Knowledge of species-specific responses to N availability can help us to understand interspecific competitive relationships. 3. We investigated the direct effects of N application on plant physiology in three Sphagnum species by exposing shoots to a range of N doses (corresponding to depositions of 0-5 6 g m) 2 year) 1), over 5 months, in a greenhouse experiment. The species investigated included one that grows high above the water-table (Sphagnum fuscum) and two that grow lower down (Sphagnum balticum and Sphagnum fallax). S. fuscum and S. balticum originate from ombrotrophic and S. fallax from minerotrophic environments. To estimate N responses, we measured the performance and light-capture kinetics of the photosynthetic apparatus (maximum photosynthetic rate and Fv/Fm), biomass production, shoot formation, and N and phosphorus (P) concentrations in the tissue. 4. Tissue nitrogen concentration generally increased with N application rate, and photosynthetic rate increased with N concentration, although S. balticum exhibited a unimodal response. With respect to production, a negative response to N application rate was found in S. fallax and S. fuscum (weak), while production in S. balticum was unrelated to application rate. S. fallax was the fastest-growing species, producing two to three times more biomass per shoot compared with the other species. 5. The mismatch between photosynthetic capacity and production could partly be explained by an increased N : P ratio following N application. Phosphorus limitation may not negatively affect photosynthetic capacity, but may hamper production. 6. The fast-growing species S. fallax is considered to benefit from increased N deposition, but we found a negative physiological response, suggesting stoichiometric constraints. Thus, we conclude that responses to N deposition cannot be predicted in a simple way from physiological traits related to growth rate without considering local environmental factors. 

National Category
Ecology
Research subject
Ecological Botany
Identifiers
urn:nbn:se:uu:diva-165127 (URN)10.1111/j.1365-2435.2011.01948.x (DOI)000302011400007 ()
Available from: 2012-01-03 Created: 2012-01-03 Last updated: 2017-12-08Bibliographically approved
4. Rapid ecosystem shifts in peatlands: Linking plant physiology and succession
Open this publication in new window or tab >>Rapid ecosystem shifts in peatlands: Linking plant physiology and succession
2010 (English)In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 91, no 10, 3047-3056 p.Article in journal (Refereed) Published
Abstract [en]

Stratigraphic records from peatlands suggest that the shift from a rich fen (calcareous fen) to an ombrotrophic bog can occur rapidly. This shift constitutes a switch from a species-rich ecosystem to a species-poor one with greater carbon storage. In this process, the invasion and expansion of acidifying bog species of Sphagnum (peat mosses) play a key role. To test under what conditions an acidifying bog species could invade a rich fen, we conducted three experiments, contrasting the bog species S. fucsum with the rich-fen species S. warnstorfii and S. teres. We first tested the effect of calcareous water by growing the three species at different constant height above the water table (HWT; 2, 7, and 14 cm) in a rich-fen pool and measured maximum photosynthetic rate and production and difference in length growth as an indicator of competition. In none of the species was the photosynthetic capacity negatively affected when placed at low HWT, but S. fuscum was a weaker competitor at low HWT. In our second experiment we transplanted the three species into microhabitats with different and naturally varying HWT in a rich fen. Here, S. fuscum nearly ceased to photosynthesize when transplanted to low HWT (brown moss carpet), while it performed similarly to the two rich-fen species at the intermediate level (S. warnstorfii hummock level). In contrast to S. fuscum, the rich-fen sphagna performed equally well in both habitats. The brown moss carpet was seasonally flooded, and in our third experiment we found that S. fuscum, but not S. teres, was severely damaged when submerged in rich-fen water. Our results suggest two thresholds in HWT affecting the ecosystem switch: one level that reduces the risk of submergence and a higher one that makes bog sphagna competitive against the rich-fen species.

Keyword
Allogenic succession; Bog; Calcareous; Catastrophic shift; Competition; Drought, Flooding, Hällefjärd, Mire, Ombrotrophication, Photosynthesis, Sphagnum spp, Sweden
National Category
Biological Sciences
Research subject
Biology with specialization in Ecological Botany
Identifiers
urn:nbn:se:uu:diva-143269 (URN)10.1890/09-2267.1 (DOI)000282654700023 ()
Available from: 2011-01-20 Created: 2011-01-20 Last updated: 2017-12-11Bibliographically approved
5. Functional traits in Sphagnum, allometry, and their impacts on carbon cycling in peatlands
Open this publication in new window or tab >>Functional traits in Sphagnum, allometry, and their impacts on carbon cycling in peatlands
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(English)Manuscript (preprint) (Other academic)
National Category
Ecology
Research subject
Ecological Botany
Identifiers
urn:nbn:se:uu:diva-165131 (URN)
Available from: 2012-01-03 Created: 2012-01-03 Last updated: 2015-04-20

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Output format
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