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Responses of peatland vegetation to enhanced nitrogen
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences.
2008 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Human alteration of the global nitrogen (N) cycle has had major impacts on naturally N-limited ecosystems worldwide. Peatlands, dominated by peat mosses, Sphagnum species, represent one such sensitive ecosystem. I have studied how this ecosystem is affected by increased N availability, using a small-scale N fertilization experiment in combination with a gradient study of three peatlands with varying N deposition.

I found both in the experiment and in the gradient a similar pattern of Sphagnum decline accompanied by an increase of vascular plants associated with enhanced N supply. For one common Sphagnum species - both in the experiment and in the gradient study - I also found an identical, linear increase in soluble amino acid N (NAA) accumulation. As soluble amino acids function as N storage compounds among Sphagna, NAA is a suitable measure for Sphagnum N status, and indicates accumulation of excess N not used for growth. My results show that NAA can be used as a sensitive indicator to signal N pollution before the slow, and gradual, regime shift from Sphagnum to vascular plant dominance is visible. In an N-uptake experiment using Sphagnum specimens from the three peatlands varying in N deposition, I found a reduced N-uptake by both investigated Sphagnum species from a high N deposition site, in south-western Sweden. This potential of Sphagna to adjust to high N loads through N uptake regulation will, however, not prevent tissue N accumulation, and as a result a shift from Sphagnum to vascular plant dominance.

In general I found similar patterns of N induced changes both in Sphagnum tissue chemistry and vegetation structure in the experiment and along the gradient study. Thus, I conclude that long-term, small-scale field experiments seem to offer reliable estimates of both the direction and strength of key vegetation responses in Sphagnum dominated peatlands. This is likely related to the key role of Sphagna as ecosystem engineers.

In the experiment I found a marked time lag in vegetation response to N application treatments. The closed Sphagnum carpet did not collapse until after eight years of continuous treatments. Another result was that dwarf shrubs, e.g. cranberry Vaccinium oxycoccos, first increased, but later declined due to severe attacks by fungal diseases. One important conclusion is that long-term, manipulative field experiments are necessary for our ability to understand how ecosystems will respond to environmental change.

Place, publisher, year, edition, pages
Umeå: Ekologi, miljö och geovetenskap , 2008. , 24 p.
Keyword [en]
Sphagnum, nitrogen, peatlands, soluble amino acids, gradient study, field experiment
National Category
Biological Sciences
Identifiers
URN: urn:nbn:se:umu:diva-1655ISBN: 978-91-7264-551-6 (print)OAI: oai:DiVA.org:umu-1655DiVA: diva2:141722
Public defence
2008-06-05, Lilla Hörsalen, KBC, SE-90187, Umeå, 10:00 (English)
Opponent
Supervisors
Available from: 2008-05-15 Created: 2008-05-15 Last updated: 2016-02-08Bibliographically approved
List of papers
1. Global change shifts vegetation and plant-parasite interactions in a boreal mire
Open this publication in new window or tab >>Global change shifts vegetation and plant-parasite interactions in a boreal mire
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2007 (English)In: Ecology, ISSN 0012-9658, E-ISSN 1939-9170, Vol. 88, no 2, 454-464 p.Article in journal (Refereed) Published
Abstract [en]

The aim of this study was to detect vegetation change and to examine trophic interactions in a Sphagnum-dominated mire in response to raised temperature and nitrogen (N) addition. A long-term global-change experiment was established in 1995, with monthly additions of N (30 kg·ha-1·yr-1) and sulfur (20 kg·ha-1·yr-1) during the vegetation period. Mean air temperature was raised by 3.6°C with warming chambers. Vegetation responses were negligible for all treatments for the first four years, and no sulfur effect was seen during the course of the experiment. However, after eight years of continuous treatments, the closed Sphagnumcarpet was drastically reduced from 100 in 1995 down to 41, averaged over all N-treated plots. Over the same period, total vascular plant cover (of the graminoid Eriophorum vaginatumand the two dwarf-shrubs Andromeda polifoliaand Vaccinium oxycoccos) increased from 24 to an average of 70 in the N plots. Nitrogen addition caused leaf N concentrations to rise in the two dwarf-shrubs, while for E. vaginatum, leaf N remained unchanged, indicating that the graminoid to a larger extent than the dwarf-shrubs allocated supplemented N to growth. Concurrent with foliar N accumulation of the two dwarf-shrubs, we observed increased disease incidences caused by parasitic fungi, with three species out of 16 showing a significant increase. Warming caused a significant decrease in occurrence of three parasitic fungal species. In general, decreased disease incidences were found in temperature treatments for A. polifoliaand in plots without N addition for V. oxycoccos. The study demonstrates that both bryophytes and vascular plants at boreal mires, only receiving background levels of nitrogen of about 2 kg·ha-1·yr-1, exhibit a time lag of more than five years in response to nitrogen and temperature rise, emphasizing the need for long-term experiments. Moreover, it shows that trophic interactions are likely to differ markedly in response to climate change and increased N deposition, and that these interactions might play an important role in controlling the change in mire vegetation composition, with implications for both carbon sequestration and methane emission.

Keyword
global warming, nitrogen deposition, oligotrophic mire, parasitic fungi, trophic interaction, vegetation change
Identifiers
urn:nbn:se:umu:diva-3213 (URN)10.1890/05-1823 (DOI)
Available from: 2008-05-15 Created: 2008-05-15 Last updated: 2017-12-14Bibliographically approved
2. Can small-scale experiments predict ecosystem responses?: An example from peatlands
Open this publication in new window or tab >>Can small-scale experiments predict ecosystem responses?: An example from peatlands
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2009 (English)In: Oikos, ISSN 0030-1299, E-ISSN 1600-0706, Vol. 118, no 3, 449-456 p.Article in journal (Refereed) Published
Abstract [en]

Oligotrophic, Sphagnum-dominated peatlands have been regarded as long-term stable ecosystems that function as carbon sinks. As a result of environmental perturbations, particularly anthropogenic N deposition, this view is now increasingly questioned. We examined whether small-scale field experiments can predict the direction and magnitude of ecosystem responses to increased N supply. We, therefore, compared data from a 10-year field experiment (involving deposition of 2, 15 and 30 kg N ha−1 year−1) with data from a gradient associated with increased N deposition (2, 8 and 12 kg N ha−1 year−1). We chose to compare: (1) the physiological response of Sphagnumbalticum, measured in the form of N accumulation as free amino acids (NAA); and (2) changes in the total Sphagnum cover, the cover of S. balticum, and vascular plant cover. In all cases we found a highly significant correlation between the two data sets. We attribute the high correspondence between the two data sets to the key function of the dominant group of organisms, the Sphagna, that monopolize N availability and control the water balance, creating an environment hostile to vascular plants. Thus the key role of Sphagna as ecosystem engineers seems to supersede the role of other, scale-dependent processes. We also conclude that NAA is a sensitive indicator that can be used to signal the slow and gradual shift from Sphagnum to vascular plant dominance.

Identifiers
urn:nbn:se:umu:diva-23292 (URN)10.1111/j.1600-0706.2008.17129.x (DOI)
Available from: 2009-06-09 Created: 2009-06-09 Last updated: 2017-12-13Bibliographically approved
3. Ecophysiological adjustment of two Sphagnum species in response to anthropogenic nitrogen deposition
Open this publication in new window or tab >>Ecophysiological adjustment of two Sphagnum species in response to anthropogenic nitrogen deposition
2009 (English)In: New Phytologist, ISSN 0028-646X, E-ISSN 1469-8137, Vol. 181, no 1, 208-217 p.Article in journal (Refereed) Published
Abstract [en]

We asked whether Sphagnum species have adjusted their N uptake in response to the anthropogenic N deposition that has drastically altered N-limited ecosystems, including peatlands, worldwide. A lawn species, S. balticum, and a hummock species, S. fuscum, were collected from three peatlands along a gradient of N deposition (2, 8 and 12 kg N ha-1 yr-1). The mosses were subjected to solutions containing a mixture of four N forms. In each solution one of these N forms was labeled with 15N (viz. 15NH4+, 15NO3- and the amino acids [15N]alanine (ALA) and [15N]glutamic acid (GLU)). We found for both species that the majority of the N taken up was from NH4+, followed by ALA, GLU, and very small amounts of NO3-. At the highest N deposition site N uptake was reduced, but this did not prevent N accumulation as free amino acids in the Sphagnum tissues. The reduced N uptake may have been genetically selected for under the relatively short period with elevated N exposure from anthropogenic sources, or may have been the result of plasticity in the Sphagnum physiological response. The negligible Sphagnum NO3- uptake may make any NO3- deposited readily available to co-occurring vascular plants.

Identifiers
urn:nbn:se:umu:diva-23284 (URN)10.1111/j.1469-8137.2008.02628.x (DOI)
Available from: 2009-06-09 Created: 2009-06-09 Last updated: 2017-12-13Bibliographically approved
4. Differing physiological responses of two Sphagnum species along a nitrogen deposition gradient
Open this publication in new window or tab >>Differing physiological responses of two Sphagnum species along a nitrogen deposition gradient
(English)Manuscript (Other academic)
Identifiers
urn:nbn:se:umu:diva-3216 (URN)
Available from: 2008-05-15 Created: 2008-05-15 Last updated: 2016-02-26Bibliographically approved

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