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Multiple feedbacks contribute to a centennial legacy of reindeer on tundra vegetation
Umeå University, Faculty of Science and Technology, Department of Ecology and Environmental Sciences. (Arcum)ORCID iD: 0000-0002-2644-2144
Ájtte, Swedish Mountain and Sami Museum, Jokkmokk, Sweden.
Department of Forest Resource Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
Department of Forest Resource Management, Swedish University of Agricultural Sciences, Umeå, Sweden.
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2018 (English)In: Ecosystems (New York. Print), ISSN 1432-9840, E-ISSN 1435-0629Article in journal (Refereed) Epub ahead of print
Abstract [en]

Historical contingency is the impact of past events, like the timing and order of species arrival, on community assembly, and can sometimes result in alternative stable states of ecological communities. Large herbivores, wild and domestic, can cause profound changes in the structure and functioning of plant communities and therefore probably influence historical contingency; however, little empirical data on the stability of such shifts or subsequent drivers of stability are available. We studied the centennial legacy of reindeer (Rangifer tarandus) pressure on arctic tundra vegetation by considering historical milking grounds (HMGs): graminoid- and forb-dominated patches amid shrub-dominated tundra, formed by historical Sami reindeer herding practices that ended approximately 100 years ago. Our results show that the core areas of all studied HMGs remained strikingly stable, being hardly invaded by surrounding shrubs. Soil nitrogen concentrations were comparable to heavily grazed areas. However, the HMGs are slowly being reinvaded by vegetative growth of shrubs at the edges, and the rate of ingrowth increased with higher mineral N availability. Furthermore, our data indicate that several biotic feedbacks contribute to the stability of the HMGs: increased nutrient turnover supporting herbaceous vegetation, strong interspecific competition preventing invasion and herbivore damage to invading shrubs. In particular, voles and lemmings appear to be important, selectively damaging shrubs in the HMGs. We concluded that HMGs provide clear evidence for historical contingency of herbivore effects in arctic ecosystems. We showed that several biotic feedbacks can contribute to subsequent vegetation stability, but their relative importance will vary in time and space.

Place, publisher, year, edition, pages
Springer, 2018.
Keywords [en]
alternative stable states, plant–herbivore interactions, historical contingency, land use legacy, plant–soil interactions, Rangifer tarandus, soil nutrients, vegetation composition
National Category
Ecology
Identifiers
URN: urn:nbn:se:umu:diva-142126DOI: 10.1007/s10021-018-0239-zOAI: oai:DiVA.org:umu-142126DiVA, id: diva2:1159499
Note

Originnally included in thesis in manuscript form

Available from: 2017-11-22 Created: 2017-11-22 Last updated: 2018-09-14Bibliographically approved
In thesis
1. Long-lasting ecological legacies of reindeer on tundra vegetation
Open this publication in new window or tab >>Long-lasting ecological legacies of reindeer on tundra vegetation
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Reindeer can have strong effects on the plant species composition and functioning of tundra ecosystems, and often promote a transition towards a graminoid-dominated vegetation type. As a result, they influence many ecological processes, such as nutrient dynamics, soil biotic composition and functioning, and carbon storage. Several studies suggest that the effect of reindeer on vegetation may follow predictable patterns and could induce an alternative stable vegetation state. However, little empirical data on the long-term stability of reindeer effects on vegetation exist, as it is inherently challenging to study these ecological processes experimentally on a sufficiently long timescale. The main objective of this thesis was therefore to gain a better understanding of the long-term ecological processes following reindeer-induced vegetation shifts.

In order to gain a more mechanistic insight in what initially drives this transition, I used a field-based grazing simulation experiment in which I separated defoliation, trampling, moss removal and the addition of feces. This allowed me to test the relative contribution of reindeer-related activities to initiating the shift from moss and heath- dominated tundra towards a graminoid-dominated vegetation state. Additionally, I studied the long-term ecological stability following such a vegetation shift. I did this by addressing historical milking grounds (HMGs): sites where high reindeer concentrations associated with historical traditional reindeer herding practices induced a vegetation transition from shrubs towards graminoids several centuries earlier, but which were abandoned a century ago. Studying HMGs allowed me to address: 1. The potential stability of reindeer-induced vegetation shifts; 2. The ecological mechanisms contributing to the long-term stability of these vegetation shifts; and 3. How such long-lasting vegetation changes influence soil carbon- and nutrient cycling.

I found that trampling by reindeer is an important mechanism by which reindeer cause vegetation change. Addressing HMGs further revealed that this vegetation change can be hightly persistent, as the studied HMGs showed only a low encroachment at the surrounding borders in the last 50 years. The vegetation in the core areas of all studied HMGs had remained strikingly stable, and were hardly invaded by surrounding shrubs. Interestingly, soil nutrient concentrations and microbial activities were still different from the surrounding area as well, and even comparable to actively grazed areas. Even after many centuries of changed vegetation composition and soil processes, there was no difference in total carbon sequestration. This suggests that the environmental conditions for microbial decomposition were more important than vegetation composition for the soil carbon stocks, in our study site.

After studying the contemporary habitat use of HMGs by reindeer and other herbivores, investigating the potential plant-soil feedbacks mechanisms and detailed soil analyses, I concluded that several ecological mechanisms contribute to the long-term stability of HMGs: first, the altered soil biotic and abiotic conditions appear to have a stronger advantage for HMG vegetation than for the surrounding tundra vegetation. Furthermore, I found a clear browsing preference of small rodents on single shrubs proliferating in HMGs, causing a strong limitation on shrub expansion. Moreover, the dense established sward of graminoids likely poses a strong direct competition for space and nutrients, hindering seedling establishment. Finally, I conclude that HMGs are highly stable on relevant ecological timescales, and propose how the concepts of historical contingency and ASS can be applied to understand stability of these reindeer-induced vegetation transitions.

Place, publisher, year, edition, pages
Umeå: Umeå University, 2017. p. 33
Keywords
Alternative stable state, Herbivory, Historical contingency, Nutrient cycling, Plant-herbivore interactions, Plant-soil feedbacks, Rangifer tarandus, Reindeer herding, Traditional land use, Tundra vegetation
National Category
Ecology
Identifiers
urn:nbn:se:umu:diva-142131 (URN)978-91-7601-788-3 (ISBN)
Public defence
2017-12-15, Carl Kempe Salen, KBC, Linneaus väg 6, Umeå, 09:30 (English)
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Supervisors
Available from: 2017-11-24 Created: 2017-11-22 Last updated: 2018-06-09Bibliographically approved

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Egelkraut, DagmarOlofsson, Johan

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