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Functional Role of Extrafloral Nectar in Boreal Forest Ecosystems under Climate Change

Department of Environmental and Biological Sciences, University of Eastern Finland, P.O. Box 1627 Kuopio, Finland
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Forests 2020, 11(1), 67; https://doi.org/10.3390/f11010067
Received: 15 December 2019 / Revised: 2 January 2020 / Accepted: 2 January 2020 / Published: 6 January 2020
(This article belongs to the Section Forest Ecology and Management)
Carbohydrate-rich extrafloral nectar (EFN) is produced in nectaries on the leaves, stipules, and stems of plants and provides a significant energy source for ants and other plant mutualists outside of the flowering period. Our review of literature on EFN indicates that only a few forest plant species in cool boreal environments bear EFN-producing nectaries and that EFN production in many boreal and subarctic plant species is poorly studied. Boreal forest, the world’s largest land biome, is dominated by coniferous trees, which, like most gymnosperms, do not produce EFN. Notably, common deciduous tree species that can be dominant in boreal forest stands, such as Betula and Alnus species, do not produce EFN, while Prunus and Populus species are the most important EFN-producing tree species. EFN together with aphid honeydew is known to play a main role in shaping ant communities. Ants are considered to be keystone species in mixed and conifer-dominated boreal and mountain forests because they transfer a significant amount of carbon from the canopy to the soil. Our review suggests that in boreal forests aphid honeydew is a more important carbohydrate source for ants than in many warmer ecosystems and that EFN-bearing plant species might not have a competitive advantage against herbivores. However, this hypothesis needs to be tested in the future. Warming of northern ecosystems under climate change might drastically promote the invasion of many EFN-producing plants and the associated insect species that consume EFN as their major carbohydrate source. This may result in substantial changes in the diet preferences of ant communities, the preventative roles of ants against insect pest outbreaks, and the ecosystem services they provide. However, wood ants have adapted to using tree sap that leaks from bark cracks in spring, which may mitigate the effects of improved EFN availability. View Full-Text
Keywords: global warming; deciduous trees; phloem sap; nectar; Formica wood ants; parasitoids; range shift; invasive species; novel ecosystems global warming; deciduous trees; phloem sap; nectar; Formica wood ants; parasitoids; range shift; invasive species; novel ecosystems
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Holopainen, J.K.; Blande, J.D.; Sorvari, J. Functional Role of Extrafloral Nectar in Boreal Forest Ecosystems under Climate Change. Forests 2020, 11, 67.

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