Abstract
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.
Highlights
Extrafloral nectar (EFN) is produced in nectaries, which are specialized plant glands or less distinctive secretory structures that contain secretory trichomes and vascularized parenchyma [1].EFN nectaries secrete carbohydrate-rich nectar and are located on plant parts other than flowers, most often on stipules, at leaf bases, or on petioles [2], but sometimes they occur on the surfaces of flower buds [3] or inside stems that have an opening to the stem surface [4]
EFN nectaries have not been observed in early angiosperms, magnoliids, or gymnosperms [2], but leaf nectaries are present on non-flowering ferns, suggesting the principal role of leaf nectaries in protective mutualism of plants before the evolution of nectar production in flowers [5]
These results suggest that in boreal forest ecosystems, aphid–ant associations promoted by EFN will more likely provide regulating Ecosystem services (ES)
Summary
Extrafloral nectar (EFN) is produced in nectaries, which are specialized plant glands or less distinctive secretory structures that contain secretory trichomes and vascularized parenchyma [1]. Ants play an important role in the control of herbivores on forest trees and may prevent insect outbreaks locally in boreal forests [51,52]. We aim to unravel and discuss the functional role of EFN, with a particular focus on that produced by deciduous woody plants (Figure 1) in boreal forest ecosystems. These ecosystems are dominated by coniferous trees that do not produce EFN, but wood ants are still keystone species in these ecosystems [56,57].
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