Environmental change, caused by nitrogen deposition and temperature increase, is predicted to affect allocation to carbon-based secondary compounds (CBSCs) in plants, due to changes in their internal carbon resources. The CBSCs are considered important for plant resistance to biotic and abiotic environmental stresses, such as herbivory, pathogen attacks, and UV radiation. To determine how allocation to putative defense compounds is affected by N deposition and increased temperature, we analyzed the composition of CBSCs in leaves of three arctic-alpine plant species: Bistorta vivipara, Dryas octopetala, and Salix reticulata after 5 years of warming (by open-top chambers) and experimental nutrient addition in an alpine Dryas heath in southern Norway. The dry weight of leaves increased after nutrient addition and warming combined with nutrient addition in all three species, while the weight of D. octopetala leaves also increased with warming alone. Individual chemical compounds or compound groups reacted to the treatments to different degrees and in different directions in the three species. The total concentration of CBSCs changed significantly only in S. reticulata, where it decreased in plots with nutrient addition combined with warming. Shading caused by taller vegetation in these plots might have bigger effects on the CBSC concentration than the direct changes in nutrient availability and temperature. Dryas octopetala had the highest concentration of CBSCs among the three species and was least affected by the treatments. Our results show that increased N availability and temperature influenced the level of carbon-based defense in some alpine plants but not others, indicating species-specific C-allocation responses to environmental change. Consequently, environmental changes may differentially affect defense abilities of alpine plant species, which could possibly contribute to future changes in interspecific competitive relationships and subsequently species composition of alpine plant communities.
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