Effects of elevation and nitrogen and phosphorus fertilization on plant defence compounds in subarctic tundra heath vegetation

1. Plant chemical and structural defence compounds are well known to impact upon herbivory of fresh leaves and in?uence decomposition rates after leaf senescence. A number of theories predict that alleviat ing nutrient limitation and reducing other environmental stressors will result in decreased production of plant chemical defences. 2. In this study, we measured plant defence properties [total polyphenols (TP), condensed tan- nins (CT) and lignin concentrations, and protein complexation capacity (PCC)] in both fresh and senesced plant leaves in a fully factorial N and P fertilization experiment set-up at each of three elevations along an elevational gradient in Swedish subarctic tundra heath vegetation. Further, we performed a decomposition of variance analysis on community-weighted averages (CWAs) of plant defence prop erties to determine the relative contributions of interspeci?c and intraspec i?c variation to the total variation observed in response to elevation and nutrient addition. 3. We hypothesized that N fertilization would reduce plant defence properties and that this reduction would be greater at higher elevat ions, while the e?ects of P fertilization would have no e?ect at any elevation. 4. At the community level, N addition reduced CT and PCC in both fresh and senesced leaves and TP in senesced leaves, while P addition had few e?ects, broadly in line with our hypo - thesis. The e?ects of N addition frequently varied with elevation, but in contrast to our hypothesis, the said e?ects were strongest at the lowest elevations. The e?ects of N addition and the interactive e?e ct of N with elevation were primarily driven by intraspeci?c, rather than interspec i?c, variation. 5. Our ?ndings suggest that as temperatures warm and N availability increases due to global climate change, secondary metabolites in subarctic heath vegetation will decline particularly within species. Our results highlight the need to consider the e?ects of both nutrient availabil- ity and temperature, and their interaction, in driving subarctic plant defence.