Data from: 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 influence decomposition rates after leaf senescence. A number of theories predict that alleviating 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 tannins (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 properties to determine the relative contributions of interspecific and intraspecific 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 elevations, while the effects of P fertilization would have no effect 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 effects, broadly in line with our hypothesis. The effects of N addition frequently varied with elevation, but in contrast to our hypothesis, the said effects were strongest at the lowest elevations. The effects of N addition and the interactive effect of N with elevation were primarily driven by intraspecific, rather than interspecific, variation. 5. Our findings 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 effects of both nutrient availability and temperature, and their interaction, in driving subarctic plant defence.

1. 众所周知，植物的化学与结构防御化合物会显著影响新鲜叶片的植食性取食，并调控叶片衰老后的分解速率。已有多项理论预测，缓解养分限制并降低其他环境胁迫，将导致植物化学防御物质的合成量下降。
2. 本研究依托瑞典亚北极冻原灌丛植被沿海拔梯度设置的3个海拔位点的全因子氮（N）、磷（P）施肥实验，测定了新鲜与衰老叶片的植物防御属性：总多酚（TP）、缩合单宁（CT）、木质素浓度以及蛋白质结合能力（PCC）。此外，我们针对植物防御属性的群落加权平均值（CWAs）开展方差分解分析，以明确种间与种内变异对海拔升高与养分添加响应中观测到的总变异的相对贡献。
3. 我们提出假说：氮素添加会降低植物防御属性，且该降低效应在高海拔位点更为显著；而磷素添加在所有海拔位点均无显著影响。
4. 在群落水平上，氮素添加降低了新鲜与衰老叶片中的缩合单宁、蛋白质结合能力，以及衰老叶片中的总多酚含量；而磷素添加几乎无影响，这与我们的假说基本一致。氮素添加的效应常随海拔发生变化，但与我们的假说相反，该效应在海拔最低的位点最为显著。氮素添加的效应及其与海拔的交互效应主要由种内变异（而非种间变异）驱动。
5. 我们的研究结果表明，随着全球气候变化导致气温升高、氮素有效性提升，亚北极灌丛植被中的次生代谢产物将出现下降，且该变化尤其体现在物种内部。本研究结果强调，在解析亚北极植物防御的驱动机制时，需要同时考虑养分有效性、温度及其交互作用的影响。