Data from: The simultaneous inducibility of phytochemicals related to plant direct and indirect defences against herbivores is stronger at low elevation

Ecological theory indicates that warmer and more stable climates should result in stronger biotic interactions. Therefore, plant species growing at lower elevations and experiencing greater herbivore pressure, should invest in higher levels of defences than those at higher elevations. Nonetheless, there are a number of studies that have found no effect of elevational gradients on plant defensive traits. Several factors might explain the lack of consistency for the altitude-defence relationships; including 1) the reduction of all defensive traits into one measure of resistance; 2) not considering plant defence as the simultaneous expression of several defensive traits; and 3) not considering the relative influence of biotic (e.g. herbivory) and abiotic (e.g. climate and soil conditions) factors associated with the ecological gradient. Here, we present a comprehensive test of the effects of elevation and its associated biotic and abiotic factors on the individual and simultaneous expression of constitutive direct and indirect defences and their inducibility (i.e. expression of defences after herbivore attack). Specifically, we estimated climatic and soil variables and measured herbivore damage and constitutive and jasmonic acid-induced glucosinolate levels in the leaves as a proxy for direct defences, and volatile emission as a proxy for indirect defences in 16 Cardamine species naturally growing along the steep elevational gradient of the Alps. Within a phylogenetic comparative framework, we found that species growing at lower elevations invested more in the simultaneous inducibility of both direct and indirect defences, whereas species growing at higher elevations invested more in constitutive direct defences. Although we found strong elevation gradients in herbivory and climatic and soil variables, these biotic and abiotic factors only partially explained elevational patterns in plant defences. Synthesis - These results highlight that the complex regulation of multiple defence traits strongly vary across elevational gradients and build towards a better understanding of the multiple mechanisms underlying trait evolution and species interactions along ecological gradients.

生态学理论指出，气候更温暖且更稳定的生境会促成更强的生物相互作用。因此，相较于高海拔分布的植物物种，分布于低海拔且承受更强植食压力（herbivore pressure）的类群，理应演化出更高水平的防御策略。然而，已有多项研究发现海拔梯度对植物防御性状并无显著影响。现有多项因素可解释海拔-防御关联的不一致性：其一，将所有防御性状简化为单一的抗性衡量指标；其二，未将植物防御视为多种防御性状的协同表达；其三，未考量与生态梯度相关的生物（如植食作用）与非生物（如气候、土壤条件）因子的相对影响。本研究针对海拔及其相关生物与非生物因子，对植物组成型直接与间接防御（constitutive direct and indirect defences）以及防御诱导性（inducibility，即植食动物攻击后防御性状的表达）的单独表达与协同表达的影响开展了全面检验。具体而言，我们针对沿阿尔卑斯山陡峭海拔梯度自然分布的16种碎米荠（Cardamine）属植物，测定了其气候与土壤变量，记录了植食损伤情况，并以叶片中组成型及茉莉酸（jasmonic acid）诱导的硫代葡萄糖苷（glucosinolate）含量作为直接防御的替代指标，以挥发性物质释放量作为间接防御的替代指标。在系统发育比较分析（phylogenetic comparative framework）框架下，我们发现：低海拔物种更注重直接与间接防御的协同诱导性投入，而高海拔物种则更倾向于组成型直接防御的投入。尽管我们在植食作用、气候与土壤变量中观测到了显著的海拔梯度特征，但这些生物与非生物因子仅部分解释了植物防御性状的海拔分布模式。综合与结论——本研究结果表明，多种防御性状的复杂调控机制随海拔梯度存在显著差异，这有助于我们更深入理解生态梯度下性状演化与物种相互作用背后的多重机制。