Data from: Latitudinal variation in resistance and tolerance to herbivory in the perennial herb Lythrum salicaria is related to intensity of herbivory and plant phenology

Both the length of the growing season and the intensity of herbivory often vary along climatic gradients, which may result in divergent selection on plant phenology, and on resistance and tolerance to herbivory. In Sweden, the length of the growing season and the number of insect herbivore species feeding on the perennial herb Lythrum salicaria decrease from south to north. Previous common-garden experiments have shown that northern L. salicaria populations develop aboveground shoots earlier in the summer, and finish growth before southern populations do. We tested the hypotheses that resistance and tolerance to damage vary with latitude in L. salicaria, and are positively related to the intensity of herbivory in natural populations. We quantified resistance and tolerance of populations sampled along a latitudinal gradient by scoring damage from natural herbivores and fitness in a common-garden experiment in the field, and by documenting oviposition and feeding preference by specialist leaf beetles in a greenhouse experiment. Plant resistance decreased with latitude of origin, whereas plant tolerance increased. Oviposition and feeding preference in the greenhouse, and leaf damage in the common-garden experiment were negatively related to damage in the source populations. The latitudinal variation in resistance was thus consistent with reduced selection from herbivores towards the northern range margin of L. salicaria. Variation in tolerance may be related to differences in the timing of damage in relation to the seasonal pattern of plant growth, as northern genotypes have developed further than southern have when herbivores emerge in early summer.

生长季长度与植食作用（herbivory）强度通常随气候梯度发生变化，这可能会对植物物候（plant phenology）、抗植食性与耐植食性产生歧化选择（divergent selection）。在瑞典，生长季长度以及取食于多年生草本千屈菜（Lythrum salicaria）的昆虫植食者物种数，均由南向北逐渐降低。此前的同质园实验（common-garden experiment）已表明，北部千屈菜种群在夏季更早抽生地上茎，且生长结束时间早于南部种群。本研究验证了两项假说：千屈菜对损伤的抗植食性与耐植食性随纬度发生变化，且与自然种群中的植食作用强度呈正相关。我们通过两项实验量化了沿纬度梯度采集的种群的抗植食性与耐植食性：其一，在野外同质园实验中统计自然植食者造成的损伤程度与植株适合度（fitness）；其二，在温室实验中记录专食性叶甲（specialist leaf beetles）的产卵偏好与取食偏好。研究结果显示，植株的抗植食性随其原产地纬度升高而降低，而耐植食性则随纬度升高而增强。温室实验中的产卵与取食偏好、同质园实验中的叶片损伤程度，均与源种群（source populations）的损伤程度呈负相关。因此，千屈菜抗植食性的纬度变异，与植食者对其北部分布区边缘（range margin）种群的选择压力降低相一致。耐植食性的变异则可能与损伤发生时间和植物生长季节动态的差异相关：初夏植食者出现时，北部基因型（genotypes）的生长阶段较南部基因型更早。