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.植物生长季长度与植食作用强度通常随气候梯度发生变化，这可能会对植物物候、抗植食性及耐植食性产生差异化选择压力。 在瑞典，取食多年生草本千屈菜（Lythrum salicaria）的昆虫植食者物种数，以及该物种的生长季长度，均由南向北递减。 此前的同质园实验（common-garden experiment）表明，北部千屈菜种群在夏季更早萌发地上茎，且生长结束时间早于南部种群。 本研究验证了两项假说：其一，千屈菜的抗植食性与耐植食性随纬度发生变异；其二，二者与自然种群中的植食作用强度呈正相关。 本研究通过两类实验量化了沿纬度梯度取样的千屈菜种群的抗植食性与耐植食性：一是开展田间同质园实验，统计自然植食者造成的叶片损伤程度与植物适合度（fitness）；二是开展温室实验，记录专食性叶甲（specialist leaf beetles）的产卵偏好与取食偏好。 植物的抗植食性随原产地纬度升高而降低，而耐植食性则随纬度升高而提升。 温室实验中的产卵偏好、取食偏好，以及同质园实验中的叶片损伤程度，均与源种群的植食损伤程度呈负相关。 由此，抗植食性的纬度变异与千屈菜北部分布边缘受到的植食者选择压力降低的结论一致。而耐植食性的纬度变异，则可能与损伤发生时间相对于植物生长季节动态的差异有关——在初夏植食者羽化时，北部基因型的生长进度已领先于南部基因型。