Geographic variation in compensation to damage in common milkweed (Asclepias syriaca)

Background and aims – Plants display a variety of resistance and tolerance responses to herbivory. Compensation, or changes in growth, allocation, and/or physiology, after damage is one way that plants tolerate herbivory, but geographic patterns in intraspecific plant compensatory responses are understudied. We aimed to study geographic variation in tolerance to herbivory to help explain geographic patterns in the distribution of resistance traits and the relationship between tolerance and resistance traits in common milkweed, Asclepias syriaca . Material and methods – We grew milkweed from 14 different populations in the greenhouse, mechanically applied 25% leaf damage to an experimental group, and compared the control and experimental groups to measure compensatory responses in final biomass, root:shoot ratios, stem investment, and relative growth rate. We compared compensatory responses across populations grouped by latitude and by temperature. Key results – Compared to controls, milkweed plants that were damaged lost mass and expressed reduced root:shoot ratios. However, the effect of damage on total mass, stem investment, and relative growth rate varied among genetic families. In regional contrasts, plants from colder climates grew larger and invested less in stems and roots than plants from warmer climates under control conditions, but they were less able to compensate for damage in terms of biomass. Plants from cold regions also showed a tendency to reduce growth rate and stem investment after damage; whereas, plants from warmer climates tended to increase their growth rate and stem investment in response to damage. Conclusion – While plants from high latitudes and colder climates were less able to compensate for damage than those from lower latitudes, we are not confident that these differences are caused by geographic differences in growth rate, or that they explain differences in resistance to herbivory. Instead, we suspect that differences in the phenology of development in plants from regions with different climates affect the impact of damage and the potential for compensatory growth. Milkweed plants from colder regions with short growing seasons grew larger during our measurement period, while those from regions with longer growing seasons invested more in stems and roots, traits which may have facilitated greater long-term growth, as well as the greater compensatory ability observed in our study. Future studies should explicitly manipulate the timing of damage applied to plants from different regions to test the relationship between phenology and compensation.