Dual-Strategy Resource Allocation in an Invasive Clonal Plant to Counter Foliar Herbivory for Rapid Recovery

Clonal integration (i.e., sharing resources via interconnected ramets) is recognized as a key mechanism enabling clonal plants to mitigate herbivory. However, how the magnitude and direction of this response vary between invasive and native clonal plants under spatially heterogeneous herbivory remains unresolved. In this study, we subjected the invasive alligator weed, Alternanthera philoxeroides, and its native congener, sessile joyweed A. sessilis, to varying feeding position treatments by the flea beetle Agasicles hygrophila (damage to apical ramets, basal ramets, or both ramets) and herbivore-free controls. We then examined the growth, biomass allocation, nitrogen (N) and ¹⁵N partitioning in both plant species. Compared with the herbivore-free controls, apical or basal herbivory reduced the fine-root: tap-root ratio of A. philoxeroides while increasing its root-to-shoot ratio and N concentrations in aboveground tissues. In contrast, A. sessilis exhibited significantly more fine roots and higher belowground N content compared to non-herbivore conditions. Although over 80% of ¹⁵N was directed to aboveground tissues in both species, A. philoxeroides preferentially allocated more ¹⁵N to herbivore-damaged tissues, whereas A. sessilis tended to direct ¹⁵N away from herbivore-damaged tissues. These results indicate that, compared to native clonal plants, clonal invaders like A. philoxeroides exhibit a more sophisticated dual resource-allocation strategy in response to foliar herbivory, by allocating more resources belowground while partitioning more N aboveground to facilitate rapid recovery after herbivore attack. Our findings highlight the importance of considering both the direct effects of herbivory and the physiological mechanisms of clonal integration when developing management strategies against clonal invaders.