Defoliation tolerance in Plantago seedlings varies with neighbour environment

Environmental selection during early ontogeny influences plant species representation in mature vegetation. Chief amongst these selective forces is the interplay of herbivore selection and seedling defence/tolerance. Although some species can recover from almost complete cotyledon loss, it remains unclear whether tolerance is impacted by other critical early life-history hazards like competition. This study examines early seedling tolerance of two sympatric Plantago species (P. lanceolata and P. major) for individuals grown in isolation, monoculture (i.e., with conspecifics) and mixed assemblages (with congenerics). The effects of 95 and 50% cotyledon removal at 14-d-old on plant growth and root:shoot allocation was established in 28-d and 100-d old plants. Although damage and neighbour environment consistently limited subsequent growth of P. major, there was no interaction between these factors. Nonetheless, individuals subjected to 95% defoliation in mixed assemblages were substantially smaller than those without neighbours. A similar response to cotyledon removal was evident for P. lanceolata at 28-d, although plants with conspecific neighbours tended to be larger. The defoliation effect disappeared at 100-d, and plants grown with neighbours were now smaller. Mortality and root:shoot allocation did not vary across treatments. While we underscore the negative impact that early ontogenetic damage and the influence of neighbour environment has on subsequent plant growth, we failed to elucidate obvious interactions between these factors. Nonetheless, we highlight how the remarkable tolerance of Plantago seedlings to almost complete defoliation in the face of intra-/inter-specific competition, may yet be impacted by anthropogenic disruption of environmental filters operating at the regeneration stage.