Environmental stress under climate change reduces plant performance, yet increases allelopathic potential of an invasive shrub

Exotic invasive plant species (EIPS) may succeed in part because they release allelochemicals that are novel to native plants (Novel Weapons Hypothesis). Plant allelopathic effects may increase under abiotic and biotic stresses based on The Stress Hypothesis of Allelopathy (TSHA). Lonicera maackii is an aggressive EIPS in the Midwestern USA, able to affect native flora by means of allelopathy. We aimed to test the role of intraspecific competition, as biotic stress, and expected future climate conditions (i.e. intense floods in spring; hotter and drier conditions in summer), as abiotic stress, on the performance and allelopathic potential of L. maackii. We grew L. maackii plants, collected from the field (field plants) or germinated from seeds in a greenhouse (seedlings), with or without intraspecific competition and under simulated present and expected future (1) spring (intense flooding vs no flooding) and (2) summer conditions. We subsequently measured plant performance variables, such as Fᵥ/Fₘ (an indicator of plant stress), plant biomass, and concentration of pigments, carbon and nitrogen; and allelopathic potential (i.e. the effect of L. maackii leaf extracts and conditioned substrates on the germination of two herb species). In accordance with TSHA, intense flooding stressed L. maackii (Fᵥ/Fₘ < 0.7), reduced its biomass, and increased its allelopathic potential. In summer, L. maackii plants were more stressed (lower average Fᵥ/Fₘ) under future summer conditions and intraspecific competition than under present conditions without competitors, but they had similar allelopathic potential. This suggests the presence of a stress threshold from which allelopathic potential does not increase further. Intraspecific competition more negatively affected the performance of seedlings (e.g. plant mass reduction) than field plants, but only increased the allelopathic potential of the latter. Our results indicated that the negative effects of intense flooding on L. maackii performance could be counteracted by the benefit gained in plant–plant competition by means of increased allelopathic potential. The complex role of abiotic and biotic conditions on L. maackii performance and allelopathy are also discussed.