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The role of allelopathy in Reynoutria japonica Houtt. ecological dominance

(2022)

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Abstract
Impacts of humanity on earth systems are tied to its dramatic technological improvements in the last centuries. Taking advantage of subsequent globalization processes, invasive species are nowadays one of the main causes of biodiversity loss worldwide. Among those, invasive Japanese knotweed (Reynoutria japonica Houtt.) is known as one of the worst invasive species in the world. While numerous hypotheses account for invasive plant species success in their invaded range, this work focuses on one of them that might explain Reynoutria japonica rampant invasion of Europe and North America: The Novel Weapons Hypothesis (NWH). The NWH states that invasive plant species are highly successful in their invaded range because of the new set of chemicals they bring with them, against which native species would be defenseless. Allelopathy is the process by which a plant (donor) inhibits the growth of another plant (target) via the release in the environment of chemical compounds and provides a theoretical frame to the NWH. This work aims at observing suggested allelopathic patterns of R. japonica in an ecologically meaningful setting to try and account for the role of allelopathy in this plant’s successful invasions. Methods used were based on theorical and experimental frames of phytotoxicity and allelopathy and on previous shortcomings identified in similar studies. A study site was selected in Ottignies (Belgium) in which Japanese knotweed was identified using indumentum criteria. Bioassays were performed on Zea mays and Trifolium incarnatum and involved tissue extracts along with invaded soil of R. japonica. Greenhouse growth experiments used a target-neighbor design with varying T. incarnatum (neighbor) densities in natural, artificial, and nutrient deficient substrates. Tissue mineral and photosynthetic pigments contents of crimson clover were also assessed. Despite identifying phytotoxic effects of tissue extracts on test species’ radicle length, we could not find strong supplementary evidence for R. japonica allelopathy in both growth experiments and bioassays, which contradicted our hypotheses. Germination of test species was not affected by tissue extracts. Biomass production and physiological parameters results of T. incarnatum plants in growth experiments were mainly explained by substrate’s abiotic conditions. We did not observe any optimal density patterns linked to Japanese knotweed phytotoxicity. Absence of observed effects could be due to resource competition, soil mediated processes or facilitation among clovers. Overall, this study did not provide arguments in favor of an ecologically important allelopathic process and of NWH in the case of R. japonica. Rather, our results support the integration of such effects in a set of other dominant processes.