Testing for allelopathy in invasive plants: it all depends on the substrate!

Abstract

Invasive plants can affect native plants through competition or allelopathy, and researchers often use pot experiments as a tool to measure the strength of these interactions. Recently, such pot experiments provided inconsistent estimates of the impact and allelopathic potential of invasive knotweed, one of the world's most successful plant invaders. We suspected that the inconsistencies may be explained by the use of different substrates in different experiments. To test this, we conducted an experiment in which knotweed competed pairwise with five common native European species in several different substrates: two compost-based potting substrates and two natural soils, with or without extra fertilizer added. To test for allelopathy, we added activated carbon to half of the pots. We found that knotweed was generally much more successful, and there was much more evidence for its allelopathy, when tested in artificial potting substrates than in natural soils. Furthermore, addition of extra fertilizer decreased the dominance of knotweed and changed patterns of allelopathy. The physicochemical properties of potting soil, such as lower bulk density, higher pore space, permeability and nitrogen content may better allow rhizomes to penetrate and/or allelochemicals to be produced and diffused. If artificial substrates generally exaggerate dominance and allelopathy also in other invasive plants, then many previous studies may have overestimated the potential impact of invaders, and the results of these experiments should be interpreted with caution. To avoid misleading results, experiments that test the competitive or allelopathic impact of invasive plants should be done with natural soils, preferably from the targeted habitats.

Effect of allelopathy on plant performance: a meta‐analysis

“…While about 90% of the soilmethod studies used natural soil, most leachate-method studies used artificial substrates. The abiotic characteristics of natural soil, such as higher bulk density, may hinder diffusion of allelochemicals (Parepa & Bossdorf 2016). Second, the effect of natural soil might also arise from the presence or higher abundance of soil microbes (Mishra et al 2013).…”

Neighbour tolerance, not suppression, provides competitive advantage to non‐native plants

“…The former bears a risk of artificially exaggerating the magnitude of the competitive effect of non‐native species due to inappropriate choice of species or environmental conditions (Vilà & Weiner ; Passioura ; Dalling et al . ; Parepa & Bossdorf ). Meanwhile, the latter is associated with the presence of non‐independence (i.e.…”

Optimising physiochemical control of invasive Japanese knotweed

“…the Enemy Release Hypothesis (ERH; Maurel et al 2013) and direct and/or indirect allelopathy through the soil biota i.e. the mutualism facilitation hypothesis (Parepa et al 2013;Parepa and Bossdorf 2016), while resource sharing through clonal rhizome integration may also aid competition and spread (You et al 2014). Such invasions displace native flora, reducing floral assemblages and modify ecosystem functioning, e.g.…”