Does the phytotoxic shrub Heterothalamus psiadioides affect a plant community through allelopathy?

Abstract

Phytotoxicity has often been proposed as evidence of allelopathy. However, few studies have investigated allelopathy in natural field conditions. In South Brazilian grasslands, Heterothalamus psiadioides, a shrub establishing in dense stands, has shown strong phytotoxicity in laboratory assays. We conducted a field study to evaluate if this species also negatively affects a plant community through allelopathy. At a shrubland site, we established a phytotoxicity gradient, consisting of plots with H. psiadioides, plots with Baccharis patens-a shrub with minimal phytotoxicity-and plots without shrubs (no phytotoxicity). We assessed plant species cover, biomass, number of seedlings, diversity, richness, and plant community characteristics. Our study did not show evidences of plant inhibition near the phytotoxic species. We observed higher plant richness and diversity in plots with H. psiadioides than in plots with B. patens and without shrubs. The highest diversity near H. psiadioides was associated with greater forb cover, due to lower cover of dominant grasses. Differences in light incidence accounted for vegetation patterns more than allelopathic activity. Therefore, our study showed that the allelopathic potential of the evaluated species was overestimated based only on laboratory studies. We emphasize the relevance of conducting allelopathy studies in field and of having a similar species with low phytotoxicity in this kind of study. Results based only on laboratory assays and visual evidence of patterns in field may be misleading and may lead to an overestimation of the role of allelopathy in plant communities.

Evidence for chemical interference effect of an allelopathic plant on neighboring plant species: A field study

“…For instance, it is known that in the field, associated soil microorganisms can metabolize chemical compounds and diminish their allelopathic effects [ 55 , 56 ]. This emphasizes that laboratory studies might overestimate the significance of allelopathy in nature, as discussed in previous studies [ 57 – 60 ]. Consequently, results obtained under controlled conditions should be interpreted carefully.…”

“…In conclusion, this study of allelopathy constitutes one of the few examples performed in natural plant communities (but see [ 57 , 65 , 66 ]) and provides novel evidence for the significance of the chemical interference by an allelopathic shrub in a semiarid plant community. Our findings indicate that under natural conditions chemical interference can result in a reduction of the vitality of some neighboring plant species ( e .…”

“…herba-alba microsite not. Having a similar species with no phytotoxicity has been considered as an adequate control to compare the effects of allelopathic species, particularly in field studies [ 57 ]. In the study area, both A .…”