Amur honeysuckle’s allelopathic effects on native plant germination are context-dependent

Background/Question/Methods

Invasive plant species can directly impact native species by releasing allelochemicals into the soil.  Recent studies suggest that the impact of allelochemicals may depend on biotic and abiotic environmental factors.  Leachate of Amur honeysuckle (Lonicera maackii), an Eastern Deciduous Forest invasive shrub, reduces the germination rate of both native and exotic herbaceous species, but whether these effects interact with abiotic or biotic environmental conditions is not known.  We first tested whether light and substrate conditions interacted with the effects of honeysuckle leachate on germination of the native woodland mint Blephilia hirsuta.  We then tested whether Amur honeysuckle leachate reduced or delayed germination of five native woodland herbaceous species and itself in live vs. sterile woodland soil.

Results/Conclusions

We found that honeysuckle leachate negatively affected Blephilia hirsuta germination, but abiotic conditions did not interact with this effect. There was an overall suppressive effect of honeysuckle extract on itself and the other five native species tested.  However, honeysuckle extract interacted with soil microbes in ways that differed with the species being tested and, in some cases, changed over time.  In some cases, soil microbes mitigated negative effects of honeysuckle extracts, but in other cases soil microbes caused increased suppression of germination.  Our findings indicate that future allelopathy studies must consider the environmental context into which allelochemicals are released.  We also suggest that under more realistic field conditions not all species will be equally vulnerable to allelopathic suppression by honeysuckle.