Soil‐plant Phytotoxicity and Its Possible Significance in Patterning of Herbaceous Vegetation in a Bottomland Forest

Plant growth inhibitors, which are known to exert synergistic effects on herbaceous vegetation, were isolated and quantified from the soils under hackberry trees. Ferulic, caffeic, and p‐coumaric acids were isolated from the soils under hackberry trees collected in January, April, and September from 0–15 and 15–30 cm soil depths. Seed germination bioassay indicated considerably higher phytotoxicity levels of individual plant growth inhibitors in January and April soils, and this toxicity level was more drastic when inhibitors were applied accumulatively. Individual phytotoxins extracted from soil in September were not very inhibitory to seed germination of selected test species; however, accumulative effects were still allelopathic to seed germination. Toxicity levels of individual compounds may reduce or disappear in a given time, but the combined action of these chemicals would still be toxic in croplands or in natural communities. Ecological implications of allelopathy in terms of soil‐plant interaction are discussed.