In search of allelopathy: an eco-historical view of the investigation of chemical inhibition in California coastal sage scrub and chamise chaparral

Allelopathy between plants, whereby one species influences another by chemical means, has been speculated upon since the Greeks. During the second half of the twentieth century, southern California was the focal point of allelopathic research by several influential investigators. Frits Went suggested Encelia farinosa, a common desert shrub, inhibited annuals from growing under its canopy by chemical inhibition. Reed Gray and James Bonner conducted further investigations. Cornelius H. Muller questioned the allelopathic explanation for Encelia, but later felt chemical inhibition was the cause for vegetation patterns found in both southern Californian coastal sage scrub, primarily around Salvia leucophylla and Artemisia californica, and Adenostoma fasciculatum (chamise) chaparral. Various investigators challenged Muller's conclusions, but Muller remained convinced allelopathy was an important ecological variable in southern California's shrublands. Muller's passionate belief in his scientific models led him to ignore contrary evidence, yet his dedication to science and the education of his students inspired many. Allelopathy remains a controversial topic today despite hundreds of investigations because of the difficulty in isolating all the possible variables affecting plant growth.