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

Bartholomew stressed the importance of holding "constant as many factors as possible and to measure the effect of single variables." Because his own data provided evidence this was not properly done in earlier work he concluded, "All of the points presented by Muller and del Moral as evidence for chemical inhibition could be subjected to experimentation. However, as presented the points are not conclusive evidence for chemical inhibition."

At the invitation of Muller's students, Bartholomew came to UC Santa Barbara to discuss his data and view the study sites on his way to Baja California. Arriving with his major professor Hal Mooney and several other graduate students in the department's field van, Bartholomew participated in lively conversations comparing data and perspectives. Muller himself was not present during the discussions and took a dim view of the entire affair, becoming angry with his students for arranging it. He viewed it as Stanford University questioning the quality of his work as a scientist, rather than a theory's validity. To the students, the visit marked a turning point in their analysis. After examining the available information, consensus was reached. Bare zone formation and maintenance was primarily the work of animals, not chemicals.

"I was in Washington when Bartholomew came down, so I wasn't part of the discussions," del Moral (pers. com. June 03) said later. "But looking back, we didn't fully appreciate the synergistic actions of all the variables."

The Turning Point. By the time of Bartholomew's research in 1970, Muller and his students had applied the concept of allelopathy to explain a wide range of vegetation patterns in California including:

-The succession of grasslands by aromatic shrub species such as Salvia leucophylla and Artemisia californica through the release of volatile inhibitors, known as the coastal sage scrub "bare zone" model. Other common species identified as toxic and capable of inhibiting growth around them included Salvia ap'iana (white sage), Salvia mellifera (black sage), Heteromeles arbutifolia (toyon), Prunus ilicifolia (holly-leafed cherry), and Artemisia tridentata (Great Basin sagebrush) (Muller 1966).

-The "deteriorated interiors of older stands" of Salvia leucophylla and Artemisia californica caused by the concentrated release of volatile toxins (auto-toxicity) (Muller 1966).

-The suppression of germination under the canopy of Adenostoma fasciculatum (chamise) chaparral (McPherson and Muller 1969). The dramatic post-fire bloom of herbaceous species is the result of heat denaturing allelopathic chemicals in the soil and the elimination of toxin-producing shrubbery (Muller et al. 1968). This explanation has been identified as the "chamise-fire cycle" model.

-The exclusion of certain herb species from grasslands by chemical inhibitors released as decomposition products of grasses like Avena fatua (wild oats) (Muller 1966).

-The invasive nature of Brassica nigra (black mustard) (Muller 1969). With David Bell, he suggested rainwater leachate from dead black mustard stalks and leaf material containing toxic chemicals could account for the plant's pure stands and the bare zones around them. Their experimental data was based on concentrated rainwash solutions and bioassays as used in previous investigations (Bell and Muller 1973).