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

Journal of the Torrey Botanical Society, Oct-Dec 2004 by Halsey, Richard W

In referring to Muller's suggestion that the shrubs were invading the grassland by way of inhibiting grass plants, Wells pointed out the top center of the photo where there were over 100 smaller shrubs scattered in the open grassland without grassless "halos." Why would isolated bushes be without bare zones? Wells wondered. This suggested the halos actually appeared long after Salvia had expanded into the grassland not as a preparatory step to their initial invasion.

Wells also noted what he saw as failures in Muller's experimental design such as use of cucumber seeds instead of natives, testing procedures remote from field conditions, and ignoring the impact of herbivores in bare zone creation. Ironically, the same criticisms Muller had leveled at Gray and Bonner. Wells concluded by saying the idea of chemical inhibition emanating from the sages was an interesting idea, "but it seems premature to imply that antibiosis ranks high in the complex of ecological factors determining the vegetation patterns depicted in the cover photograph."

As is often the case with criticisms of published papers, Muller and Muller (1964) were given the opportunity to write a response and have it printed immediately below Well's letter. The two men dismissed several of Wells' criticisms as not pertinent and repeatedly made the point that their paper was only an initial report providing "scant space for monographic completeness." They also discounted concerns over the lack of data relating to the role of cattle and herbivores because to include such information adequately would "require several pages and is being saved for the summary treatment, which will not be submitted until all possible factors have, so far as is practical, been investigated, evaluated, and fitted into the complex."

Muller and Muller were not, however, willing to ignore the comment about cowpaths. They described observations comparing the presence of cow droppings between grassland and bare zones revealing more per square meter in "remote uninhibited grassland" than any other locale. "It is clear from these figures," they wrote, "that cows go where grass is and that they do not linger about shrub patches trampling down depauperate herbs over wide areas. . . The great broad white zones of bare ground both inside and outside the shrubs are curved (not "rectilinear") and can scarcely be explained except as inhibition." The roles of smaller herbivores such as sparrows were dismissed as well because "never do they produce a bare area unaided by initial biochemical inhibition" (italics added).

In their response, Muller and Muller were treating biochemical inhibition as a forgone conclusion. According to their interpretation, inhibition was the only reasonable explanation for bare zone formation, a pattern that "never" occurs without it. In defending their hypothesis with such determination, they were demonstrating a confidence unjustified by available data. "The criticism," they continued, "that cowpaths are visible within the shrub zone and may therefore be responsible for the patterning involves the most surprising failure to observe. Steps are being taken to preserve an area of the more instructive patterning. We are desirous of showing this to anyone who will look."