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

In reviewing their preliminary findings, Muller et al. wrote, "The chaparral fire cycle thus emerges as a sequence of events consequent to the destruction of toxins and their shrub sources by fire. This is followed by the germination of seeds no longer inhibited by shrub toxins." After reviewing additional details, they concluded confidently "allelopathy is the primary basis of the herb fire cycle in California chaparral." McPherson and Muller (1969) published a complete report on what later became known as the "chamise-fire cycle" model the following spring.

McPherson and Muller's first task was to eliminate variables other than chemical. After conducting experiments and concluding nutrient depletion, shading, moisture and oxygen levels in the soil were not significant factors in preventing germination, they turned their attention toward fire. Soil under chamise shrubs was collected (with leaf litter removed) and heated at various temperatures then watered. As temperature levels were raised, more seedlings emerged, with the greatest increase in germination appearing between 80-90° C (175-195° F). "Since heating of the soil and litter mass which contains the seeds brings about increased germination," McPherson and Muller concluded, "it is strongly suggested that the heat degrades some substance in the soil which otherwise suppresses germination."

To eliminate soil as a variable and test the effect of heat alone, seeds of nine native species were heated in ovens and allowed to germinate on wet sponges covered with filter paper. After running experiments at three different temperatures (70°, 80°, 90°C, plus a control), McPherson and Muller wrote, "No species found to be stimulated to germinate in the soil heating experiment could be so stimulated by direct heating of its seeds."

The largest portion of their study focused on establishing the level of toxicity of suspected chamise allelochemicals. Bioassays were conducted in a manner similar to those used in previous experiments. All parts of the plant were tested, but leachate of intact aerial portions (leaves on branches) provided the most positive results. The solutions were prepared by sprinkling water over chamise foliage and concentrated to various strengths. Although seeds treated with normal leachate and then concentrated 1.3 times did show slightly depressed growth patterns, the results failed to demonstrate statistically significant differences when compared to seeds moistened with plain water. However, when concentrated 4 to 10 times, leachate collected during natural rainfall reduced growth of Bromus diandrus (rip-gut grass) seedlings 34% to 75% respectively. Since B. diandrus (species names reflect current taxonomy after Jepson 1993) is non-native and typically not found within undisturbed sites, additional bioassays were attempted with native herbs but the tests were rendered meaningless by mold growth.

The important question of whether or not leachate actually inhibited germination, as Muller's hypothesis suggested, was not resolved.