The California Chaparral Institute

Chemical inhibition, or allelopathy, suggests plants are capable of suppressing the growth or germination of neighboring competitors.  Although an intriguing idea, actual chemical inhibition in nature has been notoriously difficult to prove.  “To my knowledge,” wrote plant ecologist J. H. Connell in 1990, “no published field study has demonstrated direct interference by allelopathy in soil…while excluding the possibility of other indirect interactions with resources, natural enemies, or other competitors.”

This lack of scientific verification, however, has not prevented the concept from being presented as a well-understood phenomenon in science texts as well as along chaparral nature trails.  Dramatic explanations are seductive, especially if they provide interesting answers to intriguing problems.  If repeated often enough, they become dogma and influence thinking for decades.

C.H. Muller, an accomplished botanist from the University of California, Santa Barbara, suggested allelopathy explained the lack of plant growth under the canopy of mature chaparral stands in southern California (Muller, et.al. 1968).  According to his hypothesis, chemicals washed off the leaves of chamise and manzanita shrubs, suppressing the germination of seeds in the ground below.  When the chaparral burned, flames denatured the toxic substances releasing the seeds from inhibition.  This resulted in the remarkable number of shrub seedlings and wildflowers emerging in post-fire environments.  The problem with this explanation is that the soil chemicals suspected of suppressing growth actually increase after a fire.  In addition, the dormancy found in chaparral plant seeds is innate, not caused by some outside, environmental factor.  The seeds are dormant before they hit the ground.  Chaparral seed dormancy evolved because poor growing conditions under mature shrubs selected for seed traits postponing germination until those conditions improved.  Under xeric conditions, germinating under a shady canopy with hungry herbivores scurrying around is not a recipe for success.  Fire quickly removes those problems and sets the stage for chaparral renewal.  Post-fire seedling response in chaparral can be easily explained without invoking the notion of chemical inhibition.

Muller also suggested allelopathy was the cause for bare zones often found around purple sage (Salvia leucophylla) and California sagebrush (Artemisia californica) (Muller, et.al. 1964) in the coastal sage scrub community.  Later investigations revealed these bare zones are primarily the work of herbivores (Bartholomew 1971), not volatile substances from the plants themselves.  To little furry rodents like the California mouse (Peromyscus californicus) and the pacific kangaroo rat (Dipodomys agilis) the world is a dangerous place.  Cover is critical to their survival since they are on the dietary preference list of local carnivores like coyotes, snakes, and hawks.  Consequently, they have a tendency to remain under shrubbery with only occasional, quick forays into surrounding grassland to nibble on available seeds or new growth.  They will stray only as far as they can quickly leap back to safety.  Bare zones, therefore, can be viewed as “calculated-risk terrain” where rodents have a fair chance of grabbing food without getting caught.  Bare zones are bare because herbivores exploit the space to grab available snacks.

Do volatile compounds in certain coastal sage scrub plants ever play a role significant enough to make a difference in naturally occurring vegetation patterns?  “As far as I know, the question of why grasses grow within bare zones during wet years, despite animal activity, has never been adequately addressed,” Bob Muller said when reflecting upon his father’s work.  “Why don’t animals always eliminate seedlings, regardless of the level of moisture?

The explanation favored by C.H. Muller provides a reasonable hypothesis for this phenomenon; heavy rains leach toxins from the soil, removing inhibitory chemicals and permitting seedling success.  However, without further investigation the question remains unresolved.

“The critical issue,” John Harper (1975), a prominent plant population biologist from England explained, “is to determine whether such toxicity plays a role in the interactions between plants in the field.  Demonstrating this has proved extraordinarily difficult – it is logically impossible to prove that it doesn’t happen and perhaps nearly impossible to prove absolutely that it does.”