As vegetation transitions from shrubs to grassland in coastal Northern California, bare zones can be seen bordering those shrubs.
Ecologist over the decades have proposed various hypotheses for these bare zones.
One hypothesis is that the bare zones are created due to resource competition. Shrubs and grasses compete for limited resources such as water, nutrients, and sunlight. As shrubs grow larger and establish their roots, they outcompete the grasses for these resources.
The zone around the shrub, therefore, becomes bare as grasses are unable to grow in this competitive environment.
Allelopathy creates bare zones
Another hypothesis is allelopathy is response for the lack of vegetation around these shrubs. Some researchers have proposed that shrubs release allelopathic chemicals that inhibit the growth of other plants nearby.
These chemicals can suppress germination, growth, or both in the grasses around the shrub. The result is a bare zone surrounding the shrub.
In a paper published in the journal Science, UC Santa Barbara researcher Cornelius H. Muller and his co-authors (1964) argued that airborne substances produced by the leaves of Salvia leucophylla, S. apiana, and Artemisia californica inhibited the growth of oats and other seedlings in the area 60 to 90 centimeters from the canopy of these shrubs.
The creation of scurry zones
In 1970, a graduate student from Stanford named Bruce Bartholomew, published his study that looked at the role of concentrated feeding activity by rodents, rabbits, and birds in this zone.
The impact of small animals and birds on the bare zone around shrubs was something that Muller had mostly dismissed, arguing that the primary force that produce the “halo” around these shrubs was allelopathy.
Bartholomew counter-argued that rodents and birds would take advantage of the protective proximity of shrubs to feed in this zone. Native rodents like Peromyscus californicus (California mouse) and Dipodomys agilis (pacific kangaroo rat) are vulnerable to predators out in the open grasslands.
Shrubs provide easy access to cover and these rodents could quickly run out to the bare zone to scavenge for seeds and small seedlings before returning back into the bushes. Bartholomew proposed that the bare zone was a product of this feeding behavior which would naturally chew down grasses and remove seeds before they could germinate.
To show the influence of these small wildlife on the negation in this zone, Bartholomew captured animals with cages to remove them from shrubs that had been enclosed by fencing to prevent their return. After monitoring these study sites in Los Angeles and San Mateo counties for two-years, the shrubs that were completely enclosed had significant vegetation growth in the halo areas compared to shrubs that were not fenced in.
Bartholomew wrote that the results of his study reinforced his hypothesis about the influence of grazing on the areas around shrubs writing, “annuals will grow in the bare zone with either the presence or absence of volatile toxins if animal activity is excluded.”
While Bartholomew was careful to leave his conclusions open-ended, his study nevertheless set off some pushback by Muller who felt his research on the influence of allelopathy was being undermined (Halsey 2004).
Although it’s not a term used by researchers in scholarly work, some ecologists have named these bare zones “scurry zones” to reflect their use by small rodents and birds who “scurry out” to forage and then “scurry back” to the safety of the shrub.
Bartholomew, B. (1970). Bare zone between California shrub and grassland communities: the role of animals. Science, 170(3963), 1210-1212. DOI: 10.1126/science.170.3963.1210
Bradford, D. F. (1976). Space utilization by rodents in Adenostoma chaparral. Journal of Mammalogy, 57(3), 576-579. https://doi.org/10.2307/1379307
Halligan, J. P. (1973). Bare Areas Associated with Shrub Stands in Grassland: The Case of” Artemisia californica”. BioScience, 23(7), 429-432. https://doi.org/10.2307/1296544
Halsey, R. W. (2004). 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, 343-367. https://doi.org/10.2307/4126940
Hamilton, J. (2015, February 24). A landscape shaped by fear on Mount Diablo. Bay Nature. https://baynature.org/article/landscape-shaped-fear/
Muller, C. H., Muller, W. H., & Haines, B. L. (1964). Volatile growth inhibitors produced by aromatic shrubs. Science, 143(3605), 471-473.