The lack of shrub seedlings and herbaceous plants in the understory of chaparral and related shrublands has led to extensive research on the potential role of allelopathy, which is the chemical suppression by the overstory shrubs of germination (known as enforced dormancy) or growth of understory plants. Often this lack of growth extends to the edge where these shrublands meet grasslands, and forms a distinct bare zone (Figure 2). The importance of allelopathy has long been disputed, with some scientists arguing that animals in the shrub understory are the primary mechanism limiting seedlings and herbaceous

Figure 2 Bare zone between chaparral and grassland. Photo by J. E. Keeley.

species from establishing. While research has not completely ruled out the possibility of chemical inhibition, it is known that for a large portion of the flora, allelopathy has no role in seed dormancy but rather dormancy is due to innate characteristics that require signals such as heat and smoke to cue germination to postfire environments rich in nutrients and light.

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