Seed Dispersal

Shrubs can be divided into those with temporal dispersal versus those with strong spatial dispersal. The former are the fire-dependent species that accumulate dormant seed


Figure 4 Dominance-diversity curve based on cover of species in sequence from highest to lowest from postfire chaparral.

banks, which in essence disperse these shrubs in time, from one fire cycle to the next. Within this group there is limited spatial dispersal. Ceanothus have explosive capsules that shoot seeds a short distance of a meter or two from the parent shrub. Manzanitas drop most of their seeds beneath the parent plant because their small dry fruits are not attractive to birds, although a small number of the seeds are distributed further by coyotes (Canis latrans) and bears (Ursus americans, historically also included U. horribilis). Chamise produces small light fruits that may be carried tens of meters or more by the wind but it appears that most are distributed around the parent shrub.

The postfire endemic annuals also have seeds that are largely dispersed in time rather than in space. Most do not have characteristics suggestive of widespread dispersal. For example, the fire-following whispering bells (Emmenanthe penduliflora) derives its name because the flowers and fruits are pendulous and drop seeds directly beneath the parent plant. Postfire endemics in the sunflower family (Asteraceae), a family noted for well-developed dispersal with dandelion-like pappus, commonly have deciduous pappus, which ensures dispersal in time rather than in space.

Shrub species that exhibit fire-free (nonfire-dependent) reproduction have fruits highly attractive to birds and mammals, and the bulk of the seed crop appears to be dispersed by these vectors. Seedling recruitment is sensitive to desiccation and thus it is of some significance that one of the main dispersers of these fruits, the scrub jay (Aphelocoma californica), preferentially caches seeds in the shade.

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