Chaparral is a shrub-dominated vegetation with other growth forms playing minor or temporary successional roles after fire. More than 100 evergreen shrub species occur in chaparral, although sites may have as few as one or more than 20 species, depending on available moisture, slope aspect, and elevation. The most widely distributed shrub is chamise (Adenostoma fasciculatum), ranging from Baja to northern California, occurring in either pure chamise chaparral or in mixed stands. It often dominates at low elevations and on xeric south-facing slopes. The short needle-like leaves produce a sparse foliage, and soil litter layers are poorly developed and result in weak soil horizons. Chamise often forms mixed stands of vegetation with a number of species. These include the bright smooth red-barked manzanita (Arctostaphylos spp.), the sometimes spiny ceanothus, also known as buckbrush or California lilac (Ceanothus spp.). On more mesic north-facing slopes chaparral is commonly dominated by broader-leaved shrubs, including the acorn-producing scrub oak (Quercus spp.), the cathartic coffeeberry (Rhamnus californica), red-berry (R. crocea), the rather bitter chaparral cherry (Prunus ilicifolia), and chaparral holly (Heteromeles arbutifolia), from whence the film capital Hollywood derives its name.
The most common shrub species and the majority of herbaceous species have fire-dependent regeneration, meaning that seeds remain dormant in the soil until stimulated to germinate after fire (see the section titled 'Fire' below). These include chamise, manzanita, and ceanothus shrubs, which flower and produce seed most years but seldom produce seedlings without fire. Some ceanothus species are relatively short-lived or are easily shaded out by other shrubs and die after several decades. They, however, persist as a living seed pool in the soil. In addition, a large number of annual species live most of their life as dormant seeds in the soil, perhaps as long as a century or more. Also, many perennial herbs with underground bulbs, known as geophytes, may remain dormant for long periods of time between fires.
All of the other shrub species listed above are not fire dependent and produce seeds that germinate soon after dispersal; however, successful reproduction is relatively uncommon. This is because their seedlings are very sensitive to summer drought and because there are a number of herbivores that live in the chaparral understory and prey on seedlings and other herbaceous vegetation. These include deer mice (Peromyscus maniculatus), woodrats (Neotoma fuscipes), and brush rabbits (Sylvilagus bachmani). Both rodents (mice and rats) are nocturnal; however, evidence of woodrats, or packrats as they are sometimes called, is very evident in many older chaparral stands because of the several foot high nests of twigs they make under the shrub canopy. These animals not only affect community structure by consuming most seedlings and herbaceous species, but also are important vectors for disease and other health threats. For example, deer mice are host to the deadly hanta-virus and woodrats are a critical host for kissing bugs (family Reduviidae) that can cause lethal allergic responses in humans. All animals including reptiles act as hosts for Lyme disease-carrying ticks (Ixodespacificus). The browser of mature shrubs is the black-tailed deer (Odocoileus hemionus), although many are attacked by specific gall-forming wasps and aphids. Often scrub oak will have large fruit-like structures produced by gall wasp (family Cynipidae). The adult wasp oviposits on a twig, leaf, or flower and the developing larvae hijack the metabolic activities of the plant cells and force it to produce a highly nutritious spongy parenchymous tissue for the developing wasp larva.
These shrubs that reproduce in the absence of fire have successful seedling establishment largely restricted to more mesic plant communities such as adjacent woodlands, or to very old chaparral with deep litter layers that enhance the moisture holding capacity of the soil. When seedlings do establish under the shrub canopy, they typically persist for decades as stunted saplings in the understory. These saplings are heavily browsed by rodents and rabbits and often will produce a swollen woody basal burl that survives browsing and continually sprouts new shoots. If these saplings survive until fire, they are capable of resprouting from their basal burl after fire and exhibit a growth release that enhances their chances of recruiting into the mature canopy during early succession. Thus, in some sense these shrubs may be indirectly fire dependent for completion of their life cycle.
Chaparral has a number of herbaceous or woody (lianas) vines, including manroot (Marah macrocarpus) and chaparral honeysuckle (Lonicera spp.). These vines overtop the canopy of the shrubs and flower on an annual or near-annual frequency. The former produce fleshy spiny fruits with very large seeds that are highly vulnerable to predation and the latter dry capsules with light seeds that may be wind borne. Both have weak seed dormancy and often establish seedlings in the understory.
Yucca (Yucca whipplei) is a fibrous-leaved species that persists as an aboveground rosette of evergreen leaves. It often survives fire because it prefers open rocky sites with very little vegetation to fuel intense fires. Because they are monocotyledonous species they have a central meristem that is protected by the outside leaves, which can withstand severe scorching. This species flowers prolifi-cally after fire and exhibits a remarkable mutualism with the tiny yucca moth (Tegiticula maculata). Moth pupae survive in the soil and emerge in the growing season as adults that fly to yucca flowers where they collect pollen. They then instinctively fly to another yucca plant and pollinate the flower, ensuring cross-pollination, and then oviposit an egg in the base of the ovary. This egg soon hatches and the larva feeds on the developing seeds. Yucca moths only reproduce on yucca flowers and yuccas apparently require the pollinator services of this moth for successful seed production, a classic example ofsymbiosis.
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