California shrublands
California is home to many vegetation types that are dominated by shrubs. In the California Floristic Province (on the cis-montane side of California's major mountain chains) there are three main types.
Northern Coastal Scrub is dominated by evergreen species with soft, evergreen leaves. It occurs primarily along the coast from Monterrey county north. Its main dominant is Baccharis pilularis, or coyote brush. Since it does not occur close to San Bernardino, we will not discuss this shrubland type any further.
Coastal Sage Scrub occurs along the coast in the southern part of the state. It is dominated by drought-deciduous shrubs and subshrubs that include various species of sage (Salvia spp.), California sagebrush (Artemisia californica) and California buckwheat (Eriogonum fasciculatum). Because this vegetation type can occur far from the coast (such as at CSUSB), people have begun to call this vegetation "California Sage Scrub", or "CSS" (an abbreviation for both Coastal Sage Scrub and California Sage Scrub). Since the leaves of the dominant shrubs in CSS are very flexible, it has also been called "soft chaparral".
A number of species of CSS are endemic to the California Floristic Province. These endemics include many of the sages (Salvia spp.) and California sagebrush (Artemisia californica).
Chaparral is sometimes referred to as "hard chaparral". It is dominated by shrub species with hard, evergreen leaves. Examples of species that dominate chaparral in California include scrub oaks, chamise (Adenostoma fasciculatum), various species of manzanita (Arctostaphylos spp.), various species of Ceanothus, and many more (e.g., Rhus spp., Prunus ilicifolia, Heteromeles arbutifolia, etc.).
In addition to differences in the characteristics of the dominant shrubs, there are differences in the vegetation structure.
Despite the differences between chaparral and CSS, both harbor a large number of species that are endemic to the California Floristic Province.
Leaves of the dominant shrubs and subshrubs of CSS are soft and flexible. Leaves of the dominant shrubs of chaparral are sclerophyllous.
The flimsier leaves of shrubs in CSS are not built to last. Typically, these shrubs shed a lot of their leaf area during the summer drought. (They are drought-deciduous.) Many are facultatively drought-deciduous, meaning that they will retain their leaves through the summer drought if they receive water.
Coastal sage scrub dominants are usually not completely leafless during the summer. They often retain smaller, narrower leaves. An example from the bush monkey flower (Diplacus aurantiacus) is shown below. Leaves produced earlier in the spring are broad; they are dropped as the summer drought develops. Leaves produced later in the spring (under flowers near the tip of the stem) are much smaller, narrower, and their edges are cupped under.
The sclerophyllous leaves of chaparral dominants, in contrast, are tough and last much longer. These shrubs are evergreen, maintaining their leaves throughout the year.
A less obvious difference between the dominant shrubs of CSS and those of chaparral is the difference in rooting depth.
Shrubs and subshrubs of CSS tend to be much more shallowly rooted than shrubs of chaparral. Rooting profiles, studied along roadcuts and faces of gullies in the 1950s illustrate this difference. Although chaparral shrubs vary in rooting depth, many are quite deeply rooted. In contrast, the dominants of coastal sage scrub, such as the black sage (Salvia mellifera) have fairly shallow roots.
This difference affects how the shrubs see the summer drought:
The deeper-rooted chaparral dominants have access to supplies of water stored deeper in the soil profile throughout the summer. This allows them to supply some water to their leaves during the summer. This allows the leaves to remain alive through the drought, permitting an evergreen habit.
In contrast, shallowly rooted shrubs and subshrubs experience a stronger drought during the summer. They can only access the upper layers of soil that dry earlier and more thoroughly in the spring and summer. The strategy that works for these plants is to drop their larger leaves during the summer, reducing evaporative demand and water stress in the plant.
[Note: When we talk about plant "strategies", we do not mean that plants think ahead. In this context, a "strategy" is a set of traits or responses that work together to benefit the plant. In this case, a plant can be either deeply-rooted or shallowly-rooted in a mediterranean-type environment, but it can't be both shallowly rooted and evergreen. The leaf traits and rooting habits have to work together.]
Xylem water potential is a measure of the tension in the water column in the xylem. A more negative water potential indicates a greater tension. Both evaporative demand by leaves and a low water supply rate from roots can increase tension (lower water potential). The highest xylem water potential that a plant experiences during the day is right before dawn, after stomata have been shut all night, stopping evaporative demand.
Early research compared seasonal changes in the xylem water potential of Heteromeles arbutifolia (toyon), an evergreen shrub of chaparral, to that of Salvia mellifera (black sage), a drought-decidous dominant of coastal sage scrub.
Check your ability to understand these data:
Not only do drought-deciduous shrubs lose leaf area during the summer, but the photosynthetic rate of the remaining leaf area goes down (that is, the rate of carbon uptake per cm2 of leaf area per unit time). Spring leaves are not only larger, but they have a higher rate of photosynthesis per unit leaf area, than summer leaves.
Can you interpret these data?
Normally, plant growth is best when temperatures are reasonably warm (not too cold and not frying). The challenge for plants living in a mediterranean climate is that, when water is readily available, temperatures are too cool for maximal growth, but as temperatures warm up, the soil starts to dry.
Deep-rooted plants have access to more stored soil water through the summer drought, so they can continue photosynthesizing and growing later into spring and early summer.
However, because the dominants of coastal sage scrub are shallow rooted, they...
Therefore, they cannot grow as long into the late spring and summer as the evergreen chaparral dominants.
These considerations appear to have selected for differences in when different shrub types start growing in the season. Look at the figure below. It shows when different species begin producing new stems (shown as the percent of plants in a species with new shoot growth, censused repeatedly over the year).
Understand the graph above:
Evergreen sclerophyllous shrubs tend to grow a bit slower than the drought-deciduous shrubs. However, they also tend to grow taller. In places where both types of shrubs can grow and survive, faster-growing drought-deciduous shrubs may intially dominate the landscape after a disturbance. In time, however, the slower-growing evergreen shrubs may over-top them and shade them out.
Chaparral tends to dominate slightly higher elevations than coastal sage scrub.
Flat areas of campus and some of the slopes are dominated by California sage scrub, but some of the steeper north-facing slopes on campus and most of the slopes in the mountains behind campus, including much National Forest land, are covered in hard chaparral.
Küchler (1977) produced a "potential vegetation map" of California, which mapped both where different vegetation types were and where they were presumed to have been before urbanization and agricultural development eliminated them in places.
Because California sage scrub (or coastal sage scrub) is distributed on gentle slopes it has been easier to develop and turn to other uses (urban uses and agricultural uses) than land that is occupied by most other vegetation types. As a result, most of the CSS area has been lost. For that reason (and because it contains a large number of endemic species), CSS has been a focus of conservation efforts for many years now.
