There are major differences in the climates of the north and south sides of the Transverse mountain range. Think of the great temperature differences between the San Fernando Valley and the Los Angeles Basin. The south side of the Santa Monicas benefits from the marine climate and the moisture-laden breezes that blow in from the ocean. The cross-section of the mountains is generally not symmetrical. The south side, as you drive towards the Valley from Los Angeles, has much gentler slopes, while the north side slopes are much steeper. The crest of the range is offset towards the north due to the greater amount of rainfall coming in from the ocean. The streams on the south side drain south and generally are longer because of greater erosion than the streams on the north side.

This is a complex area, supporting remarkable plan communities. We are fortunate to have this wonderful region as an oasis or island in our midst, almost within walking distance away from the pressures of city life. The average rainfall in Los Angeles is 15"/38 cm. The range of precipitation is from 8" to 30" and is clearly affected by elevation. One side of the mountains will get all the precipitation and the other side very little this is called the rain shadow effect. Which side of the mountain you are on definitely affects the flora and fauna in the Transverse mountains.

Slope effect also effects the vegetation. South facing slopes must be much more drought tolerant. on the south-facing side of the Transverse mountains the chaparral extends all the way to 5000 ft or 1600m. At that elevation on the north side you will find a yellow pine forest, because water is more available because the north-facing slopes experience less evaporation. Northern exposures usually are moister than southern exposures. In the Santa Monicas, the moisture, including fog is on the south side, facing into the sea breezes. However, in the summers, the north slopes may hold the moisture longer than the south slopes.

North facing slope

As with any natural feature, you will find many exceptions to these observations. Many habitats exist in the mountains, with the major factors being exposure to the sun, moisture, slope of the surface, substrate such as the type of rock at the surface, condition of the exposed rock such as weathering and fracturing, and hardness, resistance, and type of soil formed as the rock weathers. The streams offer a riparian habitat that might be quite different from the environment in which the chaparral thrives.

South facing slope

 

Transverse Range Biota by Elevation

 

Fire is an important factor, and many plants have become adapted to survival using various strategies. Indians apparently burned the chaparral every 7-8 years to increase the annual plants and the forage. But stump sprouting can be seen after a fire, and in a few years, the community usually has returned. landslides in the broken rock hills are another factor that influences the vegetation. Chaparral plants generally have deep roots and are able to grow on steep slopes and dryer sites. Grasses are shallow rooted and tend to be seen on flat lands or gentle slopes. Geologists look for grassy slopes as possible evidence of prehistoric land slides. The disturbed soil will tend to hold more moisture, acceptable to grasses, but avoided by chaparral plants.

Different kinds of rock weather into different soils. Poorly cemented sandstones weather and slough off into sandy soils. On some sandstones, seeds are unable to take root, as the sand grains are continually breaking off the outcrop. Decomposed granite also forms sandy soils. Poorly cemented sandstones don not store water as well as clays for example. Clayey soils can develop from weathering of shales or slates. The Santa Monica slates do support chaparral, as can be seen driving through Supulveda Pass. But clays are hard to live in. They expand and contract with water. In dry areas, they form cracks that tear plant roots. Grasses have fibrous roots that can survive the changes in the soil.

The boundaries of the chaparral interfinger with other plant communities. The rock substrate is not consistent over large areas; the depth and type of soils varies like a patchwork over the mountains. In some places there are introduced plants that have thrived, affecting the native plants. The surface varies from inter mountain valleys with riparian habitats, to grassy flatlands to high, steep rocky peaks.

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In the midsummer the average high temperature is 84 and the low is 64. The climate of Southern California is a product of cold ocean water and latitude. It combines maritime and Mediterranean climates. The maritime climate usually prevails within the basin causing a consistent temperature inversion layer. This causes fog, haze and smog. All of which are common within the basin. During the summer a high pressure zone generally prevents precipitation.

Winter in California is characterized by rain and is the primary growing season. Spring is known for its fogs, summer for its haze and smog. In the autumn Santa Ana winds occur, blowing from the Mojave desert to the ocean. Maritime moisture is pushed out to sea and vegetation becomes particularly dry. This is the height of the fire season which is so closely associated with the chaparral.