In the far southeast corner of California’s South Coast Range lies a beautiful arid plain. It’s a comparatively desolate portion of California, tucked between the Caliente and Temblor mountain ranges. For much of the year the plain is stricken in “drought” – a prolonged dry season descends on the plain while a relentless sun dries a crisp layer of crust over the land. Western settlers once tried their hand at farming the plain, but mostly succumbed to the extremes of heat and aridity. Cattle and sheep grazing were also tried, but the conditions always proved too difficult for more intensive grazing operations. 

Today, the Carrizo Plain is home to “the largest single native grassland remaining in California” (1). It’s also home to an amazing diversity of rare wildlife. As such, the Carrizo Plain National Monument remains one of California’s most pristine swaths of ecology.

The Carrizo Plain lies about 60 miles inland from the California coastline and is tucked behind several other intermittent mountain ranges of California’s South Coast Range. West of the plain itself are the Caliente and La Panza ranges. To its east lies the rolling Temblor Range. 

The Carrizo Plain is effectively shadowed from winter storms that come off the Pacific Ocean. Weather patterns in California are directed by the Pacific Ocean and its interplay with the North Pacific High, which is an area of high pressure that moves up and down the west coast of North America. During Winter, the North Pacific High moves southward. This movement south allows storms developing in the Gulf of Alaska to also move south. This generally causes winter storms to move into California from the northwest, but who receives the bulk of precipitation depends on the surrounding topography. 

When winter storms brush across the northwest portion of the South Coast Range, much of the precipitation is dumped on the intermittent peaks and ridges. By the time storms reach the Carrizo Plain, there is far less precipitation left in the storm front. The town of Big Sur, which lies near the northwest corner of California's outer South Coast Range, receives about 35" of rainfall per year while the Carrizo Plain receives an average rainfall of 6" to 8”. The disparity in precipitation levels stems from the fact that the Carrizo Plain is situated within a rain shadow.  

In addition to marked differences in precipitation levels, the rain shadow effect also influences temperature extremes and evapotranspiration rates. For example, Big Sur experiences summer fog and frequent marine layers that mitigate against California's long dry summer. But, the Carrizo Plain is cut off from those temperate influences. The rain shadow blocks cool coastal fog and low-lying marine layers. As a result, the Carrizo Plain experiences many more days of sun. The abundance of sun drives up day time temperatures and plant evapotranspiration rates. This is all evident in the fact that the Carrizo Plain supports plant species more notably adapted to long periods of drought.

Map showing the South Coast Range of Central California. The Carrizo Plain National Monument lies in the southeast corner.

Since the North Pacific High drives much of California's winter weather patterns, California is also beholden to its behavior and influence over the jet stream. When the winter jet stream hangs out in the higher latitudes, only weakened winter storms will reach central and southern California. Southern California will likely accumulate a less than average winter rainfall total. And, when like conditions repeat themselves, these less than average winter rainfall totals may repeat and send southern California into a periodic drought cycle. 

Periods of sustained dry are exactly what limits the establishment of long-lived trees and shrubs in the Carrizo Plain. Long-lived woody vegetation generally requires a perennial sub-surface water source that is seasonally replenished. Additionally, a lack of maritime influence leads to high solar insolation, causing soil moisture to be lost to evaporation. This low moisture content is a limiting factor for long-lived woody vegetation, like oak trees or pine trees. 

One other distinguishing trait of the Carrizo Plain is its abundance of heavy clay soil. The abundance of clay soil is derived from the parent material throughout the Carrizo Plain. The parent material is primarily made of sandstone and shale. As the sandstone and shale materials weather, it erodes to produce layers of fine-grained clay particles. Clay soil can be a notoriously difficult soil structure for plants. When clay soils are wet, they become sticky and waterlogged, which displaces soil oxygen needed by plants for gas exchange and water uptake. Clay soils also have a high shrink swell capacity. Their fine-grained composition and smooth surface area means water easily adheres to clay particles, causing the water molecules to hold a high level of hydrostatic pressure. As such, water percolates more slowly and plants must be able to endure periods of inundation. Then they dry out. As daytime temperatures rise during Spring and Summer, the clay soils become increasingly dry.

​California's winter wet climate pattern means the clay soils retain moisture during the spring when plant evapotranspiration rates and soil evaporation rates remain relatively low, offering a great incentive for plants to grow quickly before the hot temperatures set in. It's also why many of the grasses and forbs in the Carrizo Plain are spring blooming annuals.

Annuals have evolved to respond quickly to favorable microclimatic conditions. They quickly grow, flower and set seed prior to any onset of hot and dry conditions. In a way, they've evolved to sustain dry periods, or even periodic drought cycles, by timing their growth and reproduction cycles accordingly. Their annual lifecycle allows them to quickly produce an annual seed bank, which can bloom the following Spring or lie dormant until more optimal growing conditions return.

When a series of healthy winter storms provide California with greater than average rainfall, the Carrizo Plain often benefits as well. And true to form in California's weather patterns, these greater than average rainfall years sometime follow a year or several years’ worth of drought-like conditions. Subsequent dry years followed by an intermittent wet year is the natural phenomena of the North Pacific High. When such conditions play out, the Carrizo Plain has the propensity to become a wonderland of spring time flowers. This is especially true when repeated dry years cause an abundant seed bank to build up.

Dry years are what gives the Carrizo Plain's flora the upper hand against the invasion of non-native grasses or exotic plants. It's almost as if the long term ecological health of the Carrizo Plain is dependent on periodic drought.

Ever since the non-native annual grasses were brought to California, California's flora has been fighting an uphill battle. Roughly 50% of the Carrizo Plain’s vegetation matrix today is composed of non-native vegetation. The upper hand that non-native grasses do have is their ability to support rapid short-term gain. These non-native grasses capitalize off an intermittent influx of precipitation by growing fast and flowering heavily, a trait that disperses an abundance of seed into the seed bank. Over time, by sheer number and presence, the short-term gains of non-native annuals add up. Couple that with practices like agriculture and grazing that disrupt the stasis of natural environments, non-native annual growth begins to outpace the annual growth of any native vegetation. But sometimes that upper hand only goes so far. 

Despite occurring in abundance, annual non-native grass seeds are also relatively short lived. Repeated dry years can exhaust reserves of non-native seed. On the other hand, the native seeds are more accustomed to prolonged periods of dry.

The non-native seeds outcompete native seeds by producing copious amounts of seed that capitalize off fair amounts of precipitation. But when drought sets in, the non-native seeds fail to germinate in abundance. Native seeds know how to wait patiently for optimal conditions to return. They have learned to favor the long game that is natural climate variability. Native seeds have been produced through eons of generational succession and shifting environmental parameters. This enables them to be intricately tuned to edaphic conditions in ways that non-native seeds are not. It's a large reason why above average rainfall years in the Carrizo Plain are a floral wonderland.  

Throughout California, soil type and precipitation patterns can be great equalizers. In the Carrizo Plain, these conditions may make life harsh for plants (and for that matter all biologic life), but it's also through such fitness of adversity that its ecology prevails. Quite possibly, its position within the rain shadow may be one of its greatest assets.

Additional Reading

Fremontia - Journal of the California Native Plant Society    Vol. 39 No. 2 & Vol. 39 No. 3

Additional Resources

Nature Conservancy

Carrizo Plain Conservancy

Friends of the Carrizo Plain

Bureau of Land Management

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