Ecological site group description

Provisional. A provisional ecological site description has undergone quality control and quality assurance review. It contains a working state and transition model and enough information to identify the ecological site.

Physiography

This ecological site is generally found on ridges, knobs, and the upper elevation slopes generally between 15-75% within the forested areas of LRU K. This site is generally found above 800 ft. but can be lower near the southern end of the LRU.

Climate

The average annual precipitation in this MLRA is 23 to 98 inches (585 to 2,490 millimeters), increasing with elevation inland. Most of the rainfall occurs as low-intensity, Pacific frontal storms. Precipitation is evenly distributed throughout fall, winter, and spring, but summers are dry. Snowfall is rare along the coast, but snow accumulates at the higher elevations directly inland. Fog is a significant variable that defines this MLRA from other similar MLRAs. Summer fog frequency values of greater than 35% are strongly correlated to the extent of coast redwood distribution, which is a primary indicator species in this MLRA. Nightime fog is approximately twice as common as daytime fog and seasonally, it reaches its peak frequency in early August, with the greatest occurrence of fog from June through September (Johnstone and Dawson 2010). The average annual temperature is 49 to 59 degrees F (10 to 15 degrees C). The freeze-free period averages 300 days and ranges from 230 to 365 days, decreasing inland as elevation increases.

Unlike the conifer-dominated forests of LRU I to the north, these central redwood forests in LRU K are typically more a mixture of conifers and hardwoods. Vegetation includes a multi-story canopy of redwood, Douglas-fir, tanoak, bigleaf maple, evergreen shrubs, and various grasses. The near-coastal part of the region that is influenced more by fog has more redwoods and similarities to LRU I to the north, however the drier summers and more limited duration of coastal fog limits the competitive advantage of the coastal redwoods in this LRU. This creates limited areas that remain dominated by coast redwood and a larger portion of the LRU dominated by Douglas-fir and other hardwoods, with some redwoods near the lower parts of the mountain slopes where fog still has some influence or the drainages are narrower and remain cooler in the summers, limiting evapotranspiration losses.

Soil features

Although the soils of this provisional site concept are highly varied, they all share the common variable of steeper slopes with a bedrock restriction, higher runoff rates, and loamy soil textures that provide good growing conditions for shallower rooted herbaceous species.

Vegetation dynamics

This provisional ecological site concept attempts to describe the various complicated areas of coastal prairie in LRU K. They exist in a patchy dynamic intermixed with coyotebrush shrublands, and Douglas-fir and/or redwood forests. These prairie complexes relied heavily on hundreds of years of Native American burning to maintain their dominance in an area that provided no limitations to on the growing conditions for trees that would have overtaken many of these prairies without the fires to burn them back. This concept lumps many of the unique prairie expressions into one large concept, due to limited soil mapping that successfully parses out the differences between these types and focuses primarily on the primary abiotic factors and ecological dynamics that maintain and/or alter these vegetative communities. The extent of this ecological site concept stretches primarily along the inner coastal mountain portions of LRU K.

The ustic-isomesic soil climate regime of this LRU is mainly at elevations between 500 and 2,000 feet. It is within the zone of moderate marine influence. The fog influence is less pronounced than in the udic moisture regime, but some moisture is added to the soil where the tree canopy causes water to precipitate from the fog. The fog is less dense and does not blanket this zone as frequently as in the wetter zone at the lower elevations. The soils are dry for part of the summer, and there is little variation between summer and winter soil temperatures at a depth of 20 inches. The first continuous north-south range of mountains inland from the coast that reaches 2,000 feet forms an effective barrier to the encroachment of marine air. In some drainageways, such as the Noyo River watershed, this zone extends inland 20 miles or more. In other areas, such as along Elkhorn Ridge 7 miles north of Branscomb, the marine influence stops within 8 miles of the coast.

Abiotic Factors

The primary factors that maintain these sites in either coastal prairies, coyotebrush shrublands, and Douglas-fir and/or redwoods are related to time without fire, the skeletal soils, and seed source availability.

Primary Disturbances

The primary disturbance to this ecological site concept is fire and active soil movement on the steeper slopes through surface runoff that keeps the woody species that are actively encroaching the site from establishing, allowing these grasslands to maintain the competitive advantage. When summer fog is less prevalent, tree and shrub encroachment will slow and years when summer fog is very consistent tree and shrub encroachment may be quite rapid.

Historically, prairies within this region were thought to have been dominated by native perennial bunchgrasses and numerous associated forbs. Native Americans utilized the prairies for food and cultural materials. Regular burning stimulated the growth of grasses and eliminated invading shrubs and trees, thereby attracting wildlife. The use of fire for over 5,000 years by Native Americans created a system in equilibrium that controlled the vegetative structure and composition.

With the advent of European settlements, changing land use practices significantly altered the vegetation. In the 1800s cattle and sheep grazing became widespread. Increased grazing pressure from domestic livestock and range seeding reduced the native perennials and increased the population of introduced perennials and forbs. More studies are needed to understand grazing and native plant interactions. Shifts in the annual plant community caused by grazing are difficult to document. Certain species will increase with favorable weather and grazing conditions.

Non-native grasses often outcompete natives for water, nutrients and growing space. Arrhenatherum elatius (tall oatgrass), an introduced perennial within these prairies, is considered an invasive exotic. One study indicates that early season burning may be more effective in eliminating flowers and developing seeds of tall oatgrass prior to their dispersal. However, spring burning has a negative effect on some of the native perennials, like Danthonia californica (California oatgrass). Fall burning has slowed the advance of tall oatgrass within parts of this LRU.

Prescribed burning may favor one species over another. Recent studies indicate that periodic fire may favor perennial species by reducing litter cover and eliminating other plant competition, however it may also increase the production of non-natives and exotic forbs. Long term studies are lacking to evaluate the interaction of prescribed fire, climate, and grazing on both natives and non-native species.

Historically, there was very little overlap between the prairie and conifer systems within much of this LRU. Fire exclusion in the last century has allowed for the encroachment of shrubs, and in some cases trees, into the prairies. Roads established for harvesting purposes left exposed cut and fill slopes that were rapidly invaded by Douglas-fir. Invasion of prairie by conifers has led to conversion to forest in a very short period of time.

Major Land Resource Area

MLRA 004B
Coastal Redwood Belt

Stage

Provisional

Contributors

Kendra Moseley