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

Beach dunes represent the more rapidly changing to older, more stabilized areas directly adjacent to the beaches in LRU A. They consist of actively moving to well vegetated dunes that are all oriented parallel to the prevailing winds. Many of the older, more stabilized dunes covered in coastal scrub and forest are being transitioned back to actively moving, poorly vegetated dunelands due to changing winds and human alterations and land uses. The extent of this ecological site concept stretches from the back side of the foredunes to the edge of the back dunes.

Climate

This ESG is characterized by frequent on shore winds that can carry airborne salt spray, generally mild to cool conditions with consistent temperatures and regular marine cloud cover due to the immediate proximity to the Pacific Ocean. 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 there is little rainfall in the summer. Snowfall is rare along the coast, and fog and low clouds are significant features that define this MLRA from other similar MLRAs. Nighttime 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).

This LRU is primarily influenced by hydrological processes and contains beaches, dunes, rivers, and marine terraces below 400 feet elevation. Wet forests, lakes, estuarine marshes, and tea-colored (tannic) streams are characteristic features of this LRU. Marshes and wetlands have been widely altered and/or drained with many converted to agriculture and urban developments. Soil moisture regimes are udic and aquic and soil temperatures are isomesic. Dune communities, grassland, coastal scrub, beach pine, bishop pine, and Sitka spruce on floodplain soils are more typical in this LRU, in comparison to the redwoods that dominate LRU I. Riparian areas contain red alder and salmonberry, and, in a few areas, some scattered redwoods. In California, the region includes the Crescent City Plain, and Humboldt Bay Flats and Terraces.

Soil features

The soils associated with this ESG range from isomesic to mesic depending on the degree of vegetation cover and stability. Soils that are representative of this ESG are the Lanphere and Samoa soils which are both Typic Udipsamments. They are generally deep soils that are somewhat excessively to excessively drained and have moderately rapid permeability.

Vegetation dynamics

This provisional ecological site concept covers a wide variety of dune dynamics and expressions that may need to be further refined to better represent dynamics on a smaller, more ecologically specific scale. For the purposes of this provisional concept, however, all that variation is lumped into one site concept.

This site has highly variable dynamics that include active dunes, dune mats, dune swales, and dune forests. The dominant community phase is the most stable community phase and is dominated primarily by the dune forest vegetation. These dune forests are found on the stabilized back dunes and are some of the most significant portions of intact dune forest left in California. It is dominated by two distinct and recognizable forest types, Pinus contorta spp. contorta (shore pine) dominated type, a Picea sitchensis (Sitka spruce) dominated site, and a mixed conifer type. Pseudotsuga menziesii (Douglas-fir) is found in the shore pine and spruce types, whereas Abies grandis (grand fir) is found in all three, but most dominant in the mixed conifer type. They are the most stable, having been dominated by significant tree cover for decades and have a variety of shrubs in the understory. Dominant species in the understory communities are commonly Vaccinium ovatum (California huckleberry), Myrica californica (California wax myrtle), Gaultheria shallon (salal), Arctostaphylos uva-ursi (kinnikinnick), and Lonicera involucrata (twinberry honeysuckle).

This ecological site is highly susceptible to invasives, especially in Community Phase 1.2, given the more transitory nature of these communities they are primed for openings by non-natives that can capitalize quickly on available resources much better than the native species can. The dune mat community is the most at-risk for invasion and is dominated by the aggressive Ammophila arenaria (European beach grass). It is native to Europe and is a successful sand-binder that monopolizes the dune mat communities and outcompetes the native beach grass and other native forbs. It alters sand movement, decreases invertebrate abundance and diversity, and most significantly builds a steep, continuous foredune replacing the low, hummocky foredunes that are representative of the reference state dunelands.

Abiotic Factors/Primary Disturbances

Salt spray, soil salinity, sea-water immersion and sand movement are the most critical abiotic factors for this ecological site concept, primarily in the context of geologic processes and coastline water and wind currents. The salt spray gradient is a function of wind speed, distance from the tide line, height above the ground, and microtopography. Hypertrophy (succulence) accounts for the ability of dicots to withstand salt spray, and grasses that lack this ability, rely on a thick cuticle instead. Salt spray tolerance dictates the species composition that dominates the different areas of this ecological site, but also impacts the structure of the vegetation expression as well. Although beach species are not obligate halophytes, they are tolerant of soil salinity and occasional seawater immersion. Germination and establishment phases of most beach species is when they are the most vulnerable to the soil salinity, and therefore depend on times that seawater immersion and impacts from salt water are minimal until after full establishment.

Water deposition, onshore winds, desiccation, nutrient limitations, and sand burial are the most important disturbances that naturally drive the dynamics of this ecological site concept. Adaptations to sand burial are key adaptations for vegetation in this ecological site, developing larger, heavier seeds that have the ability to emerge from much greater depths once germinated. These species have also adapted to planting themselves deeper in the sands to take advantage of higher soil moisture content and protection from the wind/sea salt spray. Other adaptations include plant morphologies and canopy densities. Different accumulations of sands occur with different plant canopies, for example plants with intermediate canopy densities with loose, cylindrical silhouettes cause a decrease in wind velocities within the plant canopy leading to accretion of fine sands and the formation of hummocks typical of this ecological site.

Desiccation is a major stressor to this ecological site, namely from intense solar radiation and evapotranspiration, seasonal drought, and low water-holding capacities of the sands typical of this ecological site. The species that dominate this ecological site are adapted to these conditions, using various rooting strategies, leaf morphologies that can withstand high levels of solar radiation, high light intensities for photosynthesis, and wind desiccation, and root and shoot strategies to ameliorate the low-fertility soil conditions.

Dune morphology begins when beaches with vegetation create shadow dunes or beach mounds. These shadow dunes occur when a plant causes wind to be deflected, slowing its speed and dropping sand particles into an elongated tongue of sand in the lee of obstacles. Changes in wind direction result in accumulations on all sides and the formation of a beach mound. These can be easily blown out by wind gusts or winter waves. However, when they are built on relatively wide areas of beach, a foredune can develop. A foredune is a vegetated ridge of sand parallel to the beach, rising above the ordinary high tides. These foredunes support greater species richness and plant cover than the upper beach, but fewer species that are found just inland on dune ridges. These incipient shadow dunes and the briefly stabilized foredunes are a part of the other ecological site, R004BA200CA Beaches, which captures the areas of the coastline that are more rapidly changing and consistently impacted by seasonal storms and weather events, shifting the site back and forth between unvegetated beaches to reasonably stabilized beach foredunes only. Anything further away from the winter storm high tide marks that can develop into more stabilized, well vegetated sand dunes are a part of this ecological site concept.

Blowouts that occur in a continuous foredune may result in incipient parabola dunes that later stabilize with vegetation, becoming relatively narrow dune ridges. Ridges alternate with depressions classified as dune swales when they are deep enough to support distinct vegetation and can become classified as deflation basins that are poorly drained and support a distinct ecological site, R004BA206CA Deflation Basins site. Where large areas of mobile sand occur in combination with a wide receptive area and unidirectional winds, transverse dunes form. Crests of these dunes are sinuous, oriented perpendicular to wind direction, and of variable length. These transverse dunes form during low sea levels with abundant sand supply from prograding shorelines and the exposed continental shelf. These dunes move too rapidly to support all but transient vegetation and they may even become superimposed over the large-scale parabola dunes that are formed in areas of stabilized vegetation. These transverse dunes, parabola dunes, and large areas of sand sheets make up the active dune portions of this ecological site concept. In the stabilized, vegetated back dunes, older ridges and stabilized parabolas support forest dune vegetation and troughs support swale vegetation or deflation basin vegetation. This dune ecological site may need to be divided into more ecological site concepts based on more locally specific site dynamics and abiotic factors, but at this time these dynamics are all included in one provisional ecological site concept.

Major Land Resource Area

MLRA 004B
Coastal Redwood Belt

Stage

Provisional

Contributors

Kendra Moseley