Ecological dynamics

This ecological site occupies marine terraces outside Trinidad, CA and Crescent City, CA and is largely contained with the coastal fog belt. This site is of limited extent, and as no late successional stands of this site remain on the landscape the reference plant community is inferred.

Redwood (Sequoia sempervirens) and to a lesser extent Sitka spruce (Picea sitchensis) dominate this site which also has a productive understory of salal (Gaultheria shallon), California huckleberry (Vaccinium ovatum), and western swordfern (Polystichum munitum). Although redwood is the dominant species on this site, the close proximity of to the coast may promote the establishment of Sitka spruce in the overstory (Franklin and Dyrness 1973).

The range of redwood is largely influenced by coastal fog, which ameliorates the effects of solar radiation on conifer transpiration rates (Daniel 1942). Fog is a critical source of water in the drier summer months for redwood, which has high transpiration rates. Fog drip and direct fog uptake by foliage may contribute significant moisture to understory species and the forest floor (Dawson 1998).

The northern range of redwoods evolved within a low to moderate natural disturbance regime, with severe fire intervals ranging from 500 to 600 years on the coast (Veirs 1979). Fires could have historically occurred by lightning ignition or deliberate setting by Native Americans to create desirable hunting habitat (Veirs 1996).

Surface fires may modify tree species composition by favoring thicker-barked redwood and killing grand fir (Abies grandis) and mature western hemlock (Tsuga heterophylla) (Veirs 1979). Redwood has the ability to resprout following fire from the root crown or from dormant buds under the bark of the bole and branches (Noss 2000), but shallow roots and thin bark make both western hemlock and Sitka spruce susceptible to fire damage (Arno 2002, Griffith 1992). However, frequent surface fire may promote establishment of western hemlock in the understory by exposing mineral-rich soil and reducing competition (Veirs 1979). In contrast, Douglas-fir seedling success may be decreased with a light fire regime (Mahony and Stuart 2000).

Moderate fire, wind disturbance, and management decisions can create a mosaic in regeneration patterns. Previous harvest and the use of fire as a slash treatment can alter species composition on many sites (Noss 2000) as repeated burning can favor resprouting of redwood and hardwoods and limit the regeneration of other conifers. Wind damage from winter storms can cause canopy top breakage which may kill individual trees or create windthrow gaps in the forest (Noss 2000). Canopy gap creation or selective redwood cutting could favor Sitka spruce growth and lead to a larger proportion of Sitka spruce in the stand.

Another mechanism through which Sitka spruce could become more firmly established in the stand is the propensity for Sitka spruce to rapidly invade adjacent coastal prairies after cessation of burning, grazing, or tilling (Franklin and Dyrness 1973). If land clearing and stump removal did occur, redwood regeneration may be slow to infill onto the site. Additionally, areas of this site on the boundary of coastal sea spray influence may have a greater composition of Sitka spruce as Sitka spruce has a high tolerance for salt spray (Zinke 1988). Some areas of this site adjacent to Lake Earl near Crescent City and Big Lagoon near Trinidad have a greater composition of Sitka spruce, suggesting the influence of both proximity to the coast and previous land clearing.

Slope may have a significant influence on the land use of this site. Some areas of this site around Crescent City with gentle (0-2%) slopes have been cleared and used for pasture. This pasture phase is not documented in the state and transition model for this site as it is of limited extent. Although of minimal acreage, this pasture land could be a distinct plant community phase if kept open by intensive grazing or clearing. As discussed in ecological site F004BX118CA, these pastures will be encroached by pioneer shrubs and conifer seedlings if grazing ceases. Additionally, sites with steeper slopes (reaching 60%) may have limitations on the type of harvesting methods that can be employed.

Red alder (Alnus rubra) is effective at rapidly colonizing disturbed landscapes following ground disturbance, harvest, or fire. Several thousand red alder per acre initially outgrow and dominate any conifers that become established in the disturbed area. Red alder is able to fix nitrogen with a symbiotic relationship with an actinomycete located on its root nodules (Bormann and Gordon 1984). These significant inputs of nitrogen to the ecosystem by red alder can increase overall stand productivity (Hart et al 1997). Shade intolerant red alder will eventually decrease in the stand as conifer regrowth reaches greater canopy heights.

California huckleberry and salal occupy a large percentage of the understory on this site. California huckleberry is a dominant shrub species across redwood ecological sites as it can thrive in both moist and dry environments. As California huckleberry is typically a fire-dependent species, sprouting can be widespread following natural fire or site preparation treatments (Tirmenstein 1990b). Salal increases significantly after harvest and can even reduce regeneration and stocking of Douglas-fir (Tirmenstein 1990a). Western swordfern can grow in a range of light conditions and can be often indicative of moist, productive forest habitat (Crane 1989).

This ecological site occupies young marine terraces near Trinidad and Crescent City. The marine terrace sequence around Trinidad demonstrates the fluctuations of sea level and tectonic uplift over the past 400,000 years. Six distinct marine terraces are identified in this area, the sediments of which we deposited during times of higher sea level (Woodward-Clyde Consultants). The youngest emergent terrace is found closest to the coast, and subsequently older terraces are found further east and at higher elevation. The oldest and highest terrace (Maple Stump) is found furthest east and exhibits the most soil development (Stephens 1982). Local eolian and colluvial deposits overlie the marine sediments on older terraces (Stephens 1982). The Savagecreek terrace, upon which this ecological site is found near Trinidad, is the second youngest of these six terraces and likely formed about 105,000 years. The marine terrace in the vicinity of Crescent City suggests similar deposition timing during sea level high stands (Lorenz and Kelsey 1999).

The effects of climate change on species distribution and viability need to be considered in this age of rapidly changed climate regimes. The western United States is already experiencing an increase in tree mortality across all tree cohort age classes, likely due to regional warming and water deficits (van Mantgem et al 2009). These forest structure changes may cause species to migrate to higher elevations, as much as 500-1000m, as temperatures increase in lower elevations (Urban et al 1993). Climate models project many different climate regimes for the north coast of California. One model predicts a warmer, wetter climate regime in which redwood may be able to expand into canyon live-oak-madrone and chaparral systems (Lenihan et al 2003). Climate change and its effects on vegetation patterns should be considered along with historical perspectives in ecological site development.