Ecological dynamics

This ecological site is found on western slopes of lower Redwood Creek in Humboldt County, CA and is largely contained with the coastal fog belt. The reference plant community for this ecological site is inferred as no late successional stands of this site remain on the landscape. Redwood (Sequoia sempervirens) dominates this site, which also has a productive understory of western swordfern (Polystichum munitum) and other shrubs and forbs of the redwood region.

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 and 150-200 years on intermediate sites further inland (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 western hemlock susceptible to fire damage (Arno 2002). 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 could 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 Douglas-fir (Psuedotsuga menziesii) and 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 regeneration of Douglas-fir and lead to a larger proportion of Douglas-fir in the stand for several centuries (Agee 1993). Aerial seeding in past decades in Redwood Creek basin has led to dense Douglas-fir dominated stands in some areas of the redwood region, skewing the natural overstory species composition (Noss 2000).

This site overlies soils and structural geology highly susceptible to debris slides. As the underlying soils have developed from debris flow colluvium derived from unstable schist bedrock, the ecological dynamics of this site are influenced by frequent ground disturbance. Redwood is able to reestablish on old landslides through infilling but also resprouting, as dormant trunk buds can produce sprouts following burial in sediment (Veirs 1996). Redwood Creek has experienced heavy sediment loading for decades, a result of the natural instability of the Franciscan Formation bedrock, a clustering of storm events, and timber harvesting (Pitlick 1981). Harvesting and upslope road building can increase the likelihood for debris slides by triggering ground disturbance, increasing overland flow of water on roads, reducing water uptake by cutting stands. Debris slide disturbances could increase in frequency if mitigating measures are taken during forest management. Current vegetation on this site includes a high percentage of red alder, which is able to rapidly colonize disturbed areas such as recent harvest blocks or debris slides.

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.

Western swordfern occupies a large percentage of the understory on this site. Western swordfern can grow in a range of light conditions and can be often indicative of moist, productive forest habitat (Crane 1989). Redwood sorrel (Oxalis oregano) is also found on the site, indicating a moist type IV redwood forest type (Popenoe 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.