California Coastal Redwood Forest

This system occurs from the Klamath Mountains south to Monterey Bay, California. The coastal redwood forest generally can be found in areas of within the fog belt. In the northern portion, it occurs on upland slopes and in riparian zones and on riverine terraces that are flooded approximately every 50-100 years. In the southern portion of the range, annual precipitation may be as little as 500 mm, and the system is limited to coves and ravines. It is commonly found on moderately well-drained marine sediments (non-metamorphosed siltstones, sandstones, etc.). This system forms the tallest forests in North America, with individuals reaching 100 m high (tallest being 106-110 m [350-360 feet]). Typically, mature stands of Sequoia sempervirens produce a deep shade, so understories can be limited, but coarse woody debris from past disturbance can be quite large. Pseudotsuga menziesii is the common associate among the large trees. Tsuga heterophylla is found in old-growth stands in northern sections, and Notholithocarpus densiflorus occurs as a subcanopy in almost all stands (possibly as a result of fire suppression). Sequoia sempervirens mixes with Arbutus menziesii, Notholithocarpus densiflorus, Pseudotsuga menziesii and Umbellularia californica. The moist, coastal Chamaecyparis lawsoniana stands from southwestern Oregon and northwestern California, often mixed with Sequoia sempervirens, Pseudotsuga menziesii, or Tsuga heterophylla, are included in this system, as ecologically they function in the same way and have a similar overall floristic composition. Shade-tolerant understory species include Rubus parviflorus, Oxalis oregana, Aralia californica, Mahonia nervosa, Gaultheria shallon, and many ferns, such as Blechnum spicant, Polystichum spp., and Polypodium spp. Historically, surface fires likely exposed mineral soil for redwood seed germination. Less frequent disturbance can result in increases in Tsuga heterophylla in northern occurrences, as it is sensitive to fire and is a decreaser with fire and flood. Fire suppression has tended to result in increasing abundance of Notholithocarpus densiflorus, Umbellularia californica, Alnus rubra, Arbutus menziesii, and Acer macrophyllum; all respond favorably to fire, flood, wind and slides, becoming more abundant in areas of frequent disturbance.

Typically, mature stands of Sequoia sempervirens produce a deep shade, so understories can be limited, but coarse woody debris from past disturbance can be quite large. Pseudotsuga menziesii is the common associate among the large trees. Tsuga heterophylla is found in old-growth stands in northern sections, and Notholithocarpus densiflorus (= Lithocarpus densiflorus) occurs as a subcanopy in almost all stands (possibly as a result of fire suppression). Sequoia sempervirens mixes with Arbutus menziesii, Notholithocarpus densiflorus, Pseudotsuga menziesii and Umbellularia californica. The moist, coastal Chamaecyparis lawsoniana stands from southwestern Oregon and northwestern California, often mixed with Sequoia sempervirens, Pseudotsuga menziesii, or Tsuga heterophylla, are included in this system, as ecologically they function in the same way and have a similar overall floristic composition. Shade-tolerant understory species include Rubus parviflorus, Oxalis oregana, Aralia californica, Mahonia nervosa (= Berberis nervosa), Gaultheria shallon, and many ferns, such as Blechnum spicant, Polystichum spp., and Polypodium spp. Historically, surface fires likely exposed mineral soil for redwood seed germination. Less frequent disturbance can result in increases in Tsuga heterophylla in northern occurrences, as it is sensitive to fire and is a decreaser with fire and flood. Fire suppression has tended to result in increasing abundance of Notholithocarpus densiflorus, Umbellularia californica, Alnus rubra, Arbutus menziesii, and Acer macrophyllum; all respond favorably to fire, flood, wind and slides, becoming more abundant in areas of frequent disturbance.

Climate is wet, mild maritime. Forests along the immediate coast experience uniformly wet and mild climate, where precipitation averages 2000-3000 mm/year (500 mm for some of the driest redwood occurrences) with frequent fog and low clouds during warmer months; additional moisture from fog-drip can be significant. The coastal redwood system generally can be found in areas of lower rainfall than other coastal rainforests in this macrogroup, but still within the fog belt. In the northern portion, it occurs on upland slopes and in riparian zones and on riverine terraces that are flooded approximately every 50-100 years. In the southern portion of the range, annual precipitation may be as little as 500 mm, and the system is limited to coves and ravines. It is commonly found on moderately well-drained marine sediments (non-metamorphosed siltstones, sandstones, etc.). Redwood forests are limited to the north by ultramafic soils of the Klamath Mountains (Sawyer 2007).

Historically, surface fires likely exposed mineral soil for redwood seed germination. Less frequent disturbance can result in increases in Tsuga heterophylla in northern occurrences, as it is sensitive to fire and is a decreaser with fire and flood. Landfire (2007a) model: Redwood forests typically burned in the summer and early fall in low- to moderate-intensity surface fires that consumed irregular patches of surface fuel and understory vegetation. The great height of the canopy and separation of surface and crown fuels resulted in a pattern where fire rarely resulted in canopy tree mortality. Fire intervals ranged from less than 10 years in interior and upland locations to 100 years or more along the coast in the fog belt. More recent research funded by Save the Redwoods League suggests that fire has been historically quite variable with much lower frequencies in the extreme north coastal portion of redwood range (as low as 1 every 500 years) and very high in the southern end where ravine redwood stands occur adjacent to California chaparral and grasslands (T. Keeler-Wolf pers. comm. 2013). Native Americans are thought to have contributed to the ignitions (perhaps as much as every 5-8 years) since lightning is relatively infrequent in the area, especially in the fog belt. Flooding events that undermine trees may be a significant disturbance, but it's not known for certain this is the case.

Conversion of this type has commonly come from logging and residential and commercial development. Logging and fire suppression have tended to result in increasing abundance of Lithocarpus densiflorus, Umbellularia californica, Alnus rubra, Arbutus menziesii, and Acer macrophyllum; all respond favorably to fire, flood, wind and slides, becoming more abundant in areas of frequent disturbance. Recent studies are finding that many coastal redwood occurrences now have multi-tiered structures with tall-shrub layers and subcanopies of a variety of other trees, creating fuel ladders which can result is severe fires and increased mortality of Sequoia sempervirens when fires occur. In addition, Sillett and Van Pelt (2000) and Sillett and Bailey (2003) report that canopies of Sequoia sempervirens support significant biomass of epiphytic ferns and shrubs that are also contributing to an altered crown structure in these forests, which is impacting the fire regime.

In northwestern California, regional climate models project mean annual temperature increases of 1.7-1.9°C (3.06-3.42°F) by 2070 (PRBO Conservation Science 2011). Regional climate models also project a decrease in mean annual rainfall of 101 to 387 mm by 2070. Currently, there is greater uncertainty about the precipitation projections than for temperature in northwestern California, but with some evidence for a slightly drier future climate relative to current conditions (PRBO Conservation Science 2011). Recent species distribution modeling and comparison of historic climate anomalies to projected climate change show that northern redwood stands appear stable while southern stands may experience the greatest changes toward a warmer and drier future, while it remains unknown how the forest may change (Save the Redwood League 2013).

Increased fire frequency with warmer temperatures and lower precipitation may result in drier, more flammable fuels, which may exacerbate the fire intensity given changes to redwood forest structure, as noted above. Forest structure does differ immensely from north to south in redwood's range and much of this has to do with fire history coupled with the gradient of warm-dry to cool-wet from south to north. Less rainfall and higher temperatures may shift species composition, to more drought-tolerant species, such as Lithocarpus densiflorus, and may also which may favor non-native species. On the other hand, Lithocarpus is likely to wink out of existence due to sudden oak death syndrome and the most likely benefactor from this in terms of the future stand composition will be California bay, which seems to be increasing relative to Lithocarpus in the central and southern portions of redwood range (T. Keeler-Wolf pers. comm. 2013). In many coastal regions, the interaction between oceanographic and terrestrial air masses may be ecologically important. Intensifying upwelling along the California coast under climate change may intensify fog development and onshore flows in summer months, leading to decreased temperatures and increased moisture flux over land (Snyder et al. 2003, Lebassi et al. 2009, as cited in PRBO Conservation Science 2011). Coastal terrestrial ecosystems could benefit from these changes. However, current trends in fog frequency along the Pacific coast from 1901-2008 have been negative (Johnstone and Dawson 2010, as cited in PRBO Conservation Science 2011), thus the effect of climate change on coastal fog remains uncertain (PRBO Conservation Science 2011). Other research on redwood ecosystems suggests that they are quite stable and persistent as long as the combination of effects of cool coastal fog and some rain persist. Climate change models have not been particularly effective in predicting the future occurrence of redwood forests since the spatial resolution of the models does not match the fine-scale topographic position of redwood stands throughout much of their range (T. Keeler-Wolf pers. comm. 2013). Redwoods have great capacity to adapt to obtaining sufficient moisture from fog-drip or from precipitation (T. Keeler-Wolf pers. comm. 2013).

This system occurs from the Klamath Mountains south to Monterey Bay, California.