Peak Health home > Plants > Coast Redwood Forests
Why Was This Indicator Chosen?
Coast redwoods are the definition of resiliency. Among the tallest trees in the world, individual redwoods may live as long as 2,000 years. Thick bark and an ability to resprout enables established adult trees to survive most wildfires, high levels of tannins make them resistant to insect and fungal infestations, and acidic soil conditions, thick duff layers, and dense shade also make coast redwood forests relatively resistant to non-native plant invasions.
Mt. Tam’s coast redwood forests provide important habitat for a number of mammals and birds, including the state and federally threatened Northern Spotted Owl (Strix occidentalis caurina). Endangered coho salmon (Oncorhynchus kisutch) and threatened steelhead trout (O. mykiss) also live in the Redwood Creek Watershed on Mt. Tam.
Redwood forest communities are also good indicators of forest management practices, wildfire regimes, disease processes, and climate change. Coast redwood trees sprout prolifically from stumps, and many of Mt. Tam’s second-growth stands have higher redwood tree densities than old-growth areas as a result of turn of the century logging (Noss, 2000). High densities of tanoak (Notholithocarpus densiflorus) and Douglas-fir (Pseudotsuga menziesii) in second-growth redwood stands in some areas reflects a history of logging, followed by fire suppression and a lack of management actions designed to favor redwood recruitment.
Wide-spread tanoak die-back and resprouting as a result of Sudden Oak Death (SOD) has led to a persistent thicket of tanoak shoots in the redwood understory. The continual re-infection of these tanoaks prevents the shoots from developing into midstory level trees, and has reduced the habitat complexity of the mountain's redwood forests.
Finally, redwoods may serve as an indicator of climate change, particularly changes in precipitation patterns and summer fog (Micheli et al., 2016). A sudden decline in such a long-lived and resilient species would signify changes on a scale likely to be detrimental to other vegetation communities on Mt. Tam.
What is Healthy?
The desired condition for old-growth redwood forests is a complex species composition and multi-aged, multi-storied stands; coarse woody debris; tree cavities; and other nesting structures such as large limbs.
In second-growth forests, the desired condition is evidence that a stand is on a trajectory toward developing old-growth characteristics. This includes a reduction in the total stem density (trees per unit area) over time as well as the development of large-diameter trees and a multi-storied stand structure (Lorimer et al., 2009). Maintaining the existing extent of redwood forests in the One Tam area of focus is considered highly desirable because of their habitat value for Northern Spotted Owls and coho salmon, their ability to store carbon and other greenhouse gases (Cobb et al., 2017), and their iconic value.
What Are the Biggest Threats?
- The death of tanoaks, an important part of the coast redwood forest structure, from Sudden Oak Death.
- Warming temperatures, and changes to fog and precipitation patterns as a result of climate change.
- Invasive, non-native species such as panic veldtgrass (Ehrharta erecta), which are able to persist in the shady redwood understory.
- Soil compaction from recreational use of redwood forests both on and off trails.
- Legacy of logging in the region.
- Fire suppression and absence of cultural burning have resulted in a buildup of fuels, particularly in second-growth stands. This has affected forest structure and diversity as well as decreased the redwood’s wildfire resilience.
What is The Current Condition?
Old-growth redwood forests are in Good condition while second-growth areas are in Fair condition. Because the vast majority of the redwood forests on Mt. Tam are second-growth, the overall trend is Fair. The mountain's second-growth stands lack the complex structure of old-growth areas, primarily as a result of the effects of SOD.
Iconic coast redwood forests (Sequoia sempervirens) are experiencing changes due to Sudden Oak Death (SOD), climate change and invasion by non-native species. The One Tam area of focus has a small amount of old-growth coast redwoods, but the majority are second-growth, having been logged at some point in the past. We have observed no detectable change in redwood forest health in the One Tam area of focus since our first assessment in 2016, and the impact of SOD on the tanoak (Notholithocarpus densiflorus) midstory appears to be slowing. Thanks to the 2018 Marin Countywide Fine Scale Vegetation Map (GGNPC et al., 2021), we now have a complete picture of where redwood stands are found throughout the county, as well as a suite of data that will allow us to assess coast redwood forest health and inform forest management into the future.
What is the Current Trend?
The trend for old-growth redwood forests is Improving, but second-growth redwood stands are Declining, again, primarily as a result of SOD.
How Sure Are We?
We have Moderate confidence in our assessments overall. For assessing old-growth structure, we are now looking at old-growth stands beyond Muir Woods and new second-growth beyond Marin Water lands, so more fieldwork is needed. For mid-canopy structure, we had high confidence in 2016 but no new data in 2022 so we assessed this indirectly with moderate confidence. For weed cover, confidence is high: target weed species mapping efforts have increased, with multiple surveys per year in some priority areas. Since 2016, the One Tam Conservation Management Team has invested in improving weed data collection protocols and data management systems, giving us increased confidence in this metric for this update.
What is This Assessment Based On?
- National Park Service 1994 vegetation map (Schirokauer et al., 2003)
- Marin Water vegetation maps from 2004, 2009, and 2014 (AIS, 2015)
- Marin Water broom mapping from 2003, 2010, and 2013 (unpublished data)
- Marin Water 2014 photo interpretation of SOD affected forest stands (AIS, 2015)
- Marin County Parks 2008 vegetation map (AIS, 2008)
- One Tam early detection and invasive plant mapping (Calflora, 2016, 2022)
- Larry Fox and Joe Saltenberger old-growth redwood data (Fox & Saltenberger, 2011)
- 2018 Fine Scale Vegetation Map (GGNPC et al., 2021)
What Don’t We Know?
Key information gaps include:
- Quantification of complex/Old-growth habitat structure including measuring and mapping coarse woody debris, tree cavities, and nesting platforms.
- Additional fieldwork is needed to connect 2018 Fine Scale Vegetation Map data (e.g., structural classifications) to on-the-ground conditions for both old-growth and second growth stands.
- A logging history study from the mid-19th to the mid-20th century would inform land managers and others of past logging operations and their continuing legacy on the landscape.
resources
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