6. Chaparral Resilience and Future Concerns 1

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Chaparral Resilience and Future Concerns 1

Thursday, October 20 at 1:00-2:40 pm, Donner Room

*note alternate instance of this session – Friday at 10am

Session Description: Chaparral contains 24 percent of California’s native plant species, and more of these plants are considered rare here than in any other plant community. Since chaparral exists in every single county, chaparral represents the most accessible native plant experience to the greatest number of Californians and visitors to our state, and it provides unique research and educational opportunities. Due to the uniqueness of chaparral it presents major challenges to fire managers concerned with balancing fire hazard reduction and resource conservation. This session will explore the remarkable biodiversity, resilience, and value that chaparral provides to all the life forms and the need to address threats to vegetation type conversion in light of many global changes.

Session Chair: Jon E. Keeley (U.S. Geological Survey, University of California, Los Angeles, CA, USA) and Tom Parker (San Francisco State University, CA, USA)

6.1 Monitoring chaparral fire properties and recovery dynamics based on Landsat image time series

Douglas Stow (San Diego State University, San Diego, CA, USA), Emanuel Story (University of New Mexico, Albuquerque, NM, USA), Krista West (San Diego State University, San Diego, CA, USA), Chandler Ross (San Diego State University, San Diego, CA, USA), Megan Jennings (San Diego State University, San Diego, CA, USA), Alexandra Syphard (Conservation Biology Institute, San Diego, CA, USA), Erin Conlisk (Point Blue Conservation Science, Petaluma, CA, USA)

Chaparral is the most prevalent biome within the coastal portions of the Southern California ecoregion, but its areal extent is declining because of increasing frequency of drought and fire, or a combination of the two. This decline is likely to continue with global climate change. Resource managers, emergency responders, and regional planners can benefit from ecoregion-wide information regarding the distribution and extent of burned areas, chaparral shrubs, and the expanding cover of non-native herbs that are replacing these shrubs, as a means for conserving and managing chaparral and wildfires. The archive of high-quality Landsat satellite surface reflectance products provides a convenient source for mapping and monitoring fire properties and vegetation recovery dynamics from 1986 to present. Using a variety of image processing and modeling approaches, our team, associated with the Connected Wildland Communities (CWC) project, generated spatial-temporal information products for several fire-related properties for the Southern California ecoregion, including: 1) burned areas and fire histories, 2) postfire recovery of chaparral, and 3) percentage of herbaceous vegetation cover. These maps of burned area, fire history, and herbaceous vegetation cover have greater spatial detail and accuracy than extant maps, including other Landsat-based products. Chaparral recovery trends were characterized for locations within the ecoregion that met specific sampling criteria and exhibit substantial spatial-temporal variability. Having open access to ecoregion-wide maps through the CWC dashboard provides land managers with information pertaining to wildfire regime and changes over several decades.

6.2 The influence of fire and disturbance frequency on chaparral shrubs in northern California

Jameson N.P. Christiansen (Stephens Lab, University of California, Berkeley, Berkeley, CA, USA)

Northern California chaparral is a unique and biodiverse ecosystem that is highly adapted to fire. Historical mean fire return intervals in these communities are estimated to have been relatively infrequent, with high severity, stand-replacing fires sweeping through every 30-70 years. However, this ecosystem has experienced significant anthropogenic alteration of its historic regime. Widespread fire suppression for the past century created an uncharacteristically long fire-free period followed by a recent dramatic increase in the number of fires burning through chaparral stands in short periods of time. It has been suggested that too infrequent incidence of fire may lead to change in plant community structure, while fire returning at too short of interval may lead to risk of type conversion to grassland vegetation. Yet few studies have tested these expectations in northern California chaparral. To better understand the implications of disturbance regime alteration in this ecosystem we resampled a network of research plots in Mendocino County’s coast range chaparral. These plots were first established in 2001 and subsequently treated as part of a study observing the long-term disturbance effects of fire and mastication. A 2018 wildland fire event then burned both treated and control plots creating patches of short (<20 years) and long (>60 year) disturbance return interval, respectively. We tested whether shrub regrowth and diversity differed between stands experiencing short vs. long disturbance intervals, as well as how initial treatment type may have impacted stand recovery. Analysis is ongoing, but preliminary work indicates that significant differences in growth and diversity were not detected between sites experiencing long vs. short disturbance free intervals or different initial treatments. This suggests that shrubs in this ecosystem may have similar recovery trajectories and may be more resilient to variable disturbance intervals than previously feared.

6.3 Understanding the role of regeneration traits and strategies in post-fire recovery in southern California chaparral

Meg Kargul (University of California Riverside, Riverside, CA, USA), Loralee Larios (University of California Riverside, Riverside, CA, USA)

Early life stage traits (regeneration traits) are vital to post-fire recovery as they may dictate successful establishment and survival under post-fire stressors, yet are understudied and may differ from adult traits. Regeneration traits can interact with post-fire regeneration strategies (e.g., resprout, seed) and environmental conditions to structure differences in the successional trajectory of communities. Understanding the mechanisms of post-fire recovery is key to prevent habitat type conversions. Therefore, our research objective was to evaluate how regeneration traits and strategies link to post-fire recovery in chaparral. We asked, how do 1) functional traits differ across life stage and aspect, 2) regeneration traits and strategies mediate survival, and 3) regeneration traits and strategies link to recovery over time? To answer this, we collected leaf functional traits in NE and SW aspects for regenerating species in the Holy Fire burn scar in southern California’s Cleveland National Forest and for adults in nearby unburned areas. We surveyed survival of tagged individuals and community composition within these different community types. We found regeneration traits were more resource-acquisitive than adult traits. Regeneration traits did not differ across aspect type but rather by regeneration strategy where resprouters were more stress-tolerant than seeders during initial recovery. Regeneration strategy rather than traits mediated survival, where resprouters had 100% survival regardless of trait value, but seeder survival varied by initial regeneration height highlighting the importance of light competition. The contribution of strategy to post-fire recovery differed over time where resprouters contributed more in year 1 but by year 2 seeders and resprouters had similar cover, suggesting regeneration strategy mediates initial recovery but not long term recovery. Our work highlights the need for long-term studies to disentangle how and when regeneration traits mediate recovery after a disturbance.

6.4 Identifying a fire frequency tipping point in northern California chaparral

Ashley Grupenhoff (University of California, Davis, Davis, CA, USA)

Fire frequency has increased exponentially in chaparral ecosystems with the rise of urbanization, warming temperatures, and invasion of non-native species. This departure from the historical fire frequency has severe effects on biodiversity and species composition, leading to exotic invasion and type conversion of shrubland to grassland. As a system approaches type conversion, we see a threshold point at which shrub regeneration drops off and nonnative grasses become the dominant herbaceous species. An increase in nonnative grasses provides highly flammable flashy fuels that have a higher probability of ignition and spread fire more easily than shrubs. Although some studies have attempted to understand the effects of fire frequency on type conversion in southern California, it remains unclear what the minimum-interval threshold that will lead to type conversion is in northern California chaparral. This study takes place on the LNU lightning complex in the coast range of northern California, one of the most frequently burned locations in the whole state. Fifty-four 250 m2 plots were surveyed in 2021 and 2022 to measure changes in plant community composition and postfire regeneration of chaparral shrubs across a large gradient of fire frequency, up to six total fires in the last 20 years. Despite the prevalence of increased fire recurrence in northern California chaparral, no studies have examined these impacts across such a large gradient of fire frequency. We will provide empirical data to understand how and when chaparral communities type convert to nonnative grassland, thus informing management, planning, and restoration efforts in this region.

6.5 Impacts of increased fire activity on biodiversity in California chaparral

Jon E. Keeley (U.S. Geological Survey, University of California, Los Angeles, CA, USA)
In recent decades we have witnessed a pronounced increase in wildfire activity, both in intensity and frequency. This is a potential threat to native plant biodiversity in some ecosystems and can result in vegetation type conversion (VTC) from native to non-native species. An ecosystem where this is a major threat is California chaparral shrublands, where many of the dominant species are sensitive to short fire intervals. Those with limited resilience are obligate seeding shrubs, largely because they lack the ability to resprout and are entirely dependent on a dormant seed bank that requires sufficient time to accumulate. Short fire-return intervals are frequently implicated in VTC although some studies cast doubt on this as a significant factor. Better understanding of VTC is needed because of an upsurge in state and federal programs to utilize prescription burning to reduce fire hazard; treatments at too short an interval may reduce or eliminate some dominant species. But how short is too short? It is commonly assumed in the literature that this threshold is less than 8 years, however, this threshold is likely to be species specific. Twenty fires from Monterey to San Diego during 2020 were sampled in the first postfire season. Fire history maps showed areas with different times since fire and our focus was on areas that overlapped with relatively recent fires, ranging from 8-20 years, as well as control areas within the fire perimeter that were much older. We sampled dominant chaparral shrubs within multiple 32 m2 plots, recording dead, resprouts, and seedlings. Throughout the state we recorded data for 48 species. Obligate seeding shrubs appear to be at risk due intervals between fires previously thought to be safe.

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