Fire and Native Plants/Habitats

Friday, October 21 at 8:00-9:40 am, Oak Room

Session Description: For many native plants and plant communities, fire is an essential ecological disturbance process, and the use of fire in management including Indigenous land management is essential for the health and longevity of forests, chaparral and so many other plant communities in Mediterranean California. This session will explore California native flora, vegetation and human relationships to fire.

Session Chair: Michelle Coppoletta (USDA Forest Service Region 5 Ecology Program, Quincy, CA, USA)

14.1 Resilience of restored coastal sage scrub and grassland systems following wildfire

Priscilla Ta (University of California, Irvine Center for Environmental Biology, Irvine, CA, USA), Michelle dela Cruz (University of California, Irvine Masters in Conservation and Restoration Science, Irvine, CA, USA), Jessica Rath (University of California, Irvine Masters in Conservation and Restoration Science, Irvine, CA, USA), Rubeen Khunkhun (University of California, Irvine Masters in Conservation and Restoration Science, Irvine, CA, USA), BryAnna Wertz (University of California, Irvine Masters in Conservation and Restoration Science, Irvine, CA, USA), Robert Freese (Irvine Ranch Conservancy, Irvine, CA, USA), Nathan Gregory (Irvine Ranch Conservancy, Irvine, CA, USA), Sarah Kimball (University of California, Irvine Center for Environmental Biology, Irvine, CA, USA)

Wildfire frequency and intensity is expected to increase due to climate change and human ignition. In Southern California, fire can lead to vegetation-type conversions through non-native invasion and impact the resiliency of native coastal sage scrub (CSS) and grassland (GL) communities. In 2020, the Silverado Canyon Fire burned through Bee Flat Canyon, located in the foothills of the Santa Ana Mountains, providing an opportunity to study the resilience of restored CSS and grassland communities after fire. We investigated the resilience of restored communities by addressing the following research questions: (1) How does the post-fire resilience of CSS, as evaluated by the abundance of shrub crown-sprouts and recruits, compare across three levels of restoration (degraded, intact, restored)? (2) How does native and non-native cover and species composition of herbaceous plants in CSS communities differ across three levels of restoration? (3) How do restored grassland communities change over time in relation to temperature, precipitation, and fire? We found that shrubs in the restored plots crown-sprouted following wildfire just as well as shrubs in intact, unrestored areas. Restored CSS plots had significantly higher cover of native forb species compared to unrestored degraded sites, but slightly less than unrestored intact areas. Restored GL plots had lower cover of native grass and higher cover of non-native grasses after the fire compared to before fire. Our results demonstrate that restored CSS communities are resilient to wildfire, while native grasslands are still highly vulnerable to post-fire invasion.

14.2 Two decades of prescribed fire within the grasslands at the Santa Rosa Plateau Ecological Reserve

Hailey Laskey (Center for Natural Lands Management, Temecula, CA, USA), Zachary Principe (The Nature Conservancy, Sacramento, CA, USA)

Of the 10,000 acres that comprise the Santa Rosa Plateau Ecological Reserve in Murrieta, California, a significant component (~4,000 acres) is grassland, primarily purple needlegrass (Stipa pulchra). Over the last three decades, late spring prescribed fire has been used with the goal of reducing non-native grass cover (Avena sp. and Bromus sp.) within 10 rotating grassland management units. Vegetation data were collected on a subset of 83 transects within the units for 22 out of the 30 burn years from 2001-2022. An initial analysis in 2006 showed prescribed fire controlled non-native grass at least two years after fire while native grass recovered within a 5-year interval. Prescribed fire continued to be used as a management tool to control non-native grasses between 2006-2021. Between 2006 and 2021 the Plateau endured significant drought and multiple wildfires. Prescribed fire as a management tool was reassessed again in 2022 when all 22 years of data were analyzed. We analyzed non-native grass, native grass, non-native forb, and native forb cover over time, and pre and post burn conditions within each management unit and across all units. Over the last 22 years across all management units, with and without the treatment of prescribed fire native grass cover decreased, non-native grass cover increased while non-native forb and native forb cover varied with precipitation. Non-native grass cover significantly decreased after prescribed fire but recovered to pre-fire cover or higher one year after fire. Native grass cover decreased after prescribed fire then recovered to pre-burn levels within five years, but never increased over time. The response of native grass to fire (wild and prescribed) was different across time and within management units, but overall native grass declined. The data is being used to inform management decisions including reducing the use of fire until further studies are completed.

14.3 Reestablishment of natural fire regimes facilitates ecological restoration of California’s red fir forests

Marc Meyer (USDA Forest Service Region 5 Ecology Program, Bishop, CA, USA), Kyle Merriam (USDA Forest Service Region 5 Ecology Program, Quincy, CA, USA), Michelle Coppoletta (USDA Forest Service Region 5 Ecology Program, Quincy, CA, USA), Becky Estes (USDA Forest Service Region 5 Ecology Program, Placerville, CA, USA), Ramona Butz (USDA Forest Service Region 5 Ecology Program, Eureka, CA, USA), Anthony Caprio (Sequoia and Kings Canyon National Parks, Three Rivers, CA, USA), Calvin Farris (National Park Service, Klamath Falls, OR, USA), Malcolm North (USDA Forest Service, Pacific Southwest Research Station, Mammoth Lakes, CA, USA)

The reestablishment of natural fire regimes may benefit forest ecosystems by restoring their fundamental structural, compositional or functional attributes. We examined the influence of fire on the structure, diversity, and health of red fir (Abies magnifica) forests by comparing twice burned and fire-excluded (unburned) stands in 29 overlapping fires across two broad geographic regions in California (Sierra Nevada and Cascade-Klamath). Burned red fir plots were characterized by lower tree densities and canopy cover, restored spatial heterogeneity, higher understory species richness, and similar densities of large trees and red fir regeneration compared to unburned plots. Forest health indicators were similar between burned and unburned sites, and red fir crown loss ratings were primarily associated with topographic variables indicative of increased moisture stress or reduced soil moisture availability (i.e. lower elevations, south-facing slopes). The effect of fire on red fir forest structure was most evident in the Sierra Nevada where the percent difference in total tree density between unburned and burned plots was significantly greater (77% difference) than in the Cascade-Klamath (53% difference), and burned plots in the Sierra Nevada had significantly lower densities of both small (<30 cm dbh) and medium sized trees (30–60 cm dbh). These stronger fire effects may be related to greater departure from historical reference fire return intervals in the Sierra Nevada, as well as the region’s warmer and drier conditions increasing the availability of fuel to burn and susceptibility of trees to fire-related mortality. Our results suggest that fire does not improve the health of red fir trees especially in areas of greater moisture stress, but it can restore red fir forest structure, increase understory diversity, and enhance adaptive capacity in landscapes with reestablished natural fire regimes across diverse montane regions of California.

14.4 Searching for good fire: does managed wildfire support both fuels reduction goals and native plant diversity?

Raphaela E. Floreani Buzbee (University of California, Berkeley, Berkeley, CA, USA), Zachary Steel (USFS Rocky Mountain Research Station, Fort Collins, CO, USA)

Fire is a fundamental ecological process for many species and plant communities across California. After over a century of fire suppression and exclusion of Indigenous fire stewardship, there is an immense effort to restore fire to landscapes by increasing the pace and scale of fuels treatments in California and across the West. Managed wildfire, alternatively referred to as wildland fire use, is one approach for restoring fire regimes at scale that also has potential to support native plant diversity. Using this approach, naturally ignited lightning fires are allowed to burn in order to accomplish resource management objectives. Most managed wildfires occur in unpopulated wilderness areas, making it a safe and cost effective form of fuels reduction. Over time, this strategy can create a mosaic of patches burned at different times and intensities thereby increasing landscape heterogeneity and decreasing the risk of extreme wildfire. We hypothesize that areas characterized by fire histories approximating the historic fire regime will have both lower fuel loads and higher native plant diversity. We compared three research areas within Yosemite and Ansel Adams wilderness in the Sierra Nevada with varied implementations of managed wildfire. In 2021 and 2022, we measured vegetative fuels (using Brown’s transects) and understory plant diversity (using transects and a timed search) at sample locations stratified across different environmental gradients such as time since last fire and previous burn severity. Ongoing data analysis will assess how fire history affects fuel loads and plant community composition. This work can help inform the design of fire restoration strategies in landscapes with multiple resource objectives as California seeks to reintroduce beneficial fire to its fire-adapted ecosystems.

14.5 Fire regime alteration in natural areas underscores the need to restore a key ecological process

Michelle Coppoletta (USDA Forest Service Region 5 Ecology Program, Quincy, CA, USA)

Research Natural Areas (RNAs) are part of a national network of federal lands set aside to protect exemplary, relatively undisturbed native plant communities from human intervention. RNAs offer some of our best examples of places where ecological processes may proceed without the influence of direct management action. Ideally, RNAs serve as properly functioning reference sites for more heavily managed landscapes. However, these natural areas are embedded within a matrix of federal lands, where landscape-scale fire regime alteration has resulted in significant changes to both the frequency and severity of wildfires. In our study, we examined 65 RNAs on Forest Service lands in California to assess the extent of modern departure from pre-Euroamerican settlement (i.e., pre-1850) fire regimes. While more than a quarter of the RNAs in California have burned in the last four years, 68% continue to exhibit moderate to high fire regime departure. Of these, 91% are burning much less frequently than they would have under the presettlement fire regime and 9% are burning more frequently. RNA plant communities that were historically characterized by frequent, low severity fire that missed multiple fire cycles tended to burn at higher fire severities than expected under the presettlement fire regime. This talk will present the findings from our study, as well as a series of case studies that illustrate how disruption of the natural fire regime and recent wildfire activity have impacted native plant communities within California’s RNAs. Fire management strategies will be presented for those tasked with managing RNAs and other protected natural areas that may require intervention to restore and maintain the natural fire regimes and the native plant communities that they were established to protect.