Soil Carbon Storage, Fuel Loads, and Runoff in Four Fire Regimes in Chaparral Shrublands of the Sierra Nevada FoothillsSoil Carbon Storage, Fuel Loads, and Runoff in Four Fire Regimes in Chaparral Shrublands of the Sierra Nevada Foothills

Jay B. NortonJay B. Norton11, William R. Horwath, William R. Horwath22, and Urszula Norton, and Urszula Norton22

1University of Wyoming Department of Renewable Resources 2University of California , Davis, Department of Land, Air, and Water Resources Chaparral Fire ManagementChaparral Fire Management

ConclusionsConclusions

AcknowledgmentsAcknowledgments

Cessation of frequent “rancher burns” in the 1970s coupled with aggressive fire suppression due to increased residential development began a rapid buildup of highly flammable fuels. While prescribed fire is the only feasible way to reduce fuel loads in rugged and isolated terrain, concerns about smoke and ecological impacts make burning increasingly difficult to achieve.

We appreciate field, laboratory, and mapping assistance from Mary Innes and Bill Schober. We are grateful to Ken Hood of the Bureau of Land Management, Kit Perlee of the City and County of San Francisco Hetch Hetchy Water and Power, and Andy McMurry of CDF for identifying and providing access to sites and fire history information. This project is funded by the University of California Kearney Foundation for Soil Science and the UC DANR Analytical Laboratory.

Contact: Jay B. Norton (jnorton4@uwyo.edu)

Nitrogen

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When averaged across soils, fire does not affect soil total N, but enhances N mineralization in all fire treatments and potential N availability in the 4-y treatment. Higher litter quality and considerable fine, near-surface root production likely contributes to soil PMN pool size on these sites.

When averaged across soils, fire does not affect soil TOC or DOC concentrations. Soil from areas burned most recently have elevated mineralizable C pools suggesting greater C availability to microbes than in FS or 20-y soils. Stores of mineralizable C in 20-y soils are intermediate between the recent fire soils and the FS soils, indicating a shift toward build up of more recalcitrant C pools derived from transition toward more woody vegetation.

Carbon

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Soil carbon, both soil types, 0- to 5-cm depth bulked from four points along 12 transects in each fire frequency area.

Fire Management and Soil QualityFire Management and Soil Quality

Fire Management and Fuel LoadsFire Management and Fuel Loads Fire Management and ErosionFire Management and ErosionCalifornia chaparral is a crown-fire ecosystem that covers 13 million

acres of California’s coast ranges and Sierra Nevada foothills. Historically infrequent (30-60 yr), stand-replacing fires stimulated resprouting and fire-obligate-reseeding brush species. Fire-adapted native shrubs and exotic annual herbaceous vegetation dominate plant communities in varying proportions depending upon time since fire. Livestock owners discovered that repeated fires in successive years can convert brush-dominated chaparral to open grasslands. This also had reduced fire danger.

The 2001 Creek Fire burned 11,000 acres, destroyed 20 homes, and threatened four up-country communities. This followed several dangerous wildfires, including the 1992 Moccasin Fire that scorched part of the highly visible Highway 120 corridor to Yosemite National Park. Rapid regrowth leading to recurring dangerous conditions led fire officials to seek an alternative strategy to permanently change fuel models in the area burned by the Creek Fire.

Vegetation and SoilsVegetation and Soils

We measured vegetation cover in 0.75-m2 (herbs) and 1-m2 plots (shrubs) along transects.

Metabasic SoilsSoils formed in residual and colluvial weathered metabasic igneous and sedimentary rocks

Rescue Series loam: Fine-loamy, mixed, thermic Mollic Haploxeralfs

Granitic SoilsSoils formed in residual and colluvial weathered granite

Auberry Series sandy loam: Fine-loamy, mixed, semiactive, thermic Ultic Haploxeralfs

As predicted by local fire managers, areas with more frequent fires have different, less dangerous, fuel models than those with less frequent fires. Shrubs in the 1-time area are rapidly growing from established root systems and will soon have closed canopies that exclude ground cover vegetation. Regrowing shrubs in the 4-y area are much farther apart because the second fire in a four-year period apparently killed many of the regrowing shrubs after the 1997 fire. In the 20-y area mature shrubs form a dense canopy, suggesting that the period between recurring fires was too long for regrowing shrubs to be killed.

We collected runoff and sediment from six 1- x 5-m plots in FS and 4-y sites on both soil types (24 plots total). Runoff water was quantified and nutrient content measured. Sediment was quantified.

FS4-y

Study site location.

Chamise -dominated chaparral

Study site location in Tuolumne County, California.

California Chaparral

Pacific Ocean

Salt Lake City

One-time burning of fire-suppressed chaparral resulted in temporary Fire Behavior Fuel Model change with much reduced rate of spread and flame height;

A second fire four years after burning of fire-suppressed chaparral changed the fuel model to more persistent grass cover with high rate of spread but very low flame height. As remaining shrubs regrow, the model will shift from 1 to 2, increasing flame height but reducing rate of spread;

Recurring fire after 20 years did not permanently change the fuel model. The area of the Moccasin Fire returned to Fuel Model 4 within 10 later;

Grass-shrub mosaics resulting from recurring fires have improved soil characteristics associated with incorporation of SOM by herbaceous vegetation

Soil parent materials impact estimates of the effects of fire management on belowground C stores, and therefore, need to be considered in determinations of ecosystem C budgets and C flow;

Photo: Al Golub

Recurring fire did not significantly impact sediment yield from the study site, but it did impact the composition of the sediments. Sediments from FS plots yielded significantly (P < 0.05) more organic C, but not N and P. FS sediments also had much higher C to N ratios and were richer in organic C and total N.

This loss of nutrients from the FS sites may be caused by accumulation of litter on the soil surface which is easily removed with runoff water. Also, though vegetation was well established on our sites when our study began, we speculate that the strong, persistent herbaceous ground cover component in 4-y would recover more rapidly following wildfire to reduce erosion. Reestablishment of perennial grasses would speed recovery and increase protection.

Ground Cover Vegetation

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Fire Suppressed (FS)20-year: ’50,’72,’92 (20-y)Four-year: ’97, ’01 (4-y)One-time burn in 2001 (1-time)

Fire RegimesFire Regimes

Study DesignStudy Design

Our study site lies in a high fire-hazard zone where recurring catastrophic fires create overlaying recovery patterns. We overlaid digital fire history and soil map layers to identify sites with four different fire histories on two soil types.

One-time burn (2001)

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Long-term fire suppressed (FS)

Photos: Ace Young

Photo: Harry Hinckley

But the ability to manage vegetation in strategic zones, on steep slopes below forest communities is crucial for fire management agencies. Our objective was to measure sustainability, fuel loading, and ecological impacts of four recent fire frequencies. We analyzed soil organic matter (SOM), plant community composition, and runoff/erosion across four fire history scenarios. Frequent burning eliminates shrubs, is good for forage production and fire management, but is also detrimental for wildlife habitat. Absence of woody roots and stems may deplete recalcitrant components in long-term SOM pools.

Conversely, long-term fire suppression eliminates the herbaceous component, which may also deplete SOM by reducing fine below-ground OM inputs, slowing turn over, and transferring OM to above-ground biomass vulnerable to loss by fire.

We believe intermediate prescribed fire frequency in high fire hazard zones will result in dynamic grass-shrub mosaics optimal for fire management, wildlife, and ecosystem functions.

The purpose of this research is to define effects of four different fire return intervals on vegetation, runoff, erosion, and soil organic matter dynamics in two predominant soil types of Sierra Nevada Foothill chaparral.

Prescribed burning to maintain lower fuel loads is much easier to achieve in such grass-shrub mosaics. Increased diversity in plant species and growth form relative to fire-suppressed shrublands may be beneficial to wildlife and may decrease smoke production during fires.

Sierra Nevada foothill chaparral occupies steep, rugged canyon walls of the Merced, Tuolumne, Stanislaus and other major river systems draining the west slope of the Sierra Nevada Range. It covers the Motherlode region of the 1849 gold rush at elevations from 300 to 1500 meters. Soils formed on a complex mixture of geological parent material in the accretionary terranes of the western foothills. Parent materials in our study area are dominated by residuum of intermixed granitic and metabasic materials. Vegetation is typically dominated by chamise (Adenostoma fasciculatum) with components of manzanita, ceonothus, toyon, yerba santa, and other shrubs. Herbaceous vegetation is dominated by annuals, both native and exotic forbs with exotic grasses. Small patches of perennial native bunches grasses remain in isolated areas.

Yield of Sediment, C, N, & P

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1-time: The Creek Fire (2001) burned over previously fire-suppressed “over-mature” brush. Dominated by rapidly regrowing chamise in skeletal remains of the previous plant community.

4-y: The Priest Fire (1997) burned over previously fire-suppressed brush, reburned by the Creek Fire in 2001. Dominated by annual herbaceous vegetation with sparse regrowing chamise.

20-y: Most recently burned in the Moccasin Fire (1992). Dominated by dense, mature chamise 1 to 3 m tall. Soil surface has very little vegetation cover and litter.

FS: Dense overstory of decadent and dead chamise 3 to 6 m tall. Under story is relatively open with sparse cover of annual grasses and thick litter layer.

Fire Behavior Fuel Models (Anderson, 1982)

Model Cover Rate of Spread (m hr-1)

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1-time 6 Chamise < 2m 156 2

4-y 1 2

Grass (<33% shrubs) Grass (33-66% shrubs)

380 170

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20-y 4 Dense chaparral > 2 m 365 6 FS 4 Dense chaparral > 2 m 365 6

N Differences by Soil Type (not shown)Metabasic soils have consistently more Total N than the sandier granitic soils but consistently less mineral N. In FS, Metabasic soils have 31% more PMN than granitic soils. Other fire treatment areas have similar levels of PMN.

C Differences by Soil Type (not shown)Metabasic soils have 50 to 80% more mineralizable C than Granitic soils. 4-y and 1-time sites have significantly more TOC than 20-y and FS only in metabasic soils.

Soil nitrogen, both soil types, 0- to 5-cm depth bulked from four points along 12 transects in each fire frequency area.