Fire-Return Intervals in Mixed-Conifer 

Forests of the Kings River Sustainable 

Forest Ecosystems Project Area1 

Catherine Phillips2 

Abstract 

Fire-return intervals were studied on six 1.4-ha plots in a 2,070-ha study area in the 

Dinkey Creek watershed. Stumps in mixed-conifer forest were examined for fire scars 

created from 1771 to 1994, with 1873 chosen as the end of the pre-Euro-American 

settlement period because the rate of fire events decreased on most plots after about that 

year. Mean intervals from 1771 to 1873 ranged from 3.19 to 5.44 years, maximum intervals 

ranged from 6 to 12 years, and minimum intervals ranged from 1 to 2 years. Differences 

between plot means were not significant during that period, and the data showed no 

influence of aspect, elevation, or slope position on return intervals. Plot means were 

shorter than those reported in other mixed-conifer areas of the central Sierra Nevada. 

Archived fire records for the study area, on file at the Sierra National Forest, revealed a 

high rate of lightning-caused fires—an average of one fire requiring suppression every 

1.36 years between 1911 and 1965. This contrasts with an average of only one lightningcaused fire requiring suppression every 9 years during a similar period in mixed-conifer 

forests at Redwood Mountain and Bearskin Creek, 40-50 km south of Dinkey Creek, 

suggesting that local factors strongly influence fire events. 

The Sierra National Forest, in collaboration with the Pacific Southwest 

Research Station, established the Kings River Administrative Project Area for 

study of sustainable forestry under two contrasting, landscape-scale ecological 

approaches (USDA 1994). The Dinkey Creek watershed was selected for 

application of a three-tiered landscape, assuming that pre-Euro-American firereturn intervals in the Dinkey Creek watershed were similar to those recorded by 

other researchers in central Sierra Nevada mixed-conifer forests (Caprio and 

Swetnam 1995, Kilgore and Taylor 1979, Swetnam 1993). It also assumed that the 

density of trees varies across a watershed in relation to landscape position and 

that vegetation is most dense in drainage bottoms, grading into a relatively open 

condition on ridges. Fire is thought to be one causal agent for this gradient 

because it is believed to be more common and has shorter return intervals on 

ridges than in valleys. 

Fire intervals had not been studied previously in the Dinkey Creek 

watershed. The goals of this study were to determine the mean fire-return 

interval and to explore relations between fire interval and landscape position in 

the mixed-conifer forest. The study was intended to provide land managers with 

site-specific information about the role of fire-return intervals in determining 

vegetation density. Results reported here came from a two-part effort that 

included the ponderosa pine forest type in the Big Creek watershed, which 

adjoins the Dinkey Creek watershed immediately to the west. Because results 

from Big Creek were inconclusive, they are not included here. 

1 An abbreviated version of this 

paper was presented at the 

Symposium on the Kings River 

Sustainable Forest Ecosystems 

Project: Progress and Current 

Status, January 26, 1998, Clovis, 

California. 

2 	Assistant Province Ecologist, 

Stanislaus National Forest, 

USDA Forest Service, 19777 

Greenley Road, Sonora, CA 

95370. 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 31 
Phillips Fire-Return Intervals in Mixed-Conifer Forests of the Kings River Sustainable Forest Ecosystems Project Area 

Study Area 

The study area covered 2,070 ha of the Dinkey Creek watershed (37o1’N, 119o7’W) 

in the Sierra National Forest. Dinkey Creek, a tributary of the North Fork of the 

Kings River, occupies a southerly trending canyon in the southern Sierra Nevada. 

Elevation in the study area ranges from 1,450 to 1,950 m. Climate is 

Mediterranean, with warm dry summers and cool wet winters. Precipitation in 

the form of rain and snow falls mainly from November to April. Lightning is 

common in the study area. 

Mixed-conifer forests of the study area are characterized by white fir (Abies 

concolor [Gordon & Glend.] Lindley) in codominance with at least two of the 

following species: sugar pine (Pinus lambertiana Douglas), incense cedar 

(Calocedrus decurrens [Torrey] Florin), ponderosa pine (P. ponderosa Laws.), or 

Jeffrey pine (P. jeffreyi Grev. & Balf.). Understory species typical for the sites are 

bearclover (Chamaebatia foliosa Benth.), Mariposa manzanita (Arctostaphylos viscida 

ssp. mariposa [Dudley] P.Wells), Sierra gooseberry (Ribes roezlii Regel), deer 

brush (Ceanothus integerrimus Hook. & Arn.), and little leaf ceanothus (C. 

parvifolius [S. Watson] Trel.). Scientific names of plants follow Hickman (1993). 

Methods 

The study area was stratified into three tiers—upper (ridge top), middle, and 

lower (riparian)—using geomorphologic designations of landscape processes. 

Upper areas were experiencing some fluvial erosion, primarily classified as 

eroding hillslopes. Middle slopes were dominated by fluvial and mass-wasting 

processes and experiencing some colluvation and debris sliding. The lower tier 

was generally an area of deposition. Two 1.4-ha study plots were located in each 

tier in mixed-conifer forest with known harvest dates within the last 11 years. 

Selected sites were examined for evidence of fire by using methods described in 

Arno and Sneck (1977). Sample plots had at least six relatively fresh stumps from 

trees more than 200 years old. All sampled stumps were located with a global 

positioning system (GPS) and labeled with metal tags. No attempt was made to 

control for plot elevation or aspect. 

I selected an average of 7.7 stumps from each plot for a total of 46 stumps 

and, with a chain saw, removed a slab from the top of each stump exhibiting a 

continuous fire record in the form of open cat faces or fire scars (Drumm 1999). 

Slabs were sanded with 500-grit sand paper and examined under 10x and 20x 

magnification. Dates of fires were determined by counting annual growth rings 

backwards in time from the known year of harvest. Correlation of ring sequences 

into a single chronology was done for each plot by using methods described in 

Arno and Sneck (1977). Missing rings, often associated with fire events, may 

have resulted in some error in the data, so dates associated with a single fire 

event were interpreted accordingly. 

Results and Discussion 

Fire dates ranged from the mid-1500’s to 1943. The clearest fire records came 

from sugar pines and generally dated from about 1770. Numbers of fire events 

per year declined in 1871, after what appeared to be a fairly active and 

widespread fire period. Thirty-two of the 46 stumps sampled recorded fire 

events in 1871, but only six recorded fire events in 1873. Caprio and Swetnam 

(1995) sampled in the Kaweah River drainage, 55 km south of my study sites; 

Wagener (1961) sampled further north, at various locations in the central Sierra 

Nevada; and Kilgore and Taylor (1979) sampled 40-50 km to the south. All 

documented a similar period of high fire activity in the early 1870s, followed by a 

decline, suggesting a regional influence on fire activity in the early 1870s. Kilgore 

32 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
 Fire-Return Intervals in Mixed-Conifer Forests of the Kings River Sustainable Forest Ecosystems Project Area 

and Taylor (1979) cited two sources that document a period of drought between 

1869 and 1871 and drew a relationship between the drought and a “larger, 

higher-intensity fire” that occurred in 1871. Swetnam (1993) found a similar 

correlation between low-rainfall years and widespread fire events along 160-km 

transects in the southern Sierra Nevada. However, the lack of correlation between 

the 1862-1864 event listed in Kilgore and Taylor (1979) and large numbers of fire 

scars in the Dinkey Creek watershed imply that local conditions vary and may 

play an overriding role in local fire behavior. 

Mean fire-return intervals from 1770 to 1871 ranged from 3.19 to 5.44 years 

(table 1), a range generally shorter than reported in other studies. Comparable 

sites sampled by Caprio and Swetnam (1995) had mean return intervals of 6.58 

and 7.82 years, and return intervals at similar elevations were 14 to 17 years, 

mainly from giant sequoia groves (Kilgore and Taylor 1979). 

The apparent shorter return intervals in the Dinkey Creek area may be a 

result of high lightning activity. Known lightning “hot spots” exist in three 

locations near the study area—Bear Mountain, Cabin Meadow, and Dinkey 

Station (fig. 1). According to Sierra National Forest fire atlases, 39 lightning fires 

occurred within the study area between 1911 and 1964, a rate of one every 1.36 

years. None of these fires was allowed to burn more than 2.5 ha, but we can 

probably assume that lightning activity in this area has not changed appreciably 

since the mid-1700’s. This level of lightning activity is significantly higher than that 

reported by Kilgore and Taylor (1979) in their giant sequoia study sites to the east of 

Dinkey Creek. They reported two fires in the area of Redwood Mountain and three 

in the Bearskin Creek area from 1921 to 1972, or an average of only one every 9 years. 

Caprio and Swetnam (1995) reported an inverse relation between elevation 

and the number of lightning-ignited fires, speculating that sites at lower 

elevations are more flammable because they support more understory vegetation 

and, hence, generate more fuels. Although the high number of lightning ignitions 

in the Dinkey Creek study area is consistent with the findings of Caprio and 

Swetnam, I did not find a significant relation between elevation and return 

intervals in the Dinkey area, possibly because of a relatively small sample size. 

Plot location did not significantly influence fire-return intervals, though 

some influence of aspect is suggested by the data. More fire scars were found per 

tree on sites with northerly aspects, even though such sites showed trends 

toward slightly lower fire frequencies. The apparent greater continuity of scar 

USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 

Phillips 

Figure 1—Areas in or near the 

Dinkey Creek watershed that 

experienced high lightning activity 

from 1911 to 1964: 1–Bear 

mountain; 2–Cabin Meadow; 3– 

Dinkey Station. 

33 
Phillips Fire-Return Intervals in Mixed-Conifer Forests of the Kings River Sustainable Forest Ecosystems Project Area 

Table 1—Fire-return intervals (years) in the Dinkey Creek watershed, Sierra National Forest, 1770-1871 

Plot Aspect Elevation Mean Min.– max. Number Tree Number

number (meters) fire-return fire-return of years2 stumps of fire

and tier1 interval interval sampled events

_________________________________________________________________________________ ± SD

P1, lower S 1,707 4.45 ± 2.2 2-11 99 8 21 

P2, lower N 1,463 3.96 ± 2.3 1-10 96 6 24 

P3, NNE 1,634 5.44 ± 2.2 2-9 99 7 18 

middle 

P4, SW 1,798 3.19 ± 1.4 1-6 100 8 33 

middle 

P5, upper SSW 1,902 3.64 ± 1.8 1-10 103 10 28 

P6, upper N 1,780 4.33 ± 2.4 2-12 102 7 20 _________________________________________________________________________________

1 Lower tier = the riparian management zone; upper tier = about 30 percent of the watershed, 

from the ridge top down; middle tier = remaining side slope between the lower and middle tiers, 

comprising about 50 percent of the watershed. 

2 Duration of the interval sampled. 

data suggests that cooler (more mesic) conditions on northerly aspects reduce the 

number of missing rings a tree may exhibit after fires, or that the cooler conditions 

may limit fire-scar formation to events that burn hotter and presumably cover a 

larger area. It is clear, however, that additional work is needed to fully understand 

the effects of landscape position on fire-return intervals. 

The maximum period between fire events in the study area ranged from 6 to 

12 years (table 1), but the distribution of return intervals on all plots was weighted 

toward shorter intervals of 2 and 3 years (fig. 2). Short fire-return intervals have 

implications for land managers in supporting the fine-grained model proposed 

by Swetnam (1994) and Chang (1996). According to these sources, a system 

under short fire-return intervals would have a fine-grained appearance of patches 

in the landscape. In fact, the landscape would be in a steady state of 

disequilibrium, one constantly changing at a small scale, but one that remains in 

a fairly stable condition, overall. Biodiversity may be high because many different 

light and structural conditions are represented over a relatively small area. 

Figure 2—Frequency distribution 

of fire-return intervals in the 

Dinkey Creek study area, 1770 to 

1871. 

34 USDA Forest Service Gen. Tech. Rep. PSW-GTR-183. 2002. 
 Fire-Return Intervals in Mixed-Conifer Forests of the Kings River Sustainable Forest Ecosystems Project Area 

Conditions today have changed enormously, and stand conditions have become 

more homogeneous at a small scale but less homogeneous at a large scale. 

Minnich (1983) and Swetnam (1994) discussed this phenomenon. Historic logging 

practices, which removed the large-tree component in much of the Sierra Nevada 

and targeted specific desirable species like sugar pine, have also contributed 

(McKelvey and Johnston 1992). 

Acknowledgments 

Mary Drumm’s assistance with the field work was indispensable. Neil Sugihara, 

Forest Service Regional Fire Ecologist, was responsible for initiating the project, 

and Mark Smith, silviculturist on the Sierra National Forest, obtained funding 

from the Sierra National Forest. Don Potter and Jared Verner provided helpful 

review comments. To all I express my deep appreciation. 

References 

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