Climatic and human influences on fire regimes in mixed conifer forests in Yosemite National Park, USA

Forest Ecology and Management 267 (2012) 144–156

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Climatic and human influences on fire regimes in mixed conifer forests in Yosemite National Park, USA Alan H. Taylor a,⇑, Andrew E. Scholl b,1 a b

Department of Geography and Earth and Environmental Systems Institute, The Pennsylvania State University, University Park, PA 16802, United States Department of Geography, The Pennsylvania State University, University Park, PA 16802, United States

a r t i c l e

i n f o

Article history: Received 8 August 2011 Received in revised form 16 November 2011 Accepted 17 November 2011 Available online 29 December 2011 Keywords: Fire regimes Climate change Native Americans Fire exclusion

a b s t r a c t The goal of this study was to identify the influence of interannual and interdecadal climate variation and changes in land use on fire regimes in fire prone mixed conifer forests in the central Sierra Nevada in Yosemite National Park, California. We quantified fire frequency, fire return interval, fire extent, and season of fire for a 400 year period using fire-scar dendrochronology. The influence of regional climate variability and land use on fire occurrence and extent was assessed by relating the fire record to regional climate reconstructions and to documentary records of settlement and land use. The timing and extent for fires was related to interannual and interdecadal variation in drought and temperature linked to variation in the Pacific North America Pattern (PNA), the Pacific Decadal Oscillation (PDO) and El Niño– Southern Oscillation (NINO) had little effect on fire. The occurrence of large fires was also influenced by interactions among climate patterns and they occurred more often than expected in PNA+ NIÑO3years. At interdecadal time scales area burned was positively correlated with temperature and the PNA. Fire occurrence and extent declined with mid-19th century Euro-American settlement and land use change and fire was nearly eliminated after 1900 when a fire exclusion policy was implemented. A two-fold increase in rate of burning in the late 18th and early 19th century corresponds with spread of non-native disease to Native American populations during the Spanish Colonial Period but the PNA was also mainly positive during this period. Fire regimes were sensitive to shifting modes of climate and land use which can lead to variable pathways of forest development and hence forest structure. Forest structure at the time of Euro-American settlement reflects this sensitivity and managers should consider presettlement conditions as only a guide for restoration planning in forests highly altered by fire exclusion under a changing climate. Moreover, considering winter PNA could provide managers with an early indication of conditions during the fire season that are conducive to widespread fire. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Fire is the most frequent and widespread disturbance in dry pine and mixed conifer forests in the western USA and fire influences the structure, composition, and dynamics of these ecosystems (Agee, 1993; Swetnam and Baisan, 2003; Schoenagel et al., 2004; Scholl and Taylor, 2010). Fire regimes (i.e., frequency, extent, season, return interval) in these forests are regulated by processes and interactions that vary across a range of temporal and spatial scales (Heyerdahl et al., 2001). For example, the bottom-up controls of vegetation type or topography (Taylor, 2000; Beaty and Taylor, 2001; Heyerdahl et al., 2001), or time since last fire (Taylor ⇑ Corresponding author. E-mail addresses: [email protected] (A.H. Taylor), [email protected] (A.E. Scholl). 1 Present address: Department of Geography, Wittenberg University, P.O. Box 720, Springfield, OH 45501, United States. 0378-1127/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2011.11.026

and Skinner, 2003; Collins et al., 2009; Scholl and Taylor, 2010) affect fire regimes at local scales by influencing fuel characteristics. At large spatial scales, fire regimes are controlled by land use practices such as fire exclusion, livestock grazing, or climate variation that can override bottom-up controls (Swetnam and Betancourt, 1998; Heyerdahl et al., 2001; Taylor and Skinner, 2003; Mori, 2011). A hallmark of fire regimes forced by variation in climate and/or land use is synchronized variation in fire activity at different locations (Swetnam and Betancourt, 1998; Schoenagel et al., 2005; Taylor et al., 2008). A fire exclusion policy synchronized reduction of fire on federal forest lands beginning in the early 20th century (Stephens and Ruth, 2005). Forests that historically burned frequently at low severity such as western USA dry pine and mixed conifer were particularly sensitive to fire exclusion. Reduced fire frequency in these forests increased forest density, shifted forest composition towards less fire tolerant species, reduced understory plant cover and species richness, and increased surface and aerial fuels (e.g., Sakulich

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and Taylor, 2007; Scholl and Taylor, 2010; Laughlin et al., 2011). These structural changes are thought to have increased the risk of insect and disease outbreaks and also high severity fire. In these highly altered forests resource managers are attempting to reduce the risk of high severity fire using prescribed fire, wildland fire and mechanical treatments and to restore fire resilient ecosystems (Covington et al., 1997; van Wagtendonk and Lutz, 2007; Schmidt et al., 2008; Collins et al., 2009; North et al., 2009). Climate variation also synchronizes fire activity in dry western forests. Years of synchronous and widespread burning are usually associated with drought and this relationship is evident both before and during the period of active fire exclusion (Swetnam and Baisan, 2003; Trouet et al., 2006; Heyerdahl et al., 2008; Morgan et al., 2008; Littell et al., 2009). But drought may not be a precondition for years of widespread burning during certain time periods (Taylor and Beaty, 2005; Taylor et al., 2008), and antecedent climatic conditions may also be important if they synchronize years of high fuel production which then burns in subsequent dry years (Swetnam and Betancourt, 1990). Shifts in the frequency and/or extent of burning that persist for decades before mid-19th century Euro-American settlement (hereafter presettlement) are also evident in tree ring records of fire activity (Grissino-Mayer and Swetnam, 2000; Sakulich and Taylor, 2007). For example, in northern California and northern Mexico, fire regime shifts in the late-18th century are related to changes in the interaction of the El Niño/Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) (Norman and Taylor, 2003; Skinner et al., 2008) which both influence the fire-climate of western North America (Swetnam and Betancourt, 1990; Schoenagel et al., 2005). Changes in land use and declines in Native American populations that used fire during the Spanish Colonial Period (1769–1849) in California are also coincident with this fire regime shift, at least in northern Mexico (Stephens et al., 2003). Later mid19th century shifts in the frequency, seasonality, or extent of fire are evident in northern California (Norman and Taylor, 2005; Stephens and Fry, 2006; Skinner et al., 2009; Taylor, 2010) and are coincident with the Gold Rush when the California Native American population was decimated and livestock grazing was expanded to feed California Gold Rush miners (Dasmann, 1965; Heizer, 1993; Trafzer and Hyer, 1999). Active fire exclusion in the 20th century significantly reduced fire frequency and extent in dry pine dominated forests in California (Stephens and Sugihara, 2006) and other dry western forests (Pyne, 1982; Stephens and Ruth, 2005). Longer term historical analyses of climate and land use on fire regimes provides key information to resource managers for identifying and refining restoration goals (Stephenson, 1999; Veblen et al., 2000; Scholl and Taylor, 2010). Moreover, determining how climate modulates fire regimes is important for understanding how forests developed before 20th century fire exclusion and how fire regimes may respond to projected 21st century climate change (Hayhoe et al., 2004; Spracklen et al., 2009). Interannual and interdecadal climatic variation that influences fire occurrence and extent in dry pine and mixed conifer forests in the Pacific coast states is related to broad scale climate teleconnection patterns generated by the El Niño/Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Pacific North America Pattern (PNA). ENSO, which is driven by variation in tropical Pacific sea surface temperatures (SST), generates high frequency (3–7 years) variation in temperature and precipitation that synchronizes drought regionally (Diaz and Markgraf, 2000). The PDO is ENSO-like but has a lower frequency (25–30 years) and is the result of variation in north Pacific SST (Mantua et al., 1997; Dettinger et al., 2000). During a warm phase (positive) ENSO and PDO conditions are wetter than normal in the Southwest (SW), while dryer than normal conditions prevail in the Pacific Northwest (PNW). These regions experience the opposite conditions

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during the negative cool phases (Gershunuv et al., 1999). The Pacific North American (PNA) is an atmospheric circulation pattern that also strongly influences Pacific coast climate and it varies on both interannual and interdecadal time scales (Barnston and Livezey, 1987; Wallace and Gutzler, 1981; Overland et al., 1999; Leathers and Palecki,1992). During a positive PNA (PNA+) an amplified ridge over western North America deflects moisture-bearing cyclonic storms from the Pacific northward (Leathers et al., 1991) resulting in above normal temperatures and below normal precipitation in the PNW (Sheridan, 2003). During the reverse PNA (PNA ), nearly zonal westerly flow off the north Pacific directs Pacific storms towards the PNW resulting in higher precipitation and lower temperatures (Leathers et al., 1991). Higher fire risk and area burned in Pacific coast forests in the 20th century are associated with PNA+ winters (Trouet et al., 2006, 2009). The goal of this study is to identify the top-down effects of climate and land use practices on fire regimes in dry mixed conifer forests in the central Sierra Nevada in Yosemite National Park (YNP). The climatic mechanisms driving fire activity in this region are complex; it is located in the pivot zone of the ENSO-PDO precipitation dipole (Dettinger et al., 1998). Fire-climate associations may be similar to the SW in some periods and the PNW in others (Westerling and Swetnam, 2003; Taylor and Beaty, 2005) and the PNA may strongly synchronize variation in fire activity in this pivot zone. This study is the first to examine fire-PNA interactions prior to fire suppression and widespread Euro-American settlement. Moreover, spread of disease to Native Americans in Yosemite in the late-18th century during the Spanish Colonial Period from California missions (Hull, 2009) may have altered Native American influences on forest fire regimes before rapid mid-19th century land use change associated with the California Gold Rush (Perlot, 1985; Rohrbough, 1997) and 20th century fire exclusion. Specifically, we address the following questions: (1) How do fire frequency and extent vary with land use (i.e., Presettlement; Spanish Colonial Period; Gold Rush-Settlement Period; Fire Exclusion Period)? (2) Was burning more widespread during warm-dry years and less extensive during cool-wet years? (3) Is variation in fire frequency and extent related to variation in ENSO, PNA, or the PDO? These questions were answered using a multi-century record of fire activity developed from fire scar dendrochronology in two forest landscapes in YNP. We then relate the fire record to documentary records of settlement and land use, and proxy records of climate at both interannual and mulitidecadal time scales.

2. Methods 2.1. Study area Fire occurrence and extent in YNP were identified in two mixed conifer forest landscapes (Fig. 1). The Big Oak Flat (BOF) site covers 2125-ha and the South Fork site (SFM) covers 1600-ha. Elevations in both landscapes range from 1300 to 2000 m. In BOF the terrain is moderately dissected and contains several small streams. SFM is bisected by the Merced River, which cuts an east–west trending valley. The climate is Mediterranean, characterized by warm dry summers and cool wet winters. Mean annual precipitation in YNP (1560 m) is 109.1 cm, and most (86%) falls as snow between November and April. Mean monthly temperatures are lowest in January (2 °C) and highest in July (18 °C). Soils are shallow (<1 m), excessively drained, medium in acidity and developed in Mesozoic aged granite (Hill, 1975; Huber, 1987). Mixed conifer forests occupy both sites and they include ponderosa pine (Pinus ponderosa), sugar pine (P. lambertiana), incense cedar (Calocedrus decurrens), Jeffrey pine (P. jeffreyii), white fir (Abies concolor), and Douglas fir (Pseudotsuga menziesii). California

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black oak (Quercus kelloggii) is an important associate on drier south-facing slopes while western dogwood (Cornus nuttallii) and bigleaf maple (Acer macrophyllum) occur on the most mesic sites. People have influenced fire regimes and vegetation conditions in YNP for thousands of years (Anderson and Carpenter, 1991; Anderson, 2005). In the presettlement period, the Miwok actively used fire to increase production of acorns, berries, roots, and fiber from certain plant species and to flush game (Barrett and Gifford, 1976; Anderson, 2005). Epidemics of non-native disease that spread from the Franciscan missions established on the California coast (1769–1823) during the Spanish Colonial Period (1769– 1849) decimated Native American populations in many parts of California (Cook, 1976). Euro-American settlers moved in large numbers into the central Sierra Nevada foothills during the California Gold Rush-Settlement Period (1849–1900) leading to further declines in the Native American populations. For example, in Mariposa County, which includes YNP, Native Americans were outlawed and killing them was legally permitted (Perlot, 1985; Trafzer and Hyer, 1999). Euro-Americans also began the practice of grazing livestock. Both the potential reduction of Native American ignitions and the reduction of fine fuels from livestock grazing may have reduced fire frequency and the potential for fire spread across the landscape. Euro-Americans first entered Yosemite Valley in 1851 and YNP was established in 1890 and it was under the stewardship of the US Army until 1914. The Army suppressed

some fires in YNP (Pyne, 1982; Meyerson, 2001) but it was not effective at reducing fires in BOF or SFM until 1900 (Scholl and Taylor, 2010), the beginning of the fire exclusion period. The National Park Service continued a policy of fire exclusion when it took jurisdiction in 1916. A policy of fire exclusion on National Forest System lands that surround YNP began in 1905 (Pyne, 1982). 2.2. Fire history Pre-fire exclusion period fire occurrence and extent in BOF and SFM were reconstructed using fire scars preserved in wood collected from live and dead fire scarred trees (Arno and Sneck, 1977). Wood samples were extracted from trees (BOF, n = 209; SFM n = 156 samples) collected in a 9 ha area surrounding grid points (BOF = 85; SFM = 64) established at 500 m intervals in each study area. An average of two samples (range 1–5), were collected at each grid point. Fire dates were determined by sanding wood samples to a high polish, cross-dating their annual growth rings (Stokes and Smiley, 1968) and recording the calendar year of each growth ring with a fire scar. The season of burn for each fire was identified from the relative position of each fire scar within an annual growth ring. Scar positions were assigned to one of five categories (Baisan and Swetnam, 1990): (1) early (first one-third of earlywood); (2) middle (second one-third); (3) late (last one-third); (4) latewood (in latewood); (5)

Fig. 1. Location of fire history study sites and weather stations in Yosemite National Park, California, USA.

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dormant (at ring boundary). In this strongly winter wet, summer dry climate, dormant season fires are interpreted to occur in late summer or fall after growth has stopped for the year, rather than before growth starts in early spring (Caprio and Swetnam, 1995). Estimates of fire occurrence and extent were made in the following way. First, a chronology of fire dates was developed for each grid point using the fire dates from samples collected at each point. Second, burn area was calculated by multiplying the proportion of recording gridpoints with a fire in a given year by study area size. Trees initially scarred by fire, or recording trees, are more likely to record a subsequent fire because of loss of protective bark. There is a strong statistical relationship between percentage of samples recording fires and fire perimeters on documentary fire maps indicating this approach provides reliable burn area estimates (Farris et al., 2010). Most fires (BOF = 81%; SFM = 83%) intersected the study area boundary suggesting that that our fire area estimates are conservative and represent a measure of relative fire extent in each study area. The accuracy of fire area estimates decreases as the number of recording gridpoints declines with time. We chose a cut-off date of 1600 for the fire regime analysis ensuring a sample of recording gridpoints of P10% which is generally adequate to characterize fire regimes parameters in forests with short fire return intervals (Caprio and Swetnam, 1995). We also estimated the annual burning rate by dividing study area size by the extent of each fire each year. Data from both sites were combined for our analysis to emphasize the influence of top–down rather than local controls (Heyerdahl et al., 2001) 2.3. Land use variation Variation in fire occurrence and extent that may be related to land use changes was identified by comparing composite fire return intervals for the presettlement (before 1769) Spanish Colonial (1769–1849), Gold Rush-Settlement (1850–1900) and fire exclusion (after 1900) periods using a Student’s t-test. To determine if land use effects were restricted to fires of a certain extent (Dietrich, 1980) comparisons of fire frequency were made for fires that burned any grid point, 10% of more of gridpoints, or 25% or more of gridpoints. Shifts in the frequency and extent of fire were also identified graphically using a 25-year moving average of fire extent centered on the middle year of the series and the timeline of land use changes identified from historical documents. 2.4. Climate record To identify the influence of instrumental PDO, ENSO and PNA on climatic conditions in YNP we calculated Spearman rank order correlation coefficients between each climate variable and average seasonal precipitation and temperature and the period reconstructed by each climate proxy for YNP climate stations (see below). Climate data for the period 1950–2010 were averaged from the three stations for the analysis (Hetch Hetchy, Yosemite Park Headquarters, Yosemite South Entrance) (Fig. 1). We used five proxies of Pacific climate variability for the fireclimate analyses. First, drought was represented by the Palmer Drought Severity Index (PDSI). PDSI integrates immediate and lagged precipitation, temperature and soil moisture data in a composite index of drought (Palmer, 1965). We used Gridpoint 47, part of a 2.5°  2.5° grid (latitude and longitude) of reconstructed PDSI from North America, to represent drought in YNP. The reconstruction used 57 drought sensitive tree ring chronologies to reconstruct PDSI at this grid point (Cook et al., 2004). Second, temperature was represented using a reconstruction of summer (April-September) temperature (TEMP) (Briffa et al., 1992). The temperature reconstruction was based on up to 53 high elevation tree ring chronologies in western North America to reconstruct

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temperature and temperature variation is expressed as a departure of the temperature from the 1951 to 1970 period. Third, we used a reconstruction of the winter (December–February) NIÑO3 index (NIÑO3) as an index of ENSO variability (Cook et al., 2008). This reconstruction captures variability in SSTs in the equatorial Pacific and was based on 23 tree ring chronologies in northern Mexico and the southwestern USA. Positive NIÑO3 values represent El Niño (warm) and negative NIÑO3 values represent La Niña (cool) conditions, respectively. Fourth, the PNA is represented by a reconstruction of the December-January index of the PNA circulation pattern (Trouet and Taylor, 2010). This reconstruction was based on three high and low elevation tree ring chronologies in the northern Rocky Mountains. Finally, we used four reconstructions of the PDO as measures of variability in northern Pacific SSTs. The PDOM reconstruction of annual PDO by MacDonald and Case (2005) was based on two moisture sensitive tree ring chronologies collected in southern California and the eastern Rocky Mountains in Alberta, Canada located at opposite ends of the PDO precipitation dipole. The PDOS reconstruction by Shen et al. (2006) was based on a drought/flood index derived from historical documents for a summer precipitation dipole in eastern China. The annual PDOD reconstruction by D’Arrigo et al. (2001) was based on temperature sensitive tree rings from nine sites in Alaska and British Columbia and moisture sensitive tree rings at two gridpoints with PDSI reconstructions by (Cook et al., 1999) in northern Mexico. The annual PDOB reconstruction by Biondi et al. (2001) was based on six moisture sensitive tree ring chronologies in southern California and northern Mexico. Each of the PDO reconstructions were strongly correlated (rs = 0.66 0.89, P < 0.0001) with annual PDO during the period of the YNP instrumental climate record. In these PDO reconstructions warm (cool) PDO conditions are represented by positive (negative) values, respectively. All climate proxies were acquired from the NOAA Paleoclimatology Program (http:// www.ncdc.noaa.gov/paleo/recons.html, last accessed 11/14/2011). 2.5. Fire-climate analysis We identified the relationships between climate and fire occurrence and extent using three complimentary approaches: (1) superposed epoch analysis (SEA); (2) correlation analysis; and (3) contingency analysis. We used period of record for each climate proxy for the fire-climate analysis and focused our analysis on the period 1600–1849 because land use changed during the Gold Rush-Settlement Period and these changes may have influenced fire-climate relationships after 1850. SEA was used to identify the relationship between interannual variation in climate and fire. SEA tests the hypothesis of no association between fire years and climate preceding, during, or following the fire year. SEA compares mean climate (PDSI, NIÑO3, TEMP) for a fire year with climate in a seven year window (i.e., four years before and two years after) the fire event. We used autocorrelation functions (ACF) and auto-regressive moving average (ARMA) models to identify and remove serial autocorrelation in PDSI, NIÑO3, and TEMP (Box et al., 2008). The SEAs were then conducted with the model residuals for each of the time series that exhibited serial autocorrelation. Monte Carlo simulations were used to calculate confidence limits for average deviations of climatic conditions for years in the window for comparison with randomly selected years in the record (Mooney and Duval, 1993). SEA was performed separately for non-fire years, years when fires burned any recording grid point, and years with large fires. Large fire years were those years when fire size exceeded the 90th percentile size of a fire. We used correlation analysis to identify fire-climate relationships at several time scales. For interannual time scales we calculated Spearman rank order correlation coefficients between each climate variable and fire extent. For interdecadal time scales, we

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calculated 10-year and 15-year non-overlapping averages for each climate variable and for fire extent. We then calculated a Spearman rank order correlation between fire extent and each climate variable for each time scale (e.g., Swetnam, 1993). We identified the influence of combinations of climate phases (above or below average) on each fire category (non, any, large) using a chi-square goodness of fit test (Sokal and Rohlf, 1994). This determines if the distribution of years for climate phases for each fire category differ from the distribution of all years for climate phase combinations regardless of fire activity. We tested for combinations of climate patterns (PNA, ENSO, PDO) and climate variables (PDSI, TEMP). We repeated combinations of climate pattern tests with each of the four PDO reconstructions. 3. Results 3.1. Fire regimes There was a long record of frequent fire in mixed conifer forest landscapes in YNP. A total of 308 fire years were recorded between 1600 and 2002 (Fig. 2(a)). 3.2. Fire synchronicity Dates of fires in BOF and SFM were synchronized. Years with fires that burned >100 ha between 1600 and 1900 co-occurred in

BOF and SFM more frequently than expected based on chance alone (X2 = 20.5, p < 0.001). This strong temporal synchrony in dates of fires between sites >45 km apart suggests a strong regional influence on fire activity in the central Sierra Nevada. 3.3. Fire return intervals The statistical description of FRI for each site includes the mean, median, and Weibull median probability interval (WMPI) as measures of central tendency. FRI distributions are often asymmetrical and the WMPI is a measure of central tendency for skewed distributions. The FRI distribution was positively skewed with more short than long fire return intervals (Table 1). The composite WMPI for fires that burned any gridpoint in BOF or SFM was 1.1 years. Composite WMPI for more widespread fires recorded by P10% or P25% of the gridpoints were longer. The per-tree WMPI was 9.7 years. 3.4. Fire season The position of fire scars within growth rings (n = 3,060 ring position determined) indicate that burns occurred mainly late in the growing season (latewood = 46.7%) or after trees have stopped growing for the year (dormant 38.4%). Few fire scars were recorded in earlywood positions (14.9%) and two thirds of these were in late earlywood.

Area burned (ha)

Recording gridpoints

a

b

25-year moving average (%)

Annual rate of burning (%)

Year

Year Presettlement Period

Spanish Colonial Period

Gold Rush Settlement Period

Fire Suppression Period

Fig. 2. Annual area burned (ha) (a) and annual rate of burning (percent study area burned per year) (b) between 1600 and 2000, Yosemite National Park, California, USA. The second y-axis is the number of points recording fires in a given year (a) and the 25-year moving average of rate of burning is centered on the middle year of the series (b). The land use time periods are presettlement (before 1769), Spanish Colonial Period (1769–1848); Gold Rush Settlement Period (1849–1900), fire suppression (after 1900).

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A.H. Taylor, A.E. Scholl / Forest Ecology and Management 267 (2012) 144–156 Table 1 Per tree (PFI) and composite fire return interval (years) statistics for period 1600–1900 for BOF and SFM mixed conifer forests, Yosemite National Park. Type of sample

Number of intervals

Mean (MFI)

Median

WMPI

SD

Min.

Max.

Skewness

Per tree (PFI) Composite P 10% Scarred P 25% Scarred

278 131 26

11.0 1.1 2.3 11.0

9.3 1 2 8

9.7 1.1 2.0 9.3

8.2 0.3 1.9 9.0

2 1 1 1

68 3 11 37

2.3 3.9 2.6 1.1

3.5. Fire extent The mean (±SD) and median burn area for the 1600–1900 period was 374 ± 384 ha and 263 ha (range 25–2765 ha), respectively (Fig. 2(a)). Burn area was positively skewed with more small than large fires and only a few fire years (n = 30) had estimated burn areas >800 ha. Interdecadal variation in the rate of burning between 1600 and 1900 is evident in the moving average graphic (Fig. 2b). The graphic indicates the rate of burning increased after 1750 and increased nearly two-fold between 1770 and ca. 1850. The burning rate then declined and approaches zero after 1900 when fire exclusion became effective. 3.6. Land use variation Fire return intervals varied by land use period (Table 2; Fig. 2b). The mean composite FRI for fires that burned any gridpoint and 10% or more of gripdoints was shorter in the Spanish Colonial Period than either the presettlement or Gold Rush settlement period. Mean composite FRIs for larger burns (P25% of gridpoints) were also shorter in this period compared to the presettlement. Composite FRI for fires that burned any grid point were longest in the fire suppression period and too few larger burns (P10% or P25% scarred) occurred in the fire suppression period for comparison of FRI with other periods. 3.7. Fire-climate relationships 3.7.1. Climate variability Interannual variation in YNP climate was associated with variation in instrumental climate indices (Table 3). Winter, summer, and annual temperature were positively correlated, and winter precipitation was negatively correlated with winter PNA. Similarly, winter and summer temperatures were positively associated with annual PDO. In contrast, winter NIÑO3 was not associated with variation in either annual or seasonal temperature and precipitation in YNP during the instrumental period. Reconstructed values of PDSI and TEMP were associated with variation in proxy indices of the PNA, NIÑO3, and the PDO during the fire-climate analysis period (1600-1850) Fig. 3a–h. NIÑO3 was positively correlated with PDSI (p < 0.001) while PNA was negatively correlated with PDSI (P < 0.01) and positively

Table 2 Composite fire-return interval statistics (years) for the presettlement (before 1769), Spanish Colonial (1769–1849), Gold Rush settlement (1850–1900), and fire suppression period (1900–2002) periods in mixed conifer forests, Yosemite National Park, USA. Values in a column with the same letter were significantly different (P < 0.05, t-test). WMPI is the Weibull median probability interval. Presettlement

Any scarred

P10% Scarred

P25% Scarred

Land use period Mean Median WMPI Range

1.1ac 1.0 1.1 1–3

2.8a 2.0 2.4 1–11

21.2a 19.0 20.7 11–37

Spanish Colonial Mean Median WMPI Range

1.0abc 1.0 1.0 1.0

1.5ab 1.0 1.4 1–4

5.9a 6.0 5.7 1–11

3.0b 3.0 2.7 1–10

11.3 11.5 10.3 3–19

Gold Rush settlement Mean 1.1bc Median 1.0 WMPI 1.1 Range 1–2 Fire suppression Mean Median WMPI Range

3.2ac 3.0 2.8 1–9

⁄

⁄

⁄

⁄

⁄

⁄

⁄

correlated with TEMP (p < 0.001). Two (PDOD, PDOM) of four proxy PDO records were positively correlated with TEMP (p < 0.001 p < 0.007) and PDOM, PDOB, and PDOS were positively correlated with PDSI (p < 0.05 p < 0.001). 3.7.2. Interannual fire-climate relationships The SEA indicates interannual fire activity was associated with variation in drought (PDSI), temperature (TEMP), and ENSO (NIÑO3). Non-fire years, and years with any size and years with large fires, occurred under moist and dry conditions as measured by PDSI (p < 0.05), respectively (Fig. 4a). Large fire years were also warm (p < 0.05) (Fig. 4b). Years with large fires were associated with variation in ENSO (p < 0.05) (Fig. 4c). In the SEA, the years after large fires were associated with La Niña conditions. There was no relationship between variation in ENSO with non-fire years or years with fires of any extent.

Table 3 Spearman rank order correlation coefficients of average annual and seasonal temperature and precipitation in Yosemite National Park and climate patterns. The climate patterns are the El Nino Southern Oscillation (NIÑO3), the Pacific Decadal Oscillation (PDO), and the Pacific North America circulation pattern (PNA) (1951–2009). Bonferonni’s correction was used to reduce Type I errors. Climate variable PNA (DJ) NIÑO3 (DJF) PDO (Annual) * **

p < 0.05. p < 0.01.

Winter (DJF) Precipitation 0.321* .051 .123

Spring (MAM) Temperature

Precipitation

0.425** .061 .319*

0.002 .095 .081

Summer (JJA) Temperature 0.065 .151 .089

Precipitation 0.009 .223 .113

Fall (SON) Temperature 0.344* .090 .037

Precipitation 0.009 .053 .071

Annual Temperature 0.065 .074 .206

Precipitation 0.041 .231 .098

Temperature 0.411* .033 .168

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Fig. 3. Reconstructed PDSI (Cook et al., 2004) (a), reconstructed western North American summer temperature (b) (Briffa et al., 1992), reconstructed El Niño–Southern oscillation (NIÑO3) (c) (Cook et al., 2008), reconstructed Pacific North America pattern (d) (Trouet and Taylor, 2010), reconstructed Pacific Decadal Oscillation (e) (MacDonald and Case, 2005), (f) reconstructed Pacific Decadal Oscillation (f) (Shen et al., 2006), reconstructed Pacific Decadal Oscillation (g) (Biondi et al., 2001), reconstructed Pacific Decadal Oscillation(h) (D’Arrigo et al., 2001). The annual series were smoothed with a 10-year spline.

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151

a

b

c

Fig. 4. Superposed epoch analysis (SEA) of reconstructed drought (PDSI) (a), western North American summer temperature (TEMP) (b), the El Niño–Southern Oscillation (NIÑO3) (c) for non-fire years, years with fires of any extent, and fires that exceeded the 90th percentile fire size for the period 1600–1849. The analysis window included up to four years before and two years after the fire event. The temperature departure is from the mean of the reference period (1951–1970) (Briffa et al., 1992). Values with filled symbols were significantly different (P < 0.05).

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Table 4 Spearman rank order product moment correlation coefficients between fire extent, drought (PDSI), western North American summer temperature (TEMP), El Niño– Southern Oscillation (NIÑO3), and the Pacific North American Pattern (PNA) (a), and four reconstructions of the Pacific Decadal Oscillation (PDO) (b) for annual and nonoverlapping ten and fifteen year periods for the period 1600–1850. Bonferonni’s correction was used to reduce Type I errors. Climate variable

* **

Annual

Ten year

Fifteen year

(a) PDSI TEMP NIÑO3 PNA

0.32** 0.36** 0.11 0.32**

0.06 0.65** 0.12 0.62*

0.03 0.66* 0.03 0.74*

(b) PDOM PDOB PDOD PDOS

.053 .105 .087 .129

0.208 0.199 0.111 0.248

.375 0.217 0.139 .260

p < 0.05. p < 0.01.

The interannual correlation analysis of fire extent and climate confirmed the SEA results. Fire extent in YNP was negatively correlated with PDSI (P < 0.01) and positively correlated with temperature (P < 0.01) and fire extent was also positively correlated with the PNA (P < 0.01) (Table 4a). Fire extent was not associated (P < 0.05) with variation in any of the PDO proxies (Table 4b). 3.7.3. Interdecadal fire-climate relationships Fire extent at interdecadal time scales was correlated with climate variation. Fire extent was positively correlated for non-overlapping 10-years periods with TEMP (P < 0.01) and PNA (P < 0.01) but there was no correlation for PDSI (P > 0.05), NIÑO3 (P > 0.05), or any of the PDO (P > 0.05) indices (Table 4a and b). The pattern of significance between fire extent and the climate variables was identical for 15-year non-overlapping periods (Table 4a and b). 3.7.4. Climate phase combinations Climate conditions (TEMP, PDSI) singularly or in combination did not differ significantly from the distribution of years for all years of those phases (p > 0.05) for the non-fire and any fire categories. However, large fires occurred more often than expected in dry (p < 0.005) or warm (p < 0.005) years and in years when dry and warm conditions coincided (p < 0.0001). The distribution of PNA for large fire years (PNA+) was different (p < 0.01) than expected for PNA alone and in combination with ENSO (PNA+ NIÑO3 ) (p < 0.05) and two (PDOB, PDOS) (PNA+ PDO ) of the four PDO reconstructions. The three way distribution of PNA, ENSO, and PDO for any category of fire year and individual ENSO and PDO indices did not differ from the distribution for all years of those phases. 4. Discussion The temporal patterns of fire occurrence in the two mixed conifer forest landscapes in YNP were strongly synchronous despite separation by > 45 km. Synchrony of fire dates was probably not caused by fires that spread continuously between the sites. Numerous terrain features (i.e., rivers, sparsely vegetated canyon walls, expanses of bare rock) known to impede fire spread occur between the sites. Synchrony of fire dates among sites is a hallmark of topdown control on fire regimes by climate or land use that influence burning conditions over wide areas (Swetnam and Betancourt, 1998; Heyerdahl et al., 2001; Taylor et al., 2008; Trouet et al., 2010).

Years with widespread burning in dry pine and mixed conifer forests in the presettlement period are strongly related to drought in the SW (Swetnam and Betancourt, 1998; Sakulich and Taylor, 2007), the Rocky Mountains (RM) (Veblen et al., 2000; Heyerdahl et al., 2008), the PNW (Heyerdahl et al., 2001), the interior west (Brown, 2006), and at other sites in California (Taylor and Beaty, 2005; Taylor et al., 2008). The effect of drought on fire in YNP was similar. Burning was more widespread in drought years with warm summers and the opposite conditions were characteristic on non-fire years. Fires in YNP were not related to antecedent climate conditions as they typically are in the SW and RM. In the SW and RM, dry pine and mixed conifer forests frequently have a welldeveloped understory of grass and forbs and antecedent wet years increase production of fine fuels and precondition forests for widespread burns in subsequent drought years. Antecedent climate conditions are associated with fire at some sites in the Sierra Nevada and southern Cascades (e.g., Taylor and Beaty, 2005; Taylor et al., 2008) suggesting that heterogeneity in forest floor vegetation may influence the temporal structure of fire-climate relationships even at local scales. In contrast, increased growth of fine fuels in wet years is not a precondition for widespread burning in the PNW (Heyerdahl et al., 2001; Hessl et al., 2004). Regional and local differences in the lagged wet-dry pattern of fire underscores the importance of vegetation type (fuels) in mediating fire-climate relationships since the drivers of interannual climatic variability among regions are similar. Interannual variability in tropical SSTs associated with ENSO is the primary driver of interannual climatic variability in the western North America (Diaz and Markgraf, 2000). In the SW (Swetnam and Betancourt, 1990), and the PNW (Heyerdahl et al., 2002) interannual variability in fire extent varies with indices of ENSO. In each region, more widespread burning tends to occur in years with the dry ENSO phase. Spatial variation in the north-south position of zonal precipitation associated with ENSO is likely to reduce the coherency of the ENSO signal on fire regimes in the pivot zone (Westerling and Swetnam, 2003; Taylor and Beaty, 2005). In parts of the southern Cascades (Taylor et al., 2008) and northern Sierra Nevada (Valliant and Stephens, 2009), higher fire activity is weakly associated with El Niño (warm). In YNP, large fires were associated with La Niña (cool) conditions which are characteristically drier in YNP. This suggests that, ENSO tends to synchronize fire activity in the central and southern Sierra Nevada with the SW, and not the PNW, an interpretation consistent with recent interregional comparisons of fire-climate relationships across the western USA (Kitzberger et al., 2007; Trouet et al., 2010). Variation in fire activity in some parts of the RM (Schoenagel et al., 2004; Sibold and Veblen, 2006; Mori, 2011) and southern Cascades (Norman and Taylor, 2003) is associated with variation in north Pacific SSTs (PDO) at least as measured by some PDO reconstructions. Yet, the relationship between historic fire activity and PDO is not consistent in the RM. Heyerdahl et al. (2008) found no association between regional fires and four reconstructions of the PDO in the northern RM. Similarly variation in fire extent in YNP was not associated with variation in any of four PDO reconstructions. In the PNW, variability in winter precipitation at interannual and multi-decadal time-scales is strongly influenced by the PDO (Mantua et al., 1997; McCabe and Dettinger, 2002) and winter temperatures are warmer and springs are dryer in the RM (Morgan et al., 2008) under PDO+ similar to conditions under PDO+ in YNP. Fire activity during the fire suppression period in western forests is associated with variation in the PDO, despite active fire suppression. Annual area burned in the 20th century in the northern RM (Morgan et al., 2008; Mori, 2011) and the Pacific Coast States (Gedalof et al., 2005; Trouet et al., 2006) is greater in PDO+ years. Warmer spring-summers in years with a warm PDO in these regions would accelerate snowmelt increasing fire season length and the

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probability of more widespread burning if ignitions occur (Westerling et al., 2006). The lack of an effect of historical PDO on historic fire activity in YNP may be due to the relatively local nature of the fire record and/or lower variation in the PDO in the past and how that is expressed in PDO proxy records (St. George and Ault, 2011). The PNA circulation pattern also strongly influences climate variability in the western USA. Winters, summers, and years with PNA+ were warmer in YNP than PNA years. Warmer temperatures generated by PNA+ would accelerate snowmelt and increase fire season length. Widespread burning in Canada’s southern RM (Johnson and Wowchuk, 1993) and higher fire risk in the PNW in the 20th century (Trouet et al., 2009) are associated with PNA+ years. PNA had a similar influence on historic fire activity in YNP. Years with large fires were associated with PNA+ and the climate signature of the PNA includes much of the western North America (Leathers et al., 1991; Sheridan, 2003; Jin et al., 2006). Given the extent of this climate signature variability in the PNA is probably an important climate mechanism synchronizing widespread fire activity in other dry western North American forests. Fire activity in YNP was also strongly affected by interdecadal climate variation and perhaps secular climate change. Decades with a high frequency of PNA+ years experienced more widespread burning than decades with a high frequency of PNA years. Burning was also more extensive in warm than cool decades and temperature increased over the period of record as cooling associated with the Little Ice Age waned (Briffa et al., 1992). The multi-decadal sensitivity of fire regimes to temperature is probably related to shifts in vegetation, fuel type, and fuel structure driven by low frequency variation in temperature (Swetnam, 1993; Kelly and Goulden, 2008; Trouet et al., 2010). This sensitivity is distinct from the high frequency response of fire activity to temperature and desiccation of fuels. In contrast to TEMP and PNA, fire activity was not related to interdecadal variation in PDSI, ENSO, or the PDO. Interactions among Pacific climate oscillations can amplify or dampened the effect of an individual teleconnection pattern. For example, interaction of the PDO with ENSO modulates the intensity and geographic expression of ENSO related climate variability (Gershunuv et al., 1999; McCabe and Dettinger, 1999). In the PNW and northern RM, drought years with high fire activity are sometimes coincident with the warm (positive) phases of ENSO and PDO (Schoenagel et al., 2005; Mori, 2011) but not always (e.g., Heyerdahl et al., 2008) while the opposite is true in the SW (Westerling and Swetnam, 2003). In YNP, the interaction of ENSO and any of the PDO reconstructions had no effect on fire activity. This may be due to the small scale record of fire activity (i.e., YNP) compared to the regional footprint of climate patterns, or YNP’s position in the ENSO-PDO pivot zone where the position of zonal precipitation has shifted on interannual and decadal time scales over the last 300 years (Dettinger et al., 1998). Phase interactions between PNA and ENSO, however, were important. Fires were larger in PNA+ ENSO (cool) (NIÑO3 ) years. Interactions of the PNA+ and PDO with fire were not consistent across PDO reconstructions and three-way interactions among the Pacific teleconnections were not evident at the scale of fire activity represented in YNP. A persistent period of PNA+ that began in the last third on the 18th century and lasted into the mid-19th century is coincident with a two-fold increase in the rate of burning (Fig. 2b). Warming associated with a persistent PNA+ conditions may have rapidly increased fire season length because of the non-linear response of snowfall frequency and snowpack development to temperature. April snowpack depth and snowpack density in the Sierra Nevada decrease rapidly below the freezing level altitude of winter storms (Barbour et al., 1991). The snowpack in the mixed conifer forest elevation belt is particularly sensitive to interannual and interdecadal temperature variability which affect snowpack duration, the drying of fuels, and fire season length (Dettinger and Cayan,

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1995; Westerling et al., 2006). This is evident in YNP in recent decades where the number of lightning caused ignitions increased exponentially with decrease in spring snowpack (Lutz et al., 2009). Larger burns in the last third of the 18th century suggest a shift to a longer period with dryer fuel conditions. Fire spread increases inversely with fuel moisture and dryer fuels increase the probability of large burns (Miller and Urban, 2000). A decline in the population of Native Americans in YNP beginning in the latter part of the 18th century could contribute to the increased rate of burning. Settlement of California by EuroAmericans began in 1769 with the establishment of the Spanish missions (Young and Levick, 1988). Disease spread quickly from missionaries to Native American populations adjacent to the missions leading to rapid declines in Native American populations (Cook, 1976). It is uncertain when disease spread from coastal tribes near the missions to tribes in interior California but it is thought to have taken several decades (Cook, 1976). Interviews of Miwok in Yosemite in 1851 when Yosemite Valley was first entered by the Mariposa Battalion include reference to an early episode of ‘‘black death’’ in the tribe (Bunnell, 1880) with a timing consistent with initial establishment of the missions in 1769 and a rapid spread of disease from the coast to the interior (Hull, 2009). Other late 18th and early 19th century expeditions note non-native disease among the Native Americans and devastating epidemics among interior tribes in the first third of the 19th century (Cook, 1976). The Miwok and other interior tribes used fire to manage natural resources (Anderson, 2005) and fire use is likely to have influenced vegetation and fuels near settlements and seasonal camps (Parker, 2002). A reduction in the frequency of Native American burning would likely increase landscape fuel continuity and the potential for larger burns. In the northern Sierra Nevada, the frequency of small fires decreased and the frequency of large fires increased after 1775 a pattern that would be consistent with a decline in Native American burning (Taylor and Beaty, 2005). Burn patterns and burn extent in YNP are sensitive to the location and timing of previous burns (Collins et al., 2009; Scholl and Taylor, 2010) and a reduction in Native American burning could increase landscape fuel continuity and the potential for larger burns. In YNP, however, the frequency of both small and large fires increased during the Spanish Colonial Period suggesting that either Native American ignitions’ increased during this period or that other factors such as climate variation contributed to the nearly two-fold increase in rate of burning between 1770 and ca. 1850. The top down controls of climate and Euro-American land use strongly influenced fire regimes in the central Sierra Nevada. Fire was virtually eliminated in YNP after 1900 as a policy of fire exclusion was implemented on Federal forest land. Similar dramatic declines in fire frequency and extent caused by fire exclusion have been identified in other California mixed conifer forests (e.g., Caprio and Swetnam, 1995; Taylor, 2000; Taylor and Skinner, 2003; Beaty and Taylor, 2008). Fire regimes were not as strongly disrupted by earlier Gold Rush exploration and settlement in the mid-19th century but fire frequency did decrease during this period compared to the Spanish Colonial Period. This suggests that a largely functioning fire regime was present in these forests until the onset of the fire exclusion period. Our historical analysis of top-down controls on fire activity demonstrate a sensitivity of fire regimes in mixed conifer forests to shifting modes of climate and land use over a 400 year period. This sensitivity probably leads to variable pathways of forest development and hence forest structure. Thus, the structure of forests at the time of Euro-American settlement reflects this sensitivity and managers using information on presettlement forests conditions should view presettlement conditions as guide for restoration planning rather than a rigid target for a restored forest. This approach is particularly appropriate given the historic sensitivity of fire regimes to climate variation

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and warming that is projected during the next 50–100 years (Allen et al., 2002; Hayhoe et al., 2004; Millar et al., 2007). The strong influence of climate, particularly TEMP and PNA, on interannual and multi-decadal variation in fire regimes suggests that future climate change will affect fire regimes in YNP and the Sierra Nevada. Forest fire extent and severity have increased with temperature in California in recent decades, despite active fire suppression (Westerling et al., 2006; Miller et al., 2009). Warming and increased fire activity is projected for the Sierra Nevada by climate and dynamic vegetation models forced by greenhouse gas emissions (Hayhoe et al., 2004; Lenihan et al., 2008). PNA+ conditions are amplified under extreme warm PDO and warm ENSO, individually and in combination (Trouet and Taylor, 2010). Thus, considering winter PNA along with indices of ENSO and PDO (e.g., Schoenagel et al., 2005) could provide managers with a more skillful long lead fire season forecast, particularly for extreme conditions conducive to widespread fire. Projected increases in ENSO variability in the tropics with climate change also has a pronounced influence on the extra-tropical PNA pattern and PNA+ follows warm ENSO more frequently in projections of climate change than in the 20th century (Mueller and Roeckner, 2006; Kug et al., 2010). The frequency, intensity, and duration of blocking ridges associated with the PNA pattern are also projected to increase under climate change (Lupo et al., 1997; Meehl and Tebaldi, 2004) conditions which would be conducive to fire over broad areas of the western USA. Acknowledgments This research was completed with the assistance of many individuals. We thank K. Painter, M. Beasely, and B. Mattos for logistical support and Charles Bache, Mike Connelly, Casey Deck, Alejandro Guarin, Andrew Greenwald, Kathleen Kelliher, Amanda McCarron, Jamie McCrory, Irene McKenna, Mike Mirobelli, Laura Rogers, and Morgan Windram for assistance in the field. Andrea Grove, Rick Carr, Dave Schmidt, Valerie Trouet, and John Sakulich assisted in the lab. C. Skinner, V. Trouet, S. Maxwell and two anonymous reviewers provided helpful comments on an earlier draft of this paper. This project was funded by a Joint Fire Sciences Grant 01-3-3-12 to A.H. Taylor and K. Paintner. References Agee, J.K., 1993. Fire Ecology of Pacific Northwest Forests. Island Press, Washington DC. Allen, C.D., Savage, M., Falk, D.A., Suckling, K.F., Swetnam, T.W., Schulke, T., Stacey, P.B., Morgan, P., Hoffman, M., Klingel, J.T., 2002. Ecological restoration of southwestern ponderosa pine ecosystems: a broad perspective. Ecol. Appl. 12, 1418–1433. Anderson, M.K., 2005. Tending the Wild: Native American Knowledge and the Management of California’s Natural Resources. University of California Press, Berkeley, CA. Anderson, R.S., Carpenter, S.L., 1991. Vegetation change in Yosemite Valley, Yosemite National Park, California during the protohistoric period. Madrono 38, 1–13. Arno, S.F., Sneck, K.M., 1977. A Method for Determining Fire History in Coniferous Forest of the Mountain West. USDA Forest General Technical Report. INT-GTR42. Baisan, C.H., Swetnam, T.W., 1990. Fire history on a desert mountain range: Rincon Mountain Wilderness, Arizona, USA. Can. J. Forest Res. 20, 1559–1569. Barbour, M.G., Berg, N.H., Kittel, T.G.F., Kunz, M.E., 1991. Snowpack and the distribution of a major vegetation ecotone in the Sierra-Nevada of California. J. Biogeogr. 18, 141–149. Barnston, A.G., Livezey, R.E., 1987. Classification, seasonality and persistence of lowfrequency atmospheric circulation patterns. Mon. Weather Rev. 115, 1083– 1126. Barrett, S.A., Gifford, E.W., 1976. Miwok Material Culture: Indian Life of the Yosemite Region. Yosemite Natural History Association, Yosemite National Park, California. Beaty, R.M., Taylor, A.H., 2001. Spatial and temporal variation of fire regimes in a mixed conifer forest landscape in the southern Cascades, California. J. Biogeogr. 28, 955–966.

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