Wieder, W. R., and N. W. Bower. 2004. Fire History of the Aiken
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THE SOUTHWESTERN NATURALIST 49(2):239±298 JUNE 2004 NOTES FIRE HISTORY OF THE AIKEN CANYON GRASSLAND-WOODLAND ECOTONE IN THE SOUTHERN FOOTHILLS OF THE COLORADO FRONT RANGE WILLIAM R. WIEDER AND NATHAN W. BOWER* Environmental Science Program, Colorado College, 14 East Cache La Poudre, Colorado Springs, CO 80903 *Correspondent: [email protected] ABSTRACT A ®re history for the Aiken Canyon Nature Conservancy preserve in Colorado is presented. Eighteen cross-sections cut from ponderosa pine recorded 20 ®re events for the period spanning 1602 to 1999. An unusually small median ®re interval (MFI) of 7.5 years was obtained for the period 1753 to 1935. A signi®cant association was found between El Nino weather patterns and the ®res at Aiken Canyon. A shorter MFI was observed in the grasslands, which are at lower elevations. Fires from railroads or cattle ranching might have caused the shorter MFI in the grass- lands between 1872 and 1935. After 1935, ®re suppression became common. RESUMEN Se presenta la historia de fuegos en la reserva de la Aiken Canyon Nature Conser- vancy en Colorado. Dieciocho secciones cortadas de Pinus ponderosa registraron 20 acontecimien- tos de fuego en el perõÂodo de 1602 a 1999. Un medio excepcionalmente pequenÄo del intervalo de fuego (MIF) de 7.5 anÄos se obtuvo por el perõÂodo de 1753 a 1935. Se encontro una asociacioÂn signi®cativa entre patrones de tiempo de El NinÄo y fuegos en el Aiken Canyon. Un MIF maÂs corto se observo en los prados que estaÂn en elevaciones maÂs bajas. Fuegos de los ferrocarriles o de ranchos de ganado pudieron haber causado el MIF corto en los prados entre 1872 y 1935. DespueÂs de 1935, la supresioÂn del fuego llego a ser comuÂn. The Nature Conservancy dedicated the Aik- Numerous ®re histories have been created en Canyon Preserve in 1994 and soon became using ponderosa pine (Pinus ponderosa) ®re concerned about the loss of grassland to wood- scars in Arizona and New Mexico (Swetnam land within this ecotone. Disturbances such as and Dieterich, 1985; Baisan and Swetnam, ®re alter landscapes, encouraging species di- 1990; Danzer et al., 1996; Fuli et al., 1997) and versity and spatial heterogeneity. Baker (1992) in northern and central Colorado (Laven et recommended managing nature preserves al., 1980; Brown et al., 1999; Veblen et al., based on landscape-scale processes rather than 2000; Donnegan et al., 2001). The nearest for individual species or particular communi- comparable sites to our study area are those of ties, noting that ecotones are worth preserving Brown and Shepperd (2001): Wet Mountain in their own right. The Nature Conservancy site, 90 km southwest (elevation 2,680 m), with considered controlled burns as a restoration 10 trees; and Manitou Demonstration Plot, 50 method at Aiken Canyon, but such manage- km north (2,400 m), with 26 trees. They ob- ment requires knowledge of past ®re distur- served median ®re-free intervals of 10 and 7.5 bance patterns in terms of their timing, sever- years covering periods from 1514 to 1908 and ity, and spatial distribution. Consequently, we 1521 to 1865, respectively. Only a few pub- conducted this study during the summer of lished ®re histories have examined the margins 1999 to examine the historic and climatic fac- of the Great Plains (Brown and Sieg, 1999), tors in¯uencing the grassland-woodland ®res but none exist for the grassland-woodland eco- at Aiken Canyon and to support management tone along the southeast-facing slopes of the decisions. Front Range of Colorado. 240 The Southwestern Naturalist vol. 49, no. 2 FIG.1 Map of The Nature Conservancy Aiken Canyon Preserve, Colorado (625 ha). Fire-scarred tree samples are indicated by dots. Ecotones composed of intermingled grass- The historical context should also be re- lands and pinÄon-juniper woodlands (Pinus ed- ported when developing ®re histories. In 1842, ulis and Juniperus monosperma), as at Aiken Can- the city of Pueblo, Colorado was founded and yon, will have scarred trees, but dif®culties in mail service along the Front Range was initi- dating pinÄon and juniper render these ®re-his- ated. Railroads reached the area in 1872, and tory studies unreliable. Instead, researchers fa- rangeland was sometimes burned to improve vor working with nearby ponderosa pine fodder for cattle (Chapman, 1924). Unlike stands, extrapolating onto adjacent plant com- much of Colorado, Aiken Canyon was never munities (Kaib et al., 1996; Brown and Sieg, logged (Colorado Natural Areas Program, 1999). 2002). Fire suppression became a policy of the Several studies exist connecting ®re frequen- Forest Service by 1910, but enforcement would cy to variations in climate at the seasonal, an- have been impractical until a Conservation nual, and decadal level for New Mexico and Corps was formed nearby in 1935 (Colorado Colorado (Swetnam and Betancourt, 1990; State Archives, 2002). Diaz and Markgraf, 1992; Swetnam and Betan- Aiken Canyon is 20 km southwest of Colo- court, 1998; Grissino-Mayer and Swetnam, rado Springs (388389N, 1048539W) on the east- 2000; Donnegan et al., 2001). For Aiken Can- ern slope of the Rampart Range, a smaller yon, such connections are of special interest range within the foothills of the Front Range because it lies on the edge of an area where (Fig. 1). Higher elevations to the west (Ram- Cole and Cook (1998) found strong correla- part Range) and north (Monument Hill/Palm- tions between El NinÄo southern oscillations er Divide) of Aiken Canyon create a barrier and local drought area indices. that supports a grassland-woodland ecotone. June 2004 Notes 241 FIG.2 Fire chronology for Aiken Canyon Preserve, Colorado, with tree sample numbers corresponding to those shown in Fig. 1. The northern limit of candelabra cactus gram FHX2 for data reduction (Grissino-May- (Opuntia imbricata) and the eastern limit of er, 2001). blue spruce (Picea pungens) are located here. The complete dendrochronology extends From 36 to 51 cm of precipitation are received from 1602 to 1999, and the results of the data over the 2,100 to 2,500 m elevation range at reduction are summarized in Fig. 2. For the this site (United States Department of Agricul- period 1753 to 1935, when at least 4 ®re- ture, 1981). The variation in precipitation, scarred trees were present, there was a median soils, and drainage creates a mosaic of tallgrass ®re interval of 7.50 years and a Weibull median and midgrass species (e.g., Andropogon gerardii 50% probability interval (WMPI) of 9.53 years, and Bouteloua gracilis), pinÄon-juniper wood- with a range of intervals from 2 to 37 years. land, and deciduous shrubland (e.g., Quercus These intervals are at the low end of the results gambelii). These grade into coniferous forest reported by Brown and Shepperd (2001). They (Abies concolor, Pinus ponderosa, and Pseudotsuga are also low compared to most of the sites of menziesii) growing above 2,200 m, where soils Donnegan et al. (2001), although their lowest are more acidic compared to the calcic sedi- elevation site (1,996 m) with 12 trees had a mentary soils of the grasslands. WMPI of 5 years for the period 1781 to 1920. Eighteen samples (all cross-sections or wedg- The 65 injuries and scars recorded corre- es, no cores) were collected and cross-dated vi- sponded to at least 20 ®res, with 5 ®re years sually using skeleton plots (Stokes and Smiley, (1753, 1839, 1859, 1892, and 1933) scarring 3 1968) for all ®re-scarred ponderosa pines that or more trees spaced over multiple hectares. could be located along the grassland-woodland Overall, the ®re interval at Aiken Canyon was ecotone. Fire-scar dates were assigned after the relatively short compared to most sites in Col- dendrochronology was completed, and scar orado. Along the northern Front Range, Veb- dates were subsequently entered into the pro- len et al. (2000) found that widespread ®re was 242 The Southwestern Naturalist vol. 49, no. 2 more than twice as frequent in the zone below Wilderness, Arizona, USA. Canadian Journal of 2,015 m compared to above 2,440 m. In our Forest Research 20:1559±1569. study, grasslands (,2,140 m) had twice the ®re BROWN, P. M., M. R. KAUFMANN, AND W. D. SHEPPERD. frequency compared to the woodlands 1999. Long-term, landscape patterns of past ®re (.2,140 m) for the period 1872 to 1935 (Fig. events in a montane ponderosa pine forest of central Colorado. Landscape Ecology 14:513± 2). 532. There was an association (x 2 5 5.15, P 5 c BROWN,P.M.,AND W. D. SHEPPERD. 2001. Fire history 0.023) between the proxy index of El NinÄo and ®re climatology along a 58 gradient in lati- events (Quinn et al., 1987) and ®res at Aiken tude in Colorado and Wyoming, USA. Paleobot- Canyon occurring 1 to 2 years later for the pe- anist 50:133±140. riod 1753 to 1935. This association was mea- BROWN,P.M.,AND C. H. SIEG. 1999. Historical vari- sured using a 2 3 2 contingency table with con- ability in ®re at the ponderosa pine northern tinuity correction for the correlation of di- Great Plains prairie ecotone, southeastern Black chotomous nominal-scale data (Zar, 1996). By Hills, South Dakota. Ecoscience 6:539±547. using this nonparametric approach with El CHAPMAN, A. 1924. The story of Colorado, out where NinÄo events and ®res scored as being present the West begins. Rand McNally and Company, New York. or absent, assumptions about underlying dis- COLE,J.E.,AND E. R. COOK. 1998. The changing tributions are minimized. Including ordinal in- relationship between ENSO variability and mois- formation about the magnitude of the events ture balance in the continental United States.