THE HISTORIC FIRE REGIME ON THE EDGE OF THE PRAIRIE: A CASE STUDY FROM THE CROSS TIMBERS OF OKLAHOMA Stacy L. Clark1,2 Department of Forestry, Oklahoma State University, 008 AGH, Stillwater, OK 74078, USA Stephen W. Hallgren Department of Forestry, Oklahoma State University, 008 AGH, Stillwater, OK 74078, USA David M. Engle3 Rangeland Ecology and Management, Department of Plant and Soil Sciences, Oklahoma State University, 368 AGH, Stillwater, OK 74078, USA David W. Stahle Department of Geosciences, University of Arkansas, 113 Ozark Hall, Fayetteville, AR 72701, USA ABSTRACT Changes in the historical disturbance regime because of Anglo-American settlement and forced removal of Native Americans have altered vegetation composition and structure of forests, particularly in mesic ecosystems. However, xeric forests have gone largely unstudied, even though they may contain some of the largest tracts of remnant old-growth forests in eastern North America. Using dendrochronological techniques, we reconstructed fire, drought, and vegetation dynamics of an old-growth forest in the Cross Timbers region of Oklahoma. We tested predictions that fires would 1) be influenced by topography, 2) have decreased in frequency since Anglo-American settlement, 3) increase in frequency during drought events, and 4) be followed by pulses of hardwood recruitment. Support for our first prediction included higher fire frequency on the most southwest-facing aspect. We found no evidence to support prediction 2; fires became more frequent following Native American and Anglo-American movement into the area. Our third and fourth predictions were both supported; large-scale fire events occurred following periods of prolonged drought and 57–100% of oak (Quercus) recruitment followed a fire event. In contrast, eastern redcedar (Juniperus virginiana), an invasive native species, recruited during fire-free periods. Our results indicate that the sequence of drought followed by fire is an important process in long-term maintenance of forest conditions in this oak ecosystem. We attribute the increase in fire frequency after settlement to increasing local anthropogenic ignitions. The local effect of settlement on the fire regime (i.e., increased fire frequency) seems to contrast with the regional effects of settlement (i.e., decreased fire frequency). keywords: Anglo-American settlement, Cross Timbers, drought, fire regime, oak recruitment, Oklahoma, stand dynamics. Citation: Clark, S.L., S.W. Hallgren, D.M. Engle, and D.W. Stahle. 2007. The historic fire regime on the edge of the prairie: a case study from the Cross Timbers of Oklahoma. Pages 40–49 in R.E. Masters and K.E.M. Galley (eds.). Proceedings of the 23rd Tall Timbers Fire Ecology Conference: Fire in Grassland and Shrubland Ecosystems. Tall Timbers Research Station, Tallahassee, Florida, USA. INTRODUCTION quency due to population pressures related to land use (Ruffner and Abrams 2002). However, by the early Fire has influenced vegetation patterns of forests 1900s, fire frequency was largely reduced from pre- in eastern North America for millennia (Clark et al. settlement levels, contributing to a decreased oak 1996, Delcourt and Delcourt 1998, Bonnicksen 2000). (Quercus) component, particularly on mesic sites Removal of Native Americans and the onset of Anglo- where competition from fire-sensitive species is in- American settlement have led to changes in the fire tense (Abrams et al. 1995, Delcourt and Delcourt regime in many eastern North American forests (Cutter 1998). and Guyette 1994, McClenahen and Houston 1998, In xeric oak woodlands and savannahs, reductions Ruffner and Abrams 2002). The early days of settle- in fire frequency have led to an increase of invasive ment often brought a temporary increase in fire fre- plant species, changes in stand structure, and decrease in spatial heterogeneity across the landscape (Jenkins and Rebertus 1994, Bo´o et al. 1997, Heikens 1999, 1 Corresponding author ([email protected]). 2 Current address: USDA Forest Service, Southern Research Sta- Fuhlendorf and Engle 2004). However, few studies tion, P.O. Box 1568, Normal, AL 35762, USA. have linked the changes in fire frequency to human 3 Current address: Department of Natural Resource Ecology and population changes and to subsequent effects on veg- Management, Iowa State University, Ames, IA 50011, USA. etation response in these ecosystems. Studies that have 40 THE HISTORIC FIRE REGIME OF THE CROSS TIMBERS 41 Fig. 1. The Cross Timbers region (adapted from Ku¨chler 1964) with approximate location of Keystone Ancient Forest Preserve (inset) and location of plots in each stand (ⅷ ϭ Southwestern slope stand; Ⅵ ϭ Northeastern slope stand; ᭡ ϭ Bench stand). Copyright 2005 by Natural Areas Association. From Natural Areas Journal, by S.L. Clark, S.W. Hallgren, D.W. Stahle, and T. Lynch, ‘‘Charac- teristics of the Keystone Ancient Forest Preserve, an old-growth forest in the Cross Timbers of Oklahoma.’’ Reprinted by permission of Natural Areas Association. reconstructed fire history in xeric oak ecosystems have The KAFP lies in the northern Cross Timbers region generally been confined to the Ozarks (Guyette and (Ku¨chler 1964), approximately 32 km west of Tulsa, Cutter 1991, Cutter and Guyette 1994, Guyette et al. Oklahoma, in southern Osage County at the confluence 2002). These studies have found that fire frequency of the Cimarron and Arkansas rivers (Figure 1). The was generally reduced following Anglo-American set- region is characterized as a mosaic of xeric oak wood- tlement and was highest during occupation by the orig- lands, savannas, and prairie openings scattered inal Native American habitants. throughout approximately 4.8 million ha, with nearly The Cross Timbers region of Oklahoma has gone half of its area in Oklahoma (Ku¨chler 1964, Hoagland largely unstudied, but is an important region from a et al. 1999). Average annual precipitation is 101 cm, biological and historical perspective (Stahle and Hehr and average winter and summer temperatures are 3Њ 1984, Therrell and Stahle 1998, Hoagland et al. 1999). and 26ЊC, respectively (Oklahoma Climatological Sur- The region was occupied by Osage Indians from the vey 2002). The KAFP has been characterized as old 1700s until the 1830s, when bands of Cherokee, growth because of the presence of post oak (Quercus Creek, Seminole, Choctaw, and Chickasaw tribes were stellata) and eastern redcedar (Juniperus virginiana) forced to leave their homes to the southeast to occupy trees greater than 300 and 500 y old, respectively, and the new Indian Territory in present-day Oklahoma the absence of anthropogenic disturbances (Therrell (Wyckoff 1984). Although many explorations took and Stahle 1998). Elevation ranges from 251 to 304 place in the 1800s (Foreman 1947, Albert and Wyck- m; topography is moderately to steeply sloping with off 1984), the Cross Timbers was largely unoccupied underlying bedrock of shale and sandstone. Soils are by Anglo-Americans until 1889 when settlement be- a Niotaze–Darnell complex, with a sandy loam surface came legal (Wyckoff 1984). layer (Bourlier et al. 1979). In this study, we examine how topography, chang- Previous research indicated that topography was es in human demographics, and climate influenced the the primary factor in identifying distinct forest vege- fire regime of an old-growth oak forest in the Cross tation communities (Roe 1998). Thus, we delineated Timbers region of Oklahoma. Our specific objectives three stands based on aspect and elevation. The 11-ha were to 1) reconstruct the fire history of this site Southwestern slope stand was located on the steep up- through dendrochronological dating of fire scars, 2) per slopes (slope ϭ 17%) with a southwestern aspect. examine the effects of topography and climate on fire The 14-ha Northeastern slope stand was located on occurrence and spread, and 3) determine the relation- steep lower slopes (slope ϭ 20%) with a northeastern ship between tree recruitment patterns and fire events. aspect and had the lowest elevation. The 5-ha Bench We had four research hypotheses: 1) fire frequency stand was located on relatively level terrain (slope ϭ would be influenced by topography, 2) fire frequency 6%) just north of the main ridgetop. has decreased since Anglo-American settlement, 3) fire events were more likely to occur during periods of prolonged drought, and 4) fire promoted oak recruit- METHODS ment. Field Sampling ϫ STUDY AREA We randomly placed three to five 55 55-m plots within each stand (Figure 1), and we randomly sam- The study area was part of The Nature Conser- pled 98 saplings (Ͼ1 m height, Ͻ10 cm diameter at vancy’s Keystone Ancient Forest Preserve (KAFP). breast height) from five species for age structure anal- 42 CLARK ET AL. ysis. We obtained cores or cross sections within 10 cm woundwood ribs at the margins of the fire scar (Smith of the root collar. The number of saplings selected and Sutherland 1999), particularly if the fire injured a within a species was consistent with species’ relative large portion of the tree’s circumference. However, densities (Clark et al. 2005). By sampling smaller trees fires sometimes caused relatively localized injury and at the base, we could examine recruitment response to did not cause formation of woundwood ribs. Fire scars recent fire events at a near-annual resolution (Villalba generally occurred during the dormant season that and Veblen 1997); all cores and cross sections either spans two calendar years (i.e., after the end of the pre- hit pith or were estimated to be within 1 or2yofpith. vious year’s growing season). The fire scars were as- Fire scars from dead tree cross sections were used signed the year in which the tree first showed a wound- to reconstruct the fire history of the KAFP. We ob- ing response to the fire (i.e., the year containing the tained 11, 8, and 8 cross sections from dead tree logs earlywood immediately following the fire scar). or snags within the Southwestern slope, Northeastern slope, and Bench stands, respectively.