A Fire History of the High Chisos, Big Bend National Park, Texas

Abstract

Fire history was studied in cypress forests and pinyon-juniper savannas. Fire dates were determined by tree ring counts from scars on Pinus cembroides. At least 10 fires occurred in the vicinity of Boot Canyon and at least eight on the Rim between 1770 and 1940. Episodic ground fires seem an important mortality factor of established pinyon and cypress saplings, but as trees grow to larger sizes, fire mortality is reduced. Longer fire intervals probably are filters that permit trees to survive into larger, fire-impervious sizes. Surface fires about every 50 years are one way to maintain the vegetation in some semblance of a natural, fire-regulated condition. The Chisos Mountains are of considerable biological interest because of their special plants, animals, and insular biogeography. Such high elevation species as Arizona cypress, Grave's oak, Chisos hornbeam, big-toothed maple, or Colima warbler present a peculiar blend of Rocky Mountain Cor- dilleran and Sierra Madrean Oriental flavors. Many visitors focus their activ- ities in these cool, forested canyons and on the high, scenic Rim. To help maintain these forest and woodland ecosystems in some kind of natural condition requires information on the role of fire, whose evidence is ubiqui- tous. One purpose of this study was to acquire field data on fire history and interpret the ecological role of fire in forest and woodland succession. Issues for the park staff were how any naturally occurring fire would behave and what changes would result under various fire management policies. This study describes dates, frequencies, intensities, and probable extent of fires in the high Chisos Mountains during the last two centuries. I will also inter- pret this fire history in terms of changing tree populations seen today (1978) at several locations. STUDY AREA.-Most of this study occurred in Boot Canyon and the scenic Rim portions of the Chisos Mountains at elevations above 1,950 m (6,400 ft). Boot Canyon is a northeasterly-trending, narrow drainage with intermittent streamflow and steep grades. Sideslopes generally are steep and marked by rhyolite outcrops and cliffs. The Rim is a plateau-like rhyolite summit whose eleva- tions exceed 2,200 m (7,200 ft). It is bounded for the most part by 60-m (200-ft) cliffs that doubtless present a fire barrier. Two structurally contrasting vegetations exist within the study area. Forests of canyon drainages feature an overstory of Arizona cypress (Cupressus arizonica) and Douglas-fir (Pseudotsuga men- ziesii) whose crowns emerge above a lower stratum of oaks (Quercus gravesii, Q. grisea), madrone (Arbutus texana), junipers (Juniperus deppeana, J. flaccida, J. pinchotti), Mexican pinyon (Pinus cembroides), and big-toothed maple (Acer grandidentatum). These cypress forests include three habitat types (Table 1). The most extensive is the Arizona cypress/Chisos bluegrass habitat type (CUAR/POIN of Table 1) along steep, north-facing canyon slopes. A diagnostic feature is the herbaceous understory dominated by Chisos bluegrass (Poa involuta) and pinyon ricegrass (Pipto- chaetium fimbriatum). The Arizona cypress/big-toothed maple habitat type (CUAR/ACGR) is res- tricted to the streamside drainages. A grassy understory is absent, but the presence of big-toothed maple is diagnostic. The Arizona cypress/juniper/pinyon ricegrass habitat type is limited to lower, gentle slopes and deep soils in Boot Canyon, but not along streamsides. Cypress is less common than in the other types, and alligator-barked juniper (Juniperus deppeana) and Mexican