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Response of herbaceous vegetation to summer fire in the western South Texas Plains.

Article Excerpt
Abstract.--With increases in wildlife related enterprises and ecological restoration efforts in southern Texas, there is an increased interest in utilizing summer fire to achieve management goals; yet, there is little data on the effects of summer burning on vegetation and wildlife. Herbaceous vegetation diversity, productivity, density and frequency were estimated on five summer burned and five nontreated sites utilizing 20 by 50 cm quadrats. Forb density and frequency was monitored for two growing-seasons postburn. Grass indices were measured three months postburn. Grass and forb yields were estimated in 0.25 [m.sup.2] plots during the first growing-season postburn. Croton (Croton sp.) responded positively to summer burning during the first growing-season postburn for all indices measured. During the second growing season postburn, Croton densities were similar among treatments. Densities of erect dayflower (Commelina erecra) and beach groundcherry (Physalis cinerascens) were greatest on burned sites throug hout the study. Silky evolvulus (Evolvulus alsinoides) and hoary blackfoot (Melampodium cinereum) were more common on nontreated sites. Grass densities were lowest on burned sites three months postburn, and yields were similar between treatments by the middle of the first postburn growing season. Summer burning does not appear to provide any additional benefits in forb response over dormant-season burning. The long-term effect of a regimented burning regime on vegetation and influence of burn season on wildlife is not clearly understood and warrants further investigation.

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The Rio Grande Plains of south Texas is the southern-most extension of the Great Plains Grasslands. Fire, along with other climatic variables such as drought, presumably maintained the honey mesquite (Prosopis glandulosa) savannas and interspersed grasslands of pre-European settlement south Texas (Scifres & Hamilton 1993). Frequency of fire appeared to be highly variable and ranged from 5-30 years (Wright & Bailey 1982). Following European settlement, suppression of fire combined with heavy livestock grazing has lead to the current thorn woodlands common throughout southern Texas (Archer et al. 1988; Archer 1994).

Beginning in the mid-twentieth century, south Texas landowners began to convert these thorn woodlands back to grasslands to enhance rangelands for livestock production. Mechanical treatments such as root plowing were commonly utilized methods for achieving this goal. Mechanical brush manipulation practices can significantly reduce woody plant cover while increasing herbaceous vegetation (Scifres et al. 1976; Bozzo et al. 1992). However, once treated rangelands are revegetated by woody species, woody plant diversity can be dramatically reduced (Fulbright & Beasom 1987; Ruthven et al. 1993), which may negatively impact diversity of wildlife species.

Land ownership and land use practices in south Texas have changed in recent years. The size of individual landholdings has decreased and revenues derived from those properties have become increasingly dependent on wildlife rather than traditional livestock operations. Many wildlife management programs are directed towards game species such as white-tailed deer (Odocoileus virgininaus) and northern bobwhite (Colinus virginianus). In the southeastern United States, prescribed fire has long been utilized to manage habitat for northern bobwhite (Landers & Mueller 1992). Woody vegetation is a primary component of white-tailed deer diet in south Texas (Drawe 1968; Taylor et al. 1997), and prescribed burning in eastern portions of the Rio Grande Plains can reduce brush cover while maintaining woody plant diversity (Box & White 1969). Dormant-season prescribed burning in southern Texas has been shown to increase herbaceous vegetation preferred by wildlife (Hansmire et al. 1988; Ruthven et al. 2000). As a result of it s reported benefits, south Texas rangeland managers are beginning to utilize prescribed fire to enhance wildlife habitat.

In addition to the rise of wildlife related enterprises, there is growing interest in restoring ecosystems to pre-European settlement conditions. Many proponents of ecological restoration promote the use of summer burning to mimic the occurrence of natural fires. Most perennial grasses generally decrease following summer burns (Scifres & Duncan 1982; Engle et al. 1993; Engle et al. 1998). In Oklahoma, summer fire can increase forb productivity (Engle et al. 1998), while in southern Texas summer prescribed burns appeared to have little affect on forbs (Mayeux & Hamilton 1988). Although effects of summer fire are documented in many ecosystems, little information is available on the response of vegetation and wildlife to growing-season fire in the more xeric areas of the western Rio Grande Plains.

The objective or this study was to determine the effects of summer prescribed fire on the diversity, density and productivity of herbaceous vegetation during the first and second growing-seasons post-treatment in the western Rio Grande Plains. It is hypothesized that prescribed burning south Texas rangelands during the growing season will result in enhanced germination and establishment of annual and perennial forbs and decreases of perennial grasses.

MATERIALS AND METHODS

The study area (Fig. 1) was located on the Chaparral Wildlife Management Area (28[degrees] 20' N, 99[degrees] 25' W) within the western South Texas Plains (Correll & Johnston 1979; Hatch et al. 1990) and northern portion of the Tamaulipan Biotic Province (Blair 1950). Climate is characterized by hot summers and mild winters with an average daily minimum winter (January) temperature of 5[degrees]C, an average daily maximum summer (July) temperature of 37[degrees]C, a growing season of 249 to 365 days, and average annual precipitation (1951-1978) of 55 cm (Stevens & Arriaga 1985). Average annual precipitation on the study site (1989-1999) was 54 cm. Precipitation patterns are bimodal with peaks occurring in late spring (May to June) and early fall (September to October).

Five sites subjected to prescribed burns were paired with five nontreated sites utilizing a randomized block design. Study sites were approximately 2 ha in size. Burned sites were located within larger areas that had been burned. Rangeland fire in southern Texas typically produces a mosaic of burned and nonburned areas as a result of uneven fuel loads (Box & White 1969). All study sites received 100% coverage by burns. Fire was applied to burn sites in August 1999. Relative humidity and air temperature, using a sling psychrometer, and surface wind speed, using the Bufort Scale, were estimated before ignition and at the completion of each fire. Weather conditions were relatively constant during all fires with a relative humidity of 32%, temperature of 39[degrees]C and wind speed of 8 kph. Wind direction was variable. Soil moisture was not recorded. All burns were conducted 3 to 5 days following a 23 cm rainfall event (Hurricane Brett) and soil moisture was considered high. Because of variable wind speed and di rection during burns and uneven fuel loads, rate of spread and flame height were highly variable and not recorded. Fuel loads...

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