West Texas Wildlife Management
Historical Perspective (Continued)
Suppression of Grassland Fires (combined with grazing effects)
Historically, fire played a major role in shaping and maintaining the Trans-Pecos grasslands (Wright and Bailey 1982, McPherson 1995, Frost 1998, Van Auken 2000), just as fire has influenced and maintained other grasslands of North America. Although periodic fire is an integral component of healthy rangelands, it is not the only process that has shaped the grasslands and savannas of the desert Southwest. Frequent drought, insects, disease, rodents, rabbits, and other browsers/grazers serve a role in maintaining grassland integrity by interacting with fire to control woody plants. In the absence of fire, grasslands gradually revert to dominance by woody plants. In arid environments, grass plants can often survive during drought and they thrive during periods of good rainfall with 2 very important provisos: 1) the density of shrubs and succulents (cholla, yucca, cacti, etc.) does not become excessive and 2) top-removal of grass plants does not occur too frequently.
Fire is a natural mechanism for controlling encroachment by woody plants and succulents, involving only periodic top-removal of herbaceous vegetation (7-12 year frequency in the higher elevations; 10-20 year frequency in the desert grasslands). If woody plants are allowed to increase, their deep, spreading root systems eventually out-compete grasses with the interacting effect of repeated droughts. If grass plants are continually defoliated (e.g., continuous heavy grazing), the photosynthetic structures (green leaves) are not allowed to replenish root with starches and carbohydrates. The result is declining root health, weakened plants, and eventual mortality, especially during drought. In addition, excessive grazing pressure prevents reproduction of herbaceous plants, especially problematic in areas of frequent and persistent drought.
The greatest impact of reduced herbaceous cover, whether through overgrazing, woody plant competition, or their combined effect, is exposure of bare soil. When the soil surface is not covered by grasses/forbs and exposed to the elements (wind and rainfall), erosion is inevitable. The immediate effect of increasing bare ground is substantial loss of water that otherwise would be conserved through soil infiltration, deep percolation, and absorption by grass roots. The loss of grass cover and increasing loss of water through runoff (reduced percolation into water table) is the primary reason that Trans-Pecos springs and creeks described in historical documents have dried up (the increasing number of water wells developed for irrigation, livestock, and human use also contributed to the problem). Another 'immediate' effect is that exposed soil quickly becomes encrusted or "capped," which hinders water infiltration, moisture retention, and seed germination. The long-term effect of increasing bare ground is soil loss through erosion, which reduces the capability of the land to support vegetation and permanently decreases the carrying capacity of the land for livestock and wildlife.
A less apparent effect of fire suppression and heavy grazing pressure in West Texas is a gradual shift in species composition of herbaceous plants. Deep-rooted perennial bunchgrasses (blue grama, sideoats grama, bluestems, Arizona cottontop, tanglehead, green sprangletop, tobosagrass) gradually give way to less desirable, shallow-rooted species (threeawn, burrograss, fluffgrass, red grama, slim tridens). Not only do the leafy bunchgrasses receive more pressure through repeated selection by grazers, but perennial bunchgrasses are fire tolerant (fire dependent, to some extent). The growing points of most bunchgrasses are protected beneath the soil, and periodic fire tends to stimulate seed germination of perennial, warm-season bunchgrasses. Timely grazing deferment and periodic fire can reverse this trend in the species composition of herbaceous plants. Although shallow-rooted species are better than bare soil, the value of maintaining deep-rooted bunchgrasses is 2-fold: 1) bunchgrasses support greater livestock numbers and greater wildlife numbers and diversity, and 2) bunchgrasses are superior in maintaining the soil hydrology (better water infiltration, retention, and deep percolation)
Today, the most common barrier to wildfire in desert grasslands is inadequate quantity and continuity of fine fuels. Livestock grazing over the past 120 years has reduced the herbaceous biomass enough to prevent the spread of fire in most years. Other constraints on the use of fire as a management tool include lack of knowledge about fire benefits, lack of experienced assistance, liability concerns, potential threat to ranch facilities and structures, and short-term financial considerations associated with grazing deferment before and after the fire. Opportunities currently exist for use of prescribed fire in desert grasslands to prevent further shrub invasion and, to some degree, reverse the trend. In many areas of the Trans-Pecos, however, a major reclamation program involving brush control and grazing deferment would be required to partially restore desert grasslands before fire could be implemented in a management program.
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