Burning season effect on four southern Chihuahuan desert plants

Use of prescribed fire to manage undesirable vegetation in the Chihuahuan Desert of Mexico, promises acceptable results, but information on plants responses to different weather conditions and fuel load availabilities is lacking in Mexico. This study investigated the effect of three burning seasons with two fuel load simulations and two plants size on plant mortality and changes in basal area of four native species of southern Chihuahuan Desert. The study was conducted in the Mexican High Plateau in Jalisco, Mexico during two consecutive years, 2005 and 2006, in a shortgrass prairie of blue grama (Bouteloua gracilis) with problematical populations of broomweed (Isocoma venetus), brickellbush (Brickellia spinulosa) and broomgrass (Muhlembergia rigida).

Fire environment was simulated using a portable propane burner calibrated to simulate time-temperature reached with 1,700 and 2,800 kg/ha fine fuel load. Size of plant was determined by height and canopy diameter in shrubs, and indicial area diameter in grasses. Shrubs with a minimum height of 25 cm and canopy diameter of 31 cm were classed as large plants. Grasses with a basal area diameter of 10 cm were classed as large plants. Plants with measurements less than these were placed in the small plant group. For each species, 50 plants were randomly treated each season at each fuel load and plant size. Plants were identified with numbered tags to evaluate mortality and basal area changes in subsequent growing season. As control, 50 plant of each species and size received no fire treatment. Plants with no live tillers one year after treatment were assumed to be dead. Basal area change was estimated based in number of pixels/cm2, using vertical photos and the Adobe Photoshop Software. Statistical analysis was performed with the GLM procedure in SAS. Mortality of blue grama plants was affected significantly by season of burning, fuel load simulation, and plant size (P<0.05). Small blue grama plants were significantly (P<0.05) more affected than large plants, mainly after spring and summer burns. Mortality of broomgrass muhly was slightly affected by burning treatments in either evaluation year, with average mortality of 1% and 2% for 2005 and 2006. Brickellbush mortality was significantly affected by burning season (P<0.05), and plant size (P<0.05). Spring and winter burns were the most detrimental seasons for small brickellbush plants. Broomweed plants were severely affected (P<0.05) by all burn treatments, regardless of fuel load simulation and plant size, with 82% average mortality. Spring and summer burning with high fuel load simulations reduced basal area of large blue grama plants. However these changes were similar (P>0.05) to unburned plants that decreased 8%. Winter burning with high fuel load had a positive effect (P<0.05) on basal area of large blue grama plants, with an increase of 37%. Small blue grama plants had a positive response to all burning treatments regardless of fuel load simulations, with average increases of 80%, 41%, and 31% for spring, summer, and winter burns respectively. Nevertheless, these changes were similar (P>0.05) with unburned plants that showed increased basal area of 41%. Basal area of muhly plants was generally reduced by all burn treatments. Basal area of large muhly plants had average reductions of 24% and 48% with summer and winter burns, while spring, summer, and winter burns reduced average basal area of small muhly plants by 2%, 4%, and 47% respectively.