In tobosagrass Hilaria mutica-honey mesquite Prosopis glundulosa var. glandulosa communities of southern USA, prescribed spring burning kills some mesquite trees, removes dead trees, and increases tobosagrass production for grazers. Two studies were undertaken, the first near Colorado City (central Texas) and the second near the town of Post to the northwest. Objectives were to measure changes in nitrate, exchangeable ammonium, total soil nitrogen, soil moisture and soil temperatures following spring burning, and to determine the extent to which each increased tobosagrass growth.
Both sites had dense stands of tobosagrass with a fairly open canopy of honey mesquite; long-term average annual precipitation was 48cm.
The first study was conducted in 1972-1973 on Spade Ranch. It examined soil temperature, moisture, nitrate and total nitrogen as possible explanations for increased tobosagrass yields following fire. Three 0.4 hectare plots within a 2 ha grazing exclosure received one of the following treatments: burned (March 10); clipped (12 to 19 March), no treatment (control).
Following from this, a second study was conducted in 1974 to evaluate the relative importance of litter removal, ash deposition, and fire heat in stimulating tobosagrass.
Removal of litter by burning or clipping increased soil temperature and nitrogen mineralization rate. Ash had no effect on these properties in 1972, but appeared to stimulate tobosagrass production in 1974. With adequate soil moisture, the higher soil temperatures on burned or clipped plots stimulated grass growth, which concomitantly, reduced soil moisture and nitrates. Suboptimal (lower) soil temperatures on partially shaded control plots appeared to limit growth, despite adequate soil moisture and nitrate.
Precipitation during 1972 was about 20% above average in Colorado City. In 1974 at Post it was 48% below average, until 1 August (11-13 cm fell during winter-spring and 40-42 cm during summer-autumn). In this dry year, soil moisture was the limiting plant growth factor and burning exhibited no beneficial effects.