On North American prairies, growing-season wildfires are relatively frequent. There is interest in using prescribed growing-season fires in management, but there is a lack of knowledge as to effects on vegetation, especially of mid-successional tallgrass prairie. A study was undertaken in Oklahoma (south-central USA) comparing vegetation composition and production on two mid-successional stage sites in response to late growing-season fire at different frequencies.
The experiment was conducted at the Samuel Roberts Noble Foundation (Love County) at two prairie sites: a ‘loamy’ (1.2 m deep loam soil) and a ‘very shallow’ (stony soil to 25 cm depth over limestone) site. Both were dominated by early- and mid-successional native grasses. Four burn treatments (no burn, or 1, 2, or 3 burns in 5 years) were applied to 10 x 20 m plots (3 replicates of each) at each site; burn dates:1990 (4-5 September), 1991 (3 October) and 1993 (9 August).
In July/August each year, production was recorded by clipping vegetation to ground level in three (0.2 x 0.5 m) quadrats per plot. Samples were separated into current year’s growth of: little bluestemSchizachyrium scoparium; tall grasses (big bluestem Andropogon gerardii, indiangrass Sorghastrum nutans and switchgrass Panicum virgatum); cool-season perennial grasses (primarily Texas wintergrass Stipa leucotricha and scribner panicum Dichanthelium oligosanthes; other warm-season perennial grasses (primarily Bouteloua and Sporobolus spp.); annual grasses (primarily prairie threeawn Aristida oligantha and annual bromes Bromus spp.); and forbs.
During the study years the area experienced above-average precipitation.
Although total production was not reduced by burning in 1990, little bluestem and perennial grasses were reduced by burning on the loamy site. Overall, forbs were more productive on burned (1,980 kg/ha) than unburned (1,290 kg/ha) plots.
Species composition and total production were similar for ‘2 consecutive-year burn’ plots compared to a single burn in 2 years. However, cool-season perennial grass production increased to almost 40% of total production on twice-burned plots (no burn 520, once burned 720 and twice burned 1,290 kg/ha). Other than this, 2 burns over 2-years had little effect beyond the first growing season after the second burn.
Twice- and 3-times burned plots produced more forbs and less perennial grasses than plots burned once or not burned. Total production was not reduced on either site, regardless of fire frequency.
Results indicate that a late-growing season burning regime in mid-successional tallgrass prairies may enhance plant diversity, with an initial reduction in production of grasses followed by an increase in forbs.