Poorly managed (e.g. overgrazed) degraded rangelands (now dominated by non-native cool-season grasses, and low yielding warm-season shortgrasses) are common in the Mixed Prairie region of central-north USA. This study evaluated the potential of a late-spring prescribed burn (near Harlan County Lake, Nebraska) to shift vegetation composition back to the mid and tallgrass species more indicative of native Mixed Prairie, such as big bluestem Andropogon gerardii, little bluestem Schizachyrium scoparium, switchgrass Panicum virgatum, and sideoats grama Bouteloua curtipendula.
The study was conducted in three native rangeland tracts of low vegetation condition on silt loam soil; dominant cool-season species were generally Kentucky bluegrass Poa pratensis and annual bromes Bromus spp., and dominant warm-season species were blue grama Bouteloua gracilis and buffalograss Buchloe dactyloides.
At each tract, treatments (autumn mowing, late-spring burning or untreated controls) were randomly allocated to plots (8 x 15 m) within each of three blocks. On autumn mowing designated plots, in September 1979 and 1980 vegetation was mown to 5 cm height and cut material removed. Prescribed burning (backfires) of burn plots took place on 25 April 1980.
Basal cover and relative species composition was measured (using point frames) in August 1979 (prior to treatment application) and subsequently in August 1980 and 1981. During the 1980 and 1981 growing seasons, plot herbage yields were determined (hand-clipping in sample quadrats) in early June and late September.
Precipitation during the 1980 growing season (April-September) was only 57% of the long-term average (550 mm).
Autumn mowing failed to produce the desired species shift; over the 2-year study period it did not significantly affect relative composition of the dominant grass species.
In two tracts supporting substantial amounts of warm-season shortgrass species, burning led to initial suppression of cool-season grasses and significantly higher yields and basal cover of the warm-season shortgrasses (reduced competition from the fire-damaged, cool-season grasses). Due to the reduction of the cool-season grasses, the soil moisture in burned plots was also relatively high at the time warm-season grasses initiated growth. On the third tract, with almost no warm-season native grass remnants, those warm-season mid and tallgrasses present did not increase but there were significant yield and cover increases of invasives.
The authors suggest that one major reason for a lack of a shift towards native mid and tallgrass species was due to the dearth of native remnants as a recolonising source. Also, precipitation in the 1980 growing season (April-September) was only 314 mm (i.e. 57% of the long-term average) which may have reduced responses to treatments.