In the Flint Hills of Geary County, eastern Kansas (central USA)in the mid-1800s (when European settlement of the area began), the region comprised bluestem prairie with tree cover mostly confined to river and stream valleys. Periodic wildfire is considered an essential component of natural bluestem prairie ecosystems, typically characterized by big bluestem Andropogon gerardi, little bluestem A.scoparius and a suite of other native perennial bunchgrasses. Extensive cattle grazing from the late 1800’s, resultant development of a year-round short sward (any grassland fires being less intense due to reduced fuel load) and a general policy of wildfire suppression, has led to tree and shrub invasion. This study investigated the rate of woody plant invasion on various soils and topographic localities of burned and unburned bluestem prairie.
At 12 study sites, using aerial photos (1937 to 1969) and field observations (‘section-line data’ over a 114-year period; 1856-1969) woody cover change over time was assessed.
Five sites had been burned regularly (3 burned annually, 1 every 2-3 years, and 1 every 5 years); five sites had been unburned (3 for at least 50 years; 2 for at least 20 years); and two unburned sites had been aerially sprayed with phenoxy herbicides in two consecutive years out of every 6 years, beginning in 1943 and 1944.
Tree cover increased by 8% from 1856 to 1969 throughout the county. On the burned sites, trees and shrubs were effectively maintained at pre-settlement cover, with only a 1% increase recorded from 1937 to 1969. On unburned sites, tree and shrub cover increased by 34% from 1937 to 1969; section-line data showed that tree cover increased 24% from 1856 to 1969.
Soil type and topography greatly influenced the rate of woody plant invasion on unburned areas. Cover increased only slightly on shallow, droughty clay loam soils characteristic of upland areas. On deeper, more permeable middle- and lower-slope soils, woody plant cover increased by more than 40% from 1937 to 1969. In 1937 trees covered 64% of lowland soils and in 1950, 89%; change was negligible thereafter.
Herbicide spraying on the two unburned sites in the longer-term, only slowed woody succession. Whilst initial spraying considerably reduced woody cover, subsequent spraying did not result in substantial reductions due to regrowth of some species, whilst others (notably red cedar Juniperus virginiana)appeared unaffected.