Shortgrass Steppe LTER

Fire Research and the Mountain Plover

By Daniel G. Milchunas

Fire Research in the Shortgrass Steppe. The U.S. Forest Service (USFS) Pawnee National Grasslands recently initiated a controlled burn program at the SGS-LTER site to manage for the mountain plover. The mountain plover is an endemic shortgrass species that recently experienced large population declines, and is being considered for listing under the Endangered Species Act. Mountain plovers require very short vegetation, a critical proportion of bare soil for nesting, and they strongly prefer heavily grazed areas. Controlled burning in late winter removes litter cover and residual old-standing dead vegetation, potentially improving nesting habitat prior to plover arrival in spring. These controlled burns are planned to continue over a five-year schedule.

Fire is common in the grasslands of the Great Plains, primarily due to its importance in grasslands with greater aboveground productivity such as the tallgrass prairie. Fire in the semiarid shortgrass steppe (SGS) was frequent historically, but possibly not as extensive as elsewhere in the Great Plains due to low aboveground productivity and herbivory by bison and prairie dogs, which reduces fuel loads. There are only a few short-term studies of fire in SGS, and these are in ecotonal areas that border pinon-juniper communities to the south or mixedgrass prairie to the north and east and are not necessarily characteristic of true shortgrass steppe.

A number of groups are involved in the controlled burn work. The USFS is required to manage for declining species such as the mountain plover, and to conduct the burns. The USGS monitors plover nesting densities and success.The local livestock associations that use grassland for grazing are concerned about burning residual forage should a dry summer follow the burn, perceiving that losses in forage production may result in over-compensatory re-sprouting of cactus. SGS-LTER scientists and staff are conducting the vegetation studies in the burn area.The prescribed burns were an opportunity for the SGS-LTER group to cooperate with government agencies, provide information to local ranchers, and study an aspect of shortgrass ecology about which we were interested. Visitors to the site often ask, “What is the role of fire in this system?” And we do not have good answers yet.

So far we have only preliminary data to report from the study. Burning has had only very small effects on aboveground primary production; a small decrease in forbs occurred in one of two years. However, the years of study thus far have coincided with years of above-average precipitation. Reports from dry areas of mixedgrass prairie and pinon-juniper grassland indicate reductions in production when a dry year follows the burn. Dry years in the shortgrass steppe are characterized as having few large rainfall events and a preponderance of small events. Under these conditions, greater bare ground exposed after burns may contribute to increases in soil temperatures, increasing evaporative loss from small events that wet only near-surface depths, thereby decreasing rain-use efficiency. Burning has had only minor effects on species composition based on cover data to date. Because of low fuel loads and relatively cool, unspectacular burns here, cactus pads called ‘cladodes’ are only initially injured by fire, and the injured cladodes quickly re-sprout. Data that are sufficiently long-term to address effects on density are not yet available. Ranchers dislike Opuntia cactus in the shortgrass steppe because they prevent grazing of areas within dense clumps, but recent studies at the SGS-LTER site show that these abundant microsites serve as refugia for some plant groups and contribute to diversity in the landscape.

A common response to fire in many systems is an increase in nitrogen concentrations in plant tissue. Increases in end-of-season nitrogen yield have not been observed. Seasonal concentrations of nitrogen in three abundant species indicates only a brief, transitory increase in nitrogen in early spring. Although preliminary, results from the burn study support a conclusion drawn from our long-term grazing studies - the shortgrass steppe is a belowground dominated system, and aboveground disturbances have relatively minor impacts compared to those systems with proportionally greater aboveground biomass.
The plovers instigated this so we should probably say something about them too. They do favor the post-burn conditions, but this varies greatly among years or locations. Nest predation is always high and one theory is that burning the cover may increase this risk, but sufficient data has not yet been collected to test this.