Exercise 4

Primary production on the prarie

Aboveground plant production in tallgrass prairie has been correlated with several climatic variables (Risser et al., 1981; Towne and Owensby, 1984; Abrams, Knapp, and Hulbert, 1986; Powell, Stadler, and Claypool, 1986). Sala et al. (1988) suggested that variability in grassland primary production at the regional scale can be accounted for by annual precipitation. However, they suggested that, as spatial scale is reduced, additional factors such as fire and soil texture need to be incorporated into predictive models. Fire is considered a necessary component for the preservation and maintenance of the North American tallgrass prairie (Hulbert, 1969; Old, 1969; Vogl, 1974; Axelrod, 1985). Without recurrent fire in the Flint Hills tallgrass prairie, litter accumulates, plant community composition changes, and woody species invade (Abrams, Knapp, and Hulbert, 1986; Knapp and Seastedt, 1986; Gibson, 1988; Hulbert, 1988; Briggs, Seastedt, and Gibson, 1989; Collins and Gibson, 1990; Sea stedt, Briggs, and Gibson, 1991; Briggs and Gibson, 1992; Knight, Briggs, and Nellis, 1994). Several studies have documented that fire can increase aboveground produc tivity in tallgrass prairie (Towne and Owensby, 1984; Abrams, Knapp, and Hulbert 1986; Briggs, Seastedt, and Gibson, 1989).

Hypotheses

1. Plant productivity is impacted by (a) burn frequency, (b) soil depth and type (related to topography), and (c) precipitation.

2. Plant productivity is impacted by the interaction of burn frequency, soil, and precipitation. For example, productivity may be impacted differently by precipitation in different soil types, and burning may have a different impact on productivity in different soil types.

• Uplands are more likely limited by water than lowlands, in which case, tallgrass prairie uplands may respond to variability in precipitation more like the shortgrass steppe (Sala et al., 1988). Burned sites are more limited by water than are unburned sites (Knapp et al., 1993).

3. Different vegetation components (grasses and forbs) respond differently to burning and/or precipitation.

• Forbs (C3 plants) have lower water use efficiency than C4 grasses.

Study location

The Konza Prairie Biological Station (KPBS), located in the Flint Hills of northeastern Kansas, USA (39°05’ N, 96°35’ W), maintains an interdisciplinary research program documenting the influences of fire, grazing, and climate on the faunal and floral ecology of the mesic prairie. KPBS was established in 1971 with land donated to The Nature Conservancy and is managed by the Division of Biology at Kansas State University. In 1981, KPBS joined the Long-Term Ecological Research (LTER) program funded by the NSF, as one of the original six field stations assembled to document temporal and spatial trends within and across biomes (www.lternet.edu).

KPBS is a 3487 hectares (ha) unplowed tallgrass prairie dominated by a few perennial warm-season grasses (big bluestem, Indian grass and little bluestem), yet supporting a species-rich community of over 500 other species consisting of herbaceous forbs, woody shrubs and trees, and both cool and warm-season grasses (Freeman 1998). KPBS experiences a temperate mid-continental climate characterized by periodic droughts and large seasonal and interannual variability in rainfall. This climate type results in cold, dry winters and warm wet summers with the majority of the annual precipitation occurring between April and September (835mm mean annual precipitation).

Historically, the Flint Hills were not cultivated because the steep hillsides and the rocky, shallow soils of the region prevented conventional tillage practices. For this reason, the region contains the largest remaining area of unplowed tallgrass prairie in North America. Another extraordinary aspect of the KPBS is its experimental design. KPBS is divided into 60 watersheds used to study three factors critical for maintaining the tallgrass prairie ecosystem: periodic fire, ungulate grazing, and a variable climate. KPBS is one of the oldest LTER sites, and experiments with fire and grazing can be carried out over long intervals (e.g., 20 year burns) and with multiple interactions. The focus of this Data Set is to compare the interaction of fire frequency and inter-annual variation in precipitation on the annual production of grasses and forbs.

Data

These data are based on analyses and measurements conducted between 1984 and 1999 at KPBS and archived online at www.konza.ksu.edu. Total aboveground productivity was estimated by quantifying the accumulation of new plant biomass at the end of the growing season along permanent transects in the experimental fire units (watersheds) of KPBS (Briggs and Knapp 1995). In general, prairie plant species in the northern Flint Hills begin growing in early April and have senesced by late September. Plants that use different photosynthetic pathways and annual differences in climate can alter this timeline, but peak biomass is typically reached by late August/early September, the date of biomass harvest each year.

Data analysis