The Konza LTER program continues to build upon a long-term database on ecological patterns and processes derived from a fully replicated watershed-level experimental design, in place since 1977 (Fig. 1). This design includes replicate watersheds subject to different fire and grazing treatments, as well as a number of long-term plot-level experiments (Table 1). The effects of climate are addressed by long-term studies encompassing the natural climatic variability, and possible directional changes, characteristic of this region, as well as manipulations of water availability in field experiments. Within core LTER watersheds, permanent sampling transects are replicated at various topographic positions (n=4/topo. position/watershed), where ANPP, plant species composition, plant and consumer populations, soil properties, and key above- and belowground processes are measured. The collection of diverse data from common sampling locations facilitates integration among our research groups. In total, the Konza LTER Program incorporates explicit study of the major factors influencing mesic grasslands in a long-term experimental setting. It has the essential components of a statistically rigorous ecological research program designed to elucidate patterns and processes inherently important in grasslands, and address the potential impacts of global change in these ecosystems. Below we elaborate on our rationale for focusing on fire, grazing and climate, and describe our core LTER experiments.

Fig. 1. Konza Prairie site experimental design, and watershed-level fire and grazing treatments. Watersheds open to bison grazing (‘N’) are highlighted in red, and cattle-grazed watersheds (‘C’) are highlighted in blue. All other watersheds are ungrazed. Numbers in watershed codes designate fire return intervals for spring-burned watersheds, and the last letter of watershed codes (A,B,C,D) is used to identify replicate watersheds of the same treatment. Watersheds subject to different seasons of burn are highlighted in yellow, and the Fire Treatment Reversal (‘R’) watersheds are highlighted green. Many of our plot-level experiments (Belowground Experimental Plots, RaMPs, Irrigation Transects) are located at the headquarters area (HQ) in the northwest portion of the site.

Table 1. Brief description of selected long-term, plot-level experiments of the Konza LTER program. Such experiments complement our long-term watershed-scale studies, and provide important information about mechanisms underlying responses at broader scales. These experiments also act as focal points for ecological studies that span multiple disciplines.

Fire Fire is essential in mesic grasslands worldwide, and human alteration of fire frequency is a key element of global change in these grasslands. Fire was important historically in tallgrass prairie, and is now managed by humans to limit the growth of woody plants and to promote the cover and productivity of C4 grasses. Fire alters the structure and function of grasslands, and our studies of the ecological consequences of fire have spanned multiple spatial and temporal scales and levels of ecological study. This hierarchical approach has identified many of the mechanisms contributing to responses at higher levels. For example, fire changes the light and soil environment of emerging plants, altering phenology and physiological responses, increasing ANPP, increasing competition for light and N. These changes contribute to reduced abundance and richness of C3 plants and a concomitant decline in biodiversity. Changes in vegetation structure,composition and tissue quality also elicit responses in aboveground consumers and affect belowground invertebrates, mycorrhizae and soil microbes.

The experimental design at Konza (Fig. 1) includes replicate watersheds (avg. size = 60 ha) that, since 1972, have been burned annually or at 2-, 4-, 10- and 20-year frequencies, encompassing a range of likely natural fire frequencies and management extremes. Most watersheds are burned at the end of the dormant season (April), when the greatest frequency of lightning and prescribed burning occurs regionally. However, fires historically occurred at other times, and during LTER IV we initiated a “season of fire” experiment, with replicate watersheds burned in the spring, summer, fall or winter. To address trajectories following a change in fire regime and the role of fire history (legacy effects), we began (in 2001) a long-term “fire treatment reversal” experiment (annual and 20 yr fire treatments switched on replicate watersheds). This new experiment will provide insights into the temporal scales over which plant and soil processes respond to changing fire regimes, and the role of site history in affecting responses to fire. We also incorporate fire into smaller plot-scale plot experiments on Konza, where mechanisms underlying effects of fire are more readily addressed.

Grazing Grazing, like fire, was historically important in tallgrass prairie, but is now largely under human control, with grazing by cattle the dominant land use in the Flint Hills. In fact, changes associated with the management of megaherbivores are among the most significant in the recent history of mesic grasslands worldwide. Understanding the role of grazers in these grasslands is crucial, To address the role of native grazers and important fire ? grazing interactions, bison were reintroduced in 1987 to a 1000-ha area of Konza that includes replicate watersheds burned at 1-, 2-, 4- and 20-year intervals and a range of topography and vegetation types. Comparative studies of native (bison) vs. introduced (cattle) ungulates are also needed to understand the impacts of changing land use in North American grasslands. Functional similarities and differences in the effects of bison and cattle on grasslands are being assessed by long-term comparisons of bison and cattle grazing at both the watershed and small enclosure (5 ha) scales.

Climate North American grasslands were formed by climate changes originating during the Miocene-Pliocene transition, and their present day distributions depend on regional temperature and precipitation gradients. Although our emphasis on fire and grazing interactions is clearly appropriate, and unique within the LTER network, >20 yrs of study has underscored the pervasive role of interannual climatic variability in this ecosystem. For mesic grasslands, the mean and extremes of precipitation (e.g., floods and droughts) affect most ecosystem processes. On a continental scale, variability in precipitation affects productivity more in grasslands than in other North American biomes. Thus, responses to altered temperature and precipitation regimes will be an important component of global change. Climate change predictions for the Central Plains include increased temperatures and increased temporal variability in rainfall (i.e., larger storm events and longer intervening dry periods. Changes in temporal patterns of precipitation are predicted impact grasslands more than changes in precipitation quantity alone. We are just beginning to understand how grasslands will respond to more extreme rainfall patterns. Longer-term studies, and those considering multiple, interacting factors (i.e., temperature and precipitation) are needed. New experiments for LTER V are assessing grassland responses to climate change over short- and long- time scales.