Predicting how ecosystems will respond to forecast environmental changes and to evaluate the consequences of those responses is a major challenge for ecologists today. One of the most prevalent contemporary land-cover changes is woody plant encroachment into grasslands and increased cover of shrubs and trees in grasslands and savannas. Causes of increased woody plant abundance may vary in different grasslands, and some likely factors include increased grazing intensity, alterations in local land management practices, and rising atmospheric CO2 concentrations. Because grassland and savanna ecosystems account for 30-35% of global terrestrial net primary production, any change in patterns or controls of C inputs and storage due to increase woody vegetation in grasslands will has important implications for regional and global nutrient cycles. In addition, woody plant expansion often decreases herbaceous species diversity and negatively impacts many consumer populations.
The KNZ LTER program has been measuring woody plant cover under different prescribed fire treatments for over 30 years. These long-term studies have revealed the major role that fire plays in determining the coexistence and balance of grasses and woody plant species in mesic grasslands. Frequent fires (every year or two) are sufficient to limit the cover and spread and woody plants, while fire exclusion can lead to shrub dominance or complete conversion to woodlands in as little as a few decades. A surprising result was that the intermediate fire frequencies (every three-four years), thought to be historically most common in these grasslands, now promote the rapid expansion of woody vegetation in tallgrass prairie. Current studies are focusing on reasons for the increase in woody plant cover in the intermediate fire regimes. In addition, we have established a long-term experiment (the Fire Reversal study) in which long-term fire treatments were ‘reversed’ on two watersheds previously burned annually and two watersheds protected from fire for ~20 yrs. This experiment is allowing us to assess the legacy effects of fire history, and to determine if a return of more frequent fires is sufficient to reverse increases in woody plant cover. After 12 years, cover of the most abundant shrub species is declining with repeated burning and increasing with fire exclusion. However, all but one woody plant species have persisted even with annual fires. These results suggest that once woody plants become established in tallgrass prairie, the return of frequent fires alone may not be sufficient to remove them from the landscape.