Fire frequency is a well-known regulator of grass and woody abundance in North American tallgrass prairie. Currently, hypothesized historical fire frequencies are insufficient to prevent woody encroachment and loss of grass production. This occurs, in part, because of fire suppression during the middle 20th Century that allowed woody vegetation to expand regionally. As a consequence, woody plants are now rapidly expanding into adjacent tallgrass prairie. We established four watershed-scale fire frequency experiments at the Konza Prairie LTER site in northeastern Kansas. One watershed has been annually burned since 1978. In a second watershed burning ended in 2001 after 20 years of annual fires to determine how rapidly woody encroachment occurs under current climatic conditions and nearby seed sources. In a third watershed, we initiated annual burning in 2001 in an area that had been burned eight times in the previous 21 years. A fourth watershed has been unburned since 1980. Twenty 10m2 permanent vegetation quadrats arrayed along four 50m long transects were established in each watershed prior to the fire reversal treatments. Vegetation was sampled in each quadrat twice each year, and the boundaries of all shrub clones in each fire reversal watershed were mapped in 2001, 2010 and 2017.
Results/Conclusions
Woody plant encroachment continues to increase rapidly in the long-term unburned watershed reaching ~100% cover by 2015. In contrast, once annual fires were re-introduced, woody cover stabilized and then decreased in the formerly infrequently burned watershed. Where annual burning stopped, cover of dominant woody species did not start to increase until 2015, 14 years after the end of the annual burning treatment. Only trace amounts of woody vegetation occur in the long-term annually burned watershed. On average, species richness was lowest in the annually burned watershed and continues to decline over time. Species richness has decreased nearly 35% in the formerly unburned watershed that is now annually burned, whereas species richness has increased by nearly 60% in the formerly annually burned watershed after the burning treatments stopped. Based on NMDS ordination, the vegetation in each fire reversal watershed is developing toward different basins of attraction compared to the vegetation found in the long-term annually burned and unburned watersheds. Our results demonstrate that high fire frequencies can reverse shrub encroachment, but one of the consequences is a significant loss of plant species diversity. Moreover, altering long-term fire frequencies causes vegetation composition to change along trajectories that differ from existing reference watersheds.