Historical variability in fire at the ponderosa pine - Northern Great Plains prairie ecotone, southeastern Black Hills, South Dakota

Ecotones are boundaries between plant assemblages that can represent a phys iological or competitive limit of species' local distributions, usually thr ough one or more biotic or abiotic constraints on species' resource require ments. However, ecotones also result from the effects of chronic or episodi c disturbances, and changes in disturbance regimes may have profound effect s on vegetation patterns in transitional areas. In this study, centuries-lo ng chronologies of surface fire events were reconstructed from fire-scarred ponderosa pine (Pinus ponderosa Dougl. ex Laws.) trees in three sites at t he ecotone between ponderosa pine forest and Northern Great Plains mixed-gr ass prairie in the southeastern Black Hills of South Dakota. The fire chron ologies provide baseline data to assess the possible role of fire in this t ransitional area and to document historical variability in fire regimes in this region of the Northern Great Plains. Regular fire events were recorded at all three sites from the beginning of the fire chronologies in the 1500 s up to the late 1800s or early 1900s, at which time spreading fires ceased . Fire frequencies derived from the fire chronologies were compared to each other and to four sites from interior ponderosa pine forest in the south-c entral Black Hills. Mean fire intervals at the savanna sites were between 1 0 to 12 years, whereas Weibull median probability intervals were one year s horter. Fire frequency at the savanna sites was twice as high as at the int erior forest sites, and most likely was due to spatial extent of fires on t he mixed-grass prairie coupled with warmer and drier climate regime. Post-s ettlement shifts in the ponderosa pine savanna during the twentieth century in this area may be largely attributed to lack of fire occurrences, althou gh grazing and other factors also likely contributed to observed changes in forest and grassland margins.