Oak expansion in the Chautauqua Hills Kansas: a regional assessment of historic change

Abstract

Woody plant expansion into grasslands and savannas is a globally occurring process which can cause loss of biodiversity and alter biogeochemical cycles. The Chautauqua Hills, in southeast Kansas, is the northernmost extent of the Cross Timbers vegetation type, Quercus stellata and Quercus marilandica are the dominant tree species. Government Land Office records from the 1860’s indicate sparse tree cover in much of this region, which is now characterized by dense oak woodlands. I use a multi-site, dendrochronological approach to address four research questions: 1) when did oak expansion occur? 2) from what landscape position did oaks expand?, 3) how have physiological differences between members of the Erythrobalanus (Q. marilandica) and leucobalanus (Q. stellata) subgenera influenced recruitment patterns?, and 4) which drivers of woody plant encroachment coincide with oak expansion in the Chautauqua Hills? Quercus stellata comprised a greater proportion of ancient (>100 years) trees than Q. marilandica at all sites. Quercus stellata age structures differed from both the normal and negative exponential distributions at all sites, while Quercus marilandica did not differ significantly from the normal distribution at three sites, and did not differ from the negative exponential distribution at two site. Three of the four study sites likely were savanna prior to Euro-American settlement, indicated by the over-representation of older age classes compared to the negative exponential distribution. Drought during the 1930’s, favorable attitudes towards trees following the dustbowl, livestock grazing, and changes in fire frequency all likely contributed to oak expansion in the Chautauqua Hills.