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

Background/Question/Methods

Woody plant expansion into grasslands and savannas is a globally occurring process with documented instances on virtually every continent.  In North America, woody plant expansion rapidly increased during the twentieth century, leading to the loss of biodiversity through alteration of landscape physiognomies and biogeochemical cycles, and the loss of forage for livestock. Many processes have been hypothesized to cause woody plant expansion, but disentangling how these processes interact in individual cases remains challenging.  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 in these communities.  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. We use a multi-site, dendrochronological approach to address four main research questions: 1) when did oak expansion begin in this region?, 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 patterns of recruitment?, and 4) which drivers of woody plant encroachment coincide with oak expansion in the Chautauqua Hills?

Results/Conclusions

Maximum ages were 192 years for Q. stellata and 115 years for Q. marilandica across the four study sites. Quercus stellata comprised a greater proportion of ancient ( >100 years) trees than Q. marilandica  at all sites, which may be  explained by its longer lifespan or historically it may have been  more prevalent on the landscape. Quercus stellata age structures differed from both the normal and negative exponential distributions at all sites due to the over-representation of older age classes.  Quercus marilandica did not differ significantly from the normal distribution at three sites, and the negative exponential at two.  At the remaining sites older age classes were over-represented.  There was no significant relationship between landscape position and tree age for either species.  Two time periods (1929-33 and 1939-43) mark the beginning of oak expansion at our study sites. Drought during the 1930’s, favorable attitudes towards woody plants following the dustbowl,  livestock grazing, and changes in fire frequency are all likely to have contributed to oak expansion in the Chautauqua Hills.  Three of the four study sites likely were savanna prior to Euro-American settlement, indicated by the over-representation of older age classes as compared to the negative exponential distribution.