Analyzing Water Isotopes in Mesic Bur Oak Forest, Homestead National Monument, Nebraska.

Chimner, Rodney; Resh, Sigrid

This study examines the the relative importance of groundwater vs. river water used by bur oaks at Homestead National Monument. In Homestead National Monument, the bur oaks grow within the historicalfloodplain of Cub Creek. However, it is likely that streambank downcutting has occurredalong Cub Creek. Because of the extent of the downcutting, questions have been raisedregarding water availability for the lowland bur oak forest and how the forest may beimpacted by management plans. Therefore, the objectives of our study were todetermine: (1) the relative importance of groundwater vs. river water used by bur oaks,and (2) whether the proportion of groundwater/river water varied based on locationrelative to Cub Creek. Plant and soil samples for isotopic analysis were collected fromthree sites during 2008 and 2009. Water table depth, soil temperature and moisture weremeasured continuously and river stage and precipitation were monitored daily byHomestead NM staff. We analyzed river, ground, twig xylem, and soil water for naturalisotopic abundance.In summary, we found that bur oaks in Homestead NM are using deep watersources, and are unlikely to be affected hydrologically by current regional downcutting ofthe river. Downcutting and alteration of the floodplain can have major affects on otherecological parameters such as floodplain vegetation communities and tree regeneration,but that is outside the scope of this study. Given that bur oaks do not appear to be waterstressed and are known to be fire tolerant, we feel that allowing fire into the bur oakstands will probably not be detrimental to the adult bur oaks. However, we suggest thatfire be reintroduced experimentally into the southern forested section first, not directlyinto the less disturbed, mature bur oak forest.