Submitted to: Forest Ecology and Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/22/2007
Publication Date: 7/16/2007
Citation: Asbjornsen, H., Tomer, M.D., Gomez-Cardenas, M., Brudvig, L.A., Greenan, C.M., Schilling, K. 2007. Tree and stand transpiration in a Midwestern bur oak savanna after elm encroachment and restoration thinning. Forest Ecology and Management. 247:209-219. Interpretive Summary: Oak savannas are included among the remaining tracts of non-cultivated land in the Midwest inhabited by historically dominant plant communities that were once maintained by repeated cycles of fire. Without fire, these savanna remnants have often been encroached by other tree species and become woodlands. There is interest in managing these areas to restore native plant communities, but the hydrologic function of these areas, particularly savannas, is not documented. In an elm-encroached savanna-woodland in Iowa, water use was similar to that found in more heavily forested areas of the Midwest. The elms were resonsible for about two thirds of the water uptake. In an adjacent stand, when elms were removed, the savanna transpired about 30% of the water used in the woodland, yet only had 11% of its sapwood area. Sunshine rather than humidity was the climatic factor most related to variation in water use. On a sapwood-area basis, elm used the least water, but competed effectively for water by having an earlier time of peak water uptake. Savanna oaks showed the greatest greater water uptake, but in both stands oak transpiration was reduced when humidity was low. Some of the water use by the woodland was supplied by groundwater, which showed a water table 15 feet deeper than the adjacent savanna. Water tables in agricultural areas are often shallow, and the potential influence of shallow water on vegetation should be considered in management of these native remnants. Managers of parks and wildlife preserves should understand hydrologic conditions when managing restoration projects. Targeting of larger remnants or use of buffers will offer these professionals the best opportunities to manage site hydrology through their restoration efforts.
Technical Abstract: Oak savannas, once common in the Midwest, are now isolated remnants within agricultural landscapes. Savanna remnants are frequently encroached by invasive trees to become woodlands. Thinning and prescribed burning can restore savanna structure, but hydrologic effects of managing these remnants are poorly understood. This study measured sap flow to quantify transpiration in an Iowa bur oak (Quercus macrocarpa) savanna encroached by elms (Ulmus americana) (woodland), and in an adjacent restored savanna after thinning to remove elms, during summer 2004. Savanna oaks had greater mean daily sap flow (35.9 L dm-2day-1) than woodland oaks (20.7 L dm-2day-1) and elms (12.4 L dm-2day-1). Sap-flow response to vapor pressure deficit (D) was unexpectedly weak, although oaks in both stands showed negative correlation between daily sap flow and daylight D for D>0.4 kPa. An earlier time of peak sap flow showed a possible advantage for water uptake by elms. As anticipated, the woodland’s stand transpiration was greater (1.23 mm day-1) than the savanna’s (0.36 mm day-1), yet the savanna achieved 30% of the woodland’s transpiration with only 11% of its sapwood area. The savanna’s reduced transpiration influenced water table depths, which were 2 m in the savanna and 6.5 m in the woodland. Regionally, row-crop agriculture has increased groundwater recharge and raised water tables, providing surplus water that perhaps facilitated elm encroachment. This may have important implications for restoration of savanna remnants. If achieving a savanna eco-hydrology is an aim of restoration, then restoration strategies may require buffers, or targeting of larger remnants.