EFFECT OF FLOOD REGIME ON TREE GROWTH IN THE FLOODPLAIN AND SURROUNDING UPLANDS OF THE WISCONSIN RIVER KATHARINE I. PREDICK, a * SARAH E. GERGEL b and MONICA G. TURNER a a Department of Zoology, University of Wisconsin, Madison, WI 53706, USA b Department of Forest Sciences, University of British Columbia, Vancouver, BC, V6T 1Z4 Canada ABSTRACT Flood regime and vegetation flood tolerance interact to influence tree growth in riverine landscapes. We studied tree growth in floodplain and upland forests of the Wisconsin River. About a century ago, levees set back from the river were constructed on this floodplain. The levee restricts some floodplain area from overbank flood events, but leaves a portion of active floodplain still inundated by floods. We addressed two questions: (1) how do growth rates of flood-tolerant and flood-intolerant tree species in the floodplain differ with flood regime? (2) At the stand level, how does growth rate differ with flood regime and between floodplain and upland areas? Annual tree growth rates from 1991 to 2000 were determined from tree increment cores for both individual species and stands. Tree growth rates of individual species varied between flood regimes. The most flood-tolerant species (Betula nigra and Fraxinus pennsylvanica) grew faster in areas with active flooding, while the growth of less flood-tolerant species (Quercus velutina and Q. ellipsoidalis) was depressed in swales and active floodplain. However, stand-level tree growth did not differ between the floodplain and upland, or between flood regimes within the floodplain. Therefore, variation in the growth of individual species may not scale up to create differences in stand-level tree growth because forest community composition varies spatially with flood regime. We suggest that growth rates are similar among sites because each community comprises of species adapted to their current flood regime. Copyright # 2008 John Wiley & Sons, Ltd. key words: basal area increment; flood tolerance; tree production; levee; microtopography; restricted flooding; radial growth; riparian forest Received 31 October 2007; Revised 11 March 2008; Accepted 17 March 2008 INTRODUCTION Flood regime is an important driver of species composition and ecosystem processes in floodplain forests. Productivity and vegetation growth vary among species and are influenced by the frequency and severity of floods in active floodplains (Brinson, 1990). Floodplains can be highly productive and are typically assumed to have higher productivities than upland forests (Brinson, 1990; Naiman and De ´camps, 1997). However, few studies directly compare floodplain and upland productivity between stands in the same river system (but see Johnson and Bell, 1976). The role of flooding is more frequently investigated by measuring productivity or growth before and after a change to the hydrologic regime. Both river regulation and dam construction have been shown to decrease floodplain tree growth and productivity (Reily and Johnson, 1982; Bakhiev and Treshkin, 1994; Middleton and McKee, 2005), and hydrologic restoration can increase productivity (Anderson and Mitsch, 2006). A review of temperate, southern bottomland forests suggests that floodplain productivity often declines due to changes in the hydrologic regime, typically because the vegetation community is not in equilibrium with the altered hydrology (Megonigal et al., 1997). Anthropogenic modifications to flood regime are common; 98% of North American rivers are influenced by human modifications (Vitousek et al., 1997) and 65% of large rivers worldwide are influenced by dams (Nilsson et al., 2005). When flood regimes are altered by dams or levees, patterns of vegetation, sediment deposition and nutrient processing are altered (Naiman and De ´camps, 1997). RIVER RESEARCH AND APPLICATIONS River. Res. Applic. 25: 283–296 (2009) Published online 9 May 2008 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/rra.1156 *Correspondence to: Katharine I. Predick, School of Natural Resources, University of Arizona, Biological Sciences East, 1311 E. 4th St., Tucson, AZ 85721-0043, USA. E-mail: kipredick@arizona.edu Copyright # 2008 John Wiley & Sons, Ltd.