Examining native and exotic species diversity in European riparian forests Annik Schnitzler a , Brack W. Hale b, *, Esther M. Alsum c a Universite ´ Paul Verlaine, Metz, LBFE, Rue du Ge ´ne ´ral Delestraint, 57 070 Metz, France b Franklin College Switzerland, Via Ponte Tresa 29, 6924 Sorengo, Switzerland c Botany Department, 430 Lincoln Drive, University of Wisconsin-Madison, Madison, WI 53706, USA ARTICLE INFO Article history: Received 6 February 2006 Received in revised form 10 March 2007 Accepted 6 April 2007 Keywords: Riparian forests Exotic species Invasive species Disturbance Europe River floodplains ABSTRACT This study performs a meta-analysis of existing studies of European riparian forests to investigate which exotic species have successfully established in intact riparian forests and which characteristics of these forests correspond with successful establishment. We used analysis of covariance models to investigate the relationship between community species richness, percent exotic species, and several environmental variables. We found a total richness of 1380 species, of which 45 (3.3%) were exotic. Species-rich communities generally had the higher percentage of exotics, but were not significantly related to latitude or environmental variables. Exotics, in contrast, were at generally higher levels at lower lat- itudes and were more abundant in large river plains and communities with intermediate levels of disturbance. These results suggest that future climate change and human actions that mimic intermediate levels of disturbance may further enhance the spread of exotic species. Ó 2007 Published by Elsevier Ltd. 1. Introduction Riparian forests are complex and dynamic ecosystems that strongly depend on flood pulses for primary productivity and biodiversity (Carbiener, 1970; Ward et al., 1999). River sys- tems naturally experience frequent, periodic flood distur- bances that are responsible for high variations in texture and drainage at local scales. Floods also transport vegetative matter and seeds from upland and upstream sources, as well as across the floodplain, thereby encouraging propagation. The abundant supply of water and the high nutrient content of freshly deposited sediments combine with the typically plentiful light availability to promote successful germination and establishment (Walter, 1979; Sanchez-Perez et al., 1993, 1999; Schnitzler et al., 2003). These conditions also promote certain types of species, such as lianas, which are often less competitive in upland forests (Schnitzler, 1995a; Allen et al., 2005). Moreover, these factors help explain why river systems provide opportunities for the establishment of exotic spe- cies—non-native species introduced intentionally or uninten- tionally by human action (Tickner et al., 2001). The expansion of exotics in river-floodplain systems char- acterized the 20th century as human alterations to these sys- tems, such as channel modifications, flow regulation, and floodplain drainage, intensified (Dynesius and Nilsson, 1994; Knutson and Klaas, 1998; Nolet and Rosell, 1998; Be ´ the ´ mont, 2000). Increasing organic pollution also favored certain types of exotics with high nutrient demand. The abandonment of traditional hay and grazing lands allowed the establishment and growth of dense stands of exotic clones (Pysek and Prach, 1993). Human activity further enhanced the expansion rates by increasing the dispersal of plant propagules into anthropo- 0006-3207/$ - see front matter Ó 2007 Published by Elsevier Ltd. doi:10.1016/j.biocon.2007.04.010 * Corresponding author: Tel.: +41 91 986 3650; fax: +41 91 994 4117. E-mail addresses: schnitz@univ-metz.fr (A. Schnitzler), bhale@fc.edu (B.W. Hale), emalsum@yahoo.com (E.M. Alsum). BIOLOGICAL CONSERVATION 138 (2007) 146 – 156 available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/biocon