Ecological Applications, 21(2), 2011, pp. 439–451 Ó 2011 by the Ecological Society of America Deer browsing and soil disturbance induce cascading effects on plant communities: a multilevel path analysis JULIEN BEGUIN, 1,2,3 DAVID POTHIER, 1 AND STEEVE D. C ˆ oTE ´ 2 1 Centre d’E ´ tude de la For ˆ et, De ´partement des Sciences du Bois et de la For ˆ et, Pavillon Abitibi-Price, 2405 Rue de la Terrasse, Universite ´ Laval, Que´bec, QC G1V0A6 Canada 2 Chaire de Recherche Industrielle CRSNG-Produits Forestiers Anticosti, De ´partement de Biologie and Centre d’E ´ tudes Nordiques, 1045 Ave. de la Me ´decine, Universite ´ Laval, Que ´bec, QC G1V 0A6 Canada Abstract. Understanding how large herbivores shape plant diversity patterns is an important challenge in community ecology, especially because many ungulate populations in the northern hemisphere have recently expanded. Because species within plant communities can exhibit strong interactions (e.g., competition, facilitation), selective foraging by large herbivores is likely not only to affect the abundance of palatable species, but also to induce cascading effects across entire plant communities. To investigate these possibilities, we first tested the effects of deer browsing and soil disturbance on herbaceous plant diversity patterns in boreal forest, using standard analyses of variance. Second, we evaluated direct and indirect effects of deer browsing and soil disturbance on the small-scale richness of herbaceous taxa using a multilevel path analysis approach. The first set of analyses showed that deer browsing and soil disturbance influenced herb richness. Path analyses revealed that deer browsing and soil disturbance influenced richness via complex chains of interactions, involving dominant (i.e., the most abundant) browsing-tolerant (DBT) taxa and white birch (Betula papyrifera), a species highly preferred by white-tailed deer (Odocoileus virginianus). We found no evidence that an increase of white birch in fenced quadrats was the direct cause of a decrease in herb richness. However, we found strong evidence that a higher abundance of DBT taxa (i.e., graminoids and Circium arvense), both in fenced and unfenced quadrats, increased herb layer richness. We propose an empirical model in which competitive interactions between white birch and DBT taxa regulate the strength of facilitative relationships between the abundance of DBT taxa and herb richness. In this model, deer browsing and the intensity of soil disturbance initiate a complex chain of cascading effects in boreal plant communities by controlling the abundance of white birch. Key words: Anticosti Island, Gulf of St. Lawrence, Canada; browsing tolerance; cascading effects; competition; deer browsing; facilitation; indirect effects; multilevel path analysis; Odocoileus virginianus; plant diversity; plant–herbivore relationships; white-tailed deer. INTRODUCTION Large herbivores can affect the vegetation physiog- nomy of many terrestrial ecosystems (van de Koppel et al. 1997, C ˆ ote´ et al. 2004), particularly in northern hemisphere forests (Horsley et al. 2003) where ungulate populations and their browsing on vegetation have recently increased (Rooney and Waller 2003). In forests, sustained overbrowsing reduces plant cover and diver- sity, alters nutrient and carbon cycling, and redirects succession to shift future overstory composition (C ˆ ote´ et al. 2004). In temperate and boreal forests, the direct effects of deer browsing on plant tissues can largely explain the decline in distribution and abundance of several palatable forest species (Brandner et al. 1990, Potvin et al. 2003), including common ( Trillium grandiflorum; Augustine and Frelich 1998) and endan- gered herbs (e.g., Panax quinquefolius; McGraw and Furedi 2005). High-density deer populations also commonly cause tree diversity to decline (Gill and Beardall 2001, Kuiters and Slim 2002, Horsley et al. 2003). However, few studies have evaluated the brows- ing impacts of large herbivores on the maintenance of diversity patterns in boreal plant communities (Gill and Beardall 2001). Mechanical site preparation (MSP) is another common disturbance used to create favorable microsite conditions for post-harvest tree seedling establishment in boreal forest. Without considering the effect of herbivores, increasing intensity of MSP was found to increase the negative impacts on boreal plant diversity (Newmaster et al. 2007; but see Haeussler et al. 1999, 2004, Peltzer et al. 2000). Although deer browsing and MSP often occur in the same sites, it is unclear whether their effects on plant diversity patterns are independent, additive, or compensatory. Understanding the mechanisms by which large herbivores and soil disturbance influence the organiza- Manuscript received 10 November 2009; revised 26 April 2010; accepted 28 April 2010. Corresponding Editor: E. M. Schauber. 3 E-mail: julien.beguin.1@ulaval.ca 439