Introduction

Foundation species represent excellent model systems for understanding the broad consequences of variation on community and ecosystem processes as they provide a focal resource upon which associated interacting species depend. As foundation species (Dayton 1972; Ellison et al. 2005), trees and other dominant plants often create stable conditions via plant traits that allow dependent communities to assemble regularly and influence ecosystem processes such as net primary productivity (NPP) and soil fertility (i.e., nutrient cycling, via accumulations of leaf or root organic matter or root exudates; Zinke 1962; Zak et al. 1986; Binkley and Giardina 1998; Bartelt-Ryser et al. 2005; Wardle 2006). Recent studies in both terrestrial and aquatic habitats have shown that intraspecific genetic variation (defined at multiple genetic scales, including introgression [movement of genes from one species to another], genotypic diversity [studies manipulating the number of genotypes in a population] and genotypic variation [variation among genotypes]) in foundation plants can have community-wide consequences. Intraspecific variation affects associated vertebrate, arthropod and microbial community composition or activity and ecosystem level processes (recently reviewed in Johnson and Stinchcombe 2007; Hughes et al. 2008; Whitham et al. 2008; Bailey et al. 2009). For example, genetic variation resulting from the introgression of genes from one species to another through the process of hybridization has been shown to have important consequences for associated species, communities and ecosystem processes in multiple hybridizing plant species, including Salix spp., Eucalyptus spp., Quercus spp. and Populus spp. (Fritz et al. 1994; Dungey et al. 2000; Hochwender and Fritz 2004; Ito and Ozaki 2005; Wimp et al. 2005; Tovar-Sanchez and Oyama 2006; Bangert et al. 2008). In the Populus system specifically, recent field and common garden studies have shown that genetic variation across a hybridizing system (P. fremontii, P. angustifolia and their natural F1 and backcross hybrids) results in shifts in plant traits, including secondary chemistry, plant water use and above- and belowground productivity (Fischer et al. 2004; Rehill et al. 2006; Schweitzer et al. 2008a; Lojewski et al. 2009). Whether due directly or indirectly to these plant traits, rates of leaf litter decomposition, total belowground carbon (C) allocation and pools of soil nitrogen (N) and rates of net N mineralization also shift along this genetic gradient (Schweitzer et al. 2004, 2008, b; LeRoy et al. 2006; Whitham et al. 2006; Lojewski et al. 2009; Fischer et al. 2007, 2010).