Ecological restoration aims to assist the recovery of ecosystems that have been damaged or destroyed. Restoration success depends on setting appropriate objectives (Cairns 2000) and the subsequent use of suitable criteria for evaluating the outcomes. The question of objectives has been the subject of much discussion (Parker & Pickett 1997), but most authors agree that success should be judged on both structural and functional aspects of ecosystems. Ecological restoration should, therefore, be based on an accurate understanding of ecosystem dynamics (Palmer, Ambrose & Poff 1997).
Community succession and assembly rules are two of the most relevant ecological concepts for ecological restoration (Young, Chase & Huddleston 2001). Community succession refers to the predictable turnover of species composition. The position of species along a successional gradient depends on their performance in colonization and biotic interaction processes (competition and/or facilitation), and any modification of the biotope that occurs during the succession (Tilman 1990). Assembly rules are based on the response of organisms to local environmental factors (biotic and abiotic) after their random arrival. To establish and survive, organisms need the appropriate ecological abilities to face the environmental filters/constraints of that biotope (Keddy 1992). Both concepts rely upon the idea that community structure can be predicted from knowledge of organisms’ traits, as these traits affect the response of the species to environmental factors, and any reciprocal effect on ecosystem functioning (Lavorel & Garnier 2002). In turn, trait assembly in local communities is recognized to influence ecosystem-level properties such as primary productivity, biochemical cycling, resistance and resilience to disturbance (Lavorel & Garnier 2002), which are useful ecosystem characteristics for evaluating success of ecological restoration (Ehrenfeld & Toth 1997). For that reason, the trait-based approach has been used to evaluate the restoration of plant communities (Hérault, Honnay & Thoen 2005) and to assess the performance of species in restored communities (Pywell et al. 2003).
Knowledge of the spatial aspects of ecological system functioning and structuring is also important. The importance of biogeographical gradients and landscape structure on the recruitment of species in degraded sites has been identified (Tong et al. 2006), indicating the need to consider a landscape ecology approach beyond the spatial scale of the restored site. However, spatial heterogeneity is also of interest at much finer, within-site scales. Indeed, the spatial heterogeneity of organisms that develops on a restored site will almost certainly reflect interacting abiotic and biotic processes, which together influence community succession and assembly at a wide range of scales (Levin 1992; Tilman 1994). Moreover, the spatial patterning of organisms has also been shown to be important in maintaining both ecosystem structure and function (Pacala & Deutschman 1995). Therefore, in recent studies of spatial heterogeneity in degraded ecosystems, some authors have emphasized its relevance for evaluating restoration success (Maestre et al. 2003; Seabloom & Van Der Valk 2003).
Recently, both trait-based and spatial approaches have allowed advances in the theoretical basis for planning and implementing ecological restoration projects. The aim of this paper is, therefore, to combine both these approaches within a model system. The model system is a large land settlement located on the banks of the river Rhône, where diverse initial restoration treatments were applied. More precisely, our objective was to assess how active restoration influenced the spatial patterning of plant traits after 25–30 years of spontaneous vegetation dynamics on the site. This study was performed by classifying the plant species into emergent groups (EGs) (Lavorel et al. 1997), i.e. species that share similar life attributes, and then analysing potential sources of their spatial heterogeneity. As spatial heterogeneity should be considered as both endogenous and exogenous, we hypothesized that: (i) the modification of environmental variables by restoration efforts induced spatial heterogeneity of the EGs at different spatial scales; and (ii) the dispersal abilities of the EGs influenced their spatial heterogeneity over the study site. These hypotheses were tested using a spatial eigenvector mapping technique. We then discuss how such an approach might be used to evaluate and monitor the effects of restoration in creating spatial heterogeneity during ecological restoration.