Comparison of two plant functional approaches to evaluate natural restoration along an old-field - deciduous forest chronosequence

Are direct and indirect trait-based approaches similar in their usefulness to synthesize species responses to successional stages? Northern hardwood forests, QuEbec, Canada (45 degrees 01'-45 degrees 08'N; 73 degrees 58'-74 degrees 21'W). Two different trait-based approaches were used to relate plant functional traits to succession on an old-field - deciduous forest chronosequence: (i) a frequently used approach based on co-occurrence of traits (emergent groups), and (ii) a new version of a direct functional approach at the trait level (the fourth-corner method). Additionally, we selected two different cut-off levels for the herb subset of the emergent group classification in order to test its robustness and ecological relevance. Clear patterns of trait associations with stand developmental stages emerged from both the emergent group and the direct approach at the trait level. However, the emergent group classification was found to hide some trait-level differences such as a shift in seed size, light requirement and plant form along the chronosequence. Contrasting results were obtained for the seven or nine group classification of the herbaceous subset, illustrating how critical is the number of groups for emergent group classification. The simultaneous use of two different trait-based approaches provided a robust and comprehensive characterization of vegetation responses in the old-field - deciduous forest chronosequence. It also underlines the different goals as well as the limitations and benefits of these two approaches. Both approaches indicated that abandoned pastures of the northern hardwood biome have good potential for natural recovery. Conversion of these lands to other functions may lead to irremediable loss of biodiversity.