Functional diversity of ground-layer plant communities in old-growth and managed northern hardwood forests

Questions: Do ecological sorting processes and functional diversity of forest ground-layer plant communities vary among mature (65-85-yr-old) even-aged, managed uneven-aged and old-growth forest stands? How does functional diversity relate to environmental variables within stands? Location: Northern temperate deciduous forests of Wisconsin and the Upper Peninsula of Michigan, USA. Methods: Ground-layer species cover and light availability were measured at each of four old-growth, even-aged second-growth, and managed uneven-aged stands (n = 12 stands total). We used mixed-effect models and fourth-corner analysis to assess relationships among forest structure, species traits and the three components of functional diversity (functional richness, evenness, divergence) based on 32 leaf, reproductive and whole plant traits from 111 species. Results: We identified differences in leaf phenology and morphology, life form and dispersal among stand types at the community level. Ground-layer plant communities of even-aged and uneven-aged stands were at opposite ends of a spectrum of strategies aimed at tolerating stressful vs competitive environments, respectively. In even-aged stands, communities were characterized by species adapted to relatively dark and closed conditions (heavy-seeded tree saplings, spring ephemerals). In contrast, managed uneven-aged stands were characterized by species with potential for quick returns on investment of nutrients and dry mass in leaves (i.e. early summer species with high specific leaf area, low leaf dry matter content and high phosphorus concentration). Old-growth stands had fewer trait associations than managed stands, and were characterized by ferns and species with either ballistic or wind-assisted seed dispersal. Functional diversity metrics were related in complex ways to light, management and soil texture. Managed stands had higher functional richness and divergence than old-growth stands, which, instead, showed higher functional evenness. Conclusions: Even-aged and managed stands support ground-layer species with a distinct set of traits relative to those found in old-growth forests. Although there is broad interest in uneven-aged management as a means to restore the structures and functions of old-growth forests, uneven-aged management does not, at least initially, produce ground-layer plant communities more similar to old-growth forests than even-aged management. We elucidated a set of life-history traits providing a synthetic view of vegetation response to forest management. Functional diversity of ground-layer plant communities was related to differences in environmental conditions related to forest structure, suggesting that habitat filtering contributes to compositional differences at the community level. We discussed these differences with reference to plants' general strategies and carbon economy