Evaluation of the ecological restoration potential of plant communities in Norway spruce plantations using a life-trait based approach.
In Europe, intensively managed coniferous plantations rarely achieve similar nature conservation functions as deciduous woodlands. The ability to identify coniferous plantations that might be successfully converted to deciduous woodland is a key goal in forest management. The herbaceous plant community composition of mature plantations may be an accurate selection criterion for stands that are most suitable for initiating the conversion process to deciduous forest. The herbaceous plant communities in Norway spruce (Picea abies) plantations were analysed using a functional group approach. All investigated forest stands were situated throughout the Grand-duché de Luxembourg, and neighbouring areas. The analysis was conducted in three steps: (i) identification of emergent groups (EG) by multivariate classification; (ii) comparison of EG abundance between coniferous and deciduous stands (i.e. the target communities); and (iii) partitioning the variation in EG abundance in coniferous plantations between forest management, local environmental and regional variables. Seven EG were identified: two core forest groups (short geophytes and zoochorous perennials), two groups from open habitats (annuals and halophytes) and three mixed groups (anemochorous perennials, graminoids and short perennials). Among the core forest groups, short geophytes were severely under-represented in plantations (because of their low dispersal abilities and specific habitat requirements), while zoochorous perennials were slightly more abundant (because of their good dispersal and competitive abilities). Regional variables largely influenced the abundance of core-forest EG. Restoration of communities rich in zoochorous perennials is far easier than restoring short geophyte-rich communities because of the different colonization abilities of these EG. Forest management variables were of secondary importance for the restoration potential of plantations. Because generalist EG were favoured by low stand densities, forest practitioners should avoid large thinning operations. Local environmental variables played a minor role in determining EG abundance. However, short geophytes preferred rather high soil pH values and were therefore negatively affected by acidification caused by coniferous litter. The performance of short geophytes is the key to conversion success of Norway spruce plantations to deciduous forest. Stands located on base-rich soils and in landscapes with high forest connectivity are the most appropriate candidates for initiating the conversion process. For other types of stands, future work should explore the possibility of restoring other land use types, such as annually mown meadows.