Is direct seeding a biologically viable strategy for restoring forest ecosystems? evidences from a meta-analysis

Abstract

Ecological restoration has become an important technique for mitigating the human impacts on natural vegetation. Planting seedlings is the most common approach to regain lost forest cover. However, these activities require a large economic investment. Direct seeding is considered a cheaper and easier alternative technique, in which tree seeds are introduced directly on the site rather than transplanting seedlings from nurseries. To evaluate the effectiveness of direct seeding, we conducted a comprehensive search of the literature using ‘restoration’, ‘direct seeding’ and ‘sowing’ as keywords, and we performed a meta-analysis using 30 papers and 89 species. We used two different measures of restoration success: seed germination probability and success probability (the chance that a seed germinates and survives until the end of the experiment). In general, restoration attempts using direct-seeding techniques were relatively unsuccessful. On average, seed germination and success prob-ability were 0·239 and 0·114, respectively, and were not affected by climate, species successional group or the application of pre-germinative treatments. Germination and success probability increased with seed size, and the use of physical protections resulted in a nearly twofold in-crease in germination probability, but this effect faded by the end of the experiments. Because of the low rate of seedling success, we suggest the use of direct seeding as a complementary technique to reduce restoration costs, particularly for species with large seeds and known high germination rates, but our results do not support direct seeding as a substitute for seedling planting.