Edges, exotics and deer: the seed bank of a suburban secondary successional temperate deciduous forest

Questions: (1) What is the utility of the soil seed bank in passive restoration of a suburban secondary successional temperate deciduous forest? (2) What is the composition and structure of the seed bank? (3) How abundant are woody and exotic species and how are they distributed in the seed bank? (4) Are edge effects evident in seed bank composition? (5) What is the influence of a suite of abiotic and biotic variables, including deer browse, on seed bank richness and abundance? Location: A 413-ha suburban forest near Baltimore, Maryland, USA. Methods: We sampled the seed bank and vegetation of 100 permanent plots in a Liriodendron-Fagus-Acer-Quercus dominated secondary successional forest. Information on soil chemistry, canopy cover, leaf litter cover and depth, deer browse severity and distance to the nearest urban edge and path was also collected for each plot. Seedling emergence was monitored in a greenhouse over 2yr. Results: A total of 76 species emerged from the seed bank, including 16 woody species and 22 exotic species. Compositional similarity between the seed bank and vegetation was low. Woody species were under-represented and exotic species were over-represented, with exotic species found in the seed bank of 94 of 100 plots. Species composition and exotic species richness and abundance were similar between edge or path and interior plots. Leaf litter cover, vegetation cover, browse severity and variables related to soil fertility were included in top models explaining patterns in seed bank richness and abundance. Conclusions: Reliance on the soil seed bank as a passive restoration strategy will be insufficient for restoring a mature secondary successional suburban forest community. However, a suite of shade-tolerant exotic shrubs and vines that are of concern to management do not appear to use the seed bank, indicating that targeted removal of these species may be successful. Increased deer browse was associated with decreased seed bank abundance and richness, suggesting that deer may directly reduce inputs to the seed bank. If deer are impacting seed bank composition, active restoration will need to include understorey herbs in addition to overstorey species.