Effect of litter cover on acorn survival, seedling emergence, and early growth of introduced Quercus rubra

Non-native tree species can pose a significant threat to ecosystems, simultaneously providing important ecosystem services. For that reason recognizing patterns of their regeneration ecology is crucial for both forestry and nature conservation. Forest litter impacts plant seed survival and seedling emergence favouring or hampering alien species regeneration and spread (invasion). Here we experimentally assessed the impact of litter cover presence, origin, and thickness (two variants: 1.5 cm and 3 cm) on the North American Quercus rubra winter survival of acorns (buried in soil) and early growth of its seedlings (in central Poland), indicating the period and rate of seedling emergency and biomass of its components (after one growth season). In a one-year common garden experiment we focused on the effects of litter from Q. rubra, Pinus sylvestris, and Q. robur. We revealed that acorns buried in soil with no litter were sufficiently protected through (even snowless) winter, showing no litter presence effect on the proportion of germinated acorns. The time and rate of seedlings' emergence and early seedling growth, however, differed significantly across experimental variants. Uncovered seedlings emerged earlier than others, and the shortest period necessary for seedlings' development was found under the thin litter cover of Q. robur. Red oak litter delayed seedling emergence but did not significantly decrease its regeneration. The thick litter cover delayed emergence, regardless of its origin, and seedlings 'breaking through' thick litter invested more resources in stem elongation. Seedlings growing from acorns buried in bare soil and covered by P. sylvestris needles had higher total biomass, than those emerging under litter of broadleaved species, biomass of seedlings from plots with red oak litter was the lowest. The effective emergence and early growth of Q. rubra seedlings under different types of litter favours its regeneration while litter thickness and origin are important for Q. rubra regeneration, indicating regeneration niche and conditions for its growth.