Effects of leaf litter depth on the emergence and seedling growth of deciduous forest tree species in relation to seed size

Leaf litter has a major impact on soil microenvironmental conditions and so can be an important influence on seedling recruitment and hence plant community structure. We conducted a greenhouse experiment to examine the impact of leaf litter from temperate deciduous forests on the emergence and growth of tree seedlings in relation to seed size. Our treatments were bare (no litter), shallow (1-2 cm deep, 140$\text{g}\cdot \text{m}^{-2}$, and deep (ca. 5 cm deep, 420$\text{g}\cdot \text{m}^{-2}$) litter. Seed sizes ranged from 0.7-3636 mg. For Betula lenta (seed mass = 0.7 mg), emergence on bare soil was greater than 50%, approximately 12% in shallow litter, and no emergence in deep litter. Percent emergence for Liquidambar styraciflua (seed mass = 6 mg) was highest in bare soil and shallow litter treatments (64-69%) and was reduced by more than 80% in the deep litter treatment. Ailanthus altissima (seed mass = 30 mg) and Quercus velutina (seed mass = 1900 mg) exhibited no significant differences in emergence among litter depth treatments whereas, for Quercus rubra (seed mass = 3636 mg), percent emergence was over 50% greater under deep litter than under shallow litter. Seedling growth also differed in response to the litter treatments. For the two smallest-seeded species, B. lenta and L. styraciflua, seedling robustness (aboveground biomass divided by seedling height) was greatest in the bare treatment. Seedling robustness of Q. velutina decreased with increased litter depth while Q. rubra had its greatest seedling robustness in the shallow litter treatment. For A. altissima, seedling robustness and root:shoot ratio both decreased with increased litter depth. Our results suggest that the two smallest-seeded species are better adapted to colonizing post-disturbance sites where there is likely to be less litter, whereas the two largest-seeded oak species are better-suited to establishing in forests with a thicker litter layer. This is consistent with the communities in which these species typically are found. The enhanced seedling growth of A.altissima in bare-to-shallow litter conditions may be one of the attributes that accounts for the success of this highly invasive non-native species in urban forests in the New York City metropolitan area which have shallower litter than nearby rural forests.