Grass and oak litter exert different effects on seedling emergence of herbaceous perennials from grasslands and woodlands

摘要

1. The effects of litter on seedling emergence (e.g. during succession from grassland to woodland) may not only depend on litter amount and environmental conditions but may also be related to litter types. We tested the effect of litter types, litter cover and soil moisture on the emergence of four congeneric pairs of grassland and woodland species. 2. We carried out a controlled pot experiment (Experiment I) using litter and species from two habitats (grassland vs. woodland), three levels of litter cover (low, medium, high) and two levels of water-addition (leading to constantly humid or intermittently dry substrate). Amounts of litter were adjusted to result in the same relative light reduction for both litter types. Consequently, we applied 2 g, 4 g and 8 g of grassland litter and 0.5 g, 1 g and 2 g of woodland (oak) litter per pot. 3. To separate the shade effect of litter from its mechanical plus chemical effects we exposed additional pots with seeds of the same species to deep shade conditions (Experiment II) corresponding to 4 g and 1 g of grass and oak litter, respectively. 4. Under intermittently dry conditions both litter types increased seedling emergence. In addition, we found a significant interaction between litter type and species origin: seedling emergence of woodland species was significantly lower from beneath grass litter than from beneath oak litter, whereas grassland species emerged equally well from beneath both litter types. Compared with seed germination under a shade cloth, seedling emergence of woodland species from beneath grass litter was reduced by 44%. Litter significantly affected soil humidity and the amplitude of diurnal temperature fluctuations. Differential effects of litter types on woodland and grassland species are probably related to litter structure. 5. Synthesis. Our experimental data present evidence that ecosystem specific litter effects slow down succession from grassland to woodland and that the effect size is controlled by the litter amount present and the environmental conditions.