Interactions between litter and water availability affect seedling emergence in four familial pairs of floodplain species

We studied seedling emergence in four familial pairs of floodplain herbs in response to the experimental manipulation of soil moisture and litter cover to analyse (i) whether the effect of litter changes from negative under humid to positive under dry conditions, and (ii) whether the response to changing water and light conditions with increasing litter cover varies among species and plant families. We carried out a controlled pot experiment using four levels of litter cover (0 g, 2 g, 4 g and 8 g litter per pot, corresponding to 0 kg m(-2), 0.2 kg m(-2), 0.4 kg m(-2) and 0.8 kg m(-2)) and two levels of water-addition, leading to constantly humid substrate or intermittently dry topsoil. Across water-additions, percentage emergence reached a peak at low levels of litter cover (0.2 kg m(-2) and 0.4 kg m(-2)). There was a significant litter x water-addition interaction in six species, with positive effects of litter under intermittently dry conditions and negative or neutral effects under constantly humid conditions. Litter lowered maximum temperature as well as amplitude, and alleviated soil humidity under low water supply, while imposing increasingly shaded conditions. Analysis of species- and family-specific responses suggested that germination under a litter cover of 0.8 kg m(-2) was significantly reduced in smaller-seeded species (i.e. those that tend to have higher light demands for germination). Our results suggest that transfer of seed-containing plant litter can aid restoration projects if applied at 0.2-0.4 kg m(-2). Below these levels, establishment of most species may be inhibited by drought, while higher amounts will increasingly suppress seedling emergence, especially of small-seeded species. In addition to facilitation effects observed between living plants, dead plant remains may also exert positive effects on establishment. The sign of the litter effect on seedling emergence depends on soil humidity, with negative effects seen above a threshold amount, which is species- and family-specific and is closely related to seed size. Whether positive litter effects in grasslands are a consequence of coevolution remains to be examined.