Effects of Casuarina pauper litter and grove soil on emergence and growth of understorey species in arid lands of South Australia

Localized effects of large persistent plants in arid lands can substantially affect the distribution of smaller plants through several mechanisms. Usually these micro-environments sustain more species and higher productivity. In arid lands of southern Australia the density of herbaceous plants inside Casuarina pauper groves is lower than in adjacent open areas. We used a series of glasshouse experiments to assess the effects of litter and different soils on the emergence and growth of two herbaceous species common in the system: Carrichtera annua, an introduced annual forb, and Danthonia caespitosa, a native grass. We also wanted to separate the physical and chemical effects of litter. Soil collected inside groves had higher contents of organic carbon and total nitrogen than soil outside groves. Soil type had no significant effect on the emergence of either species, but litter significantly reduced their emergence. The addition of artificial litter (i.e. pieces of plastic with similar physical properties) had the same effect on emergence as natural litter. Watering with litter leachates reduced the emergence of D. caespitosa, but had no effect on C. annua, suggesting a specific allelopathic effect. The results indicate both physical land chemical effects of litter. Litter had no effect on the growth of seedlings of either species, while D. caespitosa, but not C. annua, seedlings grew better in soil collected inside groves. Our results demonstrate that litter of Casuarina, and to a lesser extent soil properties, may be an important factor determining the difference in annual plant community structure inside and outside groves.