The role of trees in enhancing soil nutrient availability for native perennial grasses in open eucalypt woodlands of north-east Queensland

To investigate the effects of eucalypt trees on pasture in open eucalypt woodlands of north-eastern Queensland, 2 common native pasture species, Chrysopogon fallax and Heteropogon contortus, were grown in soil collected from under tree canopies and inter-canopy areas. These soils were collected from 2 localities that differed in soil fertility. The objective was to determine whether trees enhanced soil nutrient levels beneath their canopies and whether such changes affected pasture productivity and quality. It was hypothesised that tree effects would be greater when overall soil fertility was low. Shade and water stress treatments, which aimed to simulate field conditions, were also imposed to investigate their independent and interactive effects on plant growth. Chemical analyses showed that nutrient levels were higher in under-canopy soils, and plants grown in under-canopy soil produced 42% more biomass than plants in outside-canopy soil. This increase in biomass was significantly greater when the soil was from the low fertility site than from the high fertility site. Leaf quality, in terms of N and P concentration and dry matter digestibility (DMD), was generally higher in plants in under-canopy soil. Shading reduced plant root biomass but had no effects on above ground biomass or leaf quality. Water stress reduced above and below ground biomass and increased leaf quality. Shading and water stress effects were greater in plants in the more fertile soil types. Simulated 'under-canopy' plants (shaded, water-stressed plants in under-canopy soil) produced more biomass and had higher leaf N concentration and DMD than simulated 'outside-canopy' plants (unshaded, well-watered plants in outside-canopy soil). In a pastoral context, trees in tropical woodlands are generally considered in terms of their competitive effect on pasture productivity. This study has demonstrated that soil nutrient availability is enhanced under eucalypt canopies. The biomass results indicate that this effect is greater when overall soil fertility conditions are low. These enhanced soil nutrient conditions influence forage productivity and quality. Such positive benefits should be considered when management decisions are taken to remove or kill trees in eucalypt woodlands.