Mechanisms of competition: thermal inhibition of tree seedling growth by grass.

The relative importance of thermal interference and competition for below-ground resources in the inhibition of tree seedling growth by grass was determined under field conditions. Snow gum (Eucalyptus pauciflora) seedlings were grown in bare soil or soil covered with either live grass or straw. Covering soil with straw produced thermal conditions in soil and air that were indistinguishable from those associated with live grass. In contrast, seedlings grown in bare soil experienced more rapid increase in soil temperatures during late winter and spring, less frequent and less severe frosts, and temperature maxima that more closely followed those of the atmosphere than seedlings growing in live grass or straw. After 1 year, seedlings in bare soil had four times the biomass of those grown in grass or straw. Inhibition of seedling growth by grass was attributed to alteration of the thermal environment which caused (1) seedlings to have a short growing season largely restricted to summer, (2) temporal separation in competition for resources with consumption of below-ground resources by grass in spring reducing availability of resources to support tree seedling growth in early summer, and (3) seedlings to be more subject to stress from temperature extremes. These results show that thermal interference plays a major role in interactions between plants.