Savannah woodland vegetation in the South-East District of South Australia: the influence of evaporative aerodynamics on the foliage structure of theunderstorey invaded by introduced annuals

Evaporative aerodynamics determine the foliage projective cover of the unde rstorey of perennial tussock grasses and associated perennial herbs in the savannah woodland dominated by Eucalyptus camaldulensis on gleyed podsolic soils in the Mediterranean climate of the South-East District of South Aust ralia. By the mid 1940s, winter-spring evapotranspiration from the 'thin' l eaves (with low leaf specific weight) of introduced annual plants was deple ting surface soil water and thus reducing the annual growth of the summer-g rowing savannah understorey; perennial herbs between the tussock grasses we re the first to succumb to this competition. During spring, the percentage of the ground covered by the savannah understorey was increased by 10% in t he subhumid zone to 30% in the humid zone as the pre-European perennial her bs between the tussock grasses were replaced by introduced annuals. Applica tion of phosphatic fertilizer to the understorey increased the growth of in troduced annuals, which formed a dense stratum during their winter-spring g rowing season, increasing evapotranspiration and leading eventually to the extinction of the native perennial grasses. When the savannah understorey, invaded by introduced annuals in the mid-1940s, was converted to improved p asture, the percentage of ground covered by the seasonal foliage was increa sed by 20-30%; 100% coverage of overlapping foliage resulted in the humid z one.