A comparison of the water use by heath vegetation at Frankston, Victoria, and Dark Island Soak, South Australia

Abstract

Empirical water use equations for a perennial evergreen heath community, dominated by a relatively dense and uniform stand of Leptospermum myusinoides, growing on deep sand near Frankston, Victoria, are:

4-6 years after a fire: E_α/E_o = 0.0245(P + S) - 0.441,

7-9 years after a fire: E_α/E_o = 0.0222(P + S) - 0.439,

where E_α is the actual monthly evapotranspiration, E_o the monthly pan evaporation, P the monthly precipitation, and S the total soil water (extractable by oven-drying) stored in 6 metres of sand at the beginning of the month, all values being measured in centimetres. Both P and S contribute to a water store, partly unextractable, but most of it capable of being evaporated and transpired. The intercept on the abscissa indicates soil water normallv unextractable from the water store by the particular plant community -water normally unavailable to plants at values of soil water potential below - 15 bars, plus water potentially available but held in parts of the profile poorly explored by plant roots. Above this intercept the gradient indicates the rate of change of actual evapotranspiration, at any particular value of monthly pan evaporation, per unit change of extractable water (P + S).

The equations were compared with that for a heath community dominated by numerous emergent bushes of Banksia ornata at Dark Island Soak, South Australia:

10-14 years after a fire: E_α/E_o = 0.0433(P + S) - 0.101,

S being the total soil water stored in 2 metres of sand at the beginning of the month.

No runoff and little drainage were observed at either site. Months when drainage was observed were omitted from the calculations.

The gradient of the water use equation for Dark Island Soak is almost double that observed at Frankston. The presence of emergent bushes of Banksia ornata in the Dark Island heath may account for this marked difference. If so, this would explain the apparent ability of tall shrubs and small trees to invade heaths on deep sandy soil in south-eastern Victoria.

The negligible effect of fertilizer on water use by the Frankston heath is reported in an Appendix.