Seasonal Changes in the Water Relations of Eucalyptus behriana F. Muell. And E. microcarpa (Maiden) Maiden in the Field

Abstract

Field observations of some parameters of the water relations of the two eucalypt species E. behriana and E. microcarpa in dry sclerophyll, mallee and woodland vegetation were made at three sites from 1980 to 1983.

The mean ( n = 519) water potential measured at dawn (Ψ_dawn) was -3.07± 0.01 MPa and fluctuated seasonally with rainfall intensity over the range -2.0 ± 0, 1 to -4.4 ± 0.1 MPa ( n = 30). Both species behaved similarly and some osmotic adjustment took place.

Mean leaf conductance (g_s) varied between 0.151 ± 0.006 and 0.003 ± 0.001 mol m^-2 s^-1 . Maximum daily values of g_s were linearly related to Ψ_dawn as it fluctuated seasonally. The slope of this linear regression was not significantly different from that relating these values of g_s and Ψ, when both were measured concurrently. There were thus no indications of a distinction between the responses of g_s to long- and short-term fluctuations of Ψ or of a threshold-type response of g_s to Ψ.

Field measurements indicated that g_s was decreased at high values of vapour pressure difference (Δe). In laboratory studies with seedlings of the two species g_s decreased from 0.5 to 0.1 mol m^-2 s^-I as Δe increased from 0.5 to 3.0 kPa.

Leaf and canopy conductance were the predominant plant determinants of transpiration rate (E_r) in this type of vegetation which has the capacity to restrict E_t via the effect of water potential (Ψ) on g_s and also by the response of g_s to Δe. Some of the water relations parameters of E. behriana indicated that this species was better able to withstand drought than was E microcarpa.