Towards a mortality function based on hydraulic regulation of Eucalyptus globulus and E. smithii

Background/Question/Methods

Rising concentrations of atmospheric greenhouse gases are affecting the earth’s climate. For the worlds Eucalyptus plantations future changes will include increases in mean temperatures and the frequency and severity of drought. Considerable research has been undertaken into plant responses to drought and temperature during the last 50 years. Despite this, understanding of the role that these two variables play in tree mortality remains the subject of intense debate.  McDowell et al.(2008; New Phytologist 178, 719-739) proposed a simple conceptual model that linked the principle mechanisms of climate driven tree mortality, hydraulic failure and carbon starvation, to the intensity and duration of drought.

The E. globulus plantations of SW Australia provide an opportunity for examining hydraulic failure of a mechanism for drought mortality. Eucalyptus globulus occurs naturally in Tasmania, the Bass Strait islands and in southern Victoria (Kirkpatrick, 1974: Bot J Linn Soc Lond 69:89-104.) where rainfall generally exceeds 800 mm per annum and is distributed uniformly throughout the year; days where air saturation deficits exceed 3 kPa are rare. This contrasts strongly with the Mediterranean environments in which this species has been extensively established in commercial plantations. Using measurements in E. globulus plantations over more than 20 years and recent measurements in E. smithii (an alternative species for drought prone sites) we have characterised the relationship between whole tree conductivity and water stress for a full range of water status from well watered to dead.

Results/Conclusions

Three main stages of water stress and hydraulic regulation were observed.  We refer to these stages as (1) the normal operating range, (2) the water stressed range and (3) the severely water stressed range. We recognise that these stages are part of a continuum of responses from well-watered to critically water stressed. The main advance in this analysis is the capacity to define measurable thresholds or transition points between each of these water stress phases. Using these thresholds we have developed a mortality function in the process based model CABALA (Battaglia at al. 2004: Forest Ecology and Management 193:251-282) that predicts mortality as a function of the intensity, duration and rate of progression of water stress. We will present the functional basis for thresholds associated with increased risk of mortality, compare the responses of E. globulus and E. smithii, provide evidence from the field of hydraulic failure as a mechanism for mortality in E. globulus and present some scenarios using mortality functions in CABALA.