Death model for tussock perennial grasses: a rainfall threshold for survival and evidence for landscape control of death in drought
K. C. Hodgkinson A C and W. J. Muller BA CSIRO Sustainable Ecosystems, GPO Box 284, Canberra, ACT 2601, Australia.
B CSIRO Mathematical and Information Sciences, GPO Box 664, Canberra, ACT, 2601, Australia.
C Corresponding author. Email: ken.hodgkinson@csiro.au
The Rangeland Journal 27(2) 105-115 https://doi.org/10.1071/RJ05009
Submitted: 12 October 2004 Accepted: 24 August 2005 Published: 21 November 2005
Abstract
We investigated relationships between rainfall (and landscape, zonation and nearby grazing disturbance) and the death rates of four perennial grass species in a highly functional semi-arid wooded grassland in eastern Australia. Two grasses were palatable C3 species (Monachather paradoxa Steud. and Thyridolepis mitchelliana (Nees) S. T. Blake) and two were unpalatable C4 species (Aristida jerichoensis (Domin) Henr. var. subspinulifera Henr. and Eragrostis eriopoda Benth.). During the 10-year study the grasses were protected from large herbivore grazing within paddocks continuously grazed by sheep. Death occurred only during droughts and rates of death were species-dependent. When plotted against several water availability indices, rainfall and rainfall/evaporation during the preceding 3 months provided best predictions of death. Longer preceding periods gave inferior predictions. A 3-month rainfall total of 75 mm and a 3-month rainfall/evaporation ratio of 0.15 were survival critical thresholds below which deaths began. The 3-month rainfall totals, rainfall/evaporation and estimated water status of plants were equally reasonable predictors of deaths, but were inconsistent in their effectiveness. Rainfall was adopted for the grass death model; death begins when 3-month rainfall total declines below a threshold of 75 mm and the death rate rises with lower rainfall. Position of plants in the gently undulating landscapes influenced water status and, hence, death rates. Water status of grasses on the two water-shedding zones and the ‘flat’ zone were similar at each assessment, but higher on ‘ridge run-on’ and ‘toe-of-slope’ zones. Foliage height and diameter also influenced death rate but were species dependent. Basal diameter did not influence death rate. Survivorship of several perennial grass species at widely spaced sites in south-eastern Australia provided equivocal support for generality of the grass death model.
Additional keywords: arid, critical threshold, grassland, rangeland, temperate.
Acknowledgments
Thanks to John Barber, Sylvia Brunner, Anthony Clancy, Jaimie Cook, Brian Dixon, Alex Drew, Martin Driver, Rod Edmundson, Alex Gartmann, David Jensen, Ivan McManus, Steve Marsden, Roger Oxley and Jacqui Stol for assisting with grass measurements on one or more occasions. Special thanks to Brian Dixon for setting up the sampling, locations and assessment procedures at the CSIRO Lake Mere Research Facility and for other critical input into technical aspects of the project. John Ludwig and David Tongway suggested sampling be stratified on the zones they identified in their baseline survey of the Research Facility. Steve Marsden computed the rainfall and evaporation data. Many people generously allowed access to their pastoral properties and rainfall records and provided logistic support; Dick and Diedre Beshman of ‘Glenvue’, Michael and Anna Couglan of ‘Tarabah’, Peter Dowling of the SGSKP National Site at Carcoar, Martin Driver of ‘Barrabool’, Richard and Kathryne Harley of ‘Pretty View’, Jarvis and Margaret Hayes of ‘Wacilaly’, John and Robyn Ive of ‘Talaheni’, Kent and Noel Keith of ‘Ballanda Park’, Tony and Jacqui Mills of ‘Woodlands’, Brian and Kylie Rutledge of ‘Moble’, Andrew and Kathy Schmidt of ‘Wallen’, Jim and Annabelle Strahan of ‘Lake Mere’, Mike and Kylie Sutherland of ‘Genaren’, Roderick and Adriana Taylor of ‘Adgingbong’ and Allan and Christine Wilson of ‘Cal Col’. Comments by Sarah Bruce, Ron Hacker, Richard Morton, Jim Noble and two anonymous referees improved the paper. From 1989 to 1991, the Wool Research and Development Corporation (now Australian Wool Innovations) provided partial financial support for this long-term study.
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