Demography of two shrub species from an arid grazed ecosystem in Western Australia 1983-93

1 The demographics of two long-lived arid zone shrubs, Eremophila maitlandii. and Eremophila forrestii, were characterized using data from a grazing trial (1983-93) in arid Western Australia. Two sites were used, one dominated by annual pasture, the other by shrubs. Recruitment, mortality and size change were described from 11 annual samplings, encompassing conditions of prolonged drought and 1 year of unusually high rainfall. 2 The dynamics of these two species can be described by a combination of event-driven and continuous processes. Both recruitment and mortality were observed in all years for both species. Highest rates of recruitment were observed during the wet year and in the two subsequent years. Highest rates of mortality were observed in the 2 years following the year of lowest rainfall. 3 Stocking rate had no effect on the mortality rate of the unpalatable E. forrestii but the mortality rate of the palatable E. maitlandii was greater under high stocking. The effect of high stocking and low rainfall was additive, rather than excessive stocking being especially damaging in drought years. 4 Higher mortality rates were observed in the shorter and younger stages of both species. Tall individuals had very low mortality rates. Shrubs decreased in height in the year(s) before death. 5 Individuals of both species increased in height during the wet period and decreased during the drought. Net change in height over the trial period was small. High stocking led to higher rates of size decline during the drought. There was large height variation within all cohorts and many plants present at the start of the trial remained short. Taken together, these results suggest that caution must be exercised if size frequency distributions are used to infer population dynamics.