DEMOGRAPHY OF 2 SHRUB SPECIES FROM AN ARID GRAZED ECOSYSTEM IN WESTERN-AUSTRALIA 1983-93

1 The demographies of two long-lived arid zone shrubs, Eremophila mait landii. and Eremophila forrestii, were characterized using data from a grazing trial (1983-93) in arid Western Australia. Two sites were use d, one dominated by annual pasture, the other by shrubs. Recruitment, mortality and size change were described from 11 annual samplings, enc ompassing conditions of prolonged drought and 1 year of unusually high rainfall. 2 The dynamics of these two species can be described by a c ombination of event-driven and continuous processes, Both recruitment and mortality were observed in all years for both species. Highest rat es of recruitment were observed during the wet year and in the two sub sequent 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 mortalit y rate of the palatable E. maitlandii was greater under high stocking. The effect of high stocking and low rainfall was additive, rather tha n excessive stocking being especially damaging in drought years, 4 Hig her mortality rates were observed in the shorter and younger stages of both species. Tall individuals had very low mortality rates, Shrubs d ecreased 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. Hi gh stocking led to higher rates of size decline during the drought. Th ere was large height variation within all cohorts and many plants pres ent at the start of the trial remained short, Taken together, these re sults suggest that caution must be exercised if size frequency distrib utions are used to infer population dynamics.