Seasonal and interannual variation in vegetation composition : Implications for survey design and data interpretation

Understanding how vegetation composition varies with season and interannual climate variability is important for any ecological research that uses vegetation data derived from surveys for the basis of inference. Misunderstanding this variation can influence land management and planning decisions, leading to poor implementation of biodiversity offsetting mechanisms, for example. We monitored plots (400 m2) grazed by livestock paired with adjacent ungrazed plots in derived native pastures four times a year over 2.5 years on the North-West Slopes of New South Wales. Species density in plots varied greatly with season and interannual rainfall. Highest species density was recorded in spring, though species density in summer was not significantly lower, nor was a spring–summer peak in species density evident in the 2009 drought. Surveys in spring 2008 had the highest species density, and recorded only 60–72% of the total species recorded at each site over 2.5 years. Variation in the proportion of total site diversity represented in combinations of two or three surveys was large, though the best combinations comprised surveys from spring and summer in years of above-average rainfall, either from the same spring-summer, or from different years. Compositional differences among sites were much greater than within sites, showing that differences among sites related to broad environmental gradients were not overwhelmed by seasonal and interannual variability in site composition. When grazing was excluded, there was no evidence of competitive exclusion by the dominant grasses, and no directional shift in composition. The implications of these findings for ecological research depend on the question being addressed: if capturing a large proportion of site diversity is important, then surveys must be carefully timed, or repeat surveys must be conducted. Single surveys did not effectively capture site diversity for use in biodiversity offsetting, and the timing of repeat surveys was critical