Dynamics of the ground layer communities of tropical eucalypt woodlands of northern Queensland

Abstract

This paper presents insights from long-term monitoring into the dynamics of savanna plant species composition, specifically on the ground layer. Key questions are (1) how much variation is there from year to year in plant species abundance and composition? And (2) is there evidence of directional change in community composition? Five sites were located near Mareeba, north-eastern Australia, and sampled 18 times with fixed 500-m2 plots at the annual peak of plant species richness over 23 years. The sites were located in eucalypt communities across an altitudinal range of 460 m. Sampling involved systematic searching of all plant species visible above ground. The results show variation from year to year among satellite herbaceous species, with more than 50% of grasses and forbs observed at a given site only being detected at half or fewer of the 18 samplings. Herbaceous composition did not show clear signs of directional change relative to inter-annual variability. Over the 23 years, there were three very dry periods and 4 very wet years. Rainfall variation affected total cover more than plant species composition or richness. Several fires and episodes of grazing affected ground cover but did not have a lasting impact on cover or diversity. The cover and composition of the ground layer is consistently dominated by the core perennial grasses Themeda triandra Forssk. and Heteropogon triticeus (R.Br.) Stapf, with many species collectively contributing minimally to the overall cover but significantly to richness. Many herb species are persistent long-lived perennials which do not appear above ground every year. The number of naturalised plants in the ground layer was consistently low. Overall, the ground layer communities appear to be very stable in these woodlands that have been burnt 3–5 times in 23 years. High inter-annual variability of subdominant species is consistent with other studies, and emphasises the need for replication in time in ecological sampling.