Abstract
Andropogon gayanus Kunth. is a large African tussock grass invading Australia’s tropical savannas. Invasion results in more intense fires which increases the mortality rate of adult woody plants. Invasion may also affect community structure by altering the recruitment potential of woody plants. We investigated the effects of A. gayanus invasion on ground-level microclimate, and the carbon assimilation potential and recruitment potential of two Eucalyptus species. We compared microclimatic variables from the early wet-season and into the mid-dry season to coincide with the period of growth of A. gayanus. We assessed Eucalyptus recruitment by monitoring seedling establishment, growth and survival of experimentally sown seed, and estimating seedling density resulting from natural recruitment. A. gayanus invasion was associated with increased grass canopy height, biomass and cover. Following invasion, the understorey microclimate had significantly reduced levels of photon flux density, increased air temperatures and vapour pressure deficit. The conditions were less favourable for woody seedling with aboveground biomass of seedlings reduced by 26% in invaded plots. We estimated that invasion reduced daily carbon assimilation of woody seedlings by ~30% and reduced survivorship of Eucalyptus seedlings. Therefore, A. gayanus invasion reduces recruitment potential, contributing to the transformation of savanna to a grassland ecosystem.
Highlights
Invasive alien grasses threaten the structure and function of many of the world’s tropical savanna ecosystems[1,2,3,4]
The invasion by A. gayanus resulted in marked change in the structure of the savanna ground layer, particular by the mid-wet season
By February, grass cover was significantly higher in the invaded plots (30%) compared to the native grass plots (15%) and this continued throughout the wet season and into the mid-dry season (Table 1)
Summary
Invasive alien grasses threaten the structure and function of many of the world’s tropical savanna ecosystems[1,2,3,4]. A near complete replacement of the native understorey species occurs, resulting in significantly increased fuel loads and fires that are up to five times more intense than native grass fires[8,16] This can lead to a reduction in the cover and density of the overstorey trees[12,17]. Ground-level microclimate has a strong influence on seedling recruitment and changes caused by exotic species have affected the germination, growth and survival of woody seedlings in a range of ecosystems[20,25,26,27,28]. The aim of this study was to investigate whether A. gayanus invasion affects: (1) ground-level microclimate; and (2) the establishment, growth and survival of woody seedlings. If changes in microclimate are detected, we would expect to detect a reduction in recruitment, growth rate, and survival of Eucalyptus seedlings in invaded and native grass dominated plots
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