Disturbance intensity, timing and history interact to affect pasture weed invasion

Abstract

Past disturbance events shape future community trajectories if their ecological legacies interact with contemporary disturbance regimes to affect recruitment and reproduction. In pastures, large-scale disturbance events can become undetectable in aboveground community components but persist as augmented weed seedbanks, suggesting disturbance history may be a predictor of weed invasion magnitude following subsequent disturbance. Using a randomized block factorial design, we investigated how soil disturbance intensity, timing and history interact to affect weed recruitment, diversity and reproduction by implementing a range of disturbance intensities in April and June 2004 in pastures of differing disturbance history. Response variables were followed for two years. Disturbance timing interacted with disturbance intensity and history to affect recruitment, with some June treatment combinations having ≥48% higher weed abundance than those in April. Relative to undisturbed controls, low to intermediate disturbance intensity facilitated recruitment by ≥117% but high intensity disturbance had highest recruitment, particularly in previously disturbed pastures (≥543% increase). In both years, weed species richness was highest in intense disturbances in previously disturbed pastures. Importantly, weed reproduction was nearly one order of magnitude higher in intense disturbance patches, especially in previously disturbed pastures, and increased through time by ≥243%. These findings indicate weed recruitment, diversity and reproductive output are seed- and microsite-limited, and that intense soil disturbance may result in high long-term weed abundance. Although moderate disturbance facilitated recruitment, reproduction here was low, suggesting modest disturbance will not appreciably increase weed abundance. The most common species were Taraxacum officinale and Poa pratensis, both of which can benefit forage-livestock production, but most establishing species were of low palatability. We recommend grassland managers explicitly integrate disturbance history into dynamic management planning, consider augmenting seedbanks with ‘desirable’ weed species to help communities recover following inevitable disturbance events and do not rely exclusively on aboveground characters to evaluate their system's vulnerability to undesirable plant establishment and persistence.