Invasion of temperate grassland by a subtropical annual grass across an experimental matrix of water stress and disturbance

Abstract. An experimental matrix of water stress and disturbance was superimposed on a Lolium perenne-Trifolium repens grassland using a crossed-gradient design, and the annual subtropical grass Digitaria sanguinalis was introduced into the pasture as seeds and transplanted seedlings. Digitaria plants achieved maximum biomass at high water availability and high disturbance. Digitaria plants grown from transplanted seedlings attained greater biomass further from the conditions of high water availability and high disturbance, compared with those that had grown from seed. The biomass of the temperate species was maximized with high water availability and low to intermediate disturbance conditions. The reproductive effort of Digitaria was maximized under intermediate to high water availability and intermediate to low disturbance. Combinations of water stress and disturbance that gave rise to maximum growth of the temperate and subtropical species were consistent with those predicted by C-S-R theory. Results suggest that processes in the regenerative phase of the plant life cycle were constraining the success of Digitaria in New Zealand grassland. Addition of seed to the soil seed bank would probably be maximized in patches of grassland with high disturbance and water availability; these patches will subsequently act as foci for future invasions by Digitaria.