Downy brome (cheatgrass) is a highly successful, exotic, winter annual invader in semi-arid western North America, forming near-monocultures across many landscapes. A frequent but poorly understood phenomenon in these heavily invaded areas is periodic ‘die-off’ or complete stand failure. The fungal pathogen Pyrenophora semeniperda is abundant in cheatgrass seed banks and causes high mortality. To determine whether this pathogen could be responsible for stand failure, we quantified late spring seed banks in die-off areas and adjacent cheatgrass stands at nine sites. Seed bank analysis showed that this pathogen was not a die-off causal agent at those sites. We determined that seed bank sampling and litter data could be used to estimate time since die-off. Seed bank patterns in our recent die-offs indicated that the die-off causal agent does not significantly impact seeds in the persistent seed bank.

Arrowleaf sida (Sida rhombifolia) is a tropical weed that grows in disturbed areas. Its control by hand pulling and mowing are only partially effective because arrowleaf sida is difficult to pull and quickly sprouts after cutting. We studied the seed-bank dynamics of S. rhombifolia in the region of Campinas, São Paulo State, Brazil for 2 years, following a known number of seeds placed artificially at five burial depths. Predators and pathogens were responsible for the loss of c. 40% of the seeds during the first year of the experiment. The importance of predation declined with depth. Mortality due to fungal attack did not differ among the burial depths, except for seeds placed at the surface (where it was negligible). Seedling emergence only occurred during the first year, after the beginning of the rainy period and when the study area was still free of a dense vegetation cover. Seedlings did not emerge from depths greater than 5 cm. We conclude that seedling emergence and death are significant causes of seed losses from the seed bank of S. rhombifolia but their relative importance is highly dependent on depth of burial.