Fire effects on invasive weed seed germination

Restoring historic fire regimes is often beneficial to rangeland structure and function.  However, understanding of interactions between fire and invasive weeds is limited.  We designed an experiment to determine fire effects on germination of soil surface-deposited seeds of the invasive weeds Bromus japonicus, Centaurea biebersteinii, Acroptilon repens, and Euphorbia esula across a range of common grassland fuel loads.  We hypothesized percent germination would vary by species and decrease with increasing fuel load and heat dosage.  Fuel load explained 97% of the variation in mean heat dosage (degree-seconds).  Germination of seeds not exposed to fire varied by species, with about 100% germination for B. japonicus and C. bierbersteinii, 56% for A. repens, and 46% for E. esula.  Fire reduced percent germinated seed for all species.  Exposure to fire with 100 g m^-2 fuel loads reduced germination 80-86% relative to non-burned treatment.  Increasing fuel loads to 200 g m^-2 reduced germination 98% for B. japonicus and 95% for the other species.  Percent germinated seed was similar among species and fuel loads equal to or greater than 200 g m^-2.  However, no B. japonicus seeds germinated with 500 g m^-2 or greater fuel loads and no C. bierbersteinii seeds germinated with a 700 g m^-2 fuel load.  Results indicate high potential for fire preventing invasion and expansion of four weed species through seed mortality.  Integration of other management techniques may be required in established weed stands as adult plants of some species are known to be resistant to fire.