**AWARDS ISSUED PRIOR TO JANUARY 20, 2025, WERE FUNDED UNDER PREVIOUS ADMINISTRATIONS AND MAY NOT REFLECT THE PRIORITIES AND POLICIES OF THE CURRENT ADMINISTRATION.** NOXIOUS WEED INVASIONS REPRESENT A MAJOR THREAT TO THE PRODUCTIVITY OF RANGELANDS; IN THE U.S., INVASIVE PLANTS OCCUPY OVER 51 MILLION HECTARES OF RANGELAND AND ARE ESTIMATED TO CAUSE $5 BILLION IN ANNUAL LOSSES. INVASION BY SOME NOXIOUS WEEDS CAN REDUCE RANGELAND FORAGE PRODUCTION BY UP TO 90%. LIVESTOCK PRODUCTION PROVIDES SIGNIFICANT ECONOMIC SECURITY, PARTICULARLY FOR RURAL COMMUNITIES, WHICH ARE HEAVILY IMPACTED BY LOSS OF RANGELAND CAPACITY. LOSSES IN LIVESTOCK PRODUCTION ALSO ARE PASSED ON TO CONSUMERS AND CAUSE SUPERMARKET PRICES TO INCREASE. IN ORDER TO BETTER MANAGE INVASIVE SPECIES, WE NEED TO UNDERSTAND WHAT FACTORS PROMOTE INVASION, SO THAT THOSE FACTORS CAN BE TARGETED FOR DEVELOPING MEANS OF CONTROL.WE FOCUS ON TWO KEY FACTORS THAT ARE LIKELY TO IMPACT PLANT INVASIONS IN CALIFORNIA RANGELANDS: RAINFALL AND MICROBES. MOISTURE AVAILABILITY AFFECTS PLANT COMPETITION AND MAY INFLUENCE THE CAPABILITY OF AN INVASIVE SPECIES TO SPREAD AND DOMINATE. ALL PLANT SPECIES HAVE MICROBIOMES, MICROBES THAT LIVE ON AND IN PLANT TISSUE THAT ARE IMPORTANT DETERMINANTS OF PLANT HEALTH. INVASIVE SPECIES MAY PROMOTE BENEFICIAL MICROBES TO ENHANCE THEIR OWN RAPID GROWTH OR MAY USE PATHOGENIC MICROBES TO HARM NATIVE PLANTS. FURTHERMORE, RAINFALL AND MICROBES MAY BE INTERRELATED, FOR EXAMPLE, DROUGHT YEARS MAY NEGATIVELY AFFECT NATIVE PLANTS BUT INVADERS MAY HAVE MICROBIAL PARTNERS THAT HELP THEM RESIST DROUGHT STRESS. HERE, WE FOCUS ON HOW RAINFALL AND MICROBES AFFECT INVASION BY THE NOXIOUS WEED MEDUSAHEAD, A GRASS THAT IS NOT PALATABLE TO CATTLE AND THAT IS SPREADING RAPIDLY ACROSS CALIFORNIA AND THE WESTERN U.S.WE USE FIELD SURVEYS AND FIELD EXPERIMENTS MANIPULATING RAINFALL AND MICROBIAL COMMUNITIES TO ASSESS HOW MEDUSAHEAD RESPONDS TO DROUGHT AND HOW IT USES MICROBES TO ENHANCE INVASION. WE WILL GENERATE A LARGE DATABASE OF MICROBIAL ASSOCIATES OF MEDUSAHEAD AND DESIRABLE FORAGE GRASSES, AND WE WILL CREATE A MICROBIAL CULTURE COLLECTION IN WHICH WE WILL ISOLATE AND GROW MICROBIAL ASSOCIATES TO FURTHER STUDY THEIR FUNCTION. THIS WORK WILL IDENTIFY THE RAINFALL CONDITIONS THAT PROMOTE MEDUSAHEAD INVASION WHICH WILL BE USEFUL FOR PREDICTING WHICH YEARS ARE BEST FOR CONTROL (I.E. YEARS WHEN THE INVASIVE SPECIES DOES POORLY). WE WILL ALSO IDENTIFY THE ROLE MICROBES PLAY IN INVASION, WHICH HAS IMPLICATIONS FOR RESTORATION ACTIVITIES: IF THE INVADER ACCUMULATES SOIL PATHOGENS, RESTORATION WILL BE DIFFICULT BECAUSE NATIVE PLANTS WILL NOT GROW WELL IN THAT SOIL LEGACY. WE WILL ALSO BE ABLE TO IDENTIFY WHETHER ANY PATHOGENS AFFECT THE INVADER, WHICH COULD LEAD TO POTENTIAL BIOCONTROL AGENTS. OVERALL, BASIC UNDERSTANDING OF THE IMPACT OF RAINFALL AND MICROBES IN NOXIOUS WEED INVASION IS A FIRST STEP TOWARD MANAGEMENT STRATEGIES THAT PROMOTE HEALTHY RANGELAND AND FOOD PRODUCTION IN THE U.S.