ORAL PRESENTER CO-AUTHORS: Molly Van Appledorn, Nathan De Jager, Kristen Bouska, and Jason Rohweder, USGS TITLE: Reed canary grass habitat suitability in floodplain forest understories of the Upper Mississippi River ABSTRACT: Reed canary grass (Phalaris arundinacea L.) is one of the most common invaders of floodplains in North America. In the Upper Mississippi River floodplain, invasion by reed canary grass in forest understories can inhibit forest regeneration when gaps in the overstory form. Knowledge of where reed canary grass is likely to occur in the forest understories could help inform management actions to limit its spread or identify areas where reed canary grass may inhibit forest regeneration following forest loss. We combined reed canary grass presence and absence observations with 10 hypothesized environmental predictors of reed canary grass invasion to create models to predict the presence of reed canary grass in forest understories using three machine learning algorithms (Bayesian additive regression trees, boosted trees, and random forest). We used the outputs from the habitat suitability models to map and summarize reed canary grass habitat suitability in floodplain forests across Navigation Pools 3-13 of the Upper Mississippi River (~41,000 ha). We found variation in reed canary grass presence in forest understories across the study region that appears to be influenced by several factors, including inundation regime, propagule pressure, and light availability. Additionally, we performed a more detailed analysis of plot-level observations and recent inundation metrics to characterize the niche of reed canary grass in forest understories that may be useful in understanding drivers and thresholds of reed canary grass invasion. The results from these analyses may be used to develop more complex models that incorporate inundation patterns and forest successional processes and for refining management and restoration strategies in the future. BIO: John Delaney is a biologist at the US Geological Survey in La Crosse, WI, studying reed canary grass invasion, climate change impacts and adaptation strategies, ecological vulnerability, and developing tools to assist managers in decision making.