Reconstructing plant invasions using historical aerial imagery and pollen core analysis: Typha in the Laurentian Great Lakes

Aim Determining the spatial-temporal spread of an invasive plant is vital for understanding long-term impacts. However, invasions have rarely been directly documented given the resources required and the need for substantial foresight. One method widely used is historical photography interpretation, but this can be hard to verify. We attempt to improve this method by linking historical aerial photos to a paleobotanical analysis of pollen cores. Location Laurentian Great Lakes coastal wetlands, United States of America. Methods We chose invasive cattail ( Typha) as our model species because it is identifiable from aerial imagery and has persistent, identifiable pollen, and its ecological impacts appear to be time-dependent. We used Geographic Information Systems, aerial photo-interpretation and field verification to post-dict the invasion history of Typha in several wetland ecosystems. Using 210 Pb and 137 Cs sediment dating and pollen classification, we correlated the temporal dominance of Typha to our estimates of per cent coverage at one site. The pollen record was then used to estimate the Typha invasion dynamics for dates earlier than those for which aerial photos were available. Results Typha spread through time in all study wetlands. Typha pollen dominance increased through time corresponding with increased spatial dominance. Hybrid cattail, T. x glauca increased in pollen abundance relative to T. angustifolia pollen through time. Main conclusions This study illustrates the value of generating historical invasion maps with publically available aerial imagery and linking these maps with paleobotanical data to study recent (< 100 years) invasions. We determined rates of Typha expansion in two coastal wetland types, validated our mapping methods and modelled the relationship between pollen abundance and wetland coverage, enhancing the temporal precision and breadth of analyses. Our methodology should be replicable with similar invasive plant species. The combination of pollen records and historical photography promises to be a valuable additional tool for determining invasion dynamics.