High Winter Temperatures Facilitate Invasion of Tradescantia fluminensis in the Apalachicola River Floodplain

Temperature impacts organisms at a finer scale than that represented by weather stations and climate models. I investigated whether the expansion of a non-native herbaceous plant species, Tradescantia fluminensis (Small-leaf Spiderwort), up the slopes of the Apalachicola River, FL, was related to topography and/or surface temperature. These slopes comprise the remaining range of Torreya taxifolia (Florida Torreya), one of the rarest tree species in North America, and reports raise the concern that the development of dense patches of Small-leaf Spiderwort can inhibit the establishment of native plants and tree seedlings. I measured vegetative growth over 1 year and surface-temperature minima during the winter of 2016-2017 at the upland boundaries of Small-leaf Spiderwort patches. Temperatures were colder at higher elevations and greater distances from the floodplain, but did not reach the lethal temperature of -4.2 degrees C. Ninety four percent of patches survived. They expanded upslope an average of 1.2 m/y, and could reach the closest Florida Torreya trees in 2 y. Temperature minima were >1.3 degrees C warmer than those at the weather station in nearby Wilma, FL. Historical records for Wilma indicate that two-thirds of winters reach sufficiently cold temperatures to kill Small-leaf Spiderwort, and cold temperatures in January 2018 did kill a small proportion of plants. The lack of sufficiently cold temperature since 2011 might account for the recent expansion of Small-leaf Spiderwort in the Apalachicola floodplain.