Drivers of invasion by laurel wilt of redbay and sassafras in the southeastern US

Abstract

Context: Timely responses to mitigate economic and environmental impacts from invading species are facilitated by knowledge of the speed and drivers of invasions. Objective: Quantify changes in invasion patterns through time and factors that governed time-to-invasion by laurel wilt, one of the most damaging, non-native disturbance agents invading forests of the United States. Methods: We analyzed county-level occurrence data (2004–2021) for laurel wilt across the southeastern United States. A Cox proportional hazards modeling framework was used to elucidate drivers of invasion. Results: As of 2021, laurel wilt had been detected in 275 counties and made 72 discrete jumps (averaging 164 km ± 16 SE) into counties that did not share a border with a previously invaded county. Spread decelerated from 40 km/yr to 24 km/yr after 5 years, with a marked decline in the number of counties invaded in 2021 (16) compared with 2020 (33). The Cox proportional hazards model indicated that proxies for anthropogenic movement and habitat invasibility increased invasion risk. Conclusion: The recent decline in number of counties invaded could be due to disruptions to travel and/or surveys from the coronavirus pandemic, but exhaustion of the most suitable habitat, such as counties in the southeastern US with warm annual temperatures and high densities of host trees, could have also contributed to this trend. This work suggests that without a shift in spread driven by additional insect vectors, that rates of range expansion by laurel wilt might have peaked in 2020 and could continue decelerating.