Forest plant invasions in the eastern United States: evidence of invasion debt in the wildland-urban interface

Abstract Context Non-native invasive plants are a growing threat to forests. Meanwhile, the amount of forest within the wildland-urban interface (WUI) is increasing, with housing-associated disturbances enabling the spread of non-native plants. Objectives We tested whether (1) WUI or non-WUI forests are more invaded, (2) WUI intermix forests (houses mingling with forest) are more invaded than interface forests (housing abutting forest), (3) invasion in WUI forests is delayed (invasion debt) following housing development, and (4) WUI forest invasion is associated with land cover context (a proxy for disturbance and propagule pressure). Methods We conducted statistical comparisons of plant invasion metrics using WUI status information intersected with ~ 45,000 forest inventory plots (collected ca. 2015). We evaluated potential drivers of invasion using an ensemble learning approach and adopted a mixed-effects modeling framework to assess relationships between drivers and invasion. Results Our analyses revealed that the degree of invasion in WUI forest plots was significantly higher. We found evidence for invasion debt, including greater invasion of plots in the WUI longer. WUI interface forests were more invaded than intermix forests. Agricultural and developed land cover at medium to large scales (~ 66 − 5,300 hectares) were most highly associated with WUI forest invasion. Conclusions These findings elucidate dynamic plant invasion patterns and processes occurring within the WUI. They underscore the importance of monitoring and managing forests that have recently entered the WUI, with an emphasis on sources of exotic plant propagules at relatively large scales, before the housing invasion debt comes due.