Native species could help slow the spread of invasive plants in disturbed forests.

Nonnative invasive species (NNIS) are spreading through mature and disturbed forests leading to concerns about species diversity and ecosystem health. Though spread rates of individual nonnative invasive plants have been estimated, the spread rates of NNIS within mature and disturbed forests have not been compared. In addition, spread rates of NNIS across different land types have not been examined. Northern Research Station scientists surveyed 23 mature forests and fifteen 15-yr-old clearcut stands from three landtypes for NNIS every 5 years from 2001-2018. The mature and clearcut stands had 7.8 m² and 34.2 m² of NNIS cover per plot, with NNIS in 31 percent and 60 percent of the plots, and a spread rate of 0.49 m² per year and 2.14 m² per year, respectively. Within the mature stands, the mesic landtypes had the highest cover, frequency, and rate of invasion. Within the clearcuts, the moderately mesic landtypes showed the highest NNIS levels initially, but over time the highest levels of NNIS were observed in xeric clearcuts. The mature forest subplots with NNIS showed no change in native species. Clearcut subplots with NNIS in drier sites had a decrease in native species, while the more mesic sites showed an increase in native species. In summary, disturbed forests have NNIS invasion rates five times those of mature forests. However, harvested mesic landtypes show an increase in native species cover as well as NNIS, making drier sites more vulnerable to invasion. These findings may inform management decisions when selecting sites to harvest.