The Northern Research Station’s Forest Inventory and Analysis Program collects forest-related data throughout a 24-state region in the northeastern United States, ranging from North Dakota to Maryland and Kansas to Maine. In 2005 and 2006, we sampled for the presence and percent cover class of 25 selected non-native invasive species on all of our forested plots in the midwestern states. This paper summarizes our findings from 2 years of sampling in 7 states in this region (Indiana, Illinois, Missouri, Iowa, Minnesota, Wisconsin and Michigan).  We analyzed the data by forest type and examined the relationship between forest cover and the impact of human influences, such as forest fragmentation and distance to roads.

Results/Conclusions

Metrics of disturbance and fragmentation, such as distance to road, county percent forest or the forest intactness index, were significantly related to NNIP presence and coverage. Disturbances that initiated an invasive’s presence likely occurred several decades ago, which is why patterns of fragmentation and landscape-level forest proportions are better measures of disturbance history.

Given the history of natural and human-caused disturbance and forest types whose shade tolerance results in understory growing space that is not completely occupied, we expected to find multiple relationships between NNIP and forest and site characteristics. When looking at disturbance, we observed that multiflora rose, Japanese honeysuckle, and reed canary grass significantly benefitted from lower overstory basal areas, but this relationship did not apply to other species. Another measure of disturbance, distance to nearest road, was a significant, negative relationship with non-native bush honeysuckles and reed canary grass presence.

The percent of total land area in a county that is forest provided an indicator of historic disturbance. This metric displayed a significant negative relationship with NNIP abundance. These results are not surprising; invasive species are known to thrive on sites with more available resources. The challenge is separating the human influence from the ecological. One could easily argue that our results reflect the heavily-disturbed nature of the Midwest’s second- and third-generation forests, which either re-established following abandonment of farmland or pasture or were influenced by heavily-disturbed adjacent land. The characteristics of the landscape that influenced invasive species presence may also be a significant relationship with homestead choice by settlers. Even our disturbance measures, lower basal area, and high road density, could easily reflect the lingering influence of historic human disturbance as the microsite attributes that allowed them to thrive.