The national picture of nonnative plants in the United States according to FIA dataThis article is part of a larger document. View the larger document here.
Data collected by the U.S. Forest Service Forest Inventory and Analysis Program was assembled from each region of the United States. Occurrence, measured as the percentage of forested subplots within a county with observed nonnative invasive plant (NNIP) species, was calculated across the continental United States and Hawaii. Each region, and in some cases each state, maintains a specific watch list to constrain monitoring to only the most important NNIP species within a given area. Therefore, occurrence is based on regionally important species and is inconsistent across the United States. NNIP can be found invading forests across all of the United States. Eastern U.S. forests, however, currently exhibit high levels of NNIP occurrence. Major U.S. travel corridors and areas of considerable forest fragmentation that are often coupled with the large human population in the eastern United States can be important drivers of NNIP distributions. Travel corridors are known to play a profound role in the spread and growth of invasive plants. That fact is evident in maps of NNIP species where many major U.S. interstates are apparent. For example, the I-85 corridor from Virginia to Alabama is an area of intense invasive plant abundance. When forests are divided into smaller and smaller parcels (fragmented), the biological diversity of native animals and plants is diminished, water cycles are altered, and often nonnative invasive plants are introduced. This could help explain the high degree of plant invasions in the heavily agriculture dominated landscapes of the middle southern and middle western United States.