Biological invasion poses a serious threat to native diversity. Second only to habitat destruction, invasion is the cause for species extinction, especially for rare and endangered native species. Located in the Root River watershed, Renak Polak Woods is a mature sugar maple/beech forest remnant at risk of invasion and deserving of attention. Within its almost 60 acres, its landscape is rich with diverse habitats and canopy patches of varying regeneration stages. In the understory, an aggressive non-native species known as garlic mustard (Alliaria petiolata) is already invading the forest floor. Garlic mustard is an obligate biennial plant with a phenology typical of cool-season European plants. It invades plant communities in 34 states, and 4 Canadian provinces. In the United States, the highest densities are found in New England and the Midwest. Because of its early growing season, ephemeral species that are competing with this plant in Renak-Polak Woods include Geranium, Hepatica, Isopyrum, Arisaema triphyllum , Phlox, Smilacina racemosa, Claytonia, Dentaria, Trillium grandifolium (of special concern), Erythronium, and Asarum canadense.

Renak-Polak Woods is one of the few beech woodlands left that is not invaded by the many non-native species found in other nearby areas. In other areas, such as Chiwaukee Prairie, Lulu Lake, or Richard Bong State Recreation Area, the plant communities are already invaded by many non-native species, and now require intense management. Renak-Polak Woods is still a high quality plant community, with just the beginnings of an invasion problem, from only garlic mustard, in its rich understory. Because garlic mustard spreads so quickly, this site deserves intense and immediate attention in order to minimize bigger problems later. In addition, this site does not experience the beech blight, a potentially devastating disease caused by a wooly scale insect that feeds on the sap of beech trees. In Michigan, for example, maple-beech forests are disappearing due to this disease.