The Rise of Non-Native Invasive Plants in Wooded Natural Areas in Southwestern Ohio

During the years 1834 to 1844 Thomas G. Lea conducted a botanical survey in the Cincinnati, Ohio area and built up an herbarium (Lea 1849). A century later, Dr. E. Lucy Braun conducted a follow up botanical survey in the Cincinnati area to determine how the vegetation had changed during the intervening years (Braun 1934). In her book, The Woody Plants of Ohio, Braun provided additional information about the distribution of woody plants found in this area (Braun 1961). During the past several years, we have been conducting another botanical survey in ten wooded natural areas in the Cincinnati area. Most of the marvelous natural areas surveyed by Thomas Lea are gone, victims of human development, but a few highly disturbed “natural” areas still remain. We studied ten of these areas during the current survey. These areas include undeveloped parts of Spring Grove Cemetery and Vine Street Hill Cemetery, and several Cincinnati Park properties including Avon Woods, Burnet Woods, Parker Woods, Caldwell Preserve, LaBoiteaux Woods, Mt. Storm, Winton Commons, and banks of the Mill Creek adjacent to Salway Park. Specimens collected during the current survey are being deposited in the Margaret H. Fulford Herbarium at the University of Cincinnati.

One of the most striking differences between the botanical surveys conducted by Lea and by Braun and the current survey is the tremendous increase in the number of woody non-native invasive plant species found in wooded natural areas in the Cincinnati area, and the increase in their abundance. Most of these plants are of Asian or European origin and were deliberately introduced to North America by the horticulture trade (Dirr 2011). Due to efficient seed dispersal mechanisms, primarily animal vectors or wind, these introduced plants have escaped from cultivation and have invaded natural areas where they are rapidly spreading and displacing the native vegetation. In 1961, in Ohio, Lonicera maackii (Amur honeysuckle) was only found as an adventive in Hamilton County, where Cincinnati is located (Braun 1961). Within a few years L. maackii had invaded and spread into wooded natural areas in the Dayton, Ohio area (Conover and Geiger 1993, 1999). Lonicera maacki is now one of the most abundant woody plants found in southwestern Ohio (Gorchov and Trisel 2003, Hedeen 2006, Conover and Sisson 2016, Taylor et al. 2020). In some woodlands the L. maackii layer is so dense that the only native species remaining are the older trees whose canopy is already growing above the shrub layer. Similar scenarios occurred with other introduced plant species. For instance, in 1961 Ampelopsis brevipedunculata (porcelain berry) was only reported as an escaped plant in Ohio along railroad embankments in Lake County in the northern part of the state (Braun 1961). This species is now abundant in many natural areas in southwestern Ohio. In 1985, Euonymus fortunei (winter creeper) and Hedera helix (English ivy) which are planted as ground covers in cemeteries (Conover et al. 2020), college campuses, zoos, botanical gardens, garden centers and residential properties, were just starting to become conspicuous in wooded natural areas around Cincinnati (D. Conover, pers. obsv.). At the present time both species are frequently found climbing trees in residential and in natural areas, producing abundant seeds. As a result, they have both become common in wooded natural areas, often carpeting the forest floor (Conover et al. 2016, Swearingen 2009a, 2009b) (Supplemental Material Figure S1).

Non-native, invasive woody plants that have escaped from cultivation and which we have witnessed spreading in the ten natural wooded areas mentioned in this survey are listed in Table 1. The percentages in the table indicate the number of the ten study areas where the species is found. These numbers are also correlated with the abundance of these species in those areas since the species which have invaded multiple areas tend to be abundant within those areas. The species are listed in order of their relative abundance. For instance, L. maackii is found in all of the areas and is abundant in all of those areas, while only one plant of Actinidia chinensis var. deliciosa was found in only one of the areas (Tepe and Conover 2019). These species include in relative order of decreasing frequency the woody species: L. maackii (Supplemental Material Figure S2), E. fortunei, H. helix, Euonymus alatus (winged euonymus), Morus alba (white mulberry), Pyrus calleryana (Callery pear), Ailanthus altissima (tree of heaven), Rosa multiflora (multiflora rose), Rhamnus cathartica (European buckthorn), A. brevipedunculata, Lonicera japonica (Japanese honeysuckle), Celastrus orbiculatus (Oriental bittersweet, Supplemental Material Figure S3), Rhodotypos scandens (Japanese jetbead), Philadelphus coronarius (sweet mock orange), Ligustrum vulgare (European privet), Vinca minor (lesser periwinkle) (Conover et. al 2016), Ulmus pumila (Siberian elm), Acer platanoides (Norway maple), Acer campestre (hedge maple), Wisteria sinensis (Chinese wisteria), Prunus subhirtella var. pendula (weeping Higan cherry), Hibiscus syriacus (Rose of Sharon), Acer ginnala (Amur maple), Rubus phoenicolasius (Japanese wineberry), Elaeagnus umbellata (Autumn olive), Ulmus parvifolia (Chinese elm), Phellodendron amurense (Amur cork tree), Populus alba (white poplar), Rhamnus davurica (Dahurian buckthorn, Supplemental Material Figure S4), Paulownia tomentosa (princess tree), Ligustrum japonicum (Japanese privet), Albizia julibrissin (silk tree), Pueraria montana var. lobata (kudzu-vine), Lonicera fragrantissima (winter honeysuckle), Pachysandra terminalis (Japanese Pachysandra), Eleutherococcus sieboldianus (five-leaf aralia) (Conover 2017), Callicarpa dichotoma (Asian beauty berry), Fontanesia phillyreoides (Syrian privet), and Actinidia chinensis var. deliciosa (kiwifruit) (Tepe and Conover 2019).

In addition to the woody plants mentioned above many non-native invasive herbaceous species are also spreading in wooded natural areas in southwestern Ohio. Some of these species were introduced intentionally and subsequently escaped from cultivation. Others were introduced by accident. A few of these herbaceous species are presented in Table 2. These include Ranunculus ficaria (lesser celandine), Alliaria petiolata (garlic mustard), Microstegium vimineum (Japanese stilt grass), Fallopia japonica var. japonica (Japanese knotweed), Achyranthes japonica (Japanese chaff flower), Liriope spicata (creeping liriope), Arum italicum (Italian arum), Muscari armeniacum (grape hyacinth), Scilla siberica (Siberian squill), and Perilla frutescens (beefsteak plant).

In conclusion, the spread of non-native invasive species into wooded natural areas in southwestern Ohio seriously threatens the continued survival of native flora and fauna. Efforts by natural area managers, park staff, and volunteers to control invasive plant species has become a major part of their activities. Unfortunately, it appears that this control will be required in perpetuity and will be at great expense both monetarily and timewise, and due to collateral damage to native plants, wildlife, and humans caused by the extensive use of herbicides, chainsaws, and other mechanical equipment.