The idea of what is considered native is an important question for anyone carrying out plantings, particularly for restoration projects. Yet this isn’t always a straightforward question to answer. This chapter provides guidance on what may be considered native for an area.
Managing land often requires restoration and repair from the legacy of human impacts. Human-caused, or anthropogenic impacts range from forest clearing, to invasive species, to development, and the climate effects of burning fossil fuels. Stewardship activities to address anthropogenic stressors include the re-establishment of specific plant communities, often the reintroduction of certain native plant or animal species, and sometimes the introduction of species that are native nearby but new to the site.
Planting as part of a revegetation effort starts with the key question of what species should be planted. Hardiness, appropriate habitat, and function are among essential considerations. A harder concept to incorporate into plantings is that of nativity. The question of what is native is surprisingly complex, and the answer may even differ from one situation to another. The following are concepts in determining nativity that natural areas managers can use to guide decision-making and goal setting.
Each plant, animal, fungus, and other species has a geographic range over which it lives in the wild. Many factors have determined those ranges, and they vary dramatically in size and extent. Some species’ ranges span multiple continents while others occupy a single, limited area (endemic species). Almost all of Pennsylvania’s flora, fauna, and funga have ranges that extend beyond the state, often across a region or regions like the Northeast, Mid-Atlantic, and Midwest. However, there are many plants and animals that are limited to specific regions of Pennsylvania and are not found across the entire state even though their ranges may extend into other parts of the region or continent. Good examples are certain plants and animals of the Lake Erie region that are limited to the extreme northwest of Pennsylvania, where they are often rare, but whose ranges extend through the Great Lakes region. At a finer scale, there are local variations in the species composition of flora, fauna, and funga due to specific geology, microclimate, hydrology, or other habitat factors. Thus, a species that is abundant in one county or township or watershed may be sparse or nonexistent in an adjacent one.
Every native species has evolved within a particular set of climatic, geologic, and hydrologic conditions and boundaries along with the many other species that have evolved with them. All co-occurring species interact directly or indirectly, whether it be the close relationship between a plant and a pollinator or something less specific like deposition of leaves or other biomass that supplies food and habitat to many species. The use of native plants by land stewards takes advantage of these established relationships to restore and maintain the food web and other community and ecosystem processes.
The historical ranges of vascular plant species are mapped by the Biota of North America Program (BONAP; see Related Library Items). On the BONAP maps, native species’ historical ranges are the counties colored in green, yellow, and orange; the counties where introduced species have been found in the wild are in blue.
By its ecological definition, “native” describes a taxon, most often a species, occurring of its own accord in a given location without having been introduced by humans. One source of ambiguity in the definition comes with the word “location.” Can a plant be considered native to a site if it was historically present in the same county? The same secondary or tertiary watershed? The same ecoregion? The same state? The same contiguous group of states? When exercising the option to plant native species, there is no hard and fast rule in defining and choosing among these criteria.
Another thing left unclear by the definition is how to treat species that were planted in new places by humans before any records were kept. For most of the 15,000 years or more of human occupation in present-day Pennsylvania, it is lost to history where wild (as opposed to domestic) species such as pawpaw, black walnut, and many others were transported and planted in new locations by Native Americans. A plant’s range at the beginning of record-keeping (the arrival of Europeans) is the generally agreed-upon basis for determining specifically where it is considered native. Records of botanical exploration in southeastern Pennsylvania date back to the late 1600s and had extended statewide by the early 1800s.
Another key concept pertinent to the use of natives is genetic diversity. A species may show substantial genetic variation across its range due to evolution and adaptation to the different local climatic and soil conditions or simply due to random genetic drift as its range covers an ever-larger territory and subpopulations at the fringes become more isolated. The most pronounced variation from the norm for a species is often found at the edges of its range where conditions vary significantly from those at the center and other edges of the range. In general, the greater the genetic variation within a species across its range, the better the species’ ability is likely to be to adapt to new conditions, such as those of climate change or the introduction of new pests or diseases. Furthermore, the likelihood is high that edge-of-range subpopulations have evolved accommodations to local conditions that are absent from species members elsewhere in the range. Haphazardly planting genotypes from elsewhere risks outbreeding depression, the dilution or loss of locally adapted genetic traits. For these reasons, as a general rule plant materials should be sourced as locally as possible to a given site to help ensure that the local gene pool is maintained and not diluted or replaced.
There are two exceptions to that rule. (1) Where one or more small, disjunct populations have declined in numbers to the point where the only option for individuals is to mate with cousins or even closer relatives (this applies to plants as well as animals), inbreeding depression may result, which entails loss of genetic diversity, reproductive decline, birth defects, and eventual extirpation (local extinction) if the population is not “rescued” by the arrival of genetically diverse individuals. (2) With present-day rapid changes in climate and the increasingly frequent introduction of new pests and diseases from far away, evolution and natural migration are too slow and habitats too fragmented by human land uses for wild species to adapt on their own. Humans can help by adding individuals sourced from within a few degrees of latitude farther south (Pennsylvania’s bordering states and Virginia) or from research programs that are selectively breeding disease- or pest-resilient genotypes for introduction into wild populations.
Considerations in choosing native plant material for restoration are:
It is best to give preference to species that are common in your area and watershed. For instance, river birch may be an option for a floodplain community restoration in most of Pennsylvania, but this species is uncommon in the Ohio River Basin where natural floodplains are dominated by eastern sycamore, silver maple, and boxelder. Planting river birch would create a community that does not exist locally, thereby altering the distribution of that particular natural community and potentially impact pollinator and herbivore interactions.
For species ranked as endangered, threatened, or rare in Pennsylvania, see the Pennsylvania Natural Heritage Program’s (PNHP) resources (from the main PNHP page, click on Resources THEN Species & Natural Features THEN Search: Plants). Contact DCNR’s Plant Conservation Network to determine what specific measures should be taken for their conservation or reintroduction. Endangered, threatened, or rare species should always be sourced locally, except in some cases where genetic manipulation may be part of a conservation biology research program.
Even though there are various possible criteria for what is considered native and a candidate for planting, there are clearly species that are not native by any reasonable definition regardless of where the planting site may be in Pennsylvania, such as species that do not naturally occur in North America but instead evolved in Europe or Asia. These species should never be planted in wildlands or natural areas, even if they have not been classed as invasive. There are also species that, even though not strictly nonnative, are not good choices for planting or restoration. Except in the case of planting genotypes or species that are indigenous farther south as a climate resilience measure, avoid species that:
The Biota of North America Program (bonap.net/NAPA/Genus/Traditional/County, as of 2024)
DCNR’s Plant Conservation Network program (dcnr.pa.gov/Conservation/WildPlants/PPCN/Pages/default.aspx, as of 2024)
Pennsylvania Natural Heritage Program (naturalheritage.state.pa.us, as of 2024)