How is climate change affecting native plants in Maryland?
Whether in natural areas or in our gardens, climate change is affecting native plants. According to the Maryland Climate Summary, our temperatures are expected to increase 5⁰ F to 11⁰ F by 2100.
- Higher temperatures cause native plants to experience more heat-related stress. Heat stress causes higher water demand, a situation made worse by longer droughts.
- Higher atmospheric carbon dioxide (CO2) levels preferentially promote the growth of invasive plant species, decreasing the space needed to support natural areas.
- Elongated growing seasons cause earlier leaf out and bloom times, which in turn affects the animal species synchronized to the life cycles of native plants, especially pollinators.
Growing conditions are on the move
Graph from NOAAs Maryland Climate Summary
Each native plant species has a natural range. Within that natural range, there are specific habitats that contain the ideal combination of growing conditions for that species. With climate change, Maryland's precipitation and temperature conditions are changing.
These maps show how much the 30-year average temperature has shifted, expressed in terms of the familiar USDA Plant Hardiness Zones. Accordingly, the geographic range over which a native species' original growing conditions occur is moving. Native plant species will either adapt to new conditions, migrate to more favorable environments, or go extinct.
Over geologic time, native plant species have had to adapt and migrate as a result of climate change caused by glacial epochs. Can native plants adapt or relocate quickly enough to keep up with modern climate change?
Plant adaptation in response to climate change
Some species will evolve in response to the changing climate, allowing them to maintain or even expand their natural ranges. Native species that still thrive in your region, for example, have adapted to all the climate change that has occurred so far.
In cities, when the sun beats down on all the dark surfaces (rooftops, pavement), it creates heat. As a result, cities are much hotter than their surrounding suburbs. This makes cities good places to find native plant species that have the adaptive genetic diversity needed to cope with the big temperature increases to come. City native plants have not only adapted to all the climate change that has occurred so far, but they have also done so in just decades. Common milkweed, like the one shown thriving in Washington D.C. in the photo at top, is an example of a species that is doing well in urban areas. Herbarium studies indicate that common milkweed is even expanding its natural range southward despite climate change.
Plant migration in response to climate change
As the climate warms, the temperature conditions with which a species co-evolved will move north. But plants can't just get up and migrate the way some animals can. Plants migrate through seed dispersal. If seeds dispersed to the north find suitable growing conditions, good seedling survival will result in a successful migration. For northward migration to work, there must be large, contiguous blocks of natural area.
Plant species in Maryland's mountainous regions will find more moderate summer temperatures at higher elevations, but species already located at the top of slope have nowhere to go. Species adapted to mountaintop conditions are more likely to experience local extinctions as a result of climate change.
Plants in tidal habitats must also cope with sea level rise. As of 2018, around the Chesapeake Bay, sea level is rising at a rate of 3/4 of an inch to 1 inch every 5 years. Additionally, tidal environments are being pounded by more intense storms. Tidal plants that can disperse seeds upslope may migrate successfully. However, areas immediately above the tidal zone are often developed and therefore unsuitable for seed germination and growth.
Plant species that are adapted to very specific conditions, as many imperiled plants are, are surrounded by unsuitable growing conditions, so they will often be unable to migrate.
Where resources allow, plant ecologists, work to relocate endangered plant species to places projected to have the appropriate growing conditions in the future. This process is called assisted migration, and it is only undertaken after careful study of all potential hazards. For example, relocated species could carry pathogens or insects that could interact in the new environment in unpredictable ways, they could hybridize with related species, or they could become invasive.
Maximizing the garden performance of native plants in a changing climate
- For most species, it works best to plant native plants in growing conditions similar to those found in the habitats they evolved in. Drier or sunnier conditions will exacerbate heat stress caused by rising temperatures.
- Select species that are, for your location, in the mid to northern portions of their range. For most species that are at the southern end of their natural range, your garden is becoming increasingly inhospitable. The natural ranges of native plants are indicated by the light green counties at the Biota of North America Program website. (below under resources)
- Purchase herbaceous native plants sourced from local plant populations. Locally native plants that are still thriving in natural areas near you have adapted well to current levels of climate change.
- If you live in Western Maryland, also take elevation into consideration.
- When local commercial sources are not available, purchase plants sourced from populations in the same or similar ecoregions of Virginia.
- Plants from further south than Virginia are not better. Plants from too far south will perform poorly due to different winter conditions and daylength patterns.
- Avoid purchasing plants sourced from populations to your north.
- Trees live for decades, sometimes centuries. Successful tree selection must be based on projections of future climate conditions. Read about planting trees in our changing climate.
- Do not import southern species north of their natural range unless you are doing so as part of a large, well-researched conservation program. There can be adverse consequences for both the species itself and for local ecosystems.
Sourcing a native tree from Maryland now means that it won't be adapted to temperatures here in 2100. If you assume we begin to curb climate emissions now (left), the black arrows indicate where you should source native trees from. If we do not change our emissions behavior, the red arrows (right) indicate where you should source native trees from.
How to help save native plants
- Support the conservation of large contiguous blocks of habitat that are needed for native plants and animals to migrate in response to warming temperatures.
- Avoid destruction of natural areas on your property. Destruction of natural areas not only eliminates native plants, it results in release of carbon from plants and soils back to the atmosphere.
- Preserve as much adaptive genetic diversity in wild native plant populations as possible, this is the diversity upon which species must draw to co-evolve with the modern world's rapidly changing conditions. Generally speaking, this means that if you are using native plants, use locally native plants, as described in the section above.
- Native plants cannot migrate through natural areas that are infested with invasive plants, so help to prevent their establishment and spread.