Association of State Wetland Managers - Protecting the Nation's Wetlands.

The Compleat Wetlander: Some plants grow only in wetlands—or do they?

Rooted plants need water to grow. But lots of water for long periods depletes the oxygen in the soil creating a condition called anoxia.  Wetland plants—hydrophytic plants—are able to grow and thrive in low oxygen environments.  Sounds simple.  The boundary of a wetland is the line  where hydrophytic plants stop growing.  Right?

Nature is never so simple.

Wetlands are a tough place for a plant to live.  In addition to anoxia many wetlands vary between wet and dry periods. They have temperature extremes and changing water levels.  Salinity occurs in coastal wetlands and some inland wetlands. Ph ranges widely from one type to another.  Any experienced gardener knows that these kinds of variations are challenging for plants.  It requires a lot of energy for plants to adapt to exist in wetlands.  http://kingfish.coastal.edu/biology/sgilman/778Plants.htm

Nevertheless many plants can live in wetlands.  Some are universally recognized as wetland plants—cattails or water lilies.  But there are many others.  Roughly one third of the nation’s vascular plants (those with leaves/roots/stems) have some potential for being hydrophytes. That’s over 7500 species.  http://library.fws.gov/Wetlands/TINER_WETLANDS26.pdf

Hydrophytic plants have developed special adaptations that make it possible for them to live and even thrive in low oxygen conditions for all or part of the year. http://na.fs.fed.us/spfo/pubs/n_resource/wetlands/wetlands6_vegetation.htm These special adaptations include structural changes to store or provide access to oxygen including http://kingfish.coastal.edu/biology/sgilman/778Plants.htm

Buttressed (swollen) tree trunks
Multiple trunks
Pneumatophores (“knees”)
Adventitious roots (arising from stem above ground)
Shallow roots (often exposed at the surface)
Hypertrophied lenticels (larger than normal “bumps” on the stems)
Aerenchyma (air-filled tissue) in roots and stems
Polymorphic leaves (many or unusual shapes)

In addition hydrophytic plants adjust to changing water levels with reproductive adaptations such as  germinating only during flood conditions, surviving dormant for long periods of time, floating until a dry location is encountered, or delaying/accelerating seed production based on the presence or absence of water.

Finally vascular plants that live in salt water have specialized cells that control the import and export of salt.

Individual species of plants vary in their ability to grow in wetlands based on these adaptations.  In many wetlands, plants change from the wetter interior parts of the wetland to the drier boundaries in response to the degree an individual species can deal with the presence of water.

What this means is that wetland plants can occur in zones http://www.countrysideinfo.co.uk/wetland_survey/zones.htm and whether some plants will occur in a wetland at all can vary from one part of the country to another.

For the purpose of identifying wetlands,  plants are sorted by the frequency that they are found in wetlands.  They are placed in one of the following categories:

Obligate Wetland (OBL). Occur almost always (estimated probability, >99%) under natural conditions in wetlands.

Facultative Wetland (FACW). Usually occur in wetlands (estimated probability, 67%-99%). but occasionally, found outside wetlands.

Facultative (FAC). Equally likely to occur in wetlands or non-wetlands (estimated probability, 34%-66%),

Facultative Upland (FACU) . Usually occur in non-wetlands (estimated probability 67%-99%), but occasionally found in wetlands (estimated probability, 1%-33%).

Obligate Upland (UPL). Occur in wetlands in another region of the country, but occur almost always (estimated probability >99%) under natural conditions in non-wetlands  locally. If a species does not occur in wetlands in any region, it is not on the National Plant List https://rsgis.crrel.usace.army.mil/apex/f?p=703:1:4473315732940363.

Plants are one indication of the presence of a wetland.  But wetland delineators don’t rely on plants alone.  They look at plants and soils and hydrology to identify wetlands http://www.buzzardsbay.org/wetlands-delineation.htm.

Wetland plants vary from one part of the country to the next.  Mangroves don’t grow in North Dakota.  In Alaska there are no vascular plants but there are fungi and moss. In recent years the federal agencies, states and other partners have been developing Regional Supplements to the Corps Delineation Manual:
http://www.usace.army.mil/CECW/Pages/reg_supp.aspx


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