Duckweed Arrives

 

Duckweed (Lemna minor).
Catawba College Ecological Preserve.
Salisbury, North Carolina. 

The Emerald Ash Borer (Agrilus planipennis) has hit our area hard.  The swamp forest of the Catawba College Ecological Preserve in Salisbury, North Carolina has lost almost all its White Ash (Fraxinus amercana) trees to this invasive insect.  Emerald Ash Borers are small beetles that burrow into the trunks and kill the mature trees.   This massive die off of large trees has changed the nature of the swamp.  Where once a solid canopy of leaves shaded the swamp, now sunlight floods in.  Herbs, shrubs and trees are growing, and tiny floating plants are thriving between the trunks of the dead ash trees. 

 

Swamp forest with floating plants and dead
White Ash trees (Fraxinus americana) .
Catawba College Ecological Preserve.
Salisbury, North Carolina. 

Galleries dug by Emerald Ash Borers (Argrilis planipennis)
in a dead White Ash tree.
Catawba College Ecological Preserve,
Salisbury, North Carolina. 

Dead White Ash trees and the open canopy.
Catawba College Ecological Preserve.
Salisbury, North Carolina. 

Duckweed (Lemna  minor) is one of these small plants.  It is among the smallest of flowering plants with leaves just a few millimeters long. Each plant bears a single root that grows straight down in the water.  They carry out their whole life cycle floating on the surface of still, fresh water.  Duckweed is classified in the family Araceae, a group that also contains Philodendron and Jack-in-the Pulpit (Arisaema triphyllum).  Lemna minor is found on all continents except Antarctica with the Australian population consisting of introduced plants. 

Duckweed with its roots hanging in the water.
Catawba College Ecological Preserve.
Salisbury, North Carolina. 

Male Mallard (Anas platyrhynchos) with Duckweed.
Mecklenburg County, North Carolina. 

Three Duckweed plants on my
 fingertip each with a single root.
Catawba College Ecological Preserve,
Salisbury, North Carolina.  

While Duckweed is a flowering plant, but it rarely flowers.  Plants grow and divide asexually. It is carried to new habitats as whole plants by birds, often clinging to the feet or feathers of their namesake ducks.  Ducks are probably how Duckweed was introduced to the Catawba College Ecological Preserve swamp.  

Greater Duckweed (Spirodela polyrhiza) with the
smaller Lemna minor.
Catawba College Ecological Preserve.
Salisbury, North Carolina. 

Greater Duckweed on my
fingertip showing multiple roots. 
Catawba College Ecological Preserve, 
Salisbury, North Carolina. 

Another floating plant in the swamp is Greater Duckweed (Spirodela polyrhiza).  Its leaves are larger than Lemna minor and each plant has multiple roots.  Greater Duckweed is common in the northern hemisphere worldwide but is rarer in the southern hemisphere.  

People have found many uses for Duckweed.  It is used as a fertilizer, as a high protein animal feed and in wastewater treatment.   Duckweed is also highly efficient at removing and concentrating toxic metals from water.     

Carolina Mosquito Fern (Azolla cristata) and Duckweed.
Catawba College Ecological Preserve.
Salisbury, North Carolina. 

Another small floating plant that is now common in the swamp is the fern Azolla cristata or Carolina Mosquito Fern.  These tiny ferns have fronds that are covered with fine hairs that give them a velvety appearance.  The fronds may be green but can turn red in winter. The roots of Carolina Mosquito Fern harbor a symbiotic cyanobacterium Trichomus azollae.  This photosynthetic bacterium fixes nitrogen and supplies some of the organic nitrogen to the fern.  Because of its high nitrogen content some species of Azolla are used as biofertilizers.  

Duckweed and Greater Duckweed.
Catawba College Ecological Preserve.
Salisbury, North Carolina. 

It was tragic for this swamp to lose one of its predominant trees to an invasive insect.  But my friend, Dr. Jay Bolin has studied the effect of Emerald Ash Borers on the White Ash trees.  He says “There are winners and losers in any ecological change”.  The losers are clearly the ash trees and the winners are the the tiny aquatic plants that are thriving in the newly illuminated understory of the swamp.