Monday, November 18, 2013

A Bittersweet Story

Along the walk beside Valley Water Mill Lake, we saw the bright orange fruit of a bittersweet vine.  As Barb began the chase to determine if it was the native or the invasive oriental species, we discovered some interesting botanical sidelights.

Our native American bittersweet (Celastrus scandens) has almost been loved to death because of its bright orange berries which brighten the woods after the autumn leaves fall.  People harvest the vine for decoration, leaving less to reproduce naturally.  Bittersweet vines have either male or female flowers (called dioecious) and needs nearby male plants for the female plant to reproduce.  This complicates its reproductive ability and it is considered threatened in several states.

The berries are a favorite food for birds but poisonous to humans.  They are said to not be necessarily deadly but likely to make the prospect temporarily a desirable option until the intestinal tract settles down.  They were used by settlers and native Americans to induce vomiting and treat venereal disease (possibly making one want to avoid the source in the future?)  The bittersweet vine is stout and twines vigorously enough to strangle saplings and damage small trees.

Unfortunately, not all bittersweet is as desirable.  Celastrus orbiculatus (Oriental bittersweet) is an invasive species that is threatening not only our American variety but also the landscape it spreads in.  It was introduced as a beautiful method of roadside erosion control in the Northeastern US because it grows readily and is resistant to disease (sound like a familiar recipe for invasive spread?) and indeed it is all those things.

Oriental Bittersweet, note berries off leaf axils -
Its flowers and showy scarlet fruit arise from the leaf axils while the American species occur to the ends of the branch.  The American bittersweet capsule around the fruit is orange, the Oriental is yellow.

Oriental bittersweet vines tightly on trees with the same strangling capability as its American cousin.  Since it grows more aggressively as it climbs to find the sun it can cause more damage.  It also grows without support as a shrub, creating a dense thicket.  It naturalized, spreading across the eastern US.  It became popular in wreaths and floral arrangements which are then discarded and scavenged by birds, spreading the seeds farther.

Oriental bittersweet climbs the trees -
Although capable of growing in a wide variety of conditions, it thrives in sunlight and produces a greater amount of above ground biomass, climbing to reach the sun and blocking out less aggressive natives in their search for light.  Its ability to strangle small trees improves its access to sunlight while it also is stealing nutritional resources from the roots.

Oriental bittersweet takes over - Terrene
Oriental bittersweet vines cut at base - Terrene

Oriental bittersweet has an interesting association with mycorrhizal fungi.  C. orbiculatus is especially dependent on high levels of phosphorus.  Mycorrhizal fungi that form a mutualistic relationship with its roots provide this nutrient in phosphorus-poor soils and the bittersweet therefore can use more of its energy above ground rather than wasting it producing more root mass looking for phosphorus.  The presence of the fungi are an critical factor in the Oriental bittersweet's success.

Oriental bittersweet also hybridizes with its American cousin, outcompeting it in the landscape.  There are steps you can do to help fight the invasion.
  1. Don't plant Oriental bittersweet.
  2. If using it in decorations, destroy the berries, don't leave them in a landfill or outside  for the birds to spread.
  3. Plant only native American bittersweet from a reputable and knowledgeable dealer.
  4. Avoid harvesting American bittersweet in the wild.  It needs all the help it can get.
  5. Cut and kill Oriental bittersweet.

Oriental bittersweet base - The End- Terrene

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