Sunday, April 12, 2015

Purple (Not really) Bitter Cress (Cardamine douglassii)

Cardamine douglassii (04/08/15)
Durham Co., NC
Cardamine douglassii is sometimes called Limestone Bitter Cress, or Purple Bitter Cress. Neither name seems to be that appropriate (but common names rarely are),  First, it only rarely (maybe never?) grows directly on exposed limestone, and certainly never does so in NC. Second, the flowers are rarely purple, but nearly always appear pure white (but see below). For these provincial reasons, I prefer to call it "Douglas's Bitter Cress".

Douglas's Bitter Cress,
atypical "purple flower" variant
Granville Co., NC
Douglas's Bittercress is another plant species with restricted distribution in North Carolina, and which seems to be most abundant or widespread in the mid-western United States.  In North Carolina it occurs only in a few Piedmont counties, including Durham & Granville; these locations and the those in the Virginia Piedmont are significantly disjunct from the nearest populations to the west.  The habitat where I have observed the species is infrequently flooded floodplain forests. One of the sites supporting the species is shown below:

Cardamine douglassii habitat in Durham Co., North Carolina
Bottomland Hardwood Forest, lush cover of Spring Beauty (Claytonia virginica)
04/11/15, Cardamine just finished flowering

Like other Bitter Cress species, Cardamine douglassii is part of the Mustard or Brassicaceae family and produces slender tubular seed capsules, or "siliques", that are much longer than wide.

Cardamine douglassii in full fruit

Stems usually lack leaves for most of their length. Several accounts from other states indicate stems are hairy, but I have not observed that in North Carolina. Cauline leaves are usually found on the lower third of the stem, unlobed and usually toothed. Basal leaves are more heart shaped. Younger, non-flowering plants of these perennials produce only rounded leaves
Fortunately, the species seems to require little in the way of active management.  Localized threats to populations in North Carolina include construction & maintenance of water and sewer easements. installation of waterfowl impoundments, beaver flooding, and competition with invasive exotics, especially Japanese Honeysuckle. 

Wednesday, April 8, 2015

Plum Thicket Part 1, American Plum

American Plum flowering amidst Eastern Red Cedar, and open grassland
Durham Co., NC April 15, 2015

American Plum (Prunus americana) occurs in scattered localities across the North Carolina Piedmont & Mountains but is generally absent or infrequent in the coastal plain.  Wide ranging across North America, it may have been cultivated by some Native Americans (see Havard 1895 in Bulletin of Torrey Botanical Club)

Showy, white, highly aromatic flowers develop before the leaves, and before most other deciduous species have leafed out in our area.

American Plum thicket of dense sprouts
invading a frequently burned "prairie-like" habitat

Small American Plum tree

American Plum tree with drooping branches
Note the sparse vegetation underneath vs.
dense grass growth outside
American Plum is infamous for forming dense thickets in certain open habitats. These thickets are due, in part, to its propensity to root sprout.  In the example shown here, sprouts are 3-5' tall and leafed out more fully than nearby mature trees, but no flowers were produced. Most have developed 10' or more from the main trunk and appear to be "invading" the open area, with the shortest sprouts along the leading edge.

The origin of these sprouts appear to be large Prunus americana individuals with well developed, widely spreading, and drooping branches, above twisted,multi-trunked stems. These small trees (see images following) produce significant masses of flowers. Interestingly, the space around the trunk(s) is clear of sprouts and other woody plants, possibly due to the density of the canopy or the species may be allelopathic.

The following sequence illustrates progressive stem development.   

Prunus americana sprout elongating above surrounding stems
Note resemblance to brambles or rose canes (to which they are related) 

Prunus stem undergoing height & diameter growth,
developing vertical dimension, showing well developed spur shoots

Small diameter tree with splitting bark, few shoots emerging directly from trunk
American Plum with massive (for the species) triple trunk
Progression of American Plum sprouts to trees,
with small sprouts (left), small tree flowering (middle), large flowering tree (right)
Well-formed & floriferous American Plum
Durham County, April 08, 2015