Four Mile Run

drop it, RileyWith much schedule shuffling as a result of the recent snow disruption, we finally got out for our second field trip for winter tree ID, to a couple of spots along Four Mile Run. Proving that you don’t need a pristine natural area to learn about the world, Elizabeth started us at the dog park at Shirlington Park, a long strip of ground along the north side of the stream pinched in by the light industrial area located just to the north.

my way, birchesfast growerWe looked at natives and invasives: we keyed out an American Basswood (Tilia americana) still showing some shreds of the bracts and stalks of its fruits; we worked with a large River Birch (Betula nigra) with its scaly bark; and we found beyond the west end of the park a massive Eastern Cottonwood (Populus deltoides).

Among other trees, Elizabeth used a Black Cherry (Prunus serotina) with its “burnt cornflakes” bark for a field quiz. Her field quizzes are very effective teaching tools. We’re asked to identify a tree, by key or whatever means, to species by common and scientific name—but most importantly, we’re asked to mark three observed characteristics that led to the identification. So we’re led to a structured way of collecting information about a tree that may not yield its secrets readily. And if one of the trees is an easy ID, like an ailanthus with its brutishly heavy twigs, the rule of three keeps us looking. It might be that I go to the field guide with a known tree and then look back at it to find a bud or scar feature.

county championWe moved on a little upstream to a smallish habitat that supports a magnolia association: American Fringetree (Chionanthus virginicus), Poison Sumac (Toxicodendron vernix), and Sweetbay ( Magnolia virginiana), with Mountain Laurel (Kalmia latifolia) in an oak-heath community just up the slopes. The smooth-barked multistemmed Sweetbay in these parts retains a handful of green leaves over the winter. Farther northwest, at about mile marker 4.5 on the Four Mile Run Trail, Elizabeth showed us the Arlington County champion Green Ash (Fraxinus pennsylvanica).