Acer negundo Linnaeus, 1753

Elliott, Sara J., Grettenberger, Christen L., Donovan, Michael P., Wilf, Peter, Walter, Robert C. & Merritts, Dorothy J., 2016, Riparian and valley-margin hardwood species of pre-colonial Piedmont forests: A preliminary study of subfossil leaves from White Clay Creek, southeastern Pennsylvania, USA, Palaeontologia Electronica 13 (16), pp. 1-26 : 14-15

Acer negundo Linnaeus, 1753

Figure 9

Material. EMS 425016 ( Figure 9.1-3 )

Description. Inferred lateral leaflet, petiolulate, lamina unlobed, ovate, medially asymmetrical with basal width asymmetry. Laminar size nanophyll,

length to width ratio 2:1. Base angle obtuse, base shape convex. Apex angle acute, apex shape acuminate. Primary venation pinnate. Major secondary veins semicraspedodromous, irregularly spaced, angle decreasing proximally on one side, attachment to midvein excurrent. Intercostal tertiary veins irregular reticulate. Epimedial tertiary veins reticulate. Higher-order venation indistinct. Tooth spacing irregular with one order of teeth; principal veins present. Tooth sinuses angular to slightly rounded; both distal and proximal flanks basally convex and apically concave. Trichomes simple, restricted to midvein, secondary veins, and leaf margins, increasing in density approaching tooth bases, with no hairs directly above the tooth apex.

Discussion. As seen in the subfossil ( Figure 9 ), Acer negundo (Box Elder) is characterized by asymmetrical, irregularly toothed to shallow-lobed, petiolulate lateral leaflets and simple trichomes that occur only along the midvein, secondary veins, and at leaf margins. Juvenile Acer negundo leaflets may be mistaken for poison ivy ( Toxicodendron radicans ; United States Department of Agriculture, 2011). Leaflet morphology in both species can be highly variable, making them difficult to distinguish, and both species have marginal trichomes. However, unlike the subfossil, poison ivy leaflets that are toothed usually only have a single or few irregularly-sized teeth, and these have straight or convex, not acuminate, flanks.

Box Elder is a fast-growing, short-lived tree that reaches an average height of approximately 18 meters (United States Department of Agriculture , 2011; Overton , 1990). It has a broad geographic range throughout the eastern two-thirds of the United States and Canada, with isolated populations as far south as Guatemala ( Overton 1990). It is considered to be a facultative species with a slight affiliation to wetlands (United States Department of Agriculture, 2011). Acer negundo may grow in nearly any soil type but is frequently found in relatively flat areas near streams (Overton, 1990) .

Figures

FIGURE 9. Acer negundo (Box Elder). 1, Leaflet, showing shallow lobes and rounded sinuses, EMS 425016; 2, Trichomes along veins, EMS 425016; 3, Trichomes along tooth margin, EMS 425016, with increased density on basal side of tooth; 4, Tooth of modern A. negundo from York County, Pennsylvania (collection Y2.2 of Wilf, 1997), showing the same general trichome pattern.