Salix spp.

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 : 12-14

Salix spp.

Figure 8

Material. EMS 425022 ( Figure 8.1 ), EMS 425017- 425020, EMS 425021 ( Figure 8.2, 8.4 ), EMS 425023 ( Figure 8.3 ), EMS 425024-525027.

Description. Lamina unlobed, medially symmetrical. Apex straight, angle acute. Base convex to straight, angle acute. Primary venation pinnate. Major secondary veins semicraspedodromous, spacing and angle inconsistent, attachment to midvein excurrent. Intersecondary veins less-than-one per intercostal area, proximal course acute to midvein, distal course reticulating. Intercostal tertiary veins convex, opposite percurrent to irregular reticulate, obtuse to midvein. Epimedial tertiary veins opposite percurrent to reticulate. Areolation poorly to moderately developed, freely ending veinlets mostly one branched. Tooth spacing regular to irregular, tooth sinuses rounded. Teeth salicoid; principal veins present. Leaf surface without trichomes preserved, possibly glabrous.

Discussion. The subfossils have distinctive salicoid teeth, consisting of one round, dark, nondeciduous seta attached to the tooth apex ( Figure 8.4 ; Hickey and Wolfe, 1975), that place them confidently in Salicaceae . Only two genera of Salicaceae are native to Pennsylvania ( Salix , willows, and Populus , cottonwoods and aspens; Rhoads and Block, 2007), and these are readily distinguished because Populus leaves are generally about as wide as they are long, whereas Salix leaves are typically at least two times longer than they are wide (Rhoads and Block, 2007), as seen in the subfossils ( Figure 8.1-3 ). The specimens display both regularly and irregularly spaced teeth. The teeth also vary in orientation and size.

Willows ( Salix spp. ) are difficult to identify to the species level using leaves alone because a large amount of hybridization occurs between species, and because the typical species-level diagnostic features are not preserved here, such as leaf attachment, catkin morphology, and branching patterns ( Argus, 2006). The samples might represent a single or multiple Salix species. Salix species are widely distributed across the world and are generally affiliated with wetland conditions, but they can also grow in drier, upland environments. However, only one willow species native to Pennsylvania, the Prairie Willow ( S. humilis ), is a facul-

ELLIOTT ET AL.: PRE-COLONIAL PIEDMONT FORESTS tative-upland species ( United States Department of Agriculture, 2011). That species has variable leaf form, but it frequently has entire margins (Rhoads and Block, 2007) and a tomentose abaxial leaf surface ( Argus, 2006; Rhoads and Block, 2007), causing it to appear hairier than many other taxa within the genus. Because the subfossils appear quite different from S. humilis in these and other characteristics, we infer that the prehistoric willows were affiliated with a wetland environment.

Figures

FIGURE 8. Three Salix spp. (Willow) subfossils. 1, EMS 425022; 2, EMS 425021; 3, EMS 425023; 4, Salicoid tooth, EMS 425021.