Alnus serrulata ( Aiton, 1789 ) Willdenow, 1805

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 : 6-8

Alnus serrulata ( Aiton, 1789) Willdenow, 1805

Figure 3

Material. EMS 425002.

Description. Laminar size microphyll, length to width ratio 4:3. Laminar shape unlobed elliptic, with medial symmetry; base not preserved. Primary venation pinnate. Major secondary veins craspedodromous with excurrent attachment to midvein, spacing smoothly decreasing proximally. Intercostal tertiary veins opposite percurrent, convex or sinuous, perpendicular to midvein, angle consistent. Epimedial tertiary veins opposite percurrent, proximal course obtuse to midvein, distal course parallel to intercostal tertiary. Quaternary vein fabric irregular reticulate. Areolation moderately developed. Freely ending veinlets mostly unbranched, to 2- branched. Tooth spacing regular, tooth sinuses angular. Distal tooth flank apically concave and basally convex, tooth proximal flank convex. Leaf surface with simple stipitate glands and peltate scale trichomes, trichomes denser at vein junctions.

Discussion. The subfossil has several features of living Alnus serrulata (Hazel Alder) , including strong, non-branching, regularly spaced, approximately alternate secondary veins; very thin, opposite percurrent tertiary veins ( Figure 3.1 ); finely serrate leaf margins ( Figure 3.1, 3.4 ); and freely ending veinlets that are usually unbranched ( Figure 3.6 ). The subfossil leaves are similar in overall morphology to other species in the family Betulaceae , especially American Hornbeam ( Carpinus caroliniana ). However, the presence of both stipitate gland and peltate scale trichomes ( Figure 3.2, 3.3, 3.5 ) indicate that it is a member of the genus Alnus ( Hardin and Bell, 1986) , and its regular, finely serrate teeth distinguish it from other Pennsylvania members of this genus (Rhoads and Block, 2007).

Hazel Alder is native to eastern North America, ranging from Texas to northwest Maine, New Brunswick, and Quebec ( United States Department of Agriculture, 2011). It is considered to be an obligate wetland species that grows best at stream margins and in other wet areas including swamps and bogs, although it may also grow on well-drained upland soils (Rhoads and Block, 2007; United States Department of Agriculture, 2011). This species has a rapid growth rate and a high fire tolerance; adult individuals reach a maximum height of approximately nine meters ( United States Department of Agriculture, 2011).

Figures

FIGURE 3. Alnus serrulata (Hazel Alder), EMS 425002. 1, Whole specimen; 2, Peltate scale and simple trichomes; 3, Stipitate gland trichome; 4, Detail of teeth; 5, Trichomes at vein junction; 6, Areolation with simple and branching freely ending veinlets.