Wood Anatomy and Relationships of Betula uber

Wood anatomy of Betula uber (Ashe) Fernald is described and compared with woods of other birches belonging to series Humiles and series Costatae. Anatomically, wood of B. uber is typical of birches in general. On the basis of pore size and frequency, fiber characteristics, axial xylem parenchyma distribution, and absence of aggregate rays, it is argued that B. uber is properly classified in series Costatae. Resolution of its relationships within series Costatae is not apparent from wood data. INTRODUCTION For the most part, birches are common and abundant trees of temperate zone forests, well known to botanists, foresters, and wood anatomists. The enigmatic Virginia Round-leaf Birch, Betula uber (Ashe) Fernald, is a notable exception. This rare tree is known only from a small population of currently ca. 20 individuals in southwestern Virginia, U.S.A. (Sharik 1982). Discovered in 1914, W.W. Ashe (1918) named and described his novel round-leaved birch as a variety of the Sweet Birch, Betula lenta L. Prior to its rediscovery in 1975 (Ogle & Mazzeo 1976), B. uber was known solely from two collections (Mazzeo 1974), despite several attempts to relocate it (e.g., Johnson 1954). Nevertheless, Fernald (1945) elevated its status to that of a distinct species which he transferred from series Costatae to series Humiles. Its rediscovery was especially dramatic since, at the time, this unusual birch was presumed to be extinct (Smithsonian Institution 1974, U.S. Department oflnterior 1975). Its continued existence in the wild is, of course, precarious. Since the relationships of taxa which are rare or in danger of becoming extinct are often imperfectly known, the need for information on their morphology and anatomy can be critical. As such data clarify relationships they contribute to the bases for rational decisions on management and preservation. Many fundamental questions about the biology of Betula uber remain unanswered (Sharik 1982), including the nature of its relationships with other birches. External bark morphology and sympatry with B. lenta and B. alleghaniensi~ Britton (B. lutea of authors) suggest relationship with series Costatae, yet leaf, floral, and seed morphology led Fernald (1945) to classify B. uber with series Humiles, a group of northern shrubby birches. Anatomically, woods of series Costatae and Humiles differ in a number of respects (Hall 1952); therefore, a study of the wood of B. u.~er was initiated as a contribution towards a better understanding of the relationships of this distinctive rare birch. 26 CASTANEA VOLUME 49 MATERIALS AND METHODS Because of the small population size of Betula uber, only one wood sample was available for study. This sample came from an individual designated as "Tree No. 2," and is vouchered by Mazzeo et al. 2850 (NA). "Tree No. 2" is no longer extant, having been uprooted by stream erosion. Macerations were prepared according to the technique of Jeffrey (Johansen 1940). Prior to sectioning, wood samples were embedded in celloidin (Wetmore 1932); after sectioning, celloidin was either dissolved out (Figure 1) or retained (Figure 2) with chloroform during dehydration. Sections were cut at 20 μm on a sliding microtome and stained with Delafield's hematoxylin and safranin 0. Reported dimensions are based on: 50 measurements of pore diameters, vessel element length, and imperforate element length; 30 measurements of vessel element end wall angle; 30 counts of perforation plate bars; and 10 counts or measurements of all other features. ANATOMICAL DESCRIPTION Wood. Growth rings are present, marked by narrow bands of latewood; the sample studied possesses eccentric growth rings. The wood is diffuse porous (Figure 1). Pore distribution is 68 percent solitary, 22 percent radial multiples, and 10 percent clusters; radial multiples consist of 2-8 cells, most frequently 2 or 3 cells; clusters consist of 2-9 cells with less than 5 cells per cluster m,ost frequent. On the average there are 42 pores per mm2 which constitute an average pore area of 0.131 mm2 per mm2 of wood surface. Pore outlines are circular to slightly angular. Vessel elements have an average wall thickness of 2. 7 μm. Pore diameters range from 28-97 μm with an average of 63 μm (measured tangentially). Perforation plates are scalariform (Figure 3); the average number of bars per plate is 20 with a range of 10-35. The minute intervascular pits (Figure 4) average 2.4 μm (vertical diameter); these pits are circular to elliptic in outline with horizontally or diagonally oriented slit-like inner apertures; these pits are arranged alternately. Vessel elements have an average length of 516 μm (ranging 255-745 μm); ligules are often present. End wall angles range from 58-85° with an average inclination of 71° from the horizontal. Neither tyloses, deposits, nor spiral thickenings were observed in vessel elements. Imperforate tracheary elements are fiber-tracheids with small, faintly bordered pits. Fibers with gelatinous walls were common in the sections studied (Figure 2). The imperforate tracheary elements have an average length of 893 μm and a range of 420-1235 μm. Rays are numerous, there being an average of 9 per mm. Rays are 45 percent uniseriate and 55 percent multiseriate (Figure 4). Uniseriate rays have an average width of 9 μm; multiseriates are 2-4 cells wide with an average measured width of 22 μm. Uniseriate rays range from 3-20 cells tall, or 48-324 μm; average height is 10 cells, or 170 μm. Multiseriate rays range from 9-34 cells tall, or 159-455 μm; average height is 21 cells, or 303 μm. Rays are essentially homocellular, consisting mostly of procumbent cells with a few square cells at the margins, or occasionally, in the body of the ray. Vessel-ray pitting is circular, alternate, and about the same size as the intervascular pits (Figure 3).