The Distribution of Scrophulariaceae in the Holarctic With Special Reference to the Floristic Relationships Between Eastern Asia and Eastern North America

TLDR: Based on a general survey of the distribution of Scrophulariaceae in the Holarctic, eastern Asia is shown to be the richest in members of the family among five regions, and eastern Asia has the most tribes, the most genera, the best species, and the most endemic species.

Abstract: Based on a general survey of the distribution of Scrophulariaceae in the Holarctic, eastern Asia is shown to be the richest in members of the family among five regions. Western North America has 53% of the genera (16/30) common to eastern Asia, whereas eastern North America has 48% of the genera (I17/3 5) common to eastern Asia. Among 14 genera (excluding the cosmopolitan and pantropical genera) shared by North America and Eurasia, four distributional patterns are recognized: temperate western North America/Mediterranean disjunction (one genus); temperate Eurasia/North America disjunction (three genera); temperate eastern Asia/eastern North America disjunction (four genera), and continuous distribution through the Bering Strait or Aleutian Islands (six genera). Three phases of plant communication between North America and Eurasia are hypothesized. The origin and migration routes of some taxa are traced. Six genera are suggested to have migrated from eastern Asia to North America and four genera are considered to have migrated in the opposite direction, i.e., from western North America to eastern Asia through Beringia or the Bering Strait. The floristic affinity between eastern Asia and eastern North America was first noticed by Linnaeus (in a dissertation defended by J. Halenius; see Graham, 1972) as early as 1750 and has been the focus of much attention by plant geographers since Gray's (1840, 1846; reprinted in Graham, 1972) brilliant works. The subject has been extensively discussed over the past century (see Li, 1952; Graham, 1972, for literature). There have been, however, few, if any, discussions on the subject pertaining to individual families, probably due in part to a lack of floristic work on China, the major part of eastern Asia, in the past. This report is intended to be an initial effort in this direction. The area covered in this work is the Holarctic Kingdom (Takhtajan, 1969) or Melchior's (1964) Kingdom I. To facilitate an analysis of distribution and discussion, the kingdom is divided into five regions, i.e., eastern North America (east of the Rocky Mountains, corresponding basically to Takhtajan's region 4), western North America (Takhtajan's regions 5 plus 9), eastern Asia (Takhtajan's region 3 plus the eastern half of region 2, or Good's (1974) regions 3 plus 2B), western and central Asia (Takhtajan's region 8 or Irano-Turanian Region), and Europe and the Mediterranean together (including the Azores). GENERAL ASPECT The Scrophulariaceae are a rather large family, containing 18 tribes, about 200 genera, and 3,000 species. It is highly developed in the Holarctic Kingdom with 14 tribes, 1 10 genera, and over 2,200 species. The tribes Verbasceae, Paulownieae, Hemiphragmeae, Ellisiophylleae (if included in the family), and Collinsieae are endemic to the area, and the Antirrhineae, Veroniceae, Rhinantheae, and Digitaleae have most of their members here. Of the 110 genera, 72 are endemic. The family, however, also flourishes in the Southern Hemisphere, with the tribes Aptosimeae, Calceolarieae, Hemimerideae, Manuleae, and Aragoeae (Hong & Nilsson, unpubl.) entirely or mainly confined there. Table 1 shows that of the five regions mentioned above, eastern Asia has the most tribes, the most genera, the most endemic genera, the most species, and the most endemic species. It is the only subregion with endemic tribes (3). The least number of genera occur in western North America and the fewest species (including endemic ones) in eastern North America. North America, as a whole, is comparatively poor in scrophulariaceous plants, having only nine tribes (one endemic), 48 genera (16 endemic), and ca. 630 species (ca. 600 endemic). I I would like to express my sincere thanks to professors R.-C. Ching, T.-T. Yu, and Z.-Y. Wu for reading the manuscript and to Dr. A.-M. Lu for his suggestions. 2 Institute of Botany, Academia Sinica, Beijing, People's Republic of China. ANN. MISSOURI BOT. GARD. 70: 701-712. 1983. This content downloaded from 157.55.39.45 on Fri, 02 Sep 2016 05:18:03 UTC All use subject to http://about.jstor.org/terms 702 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 70 TABLE 1. Taxa in the five regions in North America and Eurasia. Number of Taxa in Eurasia Number of Taxa in North America Europe & E. North W. North Taxon E. Asia W. & C. Asia Medit. Total America America Total Tribe 11 (3) 8 (0) 8 (0) 12 (4) 8 (0) 8 (0) 9 (1) Genus 58 (22) 35 (5) 39 (7) 82 (43) 35 (8) 30 (6) 48 (16) Species 700 (550) 600 (350) 650 (450) 1,750 (1,650) 160 (100) 500 (450) 630 (600) Western North America/eastern Asia have 16 genera in common; and eastern North America/ eastern Asia have 17 genera in common. These paired regions share a greater number of taxa than any of the other paired regions. Thus, in the number of genera shared, the two regions in North America are both closer to eastern Asia than to Europe and Mediterranean together and to western and central Asia. All the large genera in North America have the great majority of their members in the western part of the continent, with few species (Penstemon, Castilleja, and Mimulus) or only a single species (Orthocarpus) extending into the eastern part. It is of interest to note that all four of these genera also occur in eastern Asia. Furthermore, 53% of the genera in western North America are in eastern Asia, whereas 48% of the genera in eastern North America are common to eastern Asia. It is probably reasonable to say that the relationships of the Scrophulariaceae between western North America and eastern Asia are closer than those between eastern North America and eastern Asia. THE DISTRIBUTIONAL PATTERNS OF THE TAXA SHARED BY NORTH AMERICA AND EURASIA North America and Eurasia share six tribes (Anthirrhineae, Gerardieae, Gratioleae, Rhinantheae, Scrophularieae, and Veroniceae) and 19 genera (Antirrhinum, Bacopa, Buchnera, Castilleja, Euphrasia, Gratiola, Lagotis, Limosella, Linaria, Linderia, Melampyrum, Mimulus, Orthocarpus, Pedicularis, Penstemon, Rhinanthus, Scrophularia, Veronica, and Veronicastrum) disjunctly distributed across the Atlantic and/or the Pacific. Since the genera Bacopa, Gratiola, Limosella, Linderia (all members of the tribe Gratioleae) and Buchnera (Buchnereae) are either cosmopolitan aquatics or pantropical, it is better to omit them for the analysis. The genus Mimulus, a widely distributed member of the tribe Gratioleae, is considered because it is best represented in the kingdom, particularly in western North America and shows a special relationship between western North America and eastern Asia. Different taxonomic ranks are involved in these disjunct distributions. At the tribal level, a number of paired genera are found disjunctly distributed, one member of each pair in North America and the counterpart in Eurasia. In the tribe Veroniceae, Veronica on one side and Synthyris and Besseya together on the other are such a case. Veronica is a large and polymorphic genus, with its center from the Pyrenees, along the Alps, north Anatolia, the Caucasus, the Himalayas, and the Pamirs to southwest China and Tienshan, where all eight sections and the majority of species exist. Only a few species (excluding cosmopolitan and weedy ones) mostly belonging to the section Veronica, extend into tropical Africa, Australia, and North America. Synthyris and Besseya together may be its counterpart in North America (Fig. 1). The relationships between Veronica and the other two genera are so close that no differences, except for habit and chromosome number, are useful for distinguishing them. Siphonostegia, Lesquereuxia, and Schwalbea in Rhinantheae provide another example. The first occurs in eastern Asia, the second in Greece and southern Turkey, and the third in eastern North America (Fig. 2). Siphonostegia differs from the other two in having pinnatifid leaves and subequal clayx lobes; Lesquereuxia is different from Schwalbea, mainly in its opposite leaves. The three are closely related (Hedge, 1978). The 14 genera common to the two continents may be grouped into four categories according to their distributional patterns: 1) Disjuncts across the Atlantic (with one part of the area in Europe and the Mediterranean region and the other in North America). Only one genus, Anthirrhinum, falls into this category. It consists of 30 species, 17 of them occurring in the western Mediterranean west of Italy; the othThis content downloaded from 157.55.39.45 on Fri, 02 Sep 2016 05:18:03 UTC All use subject to http://about.jstor.org/terms 1983] HONG-SCROPHULARIACEAE 703 * <** ---4--~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~e~_ i ~~I 60 ,i __W 0 ~ ~ ~ ~ ~ ~ ~~~ ~~~~~~~~~~~~~~~~~~~~3 .0 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ <~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~TX~~~~~~~~~~~~~~~~~~~~~~~~~~~~7J K I~~~~~~~~~-------------3-