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Dwarf Mistletoes This file was created by scanning the printed publication. Errors identified by the software have been corrected; however, some errors may remain. CHAPTER 1 Introduction Arceuthobium (Viscaceae) is a clearly defined ent. Control of dwarf mistletoes in some areas has group of small (generally less than 20 cm high), vari­ been hampered by inadequate knowledge of the iden­ ously colored (yellow, brown, black, or red) flowering tity of the parasite involved and the relationship to its plants that are aerial parasites only on members of the host(s). Pinaceae and Cupressaceae. The dwarf mistletoes, as Arceuthobium has been classically included in the they are commonly known, are of unusual biological subfamily Viscoideae of the Loranthaceae. Van interest because they are the most evolutionarily spe­ Tieghem (1895) considered Arceuthobium so distinct cialized genus of the Viscaceae. Some of the features from related genera that he proposed its classification that make Arceuthobium unique among mistletoes as a separate family positioned phylogenetically include: between the Viscaceae and the Santalaceae. This pro­ Extreme reduction in size-Arceuthobium posal, however, has never been followed. The sub­ minutissimum with flowering shoots only several families Loranthoideae and Viscoideae are now gener­ millimeters high is among the smallest of dicotyle­ ally agreed to warrant family status (Barlow 1964, donous plants. Thorne 1992). Members of these groups differ in floral Evolution of species that regularly cause systemic morphology, floral anatomy, pollen characteristics, patterns of witches' broom formation­ embryology, and chromosome size and numbers Arceuthobium douglasii, A. pusillum, and (Calder 1983). The previously supposed similarities A. minutissimum. between the two groups are largely the result of evo­ lutionary convergence for the aerial parasitic habit and Development of a highly effective mechanism of seed dispersal by birds, rather than the consequence explosive seed dispersal (seeds of virtually all of a common phyletic origin. We, therefore, accept other aerial mistletoes are dispersed by birds). full family status for both groups. Occurrence of bicolored fruits. Engler and Krause (1935) classified Arceuthobium Stems with variant (anomalous) patterns of sec­ as a monogeneric tribe in the Viscoideae. They placed ondary growth. Dendrophthora, Phoradendron, and Korthalsella in Sessile, ring-like anthers surrounded by a sterile the tribe Phoradendreae and separated Korthalsella in central column that in Arceuthobium ameri­ subtribe Korthalsellinae and Phoradendron in sub­ canum and A. pusillum are capable of opening tribe Phoradendrinae. Regardless of its relationship and closing in response to environmental to other Viscaceae, Arceuthobium has clearly defined changes. limits, and its generic status has never been ques­ Geographical distribution that encompasses both tioned. Kuijt (1970) suggested that the genus is sepa­ the New and Old Worlds. rable into two natural groups on the basis of branch­ ing habit (verticillate versus flabellate). The dwarf mistletoes are of immense economic The New World has a greater number of species importance because they are the single-most destruc­ (34) than the Old World (8). In the Old World, two tive pathogen of commercially valuable coniferous species are recently described from China (Arceu­ timber trees in several regions of Mexico, western thobium tibetense and A. sichuanense). Another two Canada, western United States, and parts of Asia species, A. azoricum and A. juniperiprocerae, have (Bakshi and Puri 1971, Hawksworth and Shaw 1984, been segregated from the widely distributed species Zakaullah and Badshah 1977). A.oxycedri. In the New World, dwarf mistletoes are found from southeastern Alaska, northern Canada, With the increasing recognition of dwarf mistle­ and Newfoundland through most of the western toes as destructive parasites on commercially impor­ United States and Mexico to Central America (Honduras), tant forest trees, the need for additional systematic and with an extreme outlying population on the island of other biological studies on the group became appar- Hispaniola (see fig. 5.2). Introduction 1 Chapter 1 Species diversity is greatest in northwestern examined the specimens at the major herbaria in Mexico and the western United States, where 28 of the North America and Europe. 34 New World species occur. The six New World Our goal was to develop a natural and useful clas­ species outside this area are Arceuthobium bicarina­ sification of dwarf mistletoes based on a broad eclectic tum on Hispaniola; A. aureum, A. guatemalense, A. approach involving morphology, ecology, physiology, hawksworthii, and A. hondurense in Central America; biochemistry, and genetics. Other features of evolu­ and A. pusillum in southeastern Canada, the Great tionary interest included biogeography, paleobotany, Lakes region, and the northeastern United States. reproduction, and life cycles. Finally, to provide utility Twenty species occur in Mexico, and eight of these are for practicing foresters, discussions of pathological found also in the western United States. Nineteen effects on host trees and control measures were species occur in the United States, five of which also included. occur in Canada (A. americanum, A. douglasii, A. lari­ cis, A. pusillum, and A. tsugense). Arceuthobium dou­ A glossary of some of the less familiar terms and glasii has the widest distribution in the New World. special usages is included, as well as lists of scientific This mistletoe is distributed from southern British and common names of species mentioned in the text. Columbia, Canada, southward throughout most of the western United States to southern Durango, Mexico. Taxonomic History The first dwarf mistletoe described, Arceuthobium Objectives and Scope oxycedri, was originally included in Viscum (Clusius Our initial interest in Arceuthobium was to clarify 1576). It was later segregated from Viscum by the confusion surrounding the A. campylopodum Hoffman (1808) as RazoumoJskya. Arceuthobium was complex (see Hitchcock and Cronquist 1964), which first proposed as a genus by Marschall von Bieberstein centered primarily around Gill's (1935) designation of (1819), and it was generally used until the early 1900's. host-forms in this group. However, when we discov­ The Vienna Botanical Congress in 1905 conserved ered the rich dwarf mistletoe flora in Mexico Arceuthobium over Razo umoJskya. Because the (Hawksworth and Wiens 1965, 1977, 1980, 1989), we American Code emphasized strict priority, however, expanded our investigations to include a comprehen­ most botanists in the United States continued to use sive treatment of the entire genus .. We have since stud­ RazoumoJskya. Arceuthobium finally displaced ied naturally occurring populations of all 38 known RazoumoJskya as a result of the 1930 Cambridge New World taxa, as well as 4 of the 8 known Old Botanical Congress and Gill's (1935) taxonomic revi­ World species. Beginning in 1962, our field studies of sion of the species in the United States. Arceuthobium have taken us over 800,000 kilometers Humboldt and Bonpland's collection of a dwarf by foot, hoof, wheel, and wing throughout North mistletoe on Cofre de Perote (Veracruz, Mexico, 1804) America, Central America, the Caribbean, the Azores, was apparently the first in the New World. This speci­ Europe, North and East Africa, and the Himalayas. men was designated as the type for Arceuthobium The more than 3,000 dwarf mistletoe specimens vaginatum described in 1806 by Willdenow (as we have collected in our field work, plus many early Viscum vagina tum). In 1826, David Douglas (1914) U.S. Department of Agriculture collections by]. R. discovered two dwarf mistletoes on his botanical 'Weir, G. G. Hedgcock,]. S. Boyce, andL. S. Gill, are explorations of the Pacific Northwest: A. campylopodum filed at the USDA Forest Service's Mistletoe Herbarium (on Pinus ponderosa) andA. americanum (onP. con­ at Fort Collins, Colorado. They provide the basis for torta). William Hooker (1847) first discussed the tax­ our taxonomic understanding of the genus. onomy of Arceuthobium in North America north of Duplicates of our collections are deposited in various Mexico and compared Douglas' specimens and anoth­ North American herbaria, particularly those of the er by Drummond (of A. americanum) with the University of Colorado (Boulder), Missouri Botanical European A. oxycedri, but he detected no differences Garden (St. Louis), U.S. National Museum except for color variations. George Engelmann was (Washington, DC), and Instituto de Biologia of the the first to publish (in Gray 1850) a formal description Universidad Nacional Autonoma de Mexico, Escuela of a dwarf mistletoe found in the United States Nacional de Ciencias Biologias of the Instituto (A. americanum). As the taxonomic architect of the Politecnico Nacional, and Instituto Nacional de genus in North America, Engelmann (associated with Investigaciones Forestales y Agropecuarias (Mexico the Missouri Botanical Garden) named most of the City). In addition to our own collections, we have 2 Introduction Chapter 1 American species of Arceuthobium during the latter Job Kuijt (1955, 1960a, 1960b, 1963, 1964) rejected half ofthe 19th century (Gray 1850, Watson 1880). Gill's host-form concept for Arceuthobium campy­ lopodum and A. vaginatum and considered each as a Between 1910 and 1920, G. G. Hedgcock and]. R. single variable species. He also concluded, as did Gill Weir of the Division
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