Forest Science, Vol. 34, No. I, pp. 139-151. Copyright 1988 by the Society of American Foresters Hybridization of the California Firs William B. Critchfield Abstract. Four groups of firs (sections, in the most recent classification of Abies) are represented in California. Crossing within these sections is possible and even easy, and in two of the sections intergrading populations between highly crossable taxa are wide spread in California. An exception is A. amabilis, a Northwestern fir that has not been crossed with other species in the same section {Grandes: A. concolor, A. grandis) or in other sections (e.g., Nobiles: A magnified). Crossing species in different sections is usually difficult or impossible. The genetic isolation of A. bracteata, an endemic species classified as a monotypic subgenus or section, may be nearly complete: two probable hybrids with A. concolor died at a few years of age. A few putative hybrids from inter- sectional crosses between species in Grandes and Nobiles died within months of germi nation. Intersectional crosses with firs outside California (two Mexican and four Eur asian species) all failed except A. concolor x A. religiosa, which produced numerous healthy hybrids. The common occurrence of genetic barriers in Abies is at odds with the long-held view that it is easy to hybridize fir species. For. Sci. 34(1): 139-151. Additional key words. Abies, interspecific hybrids, crossability, classification. The ability of species to hybridize has not been explored as systemati cally in the genus Abies (true firs) as it has in other genera of Pinaceae such as Pinus and Pice a. A flurry of interest in Abies hybridization more than two decades ago (Klaehn and Winieski 1962, Mergen et al. 1964, Rohmeder 1961) had two results: it generated long lists of ostensibly successful hy brids, including many between Eastern and Western Hemisphere species, and it reinforced the impression that "there is little difficulty in hybridizing fir species" (Mergen et al. 1964). These papers provided little data on species crossability, however, and nearly all of the hybrids claimed as suc cesses were unverified (Hawley and DeHayes 1985a). Mergen et al. (1964), for example, counted 18 species combinations as successful new crosses (four between Eurasian and North American species, one with both North American parents, and the rest with both Eurasian parents), but none of the putatively hybrid seedlings were compared with their parent species. The first full account of crossing behavior in a taxonomic group of firs was that of Hawley and DeHayes (1985a). Three North American species constitute section Balsameae in the classification of Liu (1971): subalpine fir (A. lasiocarpa [Hook.] Nutt.), balsam fir (A. balsamea [L.] Mill.), and Fraser fir (A. fraseri [Pursh] Poir.) Hawley and DeHayes found that these taxa were moderately to fully crossable with each other, but most crosses with white fir (A. concolor Hildebr.), a North American species in another section, either failed or produced small numbers of hybrids. This paper summarizes the results of a program of exploratory hybridiza tion in Abies carried out between 1962 and 1974 at the Forest Service's Institute of Forest Genetics (IFG), located in the Sierra Nevada foothills The author is geneticist at the USDA Forest Service Pacific Southwest Forest and Range Experiment Station, Box 245, Berkeley CA 94701. B. Burr, L. Y. Hsin, D. I. Saybold, and J. B. St. Clair studied and described fir hybrids while they were summer interns at the Institute of Forest Genetics, Placerville, CA. R. T. Bingham, J. F. Franklin, E. L. Little, Jr., and R. J. Steinhoff generously provided pollen of native firs from the Pacific Northwest, Japan, and Mexico. Manuscript received July 21, 1987. March 1988/ 139 Reprinted from the Forest Science, Vol. 34, No. 1, March 1988 near Placerville, California. Six firs native to California were one or both parents of all crosses. Most crosses were made on red fir (A. magnifica A. Murr.) or white fir in natural stands. Pollen parents included, in addition to the California firs, six species native to Mexico, Spain, or eastern Asia. Among the crosses made at IFG were the first attempts to hybridize the unique bristlecone fir (A. bracteata Poiteau), a California endemic. Our data corroborate the recent findings of Hawley and DeHayes (1985a) and Kor- mutak (1981) that genetically controlled interspecific barriers expressed during sexual reproduction are present in Abies, but the data are still too limited to establish whether these barriers are as important in this genus as they are in other genera of Pinaceae. DISTRIBUTION AND CLASSIFICATION Two of the six firs native to California are present only as small outliers in the Klamath Mountains of northern California (Griffin and Critchfield 1976): subalpine fir, which is wide-ranging in western North America outside Cali fornia, and Pacific silver fir (A. amabilis Forbes) of the Pacific Northwest. Two others, white fir and grand fir (A. grandis [D. Don] Lindl.) constitute a species-complex that is widely distributed in the western United States and well represented in California. Grand fir, a primarily Northwestern species, extends south along the California coast to near the mouth of the Russian River. White fir is distributed from California to the southern Rocky Moun tains. California white fir, sometimes designated A. concolor var. lowiana (Gord.) Lemm. to distinguish it from Rocky Mountain white fir (var. con- color), has its principal distribution at middle elevations in the Sierra Ne vada. A geographic variant of uncertain status is present in the mountains of southern California. In the mountains of northwestern California (and northeast to Idaho) are populations that intergrade between grand and white firs. In eastern California, a few western outliers of Rocky Mountain white fir grow in isolated mountain ranges of the Mojave Desert. Red fir is also part of a species-complex. In its typical form it is restricted to upper eleva tions in the Sierra Nevada, and at the lower edge of its elevational range it overlaps white fir. In northern and northwestern California, red fir inter- grades with noble fir (A. procera Rehd.) of the Pacific Northwest, and these variable populations are sometimes called Shasta red fir (A. magnifica var. shastensis Lemm.). Bristlecone fir has a limited range in the coastal moun tains of central California, where it grows at lower elevations than most other firs. The diversity of the six firs native to California is recognized in all three recent monographs of Abies (Franco 1950, Gaussen 1964, Liu 1971). They are placed in 4 of 12 groups by Franco and in 4 of 15 groups by both Gaussen and Liu (Table 1). Bristlecone fir is the only species in one of two subgenera in the classifications of Franco and Liu, and the only species in one of five unnamed sections in Gaussen's less formal classification. Subal pine fir and its eastern relatives are a separate group in all three classifica tions. The red fir-noble fir complex is also grouped separately by Liu and Franco, but Gaussen surprisingly separates red fir from a group comprising Shasta and noble firs (Table 1). Pacific silver fir is grouped with white and grand firs in all classifications. The 12 species used in the crosses at IFG provide a fairly good sample of Abies. Among Franco's (1950) 12 groups, 9 are represented, and among the 15 groups of Gaussen (1964) and Liu (1971), 9 and 10 are represented. Each 140/ Forest Science TABLE 1. Recent classifications of Abies, including species names mentioned in the text" Franco 1950 Liu 1971 Gaussen 1964 Subgen. Pseudotorreya (NA) Subgen. Pseudotorreya (NA) Sect. I + bracteata Sect. Bracteatae group 1 (NA): balsamea, Subgen. Sapinus + bracteata fraseri, + lasiocarpa Sect. Nobiles (NA) Subgen. Abies group 2 (A): sibirica, other + magnified, procera Sect. Nobiles (NA) group 3 (A): koreana, Sect. Oiamel (NA) + magnifica, procera sachalinensis, veitchii, religiosa, vejari, other Sect. Oyamel (NA) other Sect. Balsameae (NA) religiosa, other group 4 (A): homolepis, Ser. Grandes Sect. Vejarianae (NA) other + amabilis, vejari group 5 (A): firma + concolor, Sect. Grandes (NA) Sect. II (E) + grandis, other + amabilis, + concolor, group 1: cilicica, pinsapo, Ser. Lasiocarpae + grandis, other other balsamea, fraseri, Sect. Balsameae (NA) group 2: alba, + lasiocarpa balsamea, fraseri, cephalonica, Sect. Pichta (A): koreana, + lasiocarpa nordmanniana, other sachalinensis, sibirica, Sect. Pichta (A): sibirica Sect. Ill (A) veitchii, other Sect. Elate (A): koreana, group 3: forrestii, other Sect. Momi (A) veitchii, sachalinensis, group 4: mariesii, other Ser. Firmae: firma other Two other groups Ser. Homolepides: Sect. Momi (A): firma Sect. IV (NA) homolepis, marie sii, Sect. Homolepides (A) group 1: + amabilis, other homolepis, marie sii, + concolor, +grandis, Ser. Sinenses:/orr^5//7, other + magnifica, other other Sect. Elateopsis (A) group 2: religiosa, vejari, Sect. Peuce (E) forrestii, other other Ser. Albae: alba, Sect. Abies (E): alba, group 3: procera, cephalonica, cephalonica, + shastensis nordmanniana, other nordmanniana, other Sect. V (NA): + bracteata Ser. Pinsapones: Sect. Piceaster (E) cilicica, pinsapo, cilicica, pinsapo, other other Two other sections (A) One other section (A) a NA = North America, E = Europe, A = Asia, + = native to California classification is based on a series of hypotheses about relationships of the firs, and our crosses provide a preliminary test of some of these hypotheses. REPORTS OF NATURAL AND ARTIFICIAL HYBRIDIZATION Several species-complexes in Abies include intergrading populations that are usually interpreted as having arisen through secondary intergradation: hybridization and introgression of formerly allopatric taxa. According to the alternative explanation—primary intergradation—the same taxa have always been in contact and are as distinct now as they have ever been. Most evidence supports the hypothesis of secondary intergradation for the white fir-grand fir complex (Hamrick and Libby 1972, Zavarin et al. 1977) and for the intermediate populations of subalpine and balsam firs in Alberta (re viewed in Critchfield 1984b).