Flora of North America species comparison

Rhododendron viscosum is the most widespread and variable species of sect. Pentanthera and has been divided into as many as four species: R. coryi, R. oblongifolium, R. serrulatum, and R. viscosum in the narrow sense. In addition, minor varieties and forms have been described. Because correlated or geographically coherent sets of character states cannot be discerned within this species, infraspecific taxa are not recognized here (K. A. Kron 1993). Rhododendron viscosum is most closely related to R. arborescens, and possibly also R. atlanticum. Hybrids with R. cumberlandense and R. arborescens are known. Plants with white corollas and a yellow blotch may be the result of hybridization.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Genera ca. 120, species ca. 4100 (46 genera, 212 species in the flora).

The closest relatives of the broadly defined Ericaceae are Clethraceae and Cyrillaceae. Some phylogenies show Cyrillaceae as sister to Ericaceae; other analyses have Clethraceae and Cyrillaceae as closest relatives to each other, together forming the sister group to Ericaceae. Monotropa and related genera (genera 5–12 of this treatment), and Pyrola and related genera (genera 1–4 of this treatment) have been treated as families Monotropaceae and Pyrolaceae. Not all botanists agreed with this, as summarized by G. H. M. Lawrence (1951): “Many botanists (including Hutchinson) have held the view that the Pyrolaceae are not sufficiently distinct from the Ericaceae to be treated as a separate family.” Differences in habit, floral features, and pollen have helped maintain family status for Pyrolaceae and Monotropaceae in regional floras. Molecular and morphological analyses (K. A. Kron et al. 2002) show these lineages embedded within Ericaceae. Similarly, Empetraceae has been demonstrated to be nested within Ericaceae and is here included in the Ericaceae.

P. F. Stevens (2004) recognized eight subfamilies within Ericaceae; six of these are represented in the flora area. Subfamily Enkianthoideae, basal in recent phylogenies of the family, forms a sister clade to the remaining subfamilies. The subfamily includes only the single genus Enkianthus Loureiro (12 species), native to temperate eastern Asia. Enkianthus campanulatus (Miquel) G. Nicholson is cultivated occasionally in the northeastern and northwestern United States (M. A. Dirr 1998). Subfamily Styphelioideae Sweet (subfam. Epacridoideae Arnott) of the Southern Hemisphere (especially diverse in Australia with such genera as Astroloma R. Brown, Epacris Cavanilles, and Styphelia Smith), long considered a close relative of the Ericaceae, has been demonstrated as embedded within the Ericaceae. As G. H. M. Lawrence (1951) noted, distinctions between the two families are weak.

Studies in the last several decades, especially since 1990 including molecular data, have resulted in rearrangements of generic limits in the Ericaceae. These are discussed under the various genera; for the reader’s convenience they are summarized here. Ledum is included in Rhododendron; Leiophyllum and Loiseleuria are included in Kalmia; and Hypopitys is included in Monotropa. Arctous is separated from the much larger Arctostaphylos, to which it is inferred to form a sister clade. Eubotrys is segregated from Leucothoë, with which it has often been combined. Vaccinium is treated in a broad sense, to include segregates such as Oxycoccus; although Vaccinium is decidedly polymorphic, this seems a workable approach until generic limits in the Vaccinieae Reichenbach are better understood.

Most Ericaceae are evergreen shrubs. Some species are deciduous, notably in Rhododendron and Vaccinium. The propensity of members of the family to grow in acidic soils is well known. Although the family Ericaceae is generally regarded as exclusively growing on acidic substrates, some members of the family do occur in neutral or alkaline soils in North America and elsewhere.

Ericaceae are widely distributed in the Northern Hemisphere, almost ubiquitous except in desert areas. In the tropics, especially in South America, the family is diverse in upland and montane areas, and notably diverse in such genera as Bejaria and Cavendishia Lindley. Rhododendron, with centers of diversity in the Himalayas, New Guinea, and eastern North America, and Erica, diverse in southern Africa and Europe, are the largest genera in the family. The largest genus in the flora area is Arctostaphylos, with most species endemic to California and bordering states.

Species among some genera of the family enrich the human condition with edible fruits. In North America, by far the most important of these is Vaccinium. The high-bush blueberry, V. corymbosum, is cultivated in some states, notably Michigan, New Jersey, and North Carolina, and the low-bush blueberry, V. angustifolium, in Maine, Quebec, and the Canadian Maritime Provinces. The fruits of V. macrocarpon, the cranberry, are cultivated commercially in some provinces, including British Columbia, Nova Scotia, and Quebec, and some states including Massachusetts, Oregon, Rhode Island, Washington, and Wisconsin. Vaccinium vitis-idaea, lingonberry, is collected and sold in Newfoundland and Labrador as fresh fruits and preserves, and is an important addition to diet and health in the more northern areas of Canada. The leaves of Rhododendron groenlandicum (Ledum groenlandicum), Labrador tea, are used for a beverage in parts of its transcontinental range. The foliage of some genera, notably Leucothoë, Lyonia, and Rhododendron, contains andromedotoxins and is occasionally implicated in poisonings of humans, domestic pets, and livestock (J. M. Kingsbury 1964; S. D. Mancini and J. M. Edwards 1979). Kalmia also is reportedly toxic, perhaps why it is called sheep laurel, in addition to probably being allelopathic and thus detrimental to reforestation in some situations in the eastern boreal forest. Lyonia ferruginea is a valuable “forest product” in Florida. It is harvested and the stems are used in interior decoration; once silk leaves have been added, they are marketed as “artificial” plants.

Some species of Ericaceae, both native and exotic, are cultivated and of importance to the horticultural industry (M. A. Dirr 1998). Chief among these are Kalmia and Rhododendron (including many deciduous species known as “azaleas”). Other cultivated genera include Arbutus, Elliottia, Enkianthus, Leucothoë, Menziesia, Oxydendrum, and Pieris. Rock garden plants include Arctostaphylos uva-ursi, Kalmia buxifolia, K. procumbens, and Rhododendron lapponicum, as well as species of the Old World genera Calluna and Erica. Shrubs of some genera, including Gaultheria, Gaylussacia, and Vaccinium, are prominent in the understories of deciduous and evergreen forests, especially in regions of acidic soil, such as the southeastern United States. Wetlands in much of Canada and the northern United States support dense populations of ericaceous shrubs, notably Andromeda polifolia and Chamaedaphne calyculata; Kalmia spp., Rhododendron canadense, and R. groenlandicum may be as prominent depending on the region.

(Discussion copyrighted by Flora of North America; reprinted with permission.)