Flora of North America species comparison

Species 66 (62 in the flora).

Arctostaphylos is richly diverse and taxonomically challenging. Unequivocal fossils appear as far back as the middle Miocene. Many pulses of diversification and decimation may have taken place in the genus since then; evidence suggests that there has been a rapid radiation in the last 1.5 million years. Some morphological features are not clearly differentiated among taxa and appear to be mosaically distributed.

Multiple lines of evidence suggest that Arctostaphylos is a terminal branch within Arbutoideae. Arctous is treated here as a separate genus, as it is likely sister to Arctostaphylos. Only one species of Arctostaphylos, A. uva-ursi, is found outside of western North America, Mexico, and Guatemala. Taxa are concentrated within the California Floristic Province (southern Oregon to northern Baja California, Mexico) with the greatest diversity along the central California coast, where over half of the taxa are found. Along the immediate California coastline, most Arctostaphylos species are found within vegetation strongly influenced by summer fog, either within maritime chaparral, as a forest-edge species, or as part of a closed-cone conifer woodland and forest. Away from the coast, Arctostaphylos species are distributed to the desert edge in chaparral woodlands and forests.

The relationship of Arctostaphylos with poor soils correlates with a highly diverse mycorrhizal fungal community associated with these plants. Because conifers and other ectomycorrhizal trees share the ability to form mycorrhizae with a large percentage of the fungal species associated with Arctostaphylos, a long-term successional dynamic of shrub- and forest-dominated systems mediated by fire has arisen throughout the range of this genus. Habitats of Arctostaphylos species generally have nutrient-poor, usually acidic soils and relatively frequent fires. Fire has selected for different aspects of life history. Approximately one-third of the taxa have a burl—a swollen woody mass at the base of the main stem containing dormant buds (or in some species, also epicormic, forming at nodes where layering stems establish roots) from which plants can sprout after the canopy is killed by fire. The other taxa lack a burl and whole plants are killed by fire; for these species, replacement of populations is completely dependent on persistent soil seed banks.

Because of the recent divergence, reproductive barriers are not strong among some species. Introgression and species of diploid hybrid origin may be frequent. Tetraploids are hypothesized to be of cross-clade hybridization followed by polyploidization. This makes identifying both diploid and tetraploid lineages difficult.

In Arctostaphylos, one cannot overemphasize the importance of field observations for proper identification. Generally, one should carefully observe multiple individuals within a stand to determine whether individuals have burls and to determine the bark condition in older stems (described below). Other important characters include whether leaves are bifacial or isofacial, the indument patterns on young twigs, leaf shape and indument, and a number of features of the inflorescence, especially its indument, number of branches, size and shape of fruits, and whether stones within fruits are distinct or connate. Characters of the immature inflorescence (such as flowering bract size and shape and arrangement of bracts and buds) can be helpful. In post-fire habitats, plants can be readily observed sprouting from burls, but in older stands it is sometimes necessary to dig around the base of the plants to confirm the presence or absence of burls.

Older stems are most often smooth, exfoliating promptly after stem growth in the summer, to reveal reddish bark. In the foothills of the Sierra Nevada and along the California coast, some species bear bark that is usually grayish and becomes rough with persistent, flat, shredded pieces. The indument on twigs of the current year may vary widely with hairs of different lengths, types, or layers. Indument is found also on leaves, inflorescences, and fruits. Careful assessment is important; most frequently indument patterns are similar among these structures and variation from that pattern can be diagnostic.

Generally, two patterns of leaf morphology are found. In some species, leaves are bifacial; stomata are restricted to the abaxial surface. Stomata can be observed using a 10× hand lens and appear as white spots crowding the areas between leaf veinlets. When stomata are restricted abaxially, leaf surfaces typically differ in hue or indument patterns on adaxial versus abaxial surfaces. More frequently, leaves are isofacial; abaxial and adaxial surfaces are similar in hue and indument patterns, and stomata are found on both surfaces. There are exceptions to these general patterns, and it is important to examine both leaf surfaces under magnification. Arctostaphylos insularis, for example, has stomata restricted to the abaxial surface; otherwise, the leaf surfaces are similar in color and pubescence; A. pacifica has stomata on both leaf surfaces and the surfaces differ in hue.

Arctostaphylos produces a single annual growth of stems immediately after flowering. New inflorescences develop at the ends of these stems during this growth period and do not fully mature. Immature inflorescences generally remain dormant four to six months; these are critical in identifying taxa. One species (A. pringlei) initiates inflorescences immediately prior to flowering and thus flowers on current year’s stems. Inflorescences range from relatively short racemes to larger, spreading panicles. Plants that produce racemes frequently have inflorescences with a single, relatively small branch—“racemelike” structures. Usually, examination of multiple inflorescences will provide insight into whether the plant has panicle inflorescences. These structures should be examined under a hand lens for patterns of indument along the axes, bracts, and pedicels of either the flower or the fruit.

Although flower morphology varies among taxa, it is rarely important in identification. The pattern of indument on the ovary often is important. Frequently, the indument of the ovary is similar to that of mature fruit, but sometimes not; for example, Arctostaphylos canescens has densely hairy ovaries but the mature fruits are glabrescent. There are pairs of taxa for which ovary indument is the crucial distinguishing character; plant distribution can also be used in those cases.

Fruits are critical for identifying some taxa in Arctostaphylos. Fruits are usually present in a population because they are produced early in the spring, mature in the summer, and persist well into the fall and winter, often with some remaining into the next flowering season. Fruits can also be found in the litter during the latter part of the season. The fruits are a type of drupe, technically a nuculanium; the exocarp is dry at maturity, the mesocarp is dry and mealy or absent, and the fruit contains multiple hard stones. The stones are separate (distinct) in some taxa, each containing a single seed. In other taxa, stones may be connate along radial walls such that two or three stones are connate, while other stones in the same fruit remain distinct. Other species have fruits within which all radial surfaces of stones are connate, yielding a single structure, either globose or ellipsoid. With relatively few exceptions, fruits are of two general shapes: globose or nearly so, or depressed-globose and dimpled on both ends. Fruits also can be smooth and glabrous or hairy, sometimes viscid.

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

Of conservation concern.

Arctostaphylos virgata is associated mostly with Pinus muricata on the Point Reyes peninsula; populations are found at nearby sites from Bolinas Ridge, Muir Woods, and San Geronimo Ridge, Marin County.

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