Flora of North America species comparison

Cercocarpus breviflorus grows in the mountains of north-central Arizona, south-central New Mexico, and central and trans-Pecos Texas and in Mexico, over a 2000 km range. It has smaller flowers and shorter fruiting pedicels, hypanthial tubes, and stipules than C. betuloides. Flowering occurs after summer rains, providing reproductive isolation from C. betuloides.

In the mountains from east-central Arizona into north-central New Mexico, about 70–80% of plants of Cercocarpus breviflorus have pilose-hirsute to villous vestiture, and leaves tend to be thinner and more obovate with five to seven teeth in the rounded apical margins. In more arid montane regions of southern New Mexico, trans-Pecos Texas, and Mexico, leaves are more commonly sericeous (70%), often smaller, more coriaceous, with fewer teeth, in extreme being entire with a single terminal tooth. In strongly arid conditions, plants are smaller, more intricately branched, and thorny. These differences appear suitable for taxonomic recognition; a broad zone of intergradation occurs in southeastern Arizona and south-central New Mexico.

In the Big Bend region and Hill County of Texas and in the Chihuahuan Desert and mountains of eastern Mexico, plants of Cercocarpus breviflorus in more mesic chaparral habitats on north-facing slopes and drainages are taller with larger, sericeous, toothed leaves, often with the proximal teeth divergent. These plants blend completely into smaller-leaved plants in more arid sites on the same mountains. As in the C. betuloides complex, there are also plants with glabrous abaxial areoles.

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

Species 8 (4 in the flora).

Within Cercocarpus, nuclear DNA sequences and chloroplast DNA regions have been analyzed (B. D. Vanden Heuvel 2002) with support demonstrated for four clades: (1) C. betuloides clade, including all the varieties of C. betuloides; (2) C. ledifolius clade, including three varieties of C. ledifolius; (3) C. montanus clade, including C. breviflorus and C. montanus; and (4) a clade including C. fothergilloides and other Mexican species. The data sets do not resolve the relationships among individual species or varieties within the four clades.

Cercocarpus species are heteroblastic, with new growth occurring in favorable seasons via long shoots, but with most leaves produced on flower-bearing short shoots. All but one species, C. montanus, are evergreen. Long-shoot leaves are larger than short-shoot leaves. Short-shoot leaves are variable in size with the proximal smallest, more ovate, the later-formed larger, often more oblong-obovate. Plants of mesic sites typically have larger, thinner leaves than those of more xeric sites, often with variation present within a mountain range; this is not considered taxonomically important. The small leaves of drier sites are more coriaceous and thicker; the veins have bundle sheath extensions, and the stomata are in depressions. Variation also occurs in vestiture within most taxa, with some plants sericeous-strigose or with hairs ascending to wavy (more or less villous) or erect, other plants having thicker, coiled hairs. These vestiture types may be geographically distributed (and useful taxonomically) or mixed within a population. Short shoots may develop few to many flowers in a cluster; this can be a useful character or be modified by the environment and result in exceptions.

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