Flora of North America species comparison

Crataegus castlegarensis occurs from around the northern Okanagan, British Columbia, to the northern California Coast Ranges, to northwestern Montana, and the Rocky Mountains to northeast of Salt Lake City, Utah. The species occurs also in the Cypress Hills of Alberta and Saskatchewan; it is found in habitats similar to those of C. douglasii and is at least as abundant as that species in a number of parts of its range.

Crataegus castlegarensis is readily recognized by a combination of hairy inflorescence branches, pomes more or less orbicular, crimson or burgundy (with irregular earlier ripening) around the third week of August, soon becoming purple, often when nearby C. douglasii is already black, as well as a tendency to possess thorns on the young twigs branched at the base to become double, triple, or even sometimes quadruple. Such multiple thorns, though sometimes abundant on a bush, are more often few and may require searching for. Inflorescence pubescence, as in other species with this characteristic, may become sparse by fruiting. Crataegus castlegarensis is most similar to C. douglasii; its fruit is usually more orbicular, even oblately so, than is normal in that species, sometimes even with a recessed junction to the pedicel like an apple.

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

Crataegus rivulopugnensis, known only from the Cypress Hills, is widely distributed in the western part. The species is most similar to C. purpurella, though the color of the ripe fruit in the two species is quite different (red versus deep purple). They differ also in an almost allopatric distribution, habit when open-grown, and leaf size and numbers of veins per side (4 or 5 versus 3 or 4). Both species have less hairy adaxial leaf surfaces than most other sympatric hawthorns, often being glabrate adaxially at maturity.

INTERSERIAL HYBRIDS AND SPECIES OF HYBRID ORIGIN

Numerous putative interserial hybrids among North American hawthorns have been given binomials (J. B. Phipps 2005). Nearly all the named putative hybrids have proven to be rare or even apparently extinct. All of the 16 treated here satisfy at least two of the following criteria: there is a high degree of confidence that they are of interserial hybrid origin; they do not fit into any accepted series; they continue to be found long after their initial description, indicating either apomictic perpetuation, repeated creation, or both; or, they have appeared regularly as species in the literature, notably in works by E. J. Palmer. It is notable that most remain uncommon, suggesting that these are not particularly fit biotypes. Nevertheless, persisting populations of some (for example, Crataegus persimilis) may be readily found. The fifteenth example (C. turnerorum) is a well-collected, obviously persistent, local species in Texas.

The putative interserial hybrid, Crataegus florifera, however, is treated in the main text (in this case under ser. Anomalae) because it fits the serial description adequately and has significant and continuous ranges. Other examples included in the main text are some putative ser. Punctatae × ser. Crus-galli crosses that will be found associated with C. collina. The best studied interserial hybrids are two without names (though this will change with a forthcoming paper by the Dickinson group), both hybrids of the introduced C. monogyna. Here they are treated in the main text with their native parental species (C. punctata and C. suksdorfii). Neither of these hybrids is considered especially rare, and both appear to be increasing in frequency but so far there is no evidence of a fixed hybrid establishing.

Hybrids among North American native species have probably arisen multiple times (J. B. Phipps 2005); however, there was little evidence for the creation of successful new biotypes until some recent papers by the Dickinson group demonstrating modern derivatives of ancient alloploid of ser. Crataegus (E. Y. Y. Lo et al. 2009) and perhaps more recent alloploid derivatives from ser. Douglasianae (T. A. Dickinson, pers. comm.). Hybridization among more closely related apomicts may be more frequent than previously thought (Dickinson et al. 2008) but, with more cryptic characteristics, may more easily escape detection.

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