Flora of North America species comparison

Species 4 (4 in the flora).

Robinia often colonizes disturbed settings in temperate deciduous and conifer forests, woodlands, and shrubby vegetation (D. Isely and F. J. Peabody 1984; M. Lavin and M. Sousa S. 1995). Robinia pseudoacacia is planted as a landscape and timber tree far outside its natural range, from which it has naturalized extensively; the remaining shrubby species are also cultivated as ornamentals, though less commonly and with less tendency to naturalize (Isely 1998)

Robinia is distinguished from other legume genera in the flora area by its fruits that are tardily dehiscent and not very elastically (or explosively) so. Seeds are transversely arranged; the hilum faces toward the base of the fruit rather than toward the placental margin. The placental (upper) margin is usually not winged except narrowly so in R. pseudoacacia. Each seed is borne from an elongate funiculus. The Robinia raceme also differs from that of its closest relatives in originating from leaf axils of the current season’s growth and in producing flowers that reach anthesis simultaneously.

Fossil wood that is indistinguishable from that of Robinia pseudoacacia is encountered from the Late Eocene to Miocene in the south-central and western United States (U. Prakash et al. 1962; Prakash 1968; L. C. Matten et al. 1977; M. Lavin et al. 2003). Fossil leaves dating to the Oligocene have been attributed to the genus with less confidence (D. Isely 1998; Lavin et al.).

Robinia represents one of relatively few temperate North American lineages to have evolved from neotropical ancestry (as in Strophostyles; E. T. Riley-Hulting et al. 2004). Most temperate North American legume groups (such as Astragalus, Lupinus, and Vicia) originate from Eurasia. Similar to neotropical relatives, Robinia species primarily occupy disturbance-prone habitats and often root-sprout on unstable or exposed soils; they can form dense colonies that stabilize loose soil. Their growth habit ranges from shrubs to trees; R. pseudoacacia generally adopts a large-tree habit most consistently. Variation in such growth form is likely a function of disturbance. The smaller the stature of the plant, the more likely the habitat disturbance is regular or recent. Growth of Robinia species also appears to be regulated more by temperature than day length, which could be a result of a recent tropical ancestry. Consequently, Robinia seems outcompeted in undisturbed settings and is prone to die back related to cold temperatures. Apart from a variable shrub to tree growth form, this may explain the generally crooked stems and unkempt appearance of many Robinia individuals and populations.

With exception to the predominantly white-petaled and outcrossing Robinia pseudoacacia (B. C. Bongarten 1992), the three predominantly pink-petaled species (R. hispida, R. neomexicana, and R. viscosa) harbor much interpopulation variation that is often recognized formally at the species or infraspecific level (W. W. Ashe 1922). This variation is often the result of hybridization (for example, R. viscosa var. hartwigii), clonal variation due partially to reproduction involving triploidy (as in segregates of R. hispida), plasticity of growth form (R. hispida var. nana), or geographic structuring (R. neomexicana var. rusbyi) (D. Isely and F. J. Peabody 1984). None of the historically recognized intraspecific variation is treated formally here. The taxonomy of Robinia would be better served with genetic data and analyses that bear on the origins of asexual races rather than application of formal taxonomic names to every variant and intermediate form.

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