Schmaltzia serotina as a part of Rhus trilobata

Schmaltzia serotina was described in 1905 by Greene. Palmer and Steyermark (1935) reduced S. serotina to a variety of Rhus canadensis Marsh., non Mill. (=R. aromatica Ait.). Barkley (1937), in his monographic treatment of Rhus in North and Central America, transferred var. serotina to the essentially western R. trilobata Nutt. This shift was based not only on morphological differences, but also on the fact that R. aromatica flowers primarily prior to leaf development, whereas var. serotina flowers after leaf development. Although more critical observations are needed on this point, the western R. trilobata sometimes flowers before and sometimes during leaf development. Fernald (1941, p. 599-602) promptly disagreed with Barkley's treatment and presented arguments contending that both var. serotina and var. arenaria belong in the Rhus aromatica complex. His arguments were based on fruit pubescence, "stone" size and shape, abaxially glabrous floral bracts, and the size of the terminal leaflet. He held that the densely pubescent twigs were not a trait of sufficient significance. The four current eastern manuals of Fernald (1950), Gleason (1952), Steyermark (1963), and Gleason and Cronquist (1963) have all treated var. serotina as part of R. aromatica. While attempting to identify specimens of the local flora, we have become aware of these two similar, yet quite distinct, taxa growing in adjacent niches, but so close that at times their branches overlap. Over a period of six years this population has been observed and one taxon has always been found to flower some time after the other has set fruit. Phenology data, kept for the past four years, has shown that the two taxa involved are at their peak of flowering about three weeks apart. In 1964 the early flowering Rhus aromatica var. aromatica first flowered April 4 and reached its peak of flowering April 14. The first flowers of the other taxon, R. trilobata var. serotina, appeared April 27, when the leaves were half grown, and the plants were in full flower by May 4 (Fig. 1). Rhus aromatica var. aromatica in central Missouri is an undershrub that occupies the broken woodland slopes, while R. trilobata var. serotina is found only on rocky outcrops, commonly on steep bluffs, with its roots in the rock crevices, a very xeric site in a mesic deciduous woodland. On the sites observed we have not found any indication of hybridization between the two taxa. This suggests that they are genetically isolated. Examination of seeds of Rhus trilobata from California and New Mexico has revealed that they are, in many instances, as small or smaller than those of R. aromatica, an indication that seed (stone) size is of no particular value in the separation of the two taxa (Table 1). California, Colorado, and New Mexico material of R. trilobata has fruits which are both pilose and glandular viscid as well as having a pruinose covering. The pubescence of the fruit is rather easily rubbed off, an accident which may have happened to the specimens Fernald examined. We have seen two collections of R. trilobata var. trilobata fromn Kansas, however, in which the pilose hairs were very sparse, suggesting that this also is a variable character. In Missouri, the ecology of R. trilobata var. serotina is fully compatible with it being considered as an eastern extension of the western R. trilobata. An additional bit of evidence suggesting such a relationship may be noted by comparing the