Field identification of Eriogonum corymbosum vars. nilesii and aureum (Polygonaceae)

Eriogonum corymbosum vars. nilesii and aureum have been difficult to distinguish in the field due to their phenotypic similarities. Greater understanding of the ranges of these two varieties has been demonstrated by genetic analyses over the past decade. However, field identification remains problematic, and current dichotomous keys do not provide reliable traits, or trait combinations, to enable botanists to confidently distinguish these two taxa. We examined morphological characters among individuals from six populations of E. corymbosum (three of var. nilesii and three of var. aureum). We found significant differences between these two varieties in petiole length, leaf length, and the ratios of leaf-blade length to petiole length and leaf-blade width to petiole length. We also found qualitative differences in adaxial leaf color and inflorescence-stem pubescence. Petiole length was the significantly different trait with the least overlap between varieties, making it practical for distinguishing between E. corymbosum vars. nilesii and aureum in the field. Moreover, these six trait differences lend further support to genetic findings in previous studies that confirmed the existence of these two taxa and demonstrated their limited ranges and rarity.

Further examination of the geographic range of Eriogonum corymbosum var. nilesii (Polygonaceae, Eriogoneae)

The wild buckwheat Eriogonum corymbosum is widely distributed throughout the southwestern United States, forming a complex of eight varieties. E. corymbosum var. nilesii is a predominantly yellow-flowered variant reported primarily from Clark Co., Nevada. A previous genetic study by our research group found that var. nilesii is genetically distinct from other E. corymbosum varieties, based on a limited number of populations. Here, we assess genetic variation in 14 newly sampled yellow-flowered populations from southern Nevada, southern Utah, and northern Arizona, and compare them to genetic variation in six populations of previously determined E. corymbosum varieties. Of the new populations, we identified four as var. nilesii, four as var. aureum, three as var. glutinosum, two as apparent hybrids involving vars. aureum and nilesii, and one as a more distantly related admixture involving E. thompsoniae. Our results extend the range and area of E. corymbosum var. nilesii considerably from that traditionally stated in the literature. However, this extended range is confined to the Mojave Desert region of southern Nevada, and the number of known populations remains limited.