Phytogeographical relationships and analysis of the flora of South Texas Plains

Abstract

The southernmost tip of Texas is a part of the Tamaulipan Province of northeastern Mexico which some authors consider in the Madrean Subkingdom of the Holarctic Kingdom, while others the “Xerofitica Mexicana” of the Neotropical Kingdom. To shed more light on this question, a natural flora of fourteen counties in the S TX Plains ecoregion, South Texas Plains flora (S TX), was compiled, studied from a biogeographical perspective, and analyzed in this work. The analysis was based primarily on the species and genera distribution outlines. Phylogenetic literature was searched to find relationships, patterns of migrations, and geographical connections of the species of major clades. Taxonometric and geographic spectra of the S TX flora were obtained and compared with those of two other floras in southern Texas: SC Texas (EP, or Edwards Plateau) and Big Bend Region (BB). There are 1250 native species in 553 genera and 117 families in the S TX flora. These species were classified into 25 geographic (floristic) elements. Herein is presented a checklist of S TX accompanied by the geoelement descriptions, a comparison of geographical spectra of the species and genera in all three floras, and the biogeographical analysis of the S TX flora. South TX, having a flat topography and being on the crossroads of migration routes, has multiple connections with adjacent floristic centers. The Tamaulipan endemism is not particularly high (8% in S TX) in comparison with much higher Chihuahuan endemism in BB (23% in BB; 10% in S TX). Proportions of taxa of the largest families in the three southern Texas floras show much higher numbers of tropical-subtropical, Tamaulipan, Gulf Coast, and Mesoamerican (coastal) species in the S TX flora in comparison with those of EP and BB. North temperate, E North American, and Prairie species are best represented in EP (47%), followed by S TX (36%), with only 17% in BB. The majority of the BB differential species are in the Chihuahuan, SW N American, Sonoran/Apachian, and the Madrean geoelements. The genera of the largest family, Asteraceae, that are differential among the three floras show importance of the Mexican (Madrean) centers of diversity as well as migrations from the Tethyan Subkingdom in the assembly of the S TX flora. Tethyan connections are also evident among the Boraginaceae s. str. and some other groups found in the flora. The spectra of generic and specific geographical elements of S TX illustrate transitional position of its flora. It represents an ecotone between the Western (Madrean) and the Eastern (Atlantic and Gulf Coast) N American Regions, with a very high influence of the various Neotropical elements and a higher affinity to the EP flora than to that of BB.