Physaria (Lesquerella) thamnophila (Brassicaceae; henceforth Physaria) is an endangered herbaceous perennial plant found in remnant patches of xeric Tamaulipan thornscrub near the Rio Grande in south Texas. We analyzed six years of censuses of four populations of Physaria to identify any trends in population size. We also collected and analyzed quantitative vegetation data from the same four sites (a) to describe the associated plant community, (b) to test the hypothesis that too much shrub canopy cover is detrimental to this species, and (c) to guide future habitat management and restoration.
There was no overall trend in population size, suggesting that these populations will persist if the habitat remains intact. However, annual fluctuations in observed numbers of plants were extremely large: a single census cannot reliably assess Physaria population size. Surprisingly, population size was not closely related to annual precipitation. While our data confirmed that Physaria thamnophila is a species of Tamaulipan thornscrub, the vegetation of all four sites differed from previously published descriptions of this species’ habitat. Physaria appeared to coexist with native grasses, but not with either Pennisetum ciliare (buffelgrass) or Dichanthium annulatum (Kleberg bluestem), two abundant introduced species. Physaria seedling densities were positively related to shrub canopy cover, evidence against our initial hypothesis that more open microsites are more favorable for Physaria. However, there were many more Physaria plants in the brush-cleared (cut, but not bulldozed) portion of one site than in its uncleared portion. Perhaps both the roots and litter of intact shrubs and the roots and debris left after this type of brush-clearing facilitated seedling establishment by reducing movement of the highly erodible soil. Competition with shrubs later in life was suggested by qualitative observations of larger plants in more open microsites.