Clark County Nevada has proposed building an airport on a site that may adversely impact a local population of a rare relict perennial Mojave Desert forb, White-margined penstemon (Penstemon albomarginatus). Isolated populations of approximately 100,000, 4,000, 26,000 and 42,000 plants occur in NE Arizona, S California and Clark and Nye counties Nevada, respectively. However, many hectares of apparently suitable habitat in Nevada are unoccupied. The objectives of our study are to quantify soil physical and chemical properties, climatic conditions, and plant community structure and determine the influence of these variables on the lifecycle of P. albomarginatus. Most P. albomarginatus populations occur on aeolian sands, between 800 to 100 m elevation and with winter-dominated annual precipitation of 100 and 400 mm. However, in dry years the below ground portions of the plant remain dormant, consequently as much as 75% of the lifecycle of P. albomarginatus is spent as a subterranean heterotroph, including continuous intervals longer > 18 months. We selected penstemon and control sites in Clark County and Nye County in May 2007 and installed sediment transport measuring instruments, a series of soil moisture probes and meteorological data recording instruments and collected soil samples to 70 cm depth in summer 2007. Community structures (canopy heights and areas of all perennial plant species) were measured in 5x50 m transects. Growth, survival, reproduction, and recruitment of all penstemon in these transects are being documented October 2007 through April 2010. Potential root parasitism will also be determined.
Results/Conclusions
Control and penstemon sites had similar community structure, dominated by a deciduous shrub (Ambrosia dumosa) and/or a perennial grass (Pleuraphis rigida). Precipitation drives timing and amount of aboveground primary productivity and reproductive output, but only site-specific climate data is useful for predicting either. During any year, a population in full bloom may be less than 10 km distant from another population that is surviving only as underground dormant roots or seeds. Consequently, a difficult to measure fraction of P. albomarginatus genetic diversity remains buried for perhaps six or more years, based on 40 % germination of 6-year-old seed stored air-dried at room temperature. Soil electrical conductivity, phosphorus, ammonium and nitrate levels were significantly greater in the top 5 cm of control site soils. Understanding heterogeneous deposition patterns of soluble salts and nitrogen may be critical for defining appropriate unoccupied habitat and maintaining the management status of P. albomarginatus as rare, instead of threatened or endangered.
