GEOGRAPHICAL DISTRIBUTION AND SEASONAL VARIATION OF HEXALECTRIS (ORCHIDACEAE) IN TEXAS: IMPLICATIONS FOR RARITY AND CONSERVATION

Abstract

Hexalectris is a genus of mycoheterotrophic orchids found in the southwestern United States and Mexico, and some species in the genus are considered of conservation concern. Details on the evolutionary relationships between Hexalectris and symbiotic fungi are known, but relatively little is understood about the ecological factors that affect geographical distribution and seasonal variation in populations. Over a 12-year period, I (with volunteers and student assistants) studied the relationship between Hexalectris numbers and annual rainfall, and between documented orchid locations and soil–geological characteristics in Dallas County, Texas. We found a strong relationship between Hexalectris census numbers and the previous year's total rainfall. Targeted searches informed by knowledge of soil type helped to identify nine Hexalectris populations in Dallas County and a range expansion in H. grandiflora, a species previously unknown outside of western Texas. Hexalectris diversity correlates with preserve area, demonstrating that a larger preserve could help conserve more Hexalectris species. Overlapping soil surveys, geological data, and known Hexalectris locations in Texas helped to identify 64 counties with soil and geological conditions associated with Hexalectris. Many of the counties have no records of Hexalectris, showing the potential need for and value of targeted searches during wetter years to aid in identifying new populations to help fill in Hexalectris distributions and direct future conservation efforts.