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Helianthus paradoxus (Pecos sunflower) is a threatened annual sunflower endemic to salt marshes in the Chihuahuan Desert of North America. From year to year, the location of this species within the marsh changes as edaphic factors such as soil moisture and soil salinity change. In this paper, the relationship of above-ground dry mass of this species and soil surface water, subsurface water, surface salinity, subsurface salinity, and oxygen content was evaluated. H. paradoxus was found where surface water was higher, but this was dependent on the time of year. In quadrats where H. paradoxus was found, both surface and subsurface soil salinity were significantly lower than those quadrats where H. paradoxus was not found. H. paradoxus was also found in quadrats where soil oxygen content was higher later in the growing season. Regression analyses indicated that soil salinity was the most important determinant of H. paradoxus above-ground dry mass, except later in the growing season when surface moisture was the most important factor. While soil salinity appears to be the most important factor early in the growing season, the role of water may be important later in the growing season for removal and redistribution of salts in the soil profile. (c) 2005 Elsevier Ltd. All rights reserved.