The role of salinity tolerance and competition in the distribution of an endangered desert salt marsh endemic
Rare plants are often associated with distinctive soil types, and understanding why endemic species occur in unique environments is fundamental for their management. At Ash Meadows National Wildlife Refuge in southern Nevada, USA, we evaluated whether the limited distribution of endangered Amargosa niterwort (Nitrophila mohavensis) is explained by this species’ tolerance of saline soils on salt-encrusted mud flats compared with the broadly distributed desert saltgrass (Distichlis spicata var. stricta). We simultaneously explored whether niterwort distribution is restricted from expanding due to interspecific competition with saltgrass. Surface soils collected throughout niterwort’s range were unexpectedly less saline with lower extractable Na, seasonal electroconductivity, and Na absorption ratio, and higher soil moisture than in adjacent saltgrass or mixed shrub habitats. Comparison of niterwort and saltgrass growth along an experimental salinity gradient in a greenhouse demonstrated lower growth of niterwort at all but the highest NaCl concentrations. Although growth of niterwort ramets was similar when transplanted into both habitats at the refuge below Crystal Reservoir, niterwort reproductive effort was considerably higher in saltgrass compared to its own habitat, implying reallocation of resources to sexual reproduction to maximize fitness when the probability of ramet mortality increases with greater salinity stress. Saltgrass was not a demonstrated direct competitor of niterwort; however, this species is known to increase soil salinity by exuding salt ions and through litterfall. Niterwort conservation will benefit from protecting hydrological processes that reduce salinity stress and preventing saltgrass colonization into niterwort habitat.
KeywordsAmargosa niterwort Distichlis spicata var. stricta Nitrophila mohavensis Mojave desert Saltgrass
We thank the USGS staff and Student Conservation Association interns for assistance with all aspects of field, greenhouse, and laboratory work. R. Laczniak provided the Ash Meadows evaporation unit classification and R. Inman created a working model of niterwort habitat. E. Hewitt and R. Reitz at the College of Southern Nevada, Department of Horticulture generously provided greenhouse space and support. We thank S. Goodchild and C. Martinez for inviting us to work on Amargosa niterwort, and S. McKelvey, C. Baldino, and staff at Ash Meadows National Wildlife Refuge for their support and permission to collect plant tissues under federal and state permits. Reviews by J. Yee, K. Phillips, M. Brooks, K. McEachern, C. Baldino, and three anonymous reviewers significantly improved our final manuscript. Funding for this project was provided by US Fish and Wildlife Service. Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. government.
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