Juvenile shrubs show differences in stress tolerance, but no competition or facilitation, along a stress gradient

Abstract

Summary 1 We investigated experimentally differences in abiotic stress tolerance and the effects of plant–plant interactions for two desert shrubs, Chrysothamnus nauseosus and Sarcobatus vermiculatus, along a soil salinity (NaCl) and boron (B) gradient at Mono Lake, California, USA. Based on differences in natural distribution, and the classical expectation of a trade‐off between competitive ability and stress tolerance, we hypothesized that (i) Chrysothamnus would have greater competitive ability than Sarcobatus at the low salinity end of the gradient, and that (ii) Sarcobatus would be more stress tolerant than Chrysothamnus. 2 Juvenile target plants of Chrysothamnus and Sarcobatus were planted into four sites along the gradient. Biomass was determined by destructive harvests over two growing seasons. At each site, interspecific relative competitive ability was assessed as the effect of Sarcobatus neighbours on Chrysothamnus targets compared to the effect of Chrysothamnus neighbours on Sarcobatus targets. Stress tolerance was assessed as the ability of each species to survive and grow, in the absence of neighbours, at different sites along the gradient. 3 The two species did not differ in the relative strength of plant–plant interactions, providing no support for the expectation that Chrysothamnus had greater competitive ability than Sarcobatus. Furthermore, there was no evidence for competition or facilitation, either interspecific or intraspecific, at any site in either year of the study. However, fertilization treatments demonstrated nutrient limitations, soil water reached limiting levels and root systems of targets and neighbours overlapped substantially. It is therefore surprising that plant–plant interactions among juveniles apparently play little role in the growth and survival of shrubs in this saline desert habitat. 4 Sarcobatus was more stress tolerant than Chrysothamnus and the two species performed optimally at different sites along the gradient. Sarcobatus juveniles grew best at the two most saline sites and survived at all sites, whereas Chrysothamnus juveniles grew best at a low‐salinity site and did not survive at the most saline site. The difference in site of optimal performance may be due to differences in nutrient limitations or to interactions between nutrient availability and sodium (Na) and B tolerance.