Multiple resources limit plant growth and function in a saline-alkaline desert community

1 We investigated resource limitations in a saltbrush scrub community along a salinity-alkalinity gradient in the Mojave Desert of North America. Previous studies have shown that, as productivity declines with increasing soil stress, there are parallel declines in leaf Ca and Mg, suggesting availability of these resources may limit growth in addition to water, N and P limitations expected in deserts. 2 To determine which soil resources limited growth of the dominant shrub, Atriplex parryi, and whether this differed along the soil stress gradient or among life stages, water and nutrients were applied in combination at different rates to plants at high- and low-stress sites. We developed and applied a conceptual model to identify resource limitations from this experiment. We also investigated how those limitations interacted to influence A. parryi growth and physiological function at the high-stress site. 3 Availability of soil N and P limited growth at the low-stress site and N, P and Mg limited growth at the high-stress site for both adult and juvenile life stages. There was no evidence that availability of water alone or K, Ca or other nutrients, or increasing soil Na and B, limited growth along the soil stress gradient. 4 When N, P, Mg and water were supplied in combination, plant growth increased more than 16-fold. Supply of these resources interacted to influence both plant growth and function. Because of the high demand for N relative to other resources, N supply rate was the major driver of these interactions, influencing the magnitude by which P and Mg affected growth. The major mechanism behind these growth responses was an increase in allocation to leaves relative to fine roots, rather than nutrient or water effects on water relations, photosynthesis or water use efficiency. 5 Multiple, interacting resources limit the growth and distribution of A. parryi on a saline, alkaline Mojave Desert site. Similar interactions between multiple limiting resources are likely to be instrumental in shaping community and species distributions along other abiotic stress gradients.