Shading by shrubs in a desert system reduces the physiological and demographic performance of an associated herbaceous perennial

1 A 2-year field study examined the demographic consequences of association with shrubs in an herbaceous perennial, Cryptanthaflava. Physiological data were collected to evaluate whether shrub effects were mediated primarily through water, nutrient or light availability. 2 Microclimatic conditions under the north side of shrubs differed from open microhabitats, primarily in light availability. Due to little photosynthetic acclimation to light, daily photosynthesis for plants under shrubs was reduced proportionally to the light regime. 3 Shading did not reduce stomatal conductance proportionally to photosynthesis, which led to decreased water use efficiency for plants under shrubs. Few differences were found in leaf water potential between microhabitats, indicating that little competition for water was occurring. 4 There was little evidence for shrub-induced nutrient island effects. Soil nitrogen, phosphorus and organic content did not differ between open and shrub microhabitats. Leaf nitrogen content also differed little between plants in the two microhabitats. 5 Growth and flowering responses of individuals under shrubs were reduced relative to those in the open, even for plants located on the south side of shrubs. 6 Over this 2-year period of average to above-average rainfall, association of C. flava with shrubs was dominated by competition for light, rather than for water or nutrients. Future investigations will address whether this asymmetric competitive interaction changes during years with below-average rainfall to a facilitative interaction, or one of increased competition for water.