Sedges in the genus Carex (Cyperaceae) often lack mycorrhizae, but under conditions of low phosphorus availability many species facultatively form dauciform root structures that are implicated in phosphorus uptake. We assessed the functional significance of dauciform roots for sister species from 7 clades that were grown in factorial combinations of light and phosphorus availability. Sister species in subgenus Vignea did not form dauciform roots; all sister species in subgenus Carex formed dauciform roots.  Plants producing dauciform roots did so only under low-P conditions when grown in sun as opposed to shade. On average, plants in low-P growth regimes had higher root:shoot ratio, lower photosynthetic capacity, and lower productivity in both sun and shade; total root length did not differ between high- versus low-P growth regimes, but investment in fine roots increased under low-P.  Sun-grown plants had higher photosynthetic capacity, greater productivity and higher root:shoot ratio under both P regimes.  Sun-grown plants that formed dauciform roots under low-P conditions had higher root:shoot ratio, greater foliar chlorophyll content, lower photosynthetic capacity, and grew more. We discuss these results from the point of view of niche segregation between and within the wetland and upland habitats in which the sampled species typically occur.