Soils are recognized as complex biogeochemical systems where organisms, processes and physical characteristics can change dramatically with time and space, sometimes over small distances and short times. In soils, do organisms have redundant functions or do organisms specialize? This question can be studied in the rhizosphere, an important soil interface to study interactions between soil characteristics, soil organisms, and plants. We selected ectomycorrhizal tree roots to study of patterns of taxonomic and functional diversity. In a series of experiments in a northern California oak woodland, we sampled ectomycorrhizal fungi on roots of Quercus douglasii, Q. wislizeni, and Pinus sabiniana over temporal scales (seasons, years) and spatial scales (soil cores, plots, sites). Diversity was determined using molecular approaches (taxonomic diversity) and stable isotopes (functional diversity as estimated by uptake of 15-N ammonium from soils).  Taxonomic ectomycorrhizal diversity on tree roots increased with increasing spatial scale: lower in soil cores, greater around a tree, and greatest at a site. Although diversity was low within a core (mean of 6), there was little species overlap between cores. Thus, fungal species probably differed between cores. Since rates of 15-N uptake varied widely within a core, but less between cores, a few species within a core exhibited a range of 15-N uptake rates. When data from soil cores were compared, the range of 15-N uptake rates was similar, but the fungal species were different. This suggests that fungal species have redundant functions, and abilities to carry out critical soil processes like N uptake are preserved within the ectomycorrhizal community.