SPECIES INTERACTIONS AT THE LEVEL OF FINE ROOTS IN THE FIELD - INFLUENCE OF SOIL NUTRIENT HETEROGENEITY AND PLANT SIZE

Interference at the level of fine roots in the field was studied by de tailed examination of fine root distribution in small soil patches. To capture roots as they occur in natural three-dimensional soil space, we used a freezing and slicing technique for microscale root mapping. The location of individual roots intersecting a sliced soil core surfa ce was digitized and the identity of shrub and grass roots was establi shed by a chemical technique. Soil patches were created midway between the shrub, Artemisia tridentata, and one of two tussock grasses, Pseu doroegneria spicata or Agropyron desertorum. Some soil patches were en riched with nutrients and others given only deionized water (control); in addition, patches were located between plants of different size co mbination (large shrubs with small tussock grasses and small shrubs wi th large tussock grasses). The abundance of shrub and grass roots shar ing soil patches and the inter-root distances of individual fine roots were measured. Total average rooting density in patches varied among these different treatment combinations by only a factor of 2, but the proportion of shrub and grass roots in the patches varied sixfold. For the shrub, the species of grass roots sharing the patches had a prono unced influence on shrub root density; shrub roots were more abundant if the patch was shared with Pseudoroegneria roots than if shared with Agropyron roots, The relative size of plants whose roots shared the s oil patches also influenced the proportion of shrub and grass roots; l arger plants were able to place more roots in the patches than were th e smaller plants. In the nutrient-enriched patches, these influences o f grass species and size combination were amplified. At the millimeter - to centimeter-scale within patches, shrub and grass roots tended to segregate, i.e., avoid each other, based on nearest-neighbor distances . At this scale, there was no indication that the species-specific int eractions were the result of resource competition, since there were no obvious patterns between the proportion of shrub and grass roots of t he two species combinations with microsite nutrient concentrations. Ot her potential mechanisms are discussed. Interference at the fine-root level, and its species-specific character, is likely an influential co mponent of competitive success, but one that is not easily assessed.