SPATIAL DISTRIBUTIONS OF GRASS AND SHRUB ROOT SYSTEMS IN THE SHORTGRASS STEPPE

In water-limited ecosystems such as the shortgrass steppe of North America, the ability of plants to absorb sufficient water for growth and reproduction is dependent upon the characteristics of their root systems. Our objective was to evaluate the spatial distributions of roots of three plant life forms: a grass (Bouteloua gracilis) (H.B.K.) Lag., a dwarf shrub [Gutierrezia sarothrae, (Pursh.) Britt. & Rusby] and a shrub [Atriplex canescens (Pursh.) Nutt.]. Our interests were in a qualitative evaluation of evidence that different life forms obtained water from different soil layers. Root systems were excavated and evaluated in terms of horizontal and vertical extension, and proportion of total root length in each 10-cm layer of the soil profile. Bouteloua gracilis had over 80% of its roots in the top 50 cin of the soil, whereas the depth at which Atriplex canescens had over 80% of its roots was 100 cm. The corresponding depth for Gutierrezia sarothrae was 80 cm. Both of the woody plants had deeper and more widely spreading root systems than the grass. The environment of the shortgrass steppe is characterized by low and variable water availability. These conditions lead to a distribution of available water that is biased toward the upper soil layers. The greater concentration of roots of Bouteloua gracilis in the soil layers near the surface compared to the two woody species provides a potential explanation for its dominance in the shortgrass steppe.