Drought and grazing III: Root dynamics and germinable seed bank

Drought and herbivory frequently influence North American rangelands. While these influences may temporarily reduce vegetative cover, their mutual inf luence on the available seedbanks which might occupy new safe sites is uncl ear. We examine effects of drought and grazing upon pre- and post-drought p lant root distribution and germinable seed bank to determine 1) if the resp onse of root distributions to drought depends upon grazing use and 2) if th e presence of germinable seeds is altered significantly by drought and graz ing. Using twelve, 5 X 10 m nonweighing lysimeters with an automated rainou t shelter, we documented root intercepts in situ using a minirhizotron from 1993-1996, Seed bank samples were incubated in a greenhouse to determine s eedling emergence. Roots were fewer in shallow soil layers in grazed plots than ungrazed plots by the end of the study, irrespective of drought, Roots in deeper (Bw horizon) soil layers were fewer during drought, but were not influenced by grazing. Seed bank composition results suggest that perennia l grasses were a small portion of the seed bank. Cool-season annual grass s eeds accumulated after drought. Without drought, forb seed banks increased with grazing, Thus while shallow roots may decrease during drought, in the year following drought grazing may decrease aboveground net primary product ion, and allow Large accumulations of cool-season annual grass seed in a no rthern mixed grass prairie.