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United States Department of Agriculture

Agricultural Research Service

Title: Drought and Grazing Iii: Root Dynamics and Germinable Seed Bank

Authors
item Hild, Ann - UNIVERSITY OF WYOMING
item Karl, Michael - USDI-BLM
item Haferkamp, Marshall
item Heitschmidt, Rodney

Submitted to: Journal of Range Management
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 3, 2000
Publication Date: May 1, 2001
Citation: HILD, A.L., KARL, M.G., HAFERKAMP, M.R., HEITSCHMIDT, R.K. DROUGHT AND GRAZING III: ROOT DYNAMICS AND GERMINABLE SEED BANK. JOURNAL OF RANGE MANAGEMENT. p. 292-298. 2001.

Interpretive Summary: Drought and grazing are two factors that influence rangeland vegetation, yet controlled experiments examining the combined impacts of drought and grazing upon roots and soil seed banks are uncommon. We examined the combined effects of drought and grazing upon root distribution and plant species composition of seed banks. Twelve 15 by 30 foot plots with six uncovered and six covered with an automated shelter during rains from late May to mid-October 1994 were studied. Plots were either: 1) grazed both the year of the drought and the year after the drought, 2) grazed during the year of the drought and ungrazed the year after the drought, or 3) ungrazed both the year of and the year after the drought. Root densities were counted at varying soil depths in each plot during 1993- 1996. Litter and surface soil samples to a depth of 1 1/8 inches were collected in fall 1993-1996. Samples were incubated in a greenhouse to determine seedling emergence by species. Roots were fewer in shallow soil layers in grazed plots than in ungrazed plots by the end of the study. Roots in deeper soil layers were fewer during the drought, but were not influenced by grazing. Plant species composition of the seed bank suggests that cool-season annual grass seeds can accumulate if drought occurs after early spring rainfall. Without drought, forb seeds were increased with grazing. Perennial grasses were a small portion of the total seed bank. Subtle changes in seed bank composition may result in increased forb seedlings relative to cool-season annual seedlings following grazing. Drought prior to grazing may limit the amount of forb seed. Thus, drought followed by grazing may allow cool-season annuals to temporarily dominate a seed bank in a northern mixed grass prairie.

Technical Abstract: Drought and herbivory frequently influence North American rangelands. Controlled examination of the combined impacts of drought and herbivory upon roots and seed banks are not common. We examined the combined effects of drought and grazing upon post-drought plant root distribution and seed-bank species composition. Using twelve 5- X 10-m non-weighing lysimeters with an automated rainout shelter, we examined root intercepts in situ using a minirhizotron from 1993-1996. Seed-bank samples were incubated in a common greenhouse setting to determine seedling emergence by species. Roots were fewer in shallow (A horizon) soil layers in grazed plots than in ungrazed plots by the end of the study. Roots in deeper (Bw horizon) soil layers were fewer during the drought, but were not influenced by grazing. Composition of seed banks suggests that cool-season annual-grass seeds can accumulate over time. Without drought, forb seed banks were increased by grazing. Perennial grasses were a small portion of the total seed bank. Subtle transitions in seed-bank composition may result in increased forb seedlings relative to cool-season annual grass seedlings following grazing. Drought prior to grazing may limit forb seed banks. Thus, drought followed by grazing may allow cool-season annual grasses to temporarily dominate a seed bank in a northern mixed grass prairie.

Last Modified: 9/10/2014
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