|Briske, David - TEXAS A&M UNIVERSITY|
Submitted to: International Rangeland Congress
Publication Type: Proceedings
Publication Acceptance Date: June 1, 1999
Publication Date: N/A
Interpretive Summary: Do differences in plant architecture among grasses affect 1) the location where a plant accumulates nutrients like nitrogen or 2) the ability of plants to respond to simulated nutrient-rich patches in soil? Caespitose grasses are distinguished by a compact arrangement of stems, while rhizomatous grasses have a much more diffuse arrangement of stems and greater distance between stems. Caespitose grasses accumulated nutrients in soils beneath individual plants while rhizomatous grasses accumulated nutrients in belowground plant stems at both a relatively dry (north-eastern Colorado) and wet (eastern Kansas) grassland. Differences in plant architecture did not affect the ability of plants to respond to nutrient-rich patches in soils at the wet grassland. At the dry grassland, a rhizomatous grass responded more favorably than a caespitose grass to nutrient-rich patches in soils. These results have important relevance to rangeland managers by suggesting that caespitose grasses may be lost from grasslands in which management practices prevent these species from accumulating nutrients in soils. On the other hand, how a plant responds to nutrient-rich patches in the soil, such as those created by deposition of animal wastes, appears to be determined more by prevailing abiotic conditions than plant architecture.
Technical Abstract: Schizachyrium scoparium (caespitose) and Andropogon gerardii (rhizomatous) plants in a mesic, tallgrass community, and Bouteloua gracilis (caespitose) and Pascopyrum smithii (rhizomatous) plants in a semi-arid, shortgrass community were used to compare nutrient pools in long-term (>25 yrs) grazed and ungrazed sites and evaluate morphological and physiological root plasticity among growth forms. Our hypothesis were: 1) the magnitude of nutrient pools in rhizomes would be comparable to that in soils beneath caespitose grasses, 2) grazing would produce a comparable reduction in nutrient pools of both growth forms, and 3) caespitose grasses would exhibit greater physiological than morphological root plasticity. Caespitose grasses accumulated substantially greater nutrient pools in soils directly beneath plants than did rhizomatous plants in rhizomes. Nutrient pools for species of either growth form were not consistently modified by long-term grazing suggesting the occurrence of species and/or system (i.e., mesic vs. semi-arid) specific responses. Caespitose and rhizomatous grasses exhibited similar physiological and morphological root plasticity in the mesic community. However, the caespitose grass expressed less physiological, but similar morphological, root plasticity compared to the rhizomatous grass in the semi-arid community. These data suggest that architectural distinctions are manifest in distinct patterns of belowground nutrient accumulation between the two growth forms, but root plasticity appears to be mediated by prevailing abiotic variables in both growth forms.