Submitted to: Acta Oecologica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 22, 1999
Publication Date: N/A
Interpretive Summary: The process by which root systems acquire soil resources like nitrogen has two primary components. One is morphological adjustment in which a proliferation of roots occurs in a resource (water, nitrogen) rich location. The other is physiological adjustment in which roots increase the rate of uptake of soil resources when a zone of high concentration is encountered. It is speculated that rhizomatous grasses (possessing belowground stems called rhizomes) and bunchgrasses (no belowground stems) primarily utilize morphological and physiological adjustments, respectively, and that this contributes to distributional trends of these two grass growth forms over regional scales involving dry and wet grasslands. Neither growth form displayed morphological or physiological adjustments to high soil resources in a wet grassland, suggesting that aboveground constraints like light influence plant-soil relations to a much greater degree than belowground constraints. In a dry grassland, however, the rhizomatous grass displayed both morphological and physiological adjustments, and the bunchgrass displayed morphological adjustment. Therefore, it appears that belowground constraints influence plant-soil interactions to a much greater degree in a dry grassland. In addition, grass growth forms can display both morphological and physiological adjustments, suggesting that these are complimentary rather than alternative components. These findings have broad implications for resource managers because they indicate that the relative importance of aboveground and belowground constraints on plant-soil interactions differ in wet and dry grasslands.
A series of experiments were conducted to evaluate the potential tradeoff between morphological and physiological root plasticity in caespitose and rhizomatous grass growth forms in semi-arid and mesic communities. Morphological and physiological root plasticity were evaluated with in-growth cores and excised root assays, respectively. The rhizomatous grass in the semi-arid community was the only species to display significant physiological root plasticity, but all species possessed the capacity to uptake substantial amounts of 15N with increasing concentrations of (15NH4)2SO4 solution. Neither the caespitose nor the rhizomatous grass displayed morphological root plasticity in response to nitrogen addition in the mesic community. In contrast, significant morphological root plasticity occurred in species of both growth forms in the semi-arid community. These data suggest that the compact architecture and the ability to accumulate nutrients in soils directly beneath caespitose grasses did not increase selection pressure for physiological root plasticity and that the capacity for selective rhizome placement did not increase morphological root plasticity in rhizomatous grasses. Two alternative hypotheses may be inferred from these data: 1) proportional increases in the magnitude of 15N uptake over a relatively wide range of increasing solution concentrations indicate that efficient absorption kinetics are an important component of nutrient acquisition in these grasses, and 2) morphological and physiological root plasticity represent complimentary, rather than alternative, foraging strategies.