Does a tradeoff exist between morphological and physiological root plasticity? A comparison of grass growth forms

A series of experiments were conducted to evaluate the potential tradeoff b etween morphological and physiological root plasticity in caespitose and rh izomatous grass growth forms in semi-arid and mesic communities. Morphologi cal and physiological root plasticity were evaluated with in-growth cores a nd excised root assays, respectively. The rhizomatous grass in the semi-ari d community was the only species to display significant physiological root plasticity, but all species possessed the capacity to proportionally increa se N-15 uptake with increasing concentrations of ((NH4)-N-15)(2)SO4 solutio n. Neither the caespitose nor the rhizomatous grass displayed morphological root plasticity in response to nitrogen addition in the mesic community. I n contrast, significant morphological root plasticity occurred in species o f both growth forms in the semi-arid community. These data suggest that the compact architecture and the ability to accumulate nutrients in soils dire ctly beneath caespitose grasses did not increase selection pressure for phy siological root plasticity at the expense of morphological root plasticity and that the coarse grained foraging strategy and low density of large diam eter roots did not increase morphological root plasticity at the expense of physiological root plasticity in rhizomatous grasses. These preliminary da ta suggest that 1) a high maximum uptake rate for nitrogen in these perenni al grasses may minimize the expression of physiological root plasticity, 2) morphological and physiological root plasticity may represent complimentar y, rather than alternative, foraging strategies, and 3) the expression of r oot plasticity may be strongly influenced by abiotic variables within speci fic habitats. USDA-ARS (C) 1999 Published by Editions scientifiques et medi cales Elsevier SAS.