NORTH, GRETCHEN B.1*, PIERRE MARTRE2, EDWARD G. BOBICH2, and PARK S. NOBEL2. 1Department of Biology, Occidental College, Los Angeles, CA 90041; 2Department of OBEE, University of California, Los Angeles, CA 90095. - Roots and rocks: how soil rockiness affects shoot and root growth, root distribution, and other root properties for two desert species, Agave deserti (Agavaceae) and Pleuraphis rigida (Poaceae).
The effects of subterranean rocks on shoot productivity and several
root properties were investigated for the sympatric CAM succulent
Agave deserti and C4 bunchgrass Pleuraphis
rigida. Plants of both species were examined at a site in the
northwestern Sonoran Desert along a gradient of soil rockiness, with
rocks composing 5% to 60% of the total soil mass. Ten and 50 d after
August rainfalls totaling 67 mm, A. deserti had 1.7-fold more
newly unfolded leaves in sandy than in rocky soils. The greatest leaf
productivity for P. rigida also occurred in the sandiest soils.
Fifty d after the summer rain pulse, leaf number for P. rigida
had decreased by 66% and 31% in sandy and rocky soils, respectively,
but leaf number was still 2.5-fold higher in sandy than in rocky
soils. Agave deserti had more than 2-fold greater root surface
area and mean rooting depth in sandy than in rocky soils, whereas the
root surface area per plant and mean rooting depth for P.
rigida were not affected by soil rockiness. Moreover, the
hydraulic conductance for root segments of P. rigida collected
under rocks and in rock-free soil did not differ. For both species,
new root surface area represented 1% to 3% of the total root surface
area per plant in both sandy and rocky soils. The distribution of the
thick main roots of A. deserti was significantly affected by
the presence of rocks, perhaps contributing to the higher productivity
of A. deserti in sandy soils. The distribution of the finer
main roots of P. rigida was not affected by the presence of
rocks, and its greater productivity in sandy soil may reflect both the
deeper penetration of water and its greater ability to extract water
from a drying soil compared to the succulent A. deserti.
Key words: root anatomy, root hydraulic conductance, root plasticity, soil heterogeneity