PARENT SESSION
Oral Session #63: Plant Ecology: Water Relations. Presiding: J. Cleverly.
Thursday, August 9, 2001. 8:00 AM to 12:00 PM. Hall of Ideas H.
Comparison of root and stem vulnerability to xylem embolism between sprouting and non-sprouting chaparral.
McElwain, Kirsti1, Davis, Stephen1, 1
ABSTRACT- Roots are typically more vulnerability to xylem embolism than stems. Our study determined if this held true for species of chaparral having three contrasting modes of regeneration after wildfire: nonsprouting, facultative sprouting and obligate sprouting. Non-sprouting species are shallow rooted and rely exclusively on seed germination for postfire establishment, whereas obligate sprouters are deep rooted and rely exclusively on vegetative resprouting. Facultative sprouters are intermediate. We hypothesized that while sprouters may follow the typical pattern of higher root vulnerability to embolism than stems, non-sprouters may not. Selection in non-sprouters is for extreme drought tolerance, roots are shallow and non-sprouters lack a deep taproot to facilitate hydraulic redistribution. We tested the hypothesis of differential root/stem susceptibility to xylem embolism by using the centrifuge method to construct vulnerability curves for roots and stems of three co-occurring species, a non-sprouter, facultative sprouter and obligate sprouter. The species used were Ceanothus megacarpus (non-sprouter), Rhamnus crocea (obligate sprouter) and Ceanothus spinosus (facultative sprouter), all from the same taxonomic family, Rhamnaceae. We found that 50% loss in hydraulic conductivity due to embolism was not significantly different between roots and stems of the non-sprouter (root = -7.5 MPa, stem = -8.5 MPa, n = 12-14, P > 0.05) but markedly different for the obligate sprouter (root = -2.1 MPa, stem = -7.1 MPa, P < 0.0001). The facultative sprouter was intermediate (root = -4.7 MPa, stem = -6.9 MPa, P < 0.001). From these data we conclude that differences in life history characteristics, rooting depths and selective forces during seedling establishment may play a role in the resulting vulnerabilities of root and stem xylem to embolism.
KEY WORDS: cavitation, chaparral, xylem embolism, roots