Root and stem xylem embolism, stomatal conductance, and leaf turgor in Acer grandidentatum populations along a soil moisture gradient

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Abstract
The objective of this study was to determine how adjustment in stomatal conductance (gs) and turgor loss point (Ψtlp) between riparian (wet) and neighboring slope (dry) populations of Acer grandidentum Nutt. was associated with the susceptibility of root versus stem xylem to embolism. Over two summers of study (1993–1994), the slope site had substantially lower xylem pressures (Ψpx) and gs than the riparian site, particularly during the drought year of 1994. The Ψtlp was also lower at the slope (-2.9±0.1 MPa; all errors 95% confidence limits) than at riparian sites (-1.9±0.2 MPa); but it did not drop in response to the 1994 drought. Stem xylem did not differ in vulnerability to embolism between sites. Although slope-site stems lost a greater percentage of hydraulic conductance to embolism than riparian stems during the 1994 drought (46±11% versus 27±3%), they still maintained a safety margin of at least 1.7 MPa between midday Ψpx and the critical pressure triggering catastrophic xylem embolism (ΨpxCT). Root xylem was more susceptible to embolism than stem xylem, and there were significant differences between sites: riparian roots were completely cavitated at -1.75 MPa, compared with -2.75 MPa for slope roots. Vulnerability to embolism was related to pore sizes in intervessel pit membranes and bore no simple relationship to vessel diameter. Safety margins from ΨpxCT averaged less than 0.6 MPa in roots at both the riparian and slope sites. Minimal safety margins at the slope site during the drought of 1994 may have led to the almost complete closure of stomata (gs=9±2 versus 79±15 mmol m-2 s-1 at riparian site) and made any further osmotic adjustment of Ψtlp non-adaptive. Embolism in roots was at least partially reversed after fall rains. Although catastrophic embolism in roots may limit the minimum Ψ for gas exchange, partial (and reversible) root embolism may be adaptive in limiting water use as soil water is exhausted.