Variability In Resistance To Water-Stress-Induced Cavitation In Juniperus (Cupressaceae) Species

Abstract

The resistance of xylem to cavitation has been proposed to be an important component of drought tolerance. Drought tolerance may play a role in the success of Juniperus (Cupressaceae) species, many of which are increasing in density and distribution in the western and central United States. We studied interspecific variation in vulnerability to water-stress-induced cavitation across Juniperus species. Our objectives were to compare root and stem vulnerability to cavitation in Juniperus species with distributions occurring along an aridity gradient and to determine if a tradeoff exists between resistance to cavitation and hydraulic conductivity. Stem (n=6-8) and shallow root (n=4-6) samples were collected in mid-summer, 2001, and vulnerability curves were constructed using the centrifugal force method to generate cavitation at known tensions. Stems of J. ashei (Ashe juniper), which is native to semi-arid central Texas, were most resistant, with a 50% loss of conductivity (Y50%) at –12.8 (±0.6) MPa. In contrast, stems of J. virginiana (eastern redcedar), which is native to the more mesic eastern United States, had Y50% at –5.9 (±0.2) MPa. Roots were more vulnerable than stems in three of five species. There was no evidence of a tradeoff between cavitation resistance and hydraulic conductance in roots or stems among species. Differences in vulnerability to cavitation in closely-related species suggest that cavitation resistance is an adaptation to arid environments and may play a role in determining species distributions.

Background

• Juniper woodlands have expanded in historical times into drier and often lower-elevation sites previously dominated by shrubs and grasses (West 1988).
• Cavitation is the breaking of a water column under negative xylem pressure, which may occur during drought. Therefore resistance to cavitation should be related to the degree of drought experienced. The water potential corresponding to 50% loss of conductivity (Y_50%) is frequently used as a measure of vulnerability to cavitation.
• We tested the hypothesis that the degree of aridity within the distribution of a species is related to its cavitation resistance in the genus Juniperus.

Objectives

1. Assess interspecific variation in vulnerability of six Juniperus species to water-stress-induced cavitation.
2. Compare cavitation resistance between roots and stems within a species.
3. Examine the relationship between cavitation resistance and hydraulic conductivity: is there a “safety vs. efficiency” tradeoff?

Methods

Study sites: Collection sites were as follows: J. ashei stems in Austin, TX, J. deppeana, J. monosperma, J. osteosperma, and J. scopulorum stems and roots near Flagstaff, AZ, J. virginiana stems and roots in central NC. Stems (n=6-8) and shallow (30-40 cm) roots (n=4-6) were collected in mid-morning in June and July, 2001, except for J. virginiana roots, which were collected in October, 2000.  

Vulnerability curves and conductivity: A vulnerability curve describes the relationship of percent loss in hydraulic conductivity (% embolism) as a function of xylem tension (Y, MPa). We used the centrifuge technique (Pockman et al. 1995, Alder et al. 1997) to measure vulnerability of xylem to water-stress-induced cavitation. Stem and root segments were spun to sequential xylem tensions (Y, MPa), resulting in a measurable loss of hydraulic conductivity (k_h). To fit vulnerability curves and estimate Y_50% (the water potential resulting in a 50% loss of conductivity), percent loss of conductivity (PLC) was modeled as an exponential sigmoidal function of water potential (Y) (Pammenter and Vander Willigen 1998):

Statistical analyses: We estimated Y_50% by using the above equation and the nonlinear regression procedure of the S-PLUS statistical software package. Means were compared using t-tests and significance was determined by the Bonferroni correction for multiple comparisons.

Results

OBJECTIVE 1:

• STEMS: When comparing across species, J. ashei, J. monosperma, and J. osteosperma stems were similar in Y_50% and least vulnerable to water-stress-induced cavitation, followed by J. deppeana and J. scopulorum, which were intermediate in their vulnerability; J. virginiana was most vulnerable.
• ROOTS: When comparing across species, J. monosperma, J.osteosperma, and J. deppeana roots were similar in Y_50% and less vulnerable, and J. virginiana and J. scopulorum were similar and more vulnerable to water-stress-induced cavitation. Y_50% for J. ashei roots from W. T. Pockman, unpublished (see Jackson et al. 2000), suggests that J. ashei is similar to the less vulnerable group.

OBJECTIVE 2:

• Within 3 of 5 species, roots were significantly more vulnerable to cavitation than stems.

OBJECTIVE 3:

• There was no significant relationship between Y_50% and hydraulic conductivity (k_s (above), k_h, or k_l) in stems or roots.
• Roots had higher k_s and k_h than stems.

Conclusions

• Differences in vulnerability to cavitation that appear to correspond to aridity even among closely-related Juniperus species suggest that cavitation resistance is an adaptation to dry environments and may determine distributional limits.
• Junipers have roots that are highly resistant to cavitation, suggesting that they can tolerate extensive soil drought, which may explain their association with coarse or shallow soils.
• There was no evidence for a tradeoff between water transport efficiency and safety (Y50%), although there may be other costs associated with cavitation-resistant xylem.

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