Xylem structure and water transport
Resistance to water flow in the xylem is determined in part by the diameter and length of conduits responsible for axial water transport. The differences in conduit diameter that we observed among stems, shallow roots and deep roots resulted in dramatic changes in Ks (Fig. 4). Because flow in capillary systems is proportional to the fourth power of conduit radius (Zimmermann, 1983; Tyree & Ewers, 1991), increases in overall conduit diameter, as observed here, result in nonlinear increases in Ks. The large differences in Ks between stems, shallow roots and deep roots are also likely influenced by changes in conduit length. Conduit length is often positively correlated with conduit diameter (Zimmermann & Jeje, 1981; Zimmermann & Potter, 1982; Ewers et al., 1990). Although we did not measure it, we would expect that conduit length increases from stems to deep roots if the diameter–length correlation holds for the species studied here. Such changes in conduit length would contribute to the changes in conducting efficiency by reducing the number of times water moving between adjacent conduits must cross pit membranes, which are sites of high resistance to water flow (Zimmermann, 1983; Hacke et al., 2004; Sperry & Hacke, 2004).
The large vertical changes in Ks that we observed should reduce or eliminate the effect of the long path length from deep roots on whole-plant hydraulic conductance. Water absorbed by deep roots must travel a greater distance to reach the canopy than water absorbed by shallow roots near the soil surface. Although the difference in path length will depend on the geometry of the root system, in the species studied here the path length is roughly doubled for water uptake from deep roots. If Ks remained constant from stems to deep roots, the increase in path length alone would have large effects on flow rate and xylem pressure that might limit leaf gas exchange. The increase in conduit diameter and Ks that we observed from stems to shallow roots to deep roots could overcome this limitation by reducing the flow resistance in deep roots. Sinker roots of B. prionotes extending to 2 m depth have been shown to exhibit a similar pattern, with larger diameter xylem conduits, greater conduit length, and higher area-specific hydraulic conductivity than lateral roots of the same individuals (Pate et al., 1995, 1998). Interestingly, these differences were evident between deep roots 2 m below the root crown and shallow roots 2 m laterally from the root crown (Pate et al., 1995). The persistence of structural differences between shallow and deep roots located the same distance from the root crown suggests that their xylem structure was influenced by differences in their orientation, either through internally detected signals or in response to differences in the environment (e.g. increased resistance because of gravity with depth) surrounding these roots. Our data cannot address this question because our sampling of deep and shallow roots was not structured to include constant path length in each category.
In most systems variation in temperature is greatest for stems and least for deep roots. In our system, deep soil temperature varies < 1°C from the monthly mean temperature (20.3°C), and deep roots never freeze. Shallow soil is responsive to seasonal changes in climate. Despite warm winter temperatures (January MAT is 7.9°C), shallow roots may occasionally experience brief episodes of freezing and freezing-induced xylem cavitation (Martínez-Vilalta & Pockman, 2002). Although larger xylem conduits are more vulnerable to freezing-induced cavitation (Davis et al., 1999; Martínez-Vilalta & Pockman, 2002), it is unclear whether differences in freezing frequency could produce the gradient in conduit diameters that we observed.
The increased conduit diameter and Ks that we observed with depth are also important from the perspective of carbon allocation. Xylem dominated by large conduits requires less carbon than denser xylem with smaller diameter conduits (Tyree et al., 1994). Moreover, the increased Ks in deep roots means that, for a given driving force, they can carry the same amount of water as a much larger shallow root or stem. However, radial hydraulic resistance limits water uptake by roots more than axial transport because most water must cross cell membranes at the Casparian band (Steudle, 2001). Increased Ks of deep woody roots allows transport of water absorbed by a very large network of fine roots without large additional decreases in xylem water potential. In the underground stream in Powell's cave, a 1 cm diameter woody root of Bumelia or Quercus can support a network of absorbing fine roots that fills a volume roughly 5 m in length and 0.5 m in diameter (A.J.M. and W.T.P., unpublished observations). Thus the patterns that we have observed may reflect a balance between allocation to absorbing roots and woody roots that maximizes the water returned from the substantial carbon cost required to reach 20 m below the surface.
Finally, the biomechanical demands placed on wood, like wind loading and canopy support, may contribute to the anatomical differences that we observed. In above-ground tissues, wood density and hydraulic architecture have been shown to vary with biomechanical demands (Gartner, 1991); less dense, larger-diameter xylem is more efficient hydraulically but weaker mechanically than xylem dominated by smaller diameter conduits (Tyree et al., 1994). However, biomechanical stresses experienced by stems, such as vertical support of the canopy, are unlikely to affect shallow and, especially, deep roots because they exist in a matrix of supporting rocks and soil. Therefore roots likely require less mechanical strength than the canopy, and without this need for structural support can construct xylem that is more efficient for water transport.
Vulnerability to cavitation
The increasing vulnerability to cavitation with depth observed in B. lanuginosa and J. ashei indicates that deep roots are potentially more limiting to water transport than shallow roots or stems. How frequently deep roots approach the pressures required to induce cavitation depends on the range of environmental conditions that they experience. Even in the absence of any flow through the plant, cavitation can occur as plants equilibrate with drying soil (North & Nobel, 1991). Deep roots like those in our cave systems frequently exist in an environment with high water availability, or where water content and the soil Ψ that determines plant Ψ change relatively slowly. Thus, despite their increased vulnerability, deep roots with access to relatively abundant water may be unlikely to experience soil Ψ sufficient to induce cavitation. Moreover, the presence of roots in these more slowly changing environments allows deep-rooted species to maintain higher Ψ than co-occurring, shallow-rooted species which exhibit Ψ that more closely tracks the larger variations observed in shallow soil (Pockman & Sperry, 2000).
Decreases in xylem pressure during transpiration can also induce cavitation. During transpiration xylem pressure decreases below soil Ψ in a manner determined by Ks and flow rate. As a result, Ψ is highest in the soil and decreases along the flow path to the point of evaporation in the leaves. Although vulnerability to cavitation in B. lanuginosa and J. ashei increased with depth (Fig. 5), xylem sampling positions at depth may not operate any closer to critically low xylem tensions because xylem water potential in deep roots is typically higher than in stems. The safety margin between actual xylem pressure and the pressures required to induce xylem cavitation at each position in the plant are determined by the soil Ψ and the hydraulic conductance of the flow path to the point of interest. Because Ks is also greater in deep roots, any given flow rate will lead to a smaller decrease in xylem pressure in deep roots than would occur in shallow roots or stems. Thus the increased Ks associated with increased conduit diameter in deep roots may not necessarily come at a price of increased embolism, especially as embolism in roots may be easier to refill with positive root pressure (Alder et al., 1996). Detailed evaluation of the behavior of deep roots during water uptake requires consideration of the vertical profiles of xylem hydraulic architecture, root-absorbing area, soil Ψ and soil texture (Jackson et al., 2000; Sperry et al., 2002).
Despite dramatic anatomical changes with depth, we found no difference in vulnerability to cavitation among stems, shallow roots and deep roots of Q. fusiformis. Although long vessels, such as those in Q. fusiformis, can interfere with air-injection measurements of vulnerability to cavitation (Martínez-Vilalta et al., 2002), we used segments longer than the longest vessel. Furthermore, our air-injection results were consistent with measurements using the centrifuge technique. We were unable to use the dehydration method (Tyree & Sperry, 1989) because the large number of deep roots required would exceed the number that we can locate and harvest without depleting these resources for in situ physiological measurements. If the patterns we observed are supported by additional data, Q. fusiformis would be one of the few species where differences in vulnerability are not observed with changes in anatomy between roots and stems. This would require pit membrane pore diameter to remain relatively unchanged as vessel diameter increases in roots.
Conducting efficiency vs vulnerability to cavitation
Our results show a trade-off between conducting efficiency and vulnerability to cavitation across stems, shallow roots and deep roots of B. lanuginosa and J. ashei (Fig. 6). These results are consistent with the nonlinear relationships found in comparisons of the same organ across many species (Pockman & Sperry, 2000; Maherali et al., 2004) and between stems and shallow roots of the same species (Martínez-Vilalta et al., 2002).
Questions still remain about what drives this pattern of xylem anatomy and vulnerability to cavitation. Tsuda & Tyree (1997) suggested that increased Ks and vulnerability to cavitation in roots guarantees large water flow or water use when water is available, and was thought to be an adaptation to more mesic environments. However, our results show that this pattern holds in woody roots and stems in drier habitats as well. The adjustment of xylem anatomy and Ks may be more affected by water availability to all or part of a tree's root system during tissue development. Root development and subsequent hydraulic characteristics within and between trees can be affected by microsite parameters, particularly water availability (Sperry & Ikeda, 1997; Kavanaugh et al., 1999). Alder et al. (1996) found intraspecies differences in susceptibility to cavitation in root xylem between slope and riparian habitats, and suggested this was largely caused by environmental rather than genetic effects. The cave systems studied here provide an opportunity to address whether deep roots develop changes in xylem structure and function after tapping into a reliable deep water source, or whether these patterns are inherent characteristics of deep roots.