We examined functional coordination among stem and root vulnerability to xylem cavitation, plant water transport characteristics and leaf traits in 14 co-occurring temperate tree species. Relationships were evaluated using both traditional cross-species correlations and phylogenetically independent contrast (PIC) correlations. For stems, the xylem tension at which 50% of hydraulic conductivity was lost ( Ψ Ψ Ψ Ψ 50 ) was positively associated ( P < < < < 0.001) with specific conductivity ( K S ) and with mean hydraulically weighted xylem conduit diameter ( D h-w ), but was only marginally ( P = = = = 0.06) associated with leaf specific conductivity ( K L ). The PIC correlation for each of these relationships, however, was not statistically significant. There was also no relationship between root Ψ Ψ Ψ Ψ 50 and root K S in either crossspecies or PIC analysis. Photosynthetic rate ( A ) and stomatal conductance ( g s ) were strongly and positively correlated with root Ψ Ψ Ψ Ψ 50 in the cross-species analysis ( P < < < < 0.001), a relationship that was robust to phylogenetic correction ( P < < < < 0.01). A and g s were also positively correlated with stem Ψ Ψ Ψ Ψ 50 in the cross-species analysis ( P = = = = 0.02 and 0.10, respectively). However, only A was associated with stem Ψ Ψ Ψ Ψ 50 in the PIC analysis ( P = = = = 0.04). Although the relationship between vulnerability to cavitation and xylem conductivity traits within specific organs (i.e. stems and roots) was weak, the strong correlation between g s and root Ψ Ψ Ψ Ψ 50 across species suggests that there is a trade-off between vulnerability to cavitation and water transport capacity at the whole-plant level. Our results were therefore consistent with the expectation of coordination between vulnerability to xylem cavitation and the regulation of stomatal conductance, and highlight the potential physiological and evolutionary significance of root hydraulic properties in controlling interspecific variation in leaf function.

In the conifer genus Juniperus (Cupressaceae), many species are increasing rapidly in distribution, abundance, and dominance in arid and semiarid regions. To help understand the success of junipers in drier habitats, we studied hydraulic traits associated with their water stress resistance, including vulnerability to xylem cavitation, specific conductivity (K(S)), tracheid diameter, conduit reinforcement, and wood density in stems and roots, as well as specific leaf area (SLA) of 14 species from the United States and the Caribbean. A new phylogeny based on DNA sequences tested the relationships between vulnerability to cavitation and other traits using both traditional cross-species correlations and independent contrast correlations. All species were moderately to highly resistant to water-stress-induced cavitation in both roots and shoots. We found strong phylogenetic support for two clades previously based on leaf margin serration (serrate and smooth). Species in the serrate clade were 34-39% more resistant to xylem cavitation in stems and roots than were species in the smooth clade and had ∼35% lower K(S) and 39% lower SLA. Root and stem resistance to cavitation and SLA were all highly conserved traits. A high degree of conservation within clades suggests that hydraulic traits of Juniperus species strongly reflect phylogenetic history. The high resistance to cavitation observed may help explain the survival of junipers during recent extreme droughts in the southwestern United States and their expansion into arid habitats across the western and central United States.

Previous studies indicate that conifers are vulnerable to cavitation induced by drought but in many cases, not by freezing. Rarely have vulnerability to drought and freezing stress been studied together, even though both influence plant physiology and the abundance and distribution of plants in many regions of the world. We studied vulnerability to drought-and freezinginduced cavitation, along with wood density, conduit reinforcement, tracheid diameter, and hydraulic conductivity, in four Juniperus species that typically occupy different habitats, but uniquely co-occur at the same site in Arizona, AZ. We combined drought with a freeze-thaw cycle to create freezing-induced vulnerability curves. All four species demonstrated greater vulnerability to drought þ freezing-than to drought-induced cavitation alone (P < 0.0001). Mean tracheid diameter was correlated with vulnerability to drought þ freezing-induced cavitation (r 5 0.512, P 5 0.01). The vulnerability to cavitation of each species followed expected rankings based on relative moisture within each species' natural distribution. Species with naturally drier distributions showed greater resistance to both drought-and drought þ freezing-induced cavitation. Even conifer species with relatively small tracheid diameters can experience xylem embolism after a single freeze-thaw cycle when under drought stress.

Summary• Cavitation of xylem elements diminishes the water transport capacity of plants, and quantifying xylem vulnerability to cavitation is important to understanding plant function. Current approaches to analyzing hydraulic conductivity (K) data to infer vulnerability to cavitation suffer from problems such as the use of potentially unrealistic vulnerability curves, difficulty interpreting parameters in these curves, a statistical framework that ignores sampling design, and an overly simplistic view of uncertainty.• This study illustrates how two common curves (exponential-sigmoid and Weibull) can be reparameterized in terms of meaningful parameters: maximum conductivity (k sat ), water potential (-P) at which percentage loss of conductivity (PLC) = X% (P X ), and the slope of the PLC curve at P X (S X ), a 'sensitivity' index.• We provide a hierarchical Bayesian method for fitting the reparameterized curves to K H data. We illustrate the method using data for roots and stems of two populations of Juniperus scopulorum and test for differences in k sat , P X , and S X between different groups.• Two important results emerge from this study. First, the Weibull model is preferred because it produces biologically realistic estimates of PLC near P = 0 MPa. Second, stochastic embolisms contribute an important source of uncertainty that should be included in such analyses.

Two morphotypes of the evergreen shrub Artemisia tridentata Nutt. ssp. wyomingensis occur in the Shirley Basin of central Wyoming ͑USA͒, one of which was associated exclusively with Mima-like mounds generated by animal burrowing activity. Measured on a particularly dry year according to a 34-year precipitation record, plants growing on mounds ͑M͒ versus inter-mound locations ͑IM͒ were taller with greater leaf biomass and leaf area per unit ground area, and had over 90% of all inflorescences. As a result, the landscape consists of a patchy distribution of reproductive islands ͑~20-40 m -2 in size͒ separated by a mean distance of~30 m. In addition, greater photosynthesis per unit leaf area occurred for M plants when ephemeral leaves dominated total leaf area in spring and early summer, as well as during short time periods ͑ Ͻ 3 days͒ following sporadic rainfall events in summer when only perennial leaves were present. As a result, estimated total annual carbon gain was 41% greater for M plants from May to mid-June, but was not significantly different from IM plants for the remainder of the season, resulting in a total summer carbon gain that was 14% greater in M plants. Stomatal and nonstomatal conductances to CO 2 uptake were also greater for the ephemeral leaves of M plants, along with lower internal CO 2 concentrations ͑193 Ϯ 4 l l -1 vs. 209 Ϯ 8 l l -1 , respectively͒. M plants also maintained higher xylem water potentials throughout most of the growth season ͑-1.1 Ϯ 0.1 SD MPa in May, declining to Ϫ 4.4 Ϯ 0.3 SD MPa in August͒, along with higher water use efficiencies ͑photosynthesis/transpiration͒. M and IM soils did not differ significantly in total organic or nitrate contents, although leaf nitrogen content was higher in M plants when photosynthesis was also greater. Photosynthesis in M plants also responded more positively to afternoon showers greater than about 7 mm compared to IM plants. Thus, improved water and nutrient relations was associated with enhanced photosynthetic carbon gain in M plants, enabling greater flower production. Moreover, morphotypic plasticity coupled with the effects of animal burrows may have substantially increased sexual reproductive success in A. t. wyomingensis.