Xylem structure and function of two saltbush shrub species (Atriplex) from differing microhabitats

Abstract

Two formerly broadly distributed woody Atriplex species now occur as fragmented populations across a range of microhabitats in the San Joaquin Valley Desert, southern California. We hypothesized that A. lentiformis and A. polycarpa exhibit inter- and intra-specific differences in their leaf and stem structural and functional traits corresponding with differences in soil salinity and aridity. Water potential, xylem structure and function and leaf traits were compared between three populations of A. lentiformis and four populations of A. polycarpa. The two species significantly differed in their xylem traits, with A. lentiformis displaying lower xylem density, wider mean and maximum vessel diameters and higher hydraulic conductivity (K_s). They also differed in their leaf traits, such that A. lentiformis had larger leaves with higher specific leaf area (SLA) than A. polycarpa. Significant intra-specific differences occurred among leaf traits (leaf area, SLA) in A. lentiformis populations. In contrast, populations varied in their stem xylem structural traits (mean vessel wall thickness, mean vessel diameter, maximum vessel length) among A. polycarpa populations. Many of these differences were associated with soil salinity in A. lentiformis, and with minimum seasonal water potential in A. polycarpa. Overall, both saltbush species showed high intra- and inter-specific trait variation. This could be a critical consideration in understanding the evolution of these native species and has important implications for their conservation and restoration.

摘要

不同微生境两种滨藜属灌丛木质部的结构与功能

两种以前广泛分布的滨藜属(Atriplex)物种如今在南加州圣华金河谷沙漠的一系列微栖息地中以碎片化的种群出现。我们推测，由于土壤盐度和干旱程度的不同，大滨藜(A. lentiformis)和多果滨藜(A. polycarpa)在叶片和茎的结构和功能性状上存在种间和种内差异。我们比较了3个大滨藜种群和4个多果滨藜种群的水势、木质部结构功能和叶片性状。两个物种的木质部性状差异显著，其中大滨藜的木质部密度较低，导管平均直径和最大直径较宽，导水率较高。这两个物种的叶片性状也具有显著差异，其中大滨藜与多果滨藜相比，具有更大的叶片和更高的比叶面积。叶片性状在大滨藜不同种群间存在显著差异。相反，茎木质部结构特征(平均管壁厚度、平均管径和最大管长)在多果滨藜不同种群间存在差异。这些差异在很大程度上与大滨藜的土壤盐度有关，而与多果滨藜的季节性水势关系不大。总体而言，两种滨藜属灌木均表现出较高的种内和种间性状变异。这可能是理解这些本地物种进化的关键考虑因素，对它们的保护和恢复具有重要意义。

INTRODUCTION

Atriplex (Amaranthaceae) species have a nearly global distribution, including many aridland and halophytic species. Globally, these species are an important component of the ecosystems they inhabit, providing habitat, animal forage, and they are economically important for some regions (Squires and Ayoub 1994). Locally, woody Atriplex (saltbush) species are the dominant perennial vegetation of the native plant communities in the southern San Joaquin Valley, California, a region with a winter rainfall, Mediterranean-type, desert climate. Saltbush shrublands provide important habitat and food for animals within the San Joaquin Valley Desert (Germano et al. 2011). While saltbush communities in the San Joaquin Valley Desert were previously extensive, today they exist only as fragmented populations because large agricultural operations have transformed the area over the last 150 years (Coleman et al. 2021; Sankary and Barbour 1972). These remaining saltbush fragments occur across a range of microhabitats that vary in their water availability and salinity, with the two most common species being A. polycarpa (Torr.) S. Watson (cattle saltbush) and A. lentiformis (Torr.) S. Watson (big saltbush).

Structurally, these Atriplex species contain numerous characteristics that are likely adaptations to both aridity and salinity, including thick leaves with reflective, salt-filled and light-colored trichomes covering the leaf surfaces (Fig. 1; Supplementary Fig. S1) (Black 1954). Leaves display Kranz anatomy with numerous crystals located throughout the leaf mesophyll (Black 1954; Mooney et al. 1977; Supplementary Fig. S1), and they are C₄ photosynthetic (Muhaidat et al. 2007). As in other woody Atriplex species, the stem xylem is formed of anomalous secondary growth from numerous successive cambia, resulting in scattered bundles of xylem vessels and phloem occurring within a ground tissue predominantly composed of fibers (Fig. 2) (Fahn and Zimmermann 1982; Wiebe et al. 1974).

Several woody Atriplex species have the ability to alter their structural and functional traits in response to altered environmental conditions, both across natural sites and in controlled experiments. For example, leaf and stem structural changes were described in response to different substrate salinity in A. halimus (Blumenthal-Goldschmidt and Poljakoff-Mayber 1968; Boughalleb et al. 2009; Pérez-Romero et al. 2020). Leaf size and xylem density differed between populations of A. leucophylla along the coast (De Jong and Barbour 1979). Stem hydraulic traits and vessel anatomy varied across populations, ploidy level, and across habitats in A. canescens (Hao et al. 2013). Within-species variation has been identified as a key area of study (Olson 2021; Sack et al. 2016; Venturas et al. 2017), in general; and, specifically, intra-specific variation and potential for plasticity of xylem hydraulic traits has been a topic of much recent interest (Anderegg 2015; Fontes et al. 2022). This is a topic of particular concern in an era of climate change and land-use/land-cover change, with knowledge of population traits and potential variability of particular concern for conservation, potential restoration use and mitigation (Kerr et al. 2022). While previous studies have examined leaf and stem trait plasticity and adaptations of some Atriplex species to salinity, relatively little is known about A. polycarpa and A. lentiformis, the two most common and ecologically important Atriplex species found in the native plant communities of the San Joaquin Valley Desert. Intra-specific structural and functional properties of their stems and leaves have not been previously examined.

We hypothesized that an inter- and intra-specific (inter-site) variability exists between Atriplex shrubs in their xylem structure and hydraulic function, and these differences are linked to environmental differences. We examined this among A. lentiformis and A. polycarpa populations across five sites in the San Joaquin Valley Desert in southern California, two sites where both species co-occur and three additional sites were only a single species was present. Differences in leaf and stem traits may be adaptive and drive Atriplex species distributions across various microhabitats differing by their water availability and salinity. Differences in plant traits across species and microsites may be useful in identification of populations at risk, populations that could expand, and populations that could be good sources of seed for restoration projects.

MATERIALS AND METHODS

Study sites

We studied two Amaranthaceae species, A. lentiformis (Torr.) S. Watson and A. polycarpa (Torr.) S. Watson, which are both native to the San Joaquin Valley Desert of southern California. Field sites that contained one or both species were established (Table 1; Fig. 3; Supplementary Fig. S2). Three sites were located within Tejon Ranch and these included two sites located within the Chanac Creek watershed (‘Chanac East’ and ‘Chanac West’) and a site that corresponds to ‘Site 2’ previously described in Coleman et al. (2021) (referred to in the current study as ‘Coleman 2’). A fourth site was located on the California State University, Bakersfield campus within their Environmental Studies Area, corresponding with the location described in Coleman and Pratt (2019) (‘CSU Bakersfield’). Finally, a fifth site was located near Enos Lane along the Kern River Parkway (‘Enos Lane’). Within each site and for each species, six large, adult plants were selected for study and permanently tagged for repeated sampling, with a total of 42 plants tagged and used for measures across all species and sites.

Sites were established to represent those that appeared most dissimilar to one another within the region, including sites varying in visible water availability and surface salinity, shrub density, disturbance and elevation. Climate (1991–2020) and weather data from during the study period (2018–19) for the sites were gathered, including monthly precipitation, average minimum temperature, mean temperature and maximum temperature (PRISM Climate Group, Oregon State University, http://prism.oregonstate.edu, accessed 28 March 2023).We further examined and quantified environmental differences between sites by determining soil salinity, and by using water potential measures to compare plant water potential across sites (methods below). Broadly, sites may be characterized as: moist-saline (Chanac East, Chanac West, Enos Lane (Al)), moist and less saline (CSU Bakersfield) and arid and less saline (Coleman 2, Enos Lane (Ap)).

Soil salinity

Soil salinity was evaluated using an electrical conductivity (EC) method (EC_{1:5 w/v}). From each site, three soil samples were collected by core near to saltbush roots from a depth of 10–40 cm along the projection of the aerial crown circumference of the shrub. Each soil sample was then well mixed, and 1 g of dried soil was shaken with 5 mL of deionized water. The probe of a hand-held conductivity meter (Groline Soil Conductivity Tester, Hanna Instruments Inc., Woonsocket, RI, USA) was placed in the suspension, and the EC was measured.

Dry season water potentials

Midday water potentials were measured on all tagged plants (n = 6 per species per site) during the dry season (May, June, July and August) of 2019. Unbagged branchlets were collected and used to measure the leaf water potential (LWP) at midday (LWP). A second set of branches that was bagged with aluminum foil wrapped plastic bags for 2 h prior to collection (to prevent transpiration and equilibrate stems and leaves) was also collected, to measure midday stem xylem water potential (stem water potential; SWP). Water potentials were measured using a pressure chamber (Model 2000 Pressure Chamber Instrument, PMS Instruments, Corvalis, OR, USA). The lowest seasonal value of midday LWP and SWP for each individual at each site was used to calculate a mean site value for minimum seasonal water potential (P_min) at leaf and stem xylem levels.

Leaf traits

Leaf area (LA) and specific leaf area (SLA) of representative leaves were determined in August 2019. Ten leaves were collected from each tagged plant (6 individuals per species at each site). Area of the representative leaves was measured with an area meter (LI-3100 area meter, Li-COR Inc., Lincoln, NE, USA). Leaves were then placed in a drying oven at 60 °C for at least 3 days before their dry mass was measured (MSA225S100DU, Sartorius, Göttingen, Germany). SLA was calculated by dividing the LA by the leaf mass.

Leaves collected at the same time were used to determine leaf nitrogen content. Leaf nitrogen was measured as the % nitrogen (N) per dry weight. Collected leaves were dried and then ground to a fine powder with a custom ball mill. Dry leaf powder was weighed and nitrogen content per unit dry mass was analyzed using an elemental analyzer (ECS 4010, Costech Analytical Technologies Inc., Valencia, CA, USA).

Specific-hydraulic conductivity and vulnerability to embolism

Stem hydraulic parameters were measured in July–August 2019. Large branches (0.5–1 m) were removed from tagged plants while the section that was being cut was held underwater. This was done to prevent air entry into the cut end of the branch. The cut end was then transferred to a water-filled plastic centrifuge tube for transport back to the laboratory. The tube was secured to the end of the branch using plastic wrap and samples were placed in plastic bags with damp paper towels during transport.

In the laboratory, we selected straight and unbranched stem segments of approximately 6–8 mm diameter. The larger stems were trimmed underwater from both ends of the stem, until a length of 15 cm was reached. The stem ends were then further trimmed underwater with fresh razor blades (GEM single edge stainless steel Teflon-coated blades, Electron Microscopy Sciences, Hatfield, PA, USA) until a final sample length of 14 cm was reached. All samples were flushed for 1 h at 100 kPa with 20 mM degassed KCl solution that was filtered (in-line filter, Calyx Capsule Nylon 0.1 µm, GE Water & Process Technologies, Trevose, PA, USA). Maximum hydraulic conductivity (K_m) was measured using a hydraulic apparatus as described in Venturas et al. (2016). Xylem specific-hydraulic conductivity (K_s) was calculated by dividing the hydraulic conductivity by the distal xylem area.

Vulnerability to embolism curves were constructed using the standard centrifuge method and using hydration reservoirs (Tobin et al. 2013; Venturas et al. 2016). This method, particularly as employed in the current study, has been repeatedly tested and been shown to produce vulnerability curves that agree with several independent methods, including native-state embolism levels, and for species with both long and short vessel lengths (e.g. Hacke et al. 2015; Jacobsen and Pratt 2012; Tobin et al. 2013; Venturas et al. 2019). Additionally, Atriplex, including A. polycarpa, have very short mean vessel lengths when compared with angiosperms more broadly (Hacke et al. 2009). Hydraulic conductance was measured between each centrifuge spin and samples were analyzed until they had no remaining hydraulic conductivity or the pressure of −10 MPa was reached. Curves were constructed for each sample of the percentage loss in hydraulic conductivity at each pressure as fit using a Weibull curve. This curve was used to estimate the pressure at which 50% loss in hydraulic conductivity (P₅₀) occurred for each sample.

Xylem density

After hydraulics measures, a 2-cm long sample of undamaged tissue was excised from the end of each sample. Xylem tissue was saturated with water using vacuum infiltration for >12 h and the volume was calculated from the mass displacement of a sample submerged in water of known temperature and density (Analytical balance, MSA225S100DU, Sartorius, Göttingen, Germany). Samples were then dried in an oven (Thermo Scientific Precision 3051 Series, Fisher Scientific, Waltham, MA, USA) at 60 °C for ≥72 h and weighed to obtain dry mass. Xylem density was calculated as the saturated volume divided by the dry mass.

Mean diameter and mean wall thickness of xylem vessels

For anatomical measures, stem segments of diameter similar to those used for hydraulic samples (6–8 mm) were used. Thin cross-sections of stems from each of our tagged plants were made using a sledge microtome (Model 860 Microtome, American Optical Corp., Buffalo, NY, USA) or by hand (GEM single edge stainless steel Teflon-coated blades, Electron Microscopy Sciences, Hatfield, PA, USA). Cross-sections were placed on glass microscope slides and mounted with glycerol. Sections were examined and analyzed using a microscope and analysis software (Axio Imager. D2, Axiocam MRc and Axio Vision AxioVs40, v.4.8.2.0, Carl Zeiss MicroImaging GmbH, Gottingen, Germany). For vessel diameter, the vessel lumen area for at least 50 vessels was measured from each sample and these were converted to diameters using the assumption that vessel lumens were circular. From these vessels, the maximum vessel diameter (MVD) was determined, and the mean vessel diameter was calculated. We also measured the vessel wall thickness of at least 20 vessels per sample and these values were used to calculate the mean vessel wall thickness.

Maximum vessel length

Maximum vessel length was determined using an air-injection method (Greenidge 1952). Although this method likely overestimates vessel lengths, it is highly correlated to mean vessel length and therefore is useful as a comparative vessel length parameter across populations and species (Jacobsen et al. 2012). Large branches >1 m in length were harvested from plants in the field. Because of the large size of these required samples, these branches were taken from plants that were adjacent to our tagged individuals. These measures are often high variable, and so we used a larger sample size for these measures, with a minimum of six values per species and site. Stems were injected using nitrogen gas at an injection pressure of 100 kPa from branch tips toward the base. Branches were shortened at the basal end by ~5 cm increments. Maximum vessel length was estimated as the length of a branch at the first sight of streaming gas bubbles emerging from an open vessel plus half the length of the previously cut increment.

Analyses

Xylem and leaf traits were compared between species and sites using a general linear model (ANOVA) that included species and sites (species). For analyses of water potential, time of sampling was included as an additional parameter. Comparisons across sites and species were completed using a Tukey HSD analysis. Relationships among mean site traits for each species were examined using correlation analyses. Analyses were completed using Minitab (v. 17.2.1, Minitab, Inc., State College, PA, USA).

RESULTS

Soil salinity and water stress varied among sites and species

Soil salinity varied among sites (Table 1). The Enos Lane site where A. lentiformis occurred displayed the highest salinity (252 ppm), while the CSU Bakersfield A. polycarpa site was the least saline (31 ppm). Across sites, A. lentiformis populations occurred in sites with higher soil salinity, while A. polycarpa populations occurred in less saline areas.

Climatic parameters where generally similar across the sites, including longer-term averages (Supplementary Fig. S3) and conditions occurring during the study period (Supplementary Fig. S4). All sites receive relatively little precipitation, with this precipitation occurring predominantly during the winter months. The Chanac and Coleman 2 sites receive slightly more mean annual precipitation than the other sites (Table 1). Temperatures were similar across all sites.

Seasonal water stress was examined using two parameters, midday LWP and midday SWP. Both LWP and SWP varied during the seasons and between populations and between species (Figs 4 and 5). All populations of both species, except that of A. lentiformis in Enos Lane, had significantly higher water potentials during the end of the wet season (May) compared with those through the summer. During the peak of the dry season (July and August), A. polycarpa experienced lower LWP and SWP relative to A. lentiformis. There were significant differences between sites in water potentials for A. polycarpa but not for A. lentiformis (Table 2; Figs 4 and 5). The lowest water potentials were experienced by A. polycarpa shrubs at the Coleman 2 site.

Stem xylem structure and function differed among species and sites

The two saltbush species showed large differences between one another in their xylem structure and function, with significant species differences in nearly all measured traits (Table 2). Atriplex polycarpa stem-wood was significantly denser than that of A. lentiformis, and it contained narrower vessels (Figs 2 and 6). Hydraulically, A. polycarpa had much lower hydraulic efficiency (K_s) relative to A. lentiformis, although both species were similar in their vulnerability to embolism (P₅₀) (Figs 7 and 8). For A. polycarpa, P_min and P₅₀ were correlated across sites, with lower (more negative) P₅₀ occurring in the drier sites (Supplementary Fig. S5).

Across populations of A. polycarpa, we found differences in vessel wall thickness, vessel diameter and vessel length (Table 2). These differences were particularly apparent between our Chanac East and Coleman 2 sites (Fig. 6). At Chanac East, vessels were thin-walled, and narrow compared with wider and thicker-walled vessels at Coleman 2 (Fig. 6). Maximum vessel length was shortest at CSU Bakersfield and longest at Coleman 2 (Fig. 6). Across sites, differences in vessel length were correlated with P_min (Supplementary Fig. S6), but this was not the case for variation in most other traits.

Soil salinity was not associated with difference in stem xylem traits for A. polycarpa. In contrast, for A. lentiformis, both P₅₀ and mean vessel diameter (MVD) were highly correlated with soil salinity across sites (Supplementary Fig. S7).

Leaf traits varied between species and among sites

LA)and SLA varied both between species and between sites (Table 2; Fig. 9). Atriplex lentiformis leaves were much larger than those of A. polycarpa. In addition, this leaf trait showed a significant intra-specific difference between A. lentiformis populations (Fig. 9). Smaller leaves were correlated with lower site P_min for both species (Supplementary Fig. S8). However, LA was not influenced by soil salinity for both studied Atriplex species. Nevertheless, SLA was correlated with soil salinity in A. lentiformis populations. For A. polycarpa, LA was associated with P_min (Supplementary Figs S5 and S6). In both instances, SLA also decreased when the corresponding stress (salinity for A. lentiformis and water potential A. polycarpa) intensified. Species and sites did not differ in their leaf nitrogen content (Fig. 9).

DISCUSSION

Atriplex species exhibit differences in their structure and function

In the current study, two species of saltbush from the southern San Joaquin Valley Desert in California displayed large differences in their structure and function. Many of these differences appear to be associated with differences in microsite and habitat preferences, with two species occupying sites that varied in salinity and seasonal aridity. It is evident that salinity influences leaf anatomy and structure (Dolatabadian et al. 2011; Kılıç et al. 2007). Salinity affects have also been described in other species, for instance salinity significantly alters stem xylem structure in soybean (Dolatabadian et al. 2011). Atriplex lentiformis populations occurred at saline and moist sites. This species has previously been shown to tolerate waterlogging under saline conditions (Panta et al. 2016). Although saline, these moist soils likely contributed to the higher SWP and LWP of A. lentiformis populations relatively to the lower water potentials experienced by A. polycarpa at more arid and upland sites.

Both of our studied species displayed anomalous secondary xylem, with bundles of vessels and phloem surrounded by a matrix of fibers; yet, their xylem structure significantly differed. Atriplex lentiformis had lower xylem density and wider mean and MVDs compared with A. polycarpa. This corresponded with much higher hydraulic conductivity of the stem xylem in A. lentiformis, and stems that supported significantly larger leaves with higher SLA. Thus, the structure and function of these two species are well aligned with their different habitats, with A. polycarpa displaying traits associated with adaptation to increased aridity and A. lentiformis displaying more mesic traits. These patterns suggest that the saltier lowland soils occupied by A. lentiformis are more mesic, despite the presence of greater salinity.

At the leaf level, SLA decreased with the intensification of soil salinity or aridity in A. lentiformis and A. polycarpa, respectively. Soil salinity appeared to differentially influence the stem xylem structural traits of the two studied Atriplex species. In A. lentiformis, increasing salinity was associated with increases of mean vessel diameter, but the inverse pattern occurred for A. polycarpa (Supplementary Fig. S7). Similar to A. polycarpa, salinity has previously been reported to cause 25% reduction in diameter of xylem conduits in sorghum leaves (Baum et al. 2000). In addition, mean vessel wall thickness was not affected by salinity in A. lentiformis, while it decreased in A. polycarpa (Fig. 6). Inversely, other studies have found that in soybean plants and Populus euphratica seedlings salinity accelerated stem xylem development and increased wall thickness of stem xylem conduits (Dolatabadian et al. 2011; Zhou and Li 2015). Under salinity stress, the increase of mean vessel diameter and the no-impact on mean vessel wall thickness in A. lentiformis may testify to the salinity resistance of the species. These stem xylem traits might partially explain the ability of A. lentiformis to colonize moist sites having high soil salinity.

Woody Atriplex species in the arid western USA appear to vary in their hydraulic structure and function. Our A. polycarpa P₅₀, K_s and anatomical values were similar to those previously reported for this species from an arid desert site (Hacke et al. 2009; Jacobsen et al. 2007), but different from values from a third species, A. canescens. Both A. canescens and A. confertifolia were examined in a different study, with these species also differing from one another and displaying more negative P₅₀, narrower vessel diameters and higher wood density than the species we examined (Hacke et al. 2000). These inter-specific trait differences and high evolutionary plasticity may be one reason why Atriplex species are globally distributed, highly diverse and successful across arid regions (Brignone et al. 2019).

Populations exhibited intra-specific variation in xylem anatomy and leaf structure

Within our two study species, populations exhibited intra-specific differences in vessel wall thickness, vessel diameter, LA and SLA. Many of these trait differences were correlated with site differences in minimum seasonal water potential and soil salinity. This is consistent with prior studies that have found intra-specific differences in Atriplex structure and function with differences in soil type across sites (Sperry and Hacke 2002). Some Atriplex populations have been found to differ in their ploidy with concomitant changes in hydraulic traits (Hao et al. 2013). In our case, P₅₀ was highly correlated with P_min within A. polycarpa populations, and well correlated to soil salinity within A. lentiformis populations (Supplementary Figs S5 and S7). The most embolism resistant population was found at the driest site for A. polycarpa. The A. lentiformis population developing in the saltiest site was the most vulnerable to xylem embolism.

Different species and genera appear to differ in their population-level variation. Some studies have found population differences and/or evidence of plasticity (Jacobsen et al. 2014; Plavcová and Hacke 2012; Pratt et al. 2012; Sperry and Hacke 2002; Stojnić et al. 2018) while others have suggested that xylem trait variation is limited (Avila et al. 2021; Skelton et al. 2019). Leaves have also been shown to adjust their thermal tolerances among populations (Pearcy and Harrison 1974). Some species may even adjust traits seasonally in response to differing conditions (Jacobsen et al. 2007). Atriplex appear to exhibit high levels of trait change between populations, both within the current study and in prior published reports. This was particularly evident for A. polycarpa, which was able to colonize more variable microhabitats and exhibited differences in structure and function across sites. This could be valuable in selecting seed for restoration of specific sites and/or planning conservation activities to maintain microhabitat and population diversity.

CONCLUSIONS

The southern San Joaquin Valley is a Mediterranean-type climate (MTC) region desert and part of a biogeographic province characterized by high plant diversity and high threats to species (Esler et al. 2018; Mittermeier et al. 2011). Our study region has been heavily modified for agricultural uses with native vegetation existing as highly fragmented populations. Protection of existing native vegetation in microhabitats of this MTC region and restoration of affected ones could be ecologically beneficial. We found that populations of two woody Atriplex species are adapted to differing microsites. This may be attributed to their high structural and physiological plasticity, especially in A. polycarpa. Results of our study corroborate prior studies showing successful seedling recruitment of A. polycarpa in several microsites distributed along our study region (Coleman et al. 2021; Coleman and Pratt 2019). Understanding the evolutionary plasticity of the two studied Atriplex could be critical for their protection. In addition, A. polycarpa might be a promising native species for the restoration of affected microhabitats in the San Joaquin Valley Desert.

Supplementary Material

Supplementary material is available at Journal of Plant Ecology online.

Figure S1: Leaf cross-sections of A. lentiformis (left column) and A. polycarpa (right column) showing large salt-filled trichomes of the leaf epidermis, Kranz anatomy of the vascular bundles and the presence of crystals within the leaf mesophyll.

Figure S2: Study sites with representative photos of our study species from each site, including two sites where A. lentiformis and A. polycarpa co-occurred within a site (a–d) and three additional sites where only a single species was present (e and f).

Figure S3: Climate summary of the sites with data averaged over the period from 1991 to 2020, including monthly precipitation, temperature minimum, temperature mean and temperature maximum.

Figure S4: Weather conditions during the water-year leading up to the study period and continuing through the study period, including monthly precipitation, temperature minimum, temperature mean and temperature maximum.

Figure S5: Correlations between pressure at 50% loss of hydraulic conductivity (P₅₀) and minimum seasonal leaf water potential (P_min) measured in populations of A. lentiformis (a) and A. polycarpa (b) growing at different microsites in the San Joaquin Valley Desert in southern California, USA.

Figure S6: Correlations between two structural xylem traits (stem xylem density and maximum vessel length, MVL) and minimum seasonal leaf water potential (P_min) measured in populations of A. lentiformis (a and c) and A. polycarpa (b and d) growing at different microsites in the San Joaquin Valley Desert in southern California, USA.

Figure S7: Correlations between (a) pressure at 50% loss of hydraulic conductivity (P₅₀), (b) mean vessel diameter (MVD) and (c) specific leaf area (SLA) and minimum soil salinity measured in populations of A. lentiformis growing at different microsites in the San Joaquin Valley Desert in southern California, USA.

Figure S8: Correlations between two leaf traits (leaf area, LA; and specific leaf area, SLA) and minimum seasonal leaf water potential (P_min) measured in populations of A. lentiformis (a and c) and A. polycarpa (b and d) growing at different microsites in the San Joaquin Valley Desert in southern California, USA.

Funding

This work was supported by the National Science Foundation of United States of America (CREST HRD-1547784 supporting R.B.P., A.L.J., J.C.F. and V.C.) and a Fulbright Fellowship for ME (Fulbright Visiting Scholar Program 2018).

Acknowledgements

Angela Madsen is thanked for assistance with laboratory work. Tejon Ranch Conservancy and the Tejon Ranch Company are thanked for providing access to field sites and for permission to collect plant material.

Conflict of interest statement. The authors declare that they have no conflict of interest.

REFERENCES