Facultative Phreatophytes Research Articles

Growth, earlywood anatomy and wood nutrients respond to precipitation and flow in semi-arid riparian tamarisk forests

Riparian forests from semi-arid regions with saline soils are dominated by facultative phreatophytes such as tamarisks (Tamarix spp.). Tamarisks tolerate drought and salt stress by using shallow and deep soil water sources depending on precipitation and flow variability in ephemeral or permanent streams. Therefore, their radial growth and wood anatomy would differently respond to precipitation and drought severity depending on stream ephemerality. To assess these responses, tree-ring-width, earlywood anatomy and sapwood nutrient concentrations were measured in five riparian tamarisk stands located in semi-arid north-eastern Spain. These sites experienced different aridity degrees and were located near ephemeral (e.g., Lanaja-dry site) or permanent streams in intensively irrigated agricultural areas (e.g., Valcuerna). Tree rings were widest in Peñaflor (2.94 mm) and narrowest in the driest Lanaja-dry site (1.11 mm), where tree-to-tree growth coherence was higher than in the other sites. Wet conditions during the water year enhanced growth, particularly in the most responsive Lanaja-dry site and in Lopín, whereas elevated flow increased the earlywood hydraulic diameter in Valcuerna. In the Lanaja-dry site, Ca, Cu, K and Mn wood concentrations were higher than in Valcuerna, but C and N concentrations were lower. Elevated Ca wood concentrations correspond to higher drought stress in the Lanaja-dry site, whereas higher N concentrations reflect eutrophication due to extensive use of agricultural fertilizers in Valcuerna. In sites near permanent streams such as Valcuerna, depth to groundwater may be a more robust proxy of growth than in drier sites located near more ephemeral streams such as Lanaja-dry site where tamarisk growth is constrained by 12-month long spring droughts.

Seasonal and Species‐Level Water‐Use Strategies and Groundwater Dependence in Dryland Riparian Woodlands During Extreme Drought

AbstractDrought‐induced groundwater decline and warming associated with climate change are primary threats to dryland riparian woodlands. We used the extreme 2012–2019 drought in southern California as a natural experiment to assess how differences in water‐use strategies and groundwater dependence may influence the drought susceptibility of dryland riparian tree species with overlapping distributions. We analyzed tree‐ring stable carbon and oxygen isotopes collected from two cottonwood species (Populus trichocarpa and P. fremontii) along the semi‐arid Santa Clara River. We also modeled tree source water δ18O composition to compare with observed source water δ18O within the floodplain to infer patterns of groundwater reliance. Our results suggest that both species functioned as facultative phreatophytes that used shallow soil moisture when available but ultimately relied on groundwater to maintain physiological function during drought. We also observed apparent species differences in water‐use strategies and groundwater dependence related to their regional distributions. P. fremontii was constrained to more arid river segments and ostensibly used a greater proportion of groundwater to satisfy higher evaporative demand. P. fremontii maintained ∆13C at pre‐drought levels up until the peak of the drought, when trees experienced a precipitous decline in ∆13C. This response pattern suggests that trees prioritized maintaining photosynthetic processes over hydraulic safety, until a critical point. In contrast, P. trichocarpa showed a more gradual and sustained reduction in ∆13C, indicating that drought conditions induced stomatal closure and higher water use efficiency. This strategy may confer drought avoidance for P. trichocarpa while increasing its susceptibility to anticipated climate warming.

Hydraulic traits that buffer deep‐rooted plants from changes in hydrology and climate

AbstractGroundwater‐dependent ecosystems are often defined by the presence of deeply rooted phreatophytic plants. When connected to groundwater, phreatophytes in arid regions decouple ecosystem net primary productivity from precipitation, underscoring a disproportionately high biodiversity and exchange of resources relative to surrounding areas. However, groundwater‐dependent ecosystems are widely threatened due to the effects of water diversions, groundwater abstraction, and higher frequencies of episodic drought and heat waves. The resilience of these ecosystems to shifting ecohydrological–climatological conditions will depend largely on the capacity of dominant, phreatophytic plants to cope with dramatic reductions in water availability and increases in atmospheric water demand. This paper disentangles the broad range of hydraulic traits expressed by phreatophytic vegetation to better understand their capacity to survive or even thrive under shifting ecohydrological conditions. We focus on three elements of plant water relations: (a) hydraulic architecture (including root area to leaf area ratios and rooting depth), (b) xylem structure and function, and (c) stomatal regulation. We place the expression of these traits across a continuum of phreatophytic habits from obligate to semi‐obligate to semi‐facultative to facultative. Although many species occupy multiple phreatophytic niches depending on access to groundwater, we anticipate that populations are largely locally adapted to a narrow range of ecohydrological conditions regardless of gene flow across ecohydrological gradients. Consequently, we hypothesize that reductions in available groundwater and increases in atmospheric water demand will result in either (a) stand replacement of obligate phreatophytic species with more facultative species as a function of widespread mortality in highly groundwater‐dependent populations or (b) directional selection in semi‐obligate and semi‐facultative phreatophytes towards the expression of traits associated with highly facultative phreatophytes in the absence of species replacement. Anticipated shifts in the expression of hydraulic traits may have profound impacts on water cycling processes, species assemblages, and habitat structure of groundwater‐dependent woodlands and riparian forests.

Explaining the hydrological behaviour of facultative phreatophytes using a multi-variable and multi-objective modelling approach

Trees in semiarid conditions survive despite water scarcity and shallow soils because they commonly have access to subsoil water resources. Currently, conventional models do not include groundwater transpiration and the results frequently underestimate the actual evapotranspiration and overestimate the net recharge. Therefore, in this work we focus on how a multi-variable calibration with a multi-objective approach may improve model robustness leading to a more realistic closure of the water balance in two models (LEACHM and TETIS) of different conceptualisation taking into account the specific characteristics of a facultative phreatophytic forest. The results suggest that the common single-variable and single-objective calibration is not able to measure all system’s characteristics. However, the multi-variable and multi-objective calibration proved a good option to reproduce the water dynamics of a facultative phreatophytic forest and confirmed that groundwater transpiration is an important water source for them. Therefore, hydrological models should include this mechanism and both LEACHM and TETIS proved an acceptable tool to be applied in the regions covered by this species.

Hydroclimatic drivers of the growth of riparian cottonwoods at the prairie margin: River flows, river regulation and the Pacific Decadal Oscillation

Cottonwoods, riparian poplars, are facultative phreatophytes and can obtain water from shallow soil moisture originating from rainfall, or from the deeper capillary fringe above the alluvial water table that is recharged by river water infiltration. The correspondence between cottonwood growth and river flows should reveal the dependency upon alluvial groundwater and subsequently, the vulnerability to reduced river flows. To explore this association, we analyzed historic growth patterns of plains cottonwoods (Populus deltoides) along the Red Deer River (RDR), which is at the northwestern limit of the North American Great Plains. We developed chronologies of yearly radial increments (RI) and basal area increments (BAI) and explored correspondences with the environmental records from the past century. In this semi-arid region, the RI or BAI were not correlated with local precipitation while negative correlation with growth season temperature (T) (r = −0.37, p < 0.01) could reflect reduced growth with hot summers. There was correlation between growth and annual river discharge (Q, and particularly log Q that approximates river stage) and this increased with two year averaging (r = 0.51, p < 0.01), reflecting carry-over in the watershed hydrology and in the ecophysiological response. There was correspondence with the Pacific Decadal Oscillation index (PDO, r = −0.45, p < 0.01), which provides multi-decade transitions that influence Rocky Mountain headwater precipitation and other weather characteristics, and river flows. The combination of Q, PDO and T provided the strongest multiple regression model, accounting for 44% of the historic growth variation (52% correspondence for 1953–2013). The RDR was dammed in 1983, enabling winter flow augmentation, but summer flows were sustained and cottonwood growth and the streamflow correspondence persisted. This indicates that it is the pattern of dam operation and not damming per se that determines the fate of established riparian cottonwoods downstream. This study revealed that these cottonwoods are phreatophytic and dependent upon alluvial groundwater that is recharged from the river. This provides a research strategy to determine whether riparian woodlands along other regulated rivers are similarly groundwater-dependent and could be vulnerable to river flow reductions from excessive water withdrawal for irrigation or other uses, or with climate change.

Overstorey evapotranspiration in a seasonally dry Mediterranean eucalypt forest: Response to groundwater and mining

AbstractGroundwater levels in the northern jarrah forest have declined at rates up to 0.5 m year−1 owing to increased aridity in south‐western Australia in the last 40 years. The forest has also been mined and rehabilitated resulting in significant areas of postmining forest. We tested the impact of declining groundwater levels and mining on evapotranspiration by jarrah forest overstorey. We hypothesized that trees in jarrah forest are facultative phreatophytes (will use groundwater where available but are not reliant on it) and water use per unit overstorey leaf area index (Los) of postmining forest is the same as that of postharvest forest. We measured sapflow at 7 sites in the northern jarrah forest and measured rainfall interception by the canopy at 9 sites. Stemflow was measured at 3 sites. Shallow depth to groundwater was associated with a larger ratio of transpiration per unit leaf area (Eos/Los), but there was little difference in Eos/Los between postmining and postharvest jarrah forest. Eos/Los ranged from 250 ‐ 340 mm year−1 (m2 m−2)−1 at sites where depth to groundwater was &gt;15 m but was up to 400–500 mm year−1 (m2 m−2)−1 at some sites with shallow groundwater. Based on relationships between transpiration, rainfall interception, and Los, it is possible to estimate overstorey evapotranspiration in jarrah forest from Los, especially if spatial layers are available for depth to groundwater. We conclude that jarrah forest is conservative in its water use and likely to be resilient to a drying climate. Management implications for the northern jarrah forest are briefly discussed.

Water uptake by saltcedar (Tamarix ramosissima) in a desert riparian forest: responses to intra‐annual water table fluctuation

AbstractThere is considerable interest in naturalizing flow regime on managed rivers to slow the spread of saltcedar (Tamarix ramosissima) invasion in southwestern USA or to preserve riparian forests dominated by saltcedar and other species in northwestern China. However, little is known about the responses of established saltcedar in water sources to frequent intra‐annual fluctuation of water table resulting from this new, more dynamic flow regime. This study investigates how saltcedar at a riparian site in the middle reaches of the Heihe River, northwest China, responds in water sources use to intra‐annual water table fluctuations. Stable oxygen isotope was employed to determine accurate depth at which saltcedar obtains its water supply, and soil moisture monitoring was used to determine sources of plant‐available soil water. We found that the primary zone of water uptake by saltcedar were stable at 25–60 cm depth, but the water sources used by saltcedar switched between groundwater and soil moisture with the water table fluctuations. Saltcedar derived its water from groundwater when water table was at depth less than 60 cm but switched to soil moisture at 25–60 cm depth when water table declined. It is supposed that the well‐developed clay layer at 60–80 cm depth constrained lateral roots of saltcedar to the soil layers above 60 cm, while the fine‐textured soils at this site, which were periodically resaturated by rising groundwater before the stored soil moisture had become depleted, provided an important water reservoir for saltcedar when groundwater dropped below the primary zone of fine roots. The root distribution of saltcedar may also be related to local groundwater history. The quick decline in water table in the early 1980s when the riparian saltcedar had established may strand its roots in the shallow unsaturated zone. We suggested that raising the water table periodically instead of maintaining it invariably above the rooting depth could sustain desired facultative phreatophytes while maximizing water deliveries. Copyright © 2015 John Wiley &amp; Sons, Ltd.

Rapid root elongation by phreatophyte seedlings does not imply tolerance of water table decline

Despite high rates of root elongation during phreatophyte establishment, once connection to groundwater has occurred and leaf area develops, seedlings demonstrate limited capacity for root elongation in response to groundwater decline. In a water-limited environment, rapid root elongation immediately after germination can be critical for a plant to reach deeper water sources such as a water table to avoid water deficit stress. However, once plants have accessed a water table, their continued survival may depend on their ability to adapt their root distribution to changes in the depth to a water table. In glasshouse experiments using two Banksia species with contrasting water requirements, we investigated (1) the rate of root elongation by young seedlings in the presence of a shallow water table, and (2) whole plant response to rapid water table decline using older seedlings that had established root contact with a water table. The results of the first experiment agree with the hypothesis that the facultative phreatophyte, B. attenuata, has a faster rate of root elongation than the obligate phreatophyte, B. littoralis. These differences are likely related to the contrasting habitat preferences of the two species. Older seedlings in the second experiment demonstrated a water-saving response to a declining water table, rapidly closing stomata to limit water loss. Additionally, roots did not elongate to follow the water table and plants were quickly disconnected from the saturated zone. For the two phreatophytic Banksia species, the capacity for rapid root growth by young seedlings did not translate to an ability for established seedlings to adapt their root distribution to survive rapid water table decline.

Hydrotropism in lateral but not in pivotal roots of desert plant species under simulated natural conditions

Root hydrotropism has been widely studied in seedling radicles through artificial experiments that reduce the influence of gravity and soil. In this work we aimed to study hydrotropism of primary lateral and pivotal roots in developed root systems of desert plants under simulated natural conditions. We grew Bulnesia retama Griseb. (non phreatophyte), Prosopis flexuosa DC. (facultative phreatophyte) and Prosopis alpataco Phil.(obligate phreatophyte) seedlings in observation boxes with sand. Lateral and pivotal roots were stimulated by lateral water gradients and hydrotropic responses, root proliferation and root : shoot ratios were measured. We found that 65 ± 15 % of lateral roots that grew in response to water gradients in B. retama, 84 ± 8 % in P. flexuosa and 88 ± 8 % in P. alpataco displayed hydrotropism. Conversely, pivotal roots did not show hydrotropic growth. This was accompanied by root proliferation inside water patches, and biomass partitioning to shoot growth. Our results provide evidence that root hydrotropism is a relevant response that could occur in nature. Lateral and pivotal roots manifest different hydrotropic responses under the conditions assayed. The combination of hydrotropism and precise root proliferation can shape root architecture, leading to optimum water patch exploration.

Patterns, magnitude, and controlling factors of hydraulic redistribution of soil water by Tamarix ramosissima roots

Tamarix spp. (Saltcedar) is a facultative phreatophyte that can tolerate drought when groundwater is not accessed. In addition to deep water uptake, hydraulic redistribution (HR) is another factor contributing to the drought tolerance of Tamarix spp. In this study, data on soil volumetric moisture content (θ), lateral root sap flow, and relevant climate variables were used to investigate the patterns, magnitude, and controlling factors of HR of soil water by roots of Tamarix ramosissima Ledeb. in an extremely arid land in Northwest China. Results showed evident diurnal fluctuations in θ at the depths of 30 and 50 cm, indicating “hydraulic lift” (HL). θ increased remarkably at 10 and 140 cm but decreased at 30 and 50 cm and slightly changed at 80 cm after rainfall, suggesting a possible “hydraulic descent” (HD). However, no direct evidence was observed in the negative flow of lateral roots, supporting HR (including HL and HD) of T. ramosissima. The HR pathway unlikely occurred via lateral roots; instead, HR possibly occurred through adventitious roots with a diameter of 2–5 mm and a length of 60–100 cm. HR at depths of 20–60 cm ranged from 0.01–1.77 mm/d with an average of 0.43 mm/d, which accounted for an average of 22% of the estimated seasonal total water depletion at 0–160 cm during the growing season. The climate factors, particularly vapor pressure deficit and soil water potential gradient, accounted for at least 33% and 45% of HR variations with depths and years, respectively. In summary, T. ramosissima can be added to the wide list of existing species involved in HR. High levels of HR may represent a considerable fraction of daily soil water depletion and substantially improve plant water status. HR could vary tremendously in terms of years and depths, and this variation could be attributed to climate factors and soil water potential gradient.

Root architecture of riparian trees: river cut-banks provide natural hydraulic excavation, revealing that cottonwoods are facultative phreatophytes

Plant root architecture reveals the sources of water and nutrients but tree root systems are large and difficult to analyze. With riparian (floodplain) trees, river cut-banks provide natural hydraulic excavation of root systems and this presents a unique study opportunity. Subsequently, we developed the ‘Cut-bank Root Method’, a simple, quantitative approach for analyzing the distribution of coarse roots, based on analyses of photographs of river cut-banks. These reveal the vertical extent of roots and median root depths (Rd). We applied this method along six rivers draining the Canadian Rocky Mountains and observed tenfold difference in Rd. The floodplain forests were dominated by cottonwoods and from mountain to prairie zones we observed progressively deeper roots of Populus trichocarpa (black cottonwood, Rd ~ 0.3 m), P. balsamifera (balsam poplar), P. angustifolia (narrowleaf cottonwood), and P. deltoides (prairie cottonwood, Rd ~ 0.9 m), which had Rd similar to P. fremontii (Fremont cottonwood) in Nevada, USA. Roots were shallower for co-occurring facultative riparian trees, with Rd ~ 0.1 m for P. tremuloides (trembling aspen) and Picea glauca (white spruce). Across the Canadian sites, Rd for cottonwoods were strongly associated with a growth season moisture index (May through September precipitation—potential evapotranspiration; R2 = 0.97, P < 0.001). Thus, in wetter climates, riparian cottonwoods were shallow-rooted and would be more dependent upon rain than stream flow. Conversely, in the drier semi-arid regions the cottonwoods were phreatophytic, with deeper root systems in the capillary fringe above the alluvial ground-water table. These phreatophytic cottonwoods would be highly dependent upon stream flow and vulnerable to declining river flows due to river regulation or climate change.

Water stress vulnerability of four Banksia species in contrasting ecohydrological habitats on the Gnangara Mound, Western Australia

This study investigated the interspecific differences in vulnerability to xylem embolism of four phreatophytes - two facultative phreatophytes (Banksia attenuata and B. menziesii) and two obligate phreatophytes (B. ilicifolia and B. littoralis). Species differences at the same position along an ecohydrological gradient on the Gnangara Groundwater Mound, Western Australia were determined in addition to intraspecific differences to water stress between populations in contrasting ecohydrological habitats. Stem- and leaf-specific hydraulic conductivity, as well as Huber values (ratio of stem to leaf area), were also determined to support these findings. We found that where water is readily accessible, there were no interspecific differences in vulnerability to water stress. In contrast both facultative phreatophyte species were more resistant to xylem embolism at the more xeric dune crest site than at the wetter bottom slope site. B. ilicifolia did not differ in vulnerability to embolism, supporting its classification as an obligate phreatophyte. Other measured hydraulic traits (K(S), K(L) and Huber value) showed no adaptive responses, although there was a tendency for plants at the wetter site to have higher K(S) and K(L). This study highlights the influence site hydrological attributes can have on plant hydraulic architecture across species and environmental gradients.

Growth and water and nitrate uptake patterns of grazed and ungrazed desert shrubs growing over a nitrate contamination plume

Two native desert shrubs were evaluated for their growth potential and water and nitrogen uptake patterns over a nitrate-contaminated aquifer at a former uranium ore-processing facility in northeastern Arizona. Sarcobatus vermiculatus and Atriplex canescens are obligate and facultative phreatophytes, respectively, that dominate the local desert plant community. The main questions we addressed were: (1) Are these shrubs able to use water or nitrogen from the alluvial aquifer? (2) If so, does grazing interfere with that ability of shrubs? (3) What would be the ideal strategy to take up N from the plume and prevent its expansion and recharge using shrubs? δ 18O and δD isotope signatures from water in plant stem samples suggest that both species utilize mainly deep, stored soil water derived from winter rains for transpiration, rather than summer rains or plume water. δ 15N enrichment values were similar for leaves of plants growing on and off the plume and for soil and aquifer water samples, but nitrate-N levels in leaf tissues were five times higher in plants growing on the plume compared to off the plume, suggesting they may have derived at least part of their nitrogen from the contamination plume. Total leaf N was also higher for plants growing on the plume. Under present conditions, only about 5% of the area over the plume is vegetated. Plants protected from grazing inside exclosures increased in volume by 2–4-fold over three growing seasons. Transplants of A. canescens, protected from grazing and irrigated over the first summer, established readily and grew into large shrubs after 3 years. On the basis of this study, the shrub community could be increased to as high as 25% cover and could make a significant contribution to controlling recharge if the contaminated site was protected from grazing. The results suggest that deeply rooted desert shrubs can impact the subsoil water and nitrogen balance, and that this balance can be disrupted by land use practices such as overgrazing that degrade the vegetation cover.

Seedling growth of Acacia tortilis and Faidherbia albida in response to simulated groundwater tables

The growth and survival of woody seedlings in arid and semi-arid riverine forests is partly determined by the ability of their roots to maintain contact with the alluvial water table. After a flood event, the water table declines in conjunction with the river stage and the rate of decline may have species-specific effects on the regeneration of riverine trees. In this study, a growth experiment involving seedlings of Acacia tortilis and Faidherbia albida, two keystone tree species along the regulated Turkwel River in northern Kenya, was conducted to assess the effects of simulated pre- and post-dam water table dynamics. Seedlings were grown in rhizopods with constantly shallow water tables (0 cm day −1 treatment), slow rates of water table decline (5 cm day −1 treatment), rapid rates of water table decline (10 cm day −1 treatment) and no subsurface water (rainfall treatment). The last treatment corresponded to the dam-induced paucity of floodplain inundation but allowed early seedling growth in response to rainfall. Results showed that F. albida attained larger shoot growth but shorter root lengths than A. tortilis, while water table decline promoted root elongation in both species. However, F. albida seedlings were adversely affected by moisture deficits under the rapid rate of water table decline and rainfall treatments. In contrast, the growth of A. tortilis seedlings was sustained under all treatments, suggesting that A. tortilis is a drought tolerant facultative phreatophyte, while F. albida behaves more like a near obligate phreatophyte. For F. albida, the ideal stage recession rate following a flood event is 5 cm day −1 or less, which is also favourable for A. tortilis. Although more research is still needed on the hydrological preferences of woody seedlings in the Turkwel River floodplain, it is evident that the regeneration of strictly riverine trees such as F. albida depends on slow rates of water table decline in the post-flood period.

Groundwater use by vegetation in a tropical savanna riparian zone (Daly River, Australia)

Soil water matric potentials ( Ψ m) and the deuterium ( δ 2H) composition at natural abundance levels of xylem water, soil water, river water and groundwater were used to evaluate whether trees use groundwater during the dry season in the riparian zone of the Daly River (Northern Territory, Australia). Groundwater was a significant source of water for plant transpiration, probably accounting for more than 50% of the water transpired during the dry season. Groundwater use occurred either when trees used water from the capillary fringe or when low Ψ m induced by soil water uptake lifted groundwater in the vadose zone. Several water use strategies were inferred within the riparian plant community. Melaleuca argentea W. Fitzg and Barringtonia acutangula (L.) Gaertn. appeared to be obligate phreatophytes as they used groundwater almost exclusively and were associated with riverbanks and lower terraces with shallow (<5 m) water tables. Several species appeared to be facultative phreatophytes (including Cathorium umbellatum (Vahl.) Kosterm. and Acacia auriculiformis A. Cunn. ex Benth.) and tended to rely more heavily on soil water with increased elevation in the riparian zone. The levee-bound Corymbia bella K.D. Hill and L.A.S. Johnson mostly used soil water and is either a facultative phreatophyte or a non-phreatophyte. The temporal variability in groundwater utilisation by the trees is unclear because the study focused on the end of the dry season only. A decline in the regional water table as a result of groundwater pumping may affect the health of riparian zone vegetation in the Daly River because groundwater use is significant during the dry season.

Impact, Biology, and Ecology of Saltcedar (Tamarixspp.) in the Southwestern United States

Eight species ofTamarixwere first brought to North America in the 1800s from southern Europe or the eastern Mediterranean region. Many of the species escaped cultivation and by the 1920s invaded about 4,000 ha of riparian habitat in the southwestern United States. By 1987, it was estimated to have increased to at least 600,000 ha. The success of saltcedar in the southwest can be attributed to several factors related to its growth habit, reproduction, water usage, ability to tolerate highly saline conditions, and redistribution of salt from deep in the soil profile to the soil surface. The flowers produce small, numerous, and tufted seeds that can be carried long distances by wind or water. The seeds, however, have a short period of viability, and must come in contact with suitable moisture within a few weeks of dispersal. Unlike obligate phreatophytes, such as willows and cottonwoods, saltcedar is a facultative phreatophyte and is often able to survive under conditions where groundwater is inaccessible. The high evapotranspiration rates of saltcedar can lower the water table and alter the floristic composition in heavily infested areas. Mature plants are tolerant to a variety of stress conditions, including heat, cold, drought, flooding, and high salinity. Saltcedar is not an obligate halophyte but survives in areas where groundwater concentrations of dissolved solids can average 8,000 ppm or higher. In addition, the leaves of saltcedar excrete salts that are deposited on the soil surface under the plant, inhibiting germination and growth of competing species.

Water Relations and Transpiration of Honey Mesquite on 2 Sites in West Texas

Transpiration rates and internal water relationships of honey mesquite (Prosopis glandulosa) were investigated weekly during May through September 1986 on sandy loam and clay loam, both upland sites in west Texas. Average transpiration rates peaked at approximately 7 mmol m-2 s-1 at 1100 hr during wet periods and reached a plateau between 4 and 5 mmol m-2 s-1 between 1200 and 1400 hr. During dry periods, the average transpiration rates reached their maximum plateau of 2 mmol m-2 s-1 at 1000 hr and declined between 1200 and 1600 hr. The transpiration rates ranged from an average of 3.28 +/- 2.05 mmol m-2 s-1 for trees on a sandy loam site to an average of 3.85 +/- 1.94 mmol m-2 s-1 for those on a clay loam site. Stomatal closure in midsummer caused a substantial increase in leaf temperature. Mesquite has developed other means, such as leaf orientation, wax accumulation, and reduction in canopy development, to avoid drought. Stomatal conductance of mesquite is very responsive to soil water availability and dryness of the air, and is less responsive to internal water status. This research further substantiates that mesquite behaves like a facultative phreatophyte in west Texas.

‘White Storm’ and ‘Dark Storm’ Desert Willow

Abstract Desert willow [Chilopsis linearis (Cav.) Sweet.] (7, 12) is a facultative phreatophyte (a plant whose roots will extract water from underground streams, a water table, or perched sources, or the strata immediately above these sources, but which can also function well in the absence of these water supplies). It is a tall shrub to a small tree (2, 3) and is native to the American southwest, from Baja California, southern California, southern Nevada, Arizona, and New Mexico to southwestern Texas and northern Mexico (8, 9).