WATER SOURCE DETERMINATION IN COTTONWOOD/ WILLOW AND MESQUITE FORESTS ON THE SAN PEDRO RIVER IN ARIZONA

K. A. Snyder, D. G. Williams, and V. L. Gempko

University of Arizona, Tucson, Arizona, School of Renewable Natural Resources, BSE 325, Tucson, AZ 85721

e-mail: ksnyder@ag.arizona.edu

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ABSTRACT

In semi-arid and arid regions the extreme spatial and temporal variations in plant-available moisture play a critical role in determining patterns of dominant species distribution and function, and consequently have important implications for ecosystem water balance. We identified water sources (precipitation, stream, soil moisture and/or groundwater) utilized by cottonwood (Populus fremontii), willow (Salix goodingii) and mesquite (Prosopis velutina) to determine if spatial and temporal water availability influenced root water uptake behavior of key riparian species along the San Pedro River in Arizona.

Results indicate that cottonwood and mesquite trees, along a perennial reach of this river did not utilize soil moisture derived from monsoon precipitation and primarily used groundwater. However, at an ephemeral site both of these species utilized monsoon derived soil moisture. Mesquite at the ephemeral site used a greater proportion of surface soil moisture relative to cottonwood. Willow did not appear to use appreciable amounts of surface soil moisture along perennial or ephemeral stream reaches. Mesquite likewise had more negative midday water potentials than did cottonwood and willow. In contrast, cottonwood and willow maintained midday water potentials around -1.5 MPa, suggesting that different levels of leaf water potentials among these dominant riparian trees limit transpiration.

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INTRODUCTION

Human alterations of hydrology and regional climate that impact terrestrial vegetation are of great concern. One such example is the alteration of riparian ecosystems by groundwater pumping and surface water diversions. In semi-arid and arid regions of the world, these impacts have produced dramatic changes in stand structure and species composition of riparian ecosystems (Stromberg and Patten 1990). The dynamics between groundwater and surface water in riparian ecosystems results in very heterogeneous distributions of plant-available moisture in space and time. Understanding plant responses to changes in this spatial and temporal moisture availability is crucial for predicting vegetation response in arid environments.

Recent isotopic studies in arid environments have shown that plants may not be using water from all potential sources (Dawson and Ehleringer 1991, Lin et al. 1996). Depth of water extraction has been found to vary among species (Ehleringer and Dawson 1992, Flanagan et al. 1992), as well as within a species (Donovan and Ehleringer 1994). Several studies have shown that obligate phreatophytes rarely utilize surface soil moisture (Dawson and Ehleringer 1991, Busch 1992, Kolb et al. 1997), but under some circumstances these species can switch from groundwater usage to water uptake from surface soil moisture (Dawson and Pate 1996). The physiological explanation for why some populations and species of trees are able to utilize surface water and others do not is unresolved. We hypothesized that species with access to a stable water source, in this case groundwater or perennial streamwater, would be less likely to expend carbon to grow lateral surface roots to acquire sporadic precipitation. We investigated seasonal patterns of water source utilization and water stress in semi-arid riparian tree species, to determine how variations in the spatial and temporal dynamics of moisture availability influence patterns of tree water-use (Fig. 1). Sites with different streamflow regimes and groundwater characteristics were selected to determine if variations in groundwater or stream water availability influence plant uptake of surface soil moisture.

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HYPOTHESES

• Species confined to riparian habitats (obligate phreatophytes) will not use surface soil moisture, whereas species that survive in riparian and non-riparian habitats (facultative phreatophytes) can switch to utilization of surface soil moisture during the summer monsoon season.
• Within a species, populations at sites with declining groundwater or streamwater availability will exhibit enhanced use of surface soil moisture. In contrast, populations with access to a stable source of water, in this case groundwater and perennial streamwater will be less likely to utilize monsoon-derived surface soil moisture.

To examine these hypotheses we contrasted the obligate phreatophytes, cottonwood (Populus fremontii) and willow (Salix goodingii), with a facultative phreatophyte, mesquite (Prosopis velutina) growing along ephemeral and perennial stream reaches.

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METHODS

In 1997, a study was conducted along the San Pedro River in southeastern Arizona. Sites in close proximity along the San Pedro river were selected to encompass varying hydrologic regimes and with the target species (cottonwood, willow and mesquite) present. The Lewis Springs (LS) site was located along a perennial reach of the San Pedro, where groundwater recharged the stream (see Pic.1). The Escapule Wash (EW) site was an ephemeral wash and streamflow recharged the groundwater aquifer (see Pic. 2). The ephemeral site appeared to be the boundary of willow and cottonwood. At EW the stand of cottonwood and willow was sparse and graded into a mesquite community upstream.

At each site 7-10 individuals of each species were randomly selected as study plants. Each individual was repeatedly sampled at key times (spring, summer drought and monsoon season) throughout the growing season to determine seasonal patterns of water source use. Stable isotopes of oxygen in xylem water extracted from twig samples were used as a natural tracer for measuring plant fractional uptake from groundwater, soil moisture, streamwater and precipitation (Ehleringer and Dawson 1992, Brunel et al. 1995). During each sample period, soils for isotopic analysis were collected at each site from 5, 10, 25, 50 and 100-cm depths. At each sampling period and at all sites, streamwater was collected and groundwater was sampled from wells with a peristaltic pump. Precipitation was collected at all sites in standard rain gages containing a layer of mineral oil to minimize evaporation. Precipitation was collected monthly throughout 1997 and more frequently during the monsoon season (July - September). Using a pressure chamber, water stress was quantified with measurements of predawn leaf water potential and midday leaf water potential .

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RESULTS

• Contrary to our predictions, all species, including mesquite, did not appear to use appreciable amounts of surface soil moisture along the perennial reach.
• Along the ephemeral reach, water stress was greater, and cottonwood and mesquite used surface soil moisture. As hypothesized, mesquite utilized the greatest proportion of surface soil moisture. In contrast, willow did not use surface soil moisture along either stream reach.

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RESULTS & DISCUSSION

Monsoon rains had values ranging from 1.8 to -3.0 (‰), and groundwater had stable values throughout the year, averaging -8.3 ‰ at LS, and -8.5‰ at EW. We found little variation in the values of cottonwood xylem water sampled at LS between the June drought and after a significant input of monsoon precipitation (26.7 mm) in August. This indicates that cottonwood did not utilize surface soil moisture at the perennial site (LS), and relied primarily on groundwater (Fig. 2), even after substantial monsoon rain. However, at the ephemeral site of cottonwood xylem water showed enrichment in August relative to xylem water sampled in June, indicating that a fraction of xylem water was derived from soil moisture. Cottonwood trees at EW experienced greater water stress with mean declining to -2.0 MPa during the summer drought, while cottonwoods at LS maintained mean of -1.5 MPa (Fig. 3).

Mesquite xylem water, sampled in August, showed substantial enrichment relative to xylem water sampled in June only at the ephemeral site (Fig. 2). Mesquite appeared to respond more dramatically than cottonwood to rainfall events at the ephemeral site with xylem water being enriched by 3.7‰ and 2.6‰, respectively. Indicating that mesquite derived a greater proportion of its total xylem water from surface moisture. Willow did not appear to use surface soil moisture at LS and showed only a small enrichment of (1.4‰) at EW. Willow experienced greater water stress during the summer drought at EW relative to LS, with average of -2.0 MPa and -1.0 MPa respectively (Fig. 3).

Leaf water potential patterns differed between the obligate phreatophytes and mesquite. The relationship between and at LS (Fig. 4) indicates that regardless of , are maintained at near -1.5 MPa for both willow and cottonwood. In contrast, mesquite exhibited considerable variation in . are a measure of soil moisture conditions with more negative values reflecting decreased soil moisture availability. The negligible slope of the relationship for cottonwood and willow indicates that these species maintain a critical level of as soil moisture availability. Maintenance of stable under conditions of decreased water availability may be accomplished either by 1) dynamic regulation of transpirational water loss by stomatal regulation 2) increased soil-to-leaf hydraulic conductance, or 3) decreased leaf area. Further investigations are needed to determine which of these mechanisms are controlling the observed patterns in leaf water potentials of these species.

CONCLUSIONS

Site hydrology influences the functional depth of riparian tree roots systems; all sources of available water are not used equally. The highly plastic behavior of mesquite must be considered in relation to water availability. Similarly, even obligate phreatophytes may exhibit flexibility in water sources at ecotones where the conditions for their survival are marginal. A high degree of reliance on a single water source, while other sources of water are available may be indicative of plants that are extremely vulnerable to xylem cavitation and exhibit highly regulated stomatal behavior. Previous research found that cottonwood and willow shoots could reach complete cavitation by xylem pressure of -2.0 MPa (Pockman et al. 1995). Cottonwood and willow at perennial site may be regulating to avoid xylem cavitation. At the ephemeral site cottonwood utilized precipitation derived soil moisture, perhaps because of the decrease in to -2.0 MPa, a level which is likely causing cavitation. Cottonwood and willow appear to rely on groundwater until irreversible cavitation levels are reached. Their tolerance of low water potentials is substantially less than that of mesquite, and they must maintain a high degree of transpiration regulation. These data indicate that declining water tables will have a disproportionate effect on the sustainability of obligate riparian trees, such as cottonwood and willow, which are critically tied to groundwater levels, and that the dynamics of water uptake of the dominant riparian species differs between stream types.

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LITERATURE CITED

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Dawson, T.E., and J.S. Pate, 1996: Seasonal water uptake and movement in root systems of phraeatophytic plants of dimorphic root morphology: a stable isotope investigation. Oecologia, 107, 13-20.

Donovan, L.A. , and J.R. Ehleringer, 1994: Water stress and use of summer precipitation in a Great Basin shrub community. Functional Ecology , 8, 289-297.

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Flanagan, L.B., J.R. Ehleringer, and J.D. Marshall, 1992: Differential uptake of summer precipitation among co-occurring trees and shrubs in a pinyon juniper woodland. Plant, Cell and Environment, 15, 831-836.

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Lin, G., S.L. Phillips, and J.R. Ehleringer, 1996: Monsoonal precipitation responses of shrubs in a cold desert community on the Colorado Plateau. Oecologia, 106, 8-17.

Pockman, W.T., J.S. Sperry, and J.W. O'Leary, 1995: Sustained and significant negative water pressure in xylem. Nature, 378, 715-716.

Stromberg J.C., and D. T. Patten, 1990: Riparian vegetation instream flow requirements: a case study from a diverted stream in the eastern Sierra Nevada, California. Environmental Management, 14, 185-194

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ACKNOWLEDGMENTS

Financial support from the USDA-ARS Global Change Research Program, NASA grant W-18,997 and USDA National Research Initiative Grant Program is gratefully acknowledged.