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Overland water flow contributes little to survival, growth, reproduction, and ecophysiology of Olneya tesota (desert ironwood) trees.

Phreatophytes are deeply rooted plants that use groundwater to fulfill parts of their water needs (Thomas, 2014). In deserts, phreatophytes are typically found in washes, where runoff provides substantial amounts of water close to the surface for at least parts of the year. This suggests that these phreatophytes depend on surface runoff at least for their establishment phase. Do they also need it for survival as mature plants? That question is becoming important in many deserts worldwide, as construction of roads and other infrastructure disrupts and rechannels surface flow. In California and elsewhere, construction of enormous solar energy plants in the desert has potentially large effects on runoff patterns as well as on groundwater tables, either through redirection of surface flow or direct use of groundwater by many of these facilities.

There is abundant evidence for strong effects of overland water runoff on desert plants (Walter, 1963). Higher plant water potentials and increased stomatal conductance were observed in shrubs in washes and areas that received overland water runoff compared with areas that did not (Schlesinger and Jones, 1984; Ehleringer and Cooper, 1988; Schlesinger et al., 1989; Monson et al., 1992; Smith et al., 1995). Such effects allow for greater carbon uptake and plant growth. Increased seed production can result in more germination and in increased plant densities (Schlesinger and Jones, 1984; Siemens, 1994; Siemens and Johnson, 1995).

Previous research on responses of desert plants to overland flow has focused on relatively shallow-rooted shrub species, and it is unknown whether deeply rooted desert trees are similarly affected by the anthropogenic diversion of such flow. our study compared Olneya tesota (desert ironwood, Fabaceae) trees with and without access to overland flow. Olneya tesota is a common and characteristic legume tree in the Sonoran Desert of southwestern North America. As with most desert trees, this species is largely restricted to washes, at least in the western part of its range (Turner et al., 1995); and therefore, it appears to require a relatively stable water source that is, in part, provided by runoff that accumulates in washes. It is semideciduous (Dimmitt, 2000), dropping its leaves during freezing temperatures or before flowering, and is deeply rooted as are most desert tree species (Cannon, 1911; Daubenmire, 1947; Phillips, 1963; Jennings, 1974; Canadell et al., 1996; Schenk and Jackson, 2002; Stromberg, 2013); however, its maximum rooting depth is unknown. Desert ironwood trees are estimated to live [greater than or equal to] 300-800 years (Dimmitt, 2000), usually have multibranched trunks, and can grow to heights of 6-10 m, with branches often flexing down to the ground. Olneya flowers develop between March and June, but large blooms occur only 2 out of every 5 years (Dimmitt, 2000). The pollinators are bees, mainly Centris pallida (Simpson, 1977). The result of this pollination is a legume, which contains only one seed 70% of the time (Siemens and Johnson, 1995).

In the study area, Olneya trees grow in dry washes that have been bisected by man-made sand berms for [greater than or equal to] 60 years . A large solar farm under construction close to the study site will use local groundwater, potentially threatening the survival of these trees if they rely largely on groundwater. The berms divert overland water runoff, thereby creating areas upstream from the berms with uninterrupted overland flow and downstream from the berms where the supply of overland flow is highly restricted. Because overland flow can be a very substantial source of water in deserts (Walter, 1963; Schlesinger and Jones, 1984; Schlesinger et al., 1989), we hypothesized that mature Olneya trees with access to it would have higher leaf water potentials, lower leaf carbon isotope ratios, lower leaf mass per leaf area, as well as higher branch growth, flower and seed production, and higher population density.

METHODS--Site Description--We conducted our study between July 2005 and June 2006 in the Sonoran Desert near Chuckwalla Valley Road, Riverside County, California (115[degrees]10'00"N, 33[degrees]38'00"W) at 250 m above sea level. This site is located next to Interstate Highway 10, 17 miles (27.4 km) east of the town of Desert Center.

We used O. tesota trees that were located in dry washes on the toe portion of an alluvial fan on Quarternary alluvium. Manmade sand berms, approximately 1.5 m high, dissect the dry washes and redirect runoff to channels underneath the I-10 Highway, which was constructed after the berms that were created in the early 1940s to prevent stormwater from washing over the old U.S. Route 60 (M. Dimmitt, pers. comm.). Olneya trees upstream of the berms receive overland runoff, while trees downstream from the berms do not. The depth to groundwater at the study site appears to be below 18 m (California State Water Resources Control Board, in litt.), but at the nearby Corn Springs palm oasis it is as shallow as 6 m (Lewis and Morris, 2002; P. E. Godfrey, Hydrologist, BLM California Desert District, pers. comm.).

Wash and Tree Selection--We paired trees upstream (+ overland flow) and downstream from the berms (- overland flow) in 8 washes that we randomly selected from 20 suitable washes in which water once flowed normally, prior to being bisected by the installation of a berm. We selected washes that contained five acceptable trees in the upstream and downstream portions, which were [greater than or equal to] 2 m tall, alive, with no damage due to human activity, and no closer to the berm than 50 m. We randomly chose each sampled tree from the set of five trees per wash section, for eight total pairs of trees, one each from the upstream and downstream portions of the eight randomly selected washes.

Tree Population Densities--We examined tree population densities in 250-[m.sup.2] areas from satellite images with a resolution of 15 m. We took images from Google maps (http://maps. google.com) in october 2006, and counted trees in 16 250-[m.sup.2] areas, 8 upstream and 8 downstream from berms.

Leaf Mass Per Leaf Area--We measured leaf mass per leaf area by collecting 30 leaves randomly from the northern side and outer portion of each tree in order to limit variability in light exposure. We determined the area of each leaf using a Delta-T Area Meter (Delta-T Devices Ltd., Cambridge, United Kingdom); we then oven-dried leaves for 48 h at 65[degrees]C, and weighed them to the nearest 0.01 mg.

Branch Growth--We randomly chose 15 secondary, greenbarked branches per tree. We randomly selected 10 of these to determine growth in length (cm) of branches over one growing season (June 2005 to September 2005).

Flower Production--We used a cube-shaped, 0.25-[m.sup.3] framing device to estimate flower production per volume of canopy during a flowering event in May 2006. We randomly inserted the device into three different locations on the outer portion of the trees, and counted every flower observed within it.

Seed Production--We determined seed production during the same flowering event as for flower production; we accomplished this by collecting 10 mature pods and dissecting them in the lab to determine the number of seeds per pod. We classified pods as mature when they had turned from green to a reddish-brown or brown color.

Water Potentials--We measured predawn and midday leaf water potentials with a pressure chamber (PMS model 1000; PMS Instrument Company, Albany, oregon) using standard procedures. We measured water potentials once per month, starting in July 2005 and ending in June 2006.

Carbon Isotope Ratios--The leaves that we used to measure leaf mass per area we also used to determine leaf carbon isotope ratios. We combined leaves from one tree into one sample, placed the sample in a mortar, and ground it until a homogenous mixture was formed. We half-filled a 1.0-mL container with the homogenous leaf mixture and placed it into a Crescent Wig-L-Bug grinder (Dentsply, York, Pennsylvania). We ground the mixture for 30 s, resulting in a fine powder. From the powder, we placed 1.5 mg [+ or -] 0.1 mg of the leaf powder into 5 x 9-mm tin cups. We then sent the leaf samples to University of California, Irvine, for analysis of carbon isotope ratios via mass spectrometry (Dawson et al., 2002).

Statistical Analysis--Except for water potentials, we statistically analyzed all data using paired t-tests to test for differences between paired trees located in the same washes upstream and downstream from berms. Alternative analyses of the same data using two-sample t-tests, which assume statistical independence of trees located in the same washes, did not change any of the results, so we herein report only the results from paired t-tests. We analyzed water potentials with repeated-measures analysis of variance using SYSTAT (version 9.01; SPSS Inc., Chicago, Illinois), followed by planned contrast comparisons. In all cases, we compared upstream trees with downstream trees.

RESULTS--Tree population densities showed a slight difference with upstream areas having a higher density than downstream (P = 0.54). Trees in upstream areas had branches that were significantly longer than those on trees found in the downstream areas (P = 0.02). All other morphological measures did not differ significantly between upstream and downstream trees.

Upstream trees had slightly higher predawn leaf water potentials than downstream trees (P = 0.02), with significant differences between upstream and downstream trees in July (P < 0.01) and March (P = 0.04; Fig. 1). Midday water potentials did not differ consistently, but we found significant differences during the months of July 2005 (upstream lower, P < 0.001), October 2005 (upstream higher, P = 0.04), and March 2006 (upstream lower, P = 0.01).

Leaf carbon isotope ratios were not statistically different between trees in upstream and downstream treatments (P = 0.28), indicating that there was no significant, long-term difference in water use efficiencies.

DISCUSSION--Overland flow in desert washes is such an obvious source of water for plants that grow in these washes that we hypothesized that mature O. tesota trees would use this water in addition to water accessed via their deep roots. However, our findings show that overland flow contributed very little to survival, growth, reproduction, and ecophysiology of these trees. Of all the parameters measured, only branch growth and predawn water potentials showed slight positive effects of access to overland flow.

Predawn water potentials of leaves closely matched precipitation patterns (Fig. 1), as noted in previous studies of O. tesota and other perennial desert plant species (Monson and Smith, 1982; Nilsen et al., 1984). Not surprisingly, upstream trees had higher water potentials than downstream trees during months with precipitation, including July 2005 and March 2006 (Fig. 1). Although this difference disappeared during dry periods, the overall difference remained significant. Comparing predawn and midday water potentials (Figs. 1 and 2), one can see that water potentials tended to converge toward the same low midday values in both areas, suggesting that trees transpired until they reached similar water potentials. Either trees in both areas experienced the same midday water stress or stomatal regulation in this species adjusts to a certain midday leaf water potential, regardless of water availability. Although there were significant differences in predawn water potentials, indicating temporarily increased water stress in the downstream trees, we found no difference in leaf carbon isotope ratios. It was hypothesized that the downstream trees would have a lower [[delta].sup.13] in comparison to the upstream trees. This was anticipated because the carbon-fixing enzyme in plants (Rubisco) has a higher affinity for the lighter [sup.12]C isotope and discriminates against the heavier [sup.13]C (Ehleringer, 1993; Dawson et al., 2002). Under water-stressed conditions, stomatal closure leads to a reduction in C[O.sub.2] intake (Szarek and Woodhouse, 1976), causing depletion of [sup.12]C inside leaves and causing more [sup.13]C to be fixed.

The lack of statistically significant differences in leaf carbon isotope ratios, average leaf mass per leaf area, flower counts per unit of canopy volume, mature seed counts, and tree population densities could be interpreted in two ways: either this desert phreatophyte is consistently able to obtain sufficient water from deep below the soil surface; or water availability might be temporarily similar upstream and downstream from berms during periods when most photosynthesis, leaf, and flower production occurs. Specifically, this might be the case for leaf production. It would be helpful to know when and under what conditions the leaves of the study trees had been produced, but unfortunately this information was not available.

Leaf age in Olneya is difficult to determine because the tree is semideciduous (Dimmitt, 2000). Olneya tesota has been found to drop some of its leaves during the winter if the temperature drops below 0[degrees]C or just before flowering, which only occurs on average 2 out of every 5 years (Dimmitt, 2000). However, defoliating events do not occur every year, nor do either of these events cause all leaves to drop off (Klikoff, 1967). Therefore, some leaves could remain on the tree for more than a single year. If, in this study, the leaves had been on the trees for >1 year, then the production of these leaves would have been during a period of drought, which places both trees under the similar stressed conditions.

Timing of rainfall probably affected many of the measured plant traits. We hypothesized that branch growth of the upstream trees over one growing season would be significantly longer than that of the downstream trees. This hypothesis was based on previous findings that O. tesota undergoes vegetation growth during the monsoonal season from June to September (Turner, 1963), when summer thunderstorms occasionally contribute large amounts of rainfall. The monsoonal season that these trees experienced provided a total of 7.6 cm of rain (Fig. 1). During this time, upstream trees had access to rainfall, overland water flow, and groundwater; whereas, the downstream trees only had access to rainfall and groundwater. Presumably, upstream trees were exposed to the additional water source of overland water runoff, which might account for their higher branch growth.

[FIGURE 1 OMITTED]

Timing of rainfall in relation to plant phenology could have also affected flower and seed production. Flowers began to bloom during the middle of May 2006 and began to fruit by the beginning of June 2006. During April, May, and June of 2006 there was no precipitation (Fig. 1), and both treatments experienced the same drought conditions, which is supported by the predawn water potentials. Therefore, it is not surprising that there was no difference in the number of flowers or seeds produced during that period.

Because of the life history of the tree, differences found between reproductive and branch growth were not entirely unexpected. In O. tesota, vegetation growth occurs during the wet monsoonal season and reproductive growth during the dry spring (Turner, 1963; Dimmitt, 2000). This phenological pattern would result in a significant difference in branch growth if there were a difference between the amounts of monsoonal overland runoff in the two treatments. In contrast, the same reproductive rates would tend to occur in both treatments because reproduction in this species tends to happen during the dry season when water availability is limited to deep subsurface water. Olneya tesota is not the only species that has exhibited these types of results. Evans and Black (1993) reported similar findings for Artemisia tridentata, in which vegetation and reproductive growth were also temporally separated. Vegetation growth occurred during the wetter seasons, whereas reproductive growth took place during the dry season. Evans and Black (1993) also found no significant difference in reproductive growth between two treatments, but did find a significant difference in vegetation growth.

The original plan for this study was to include seedlings, saplings, and dead trees in tree population density counts. However, we found no saplings or seedlings in the downstream areas, and we found no seedlings and only a few saplings upstream, all of which were found outside the study area. Finding no or few saplings and seedlings is likely a result of seedlings lacking the deep root system that would allow them to access deeper groundwater, and seedling survival requiring a large amount of precipitation during the monsoonal season after seeds dehisce from the pods (Dimmitt, 2000). The precipitation requirements for seedlings, the sporadic flowering (2 out of every 5 years), and the longevity of the species ([greater than or equal to] 300-800 years) together imply that recruitment events might be rare but sufficient to maintain the population. Therefore, this critical phase of the life cycle of the desert ironwood would be more likely to be significantly influenced by wash water, and the presence of the man-made sand berms may preclude the establishment of new seedlings in the area.

[FIGURE 2 OMITTED]

In the absence of recent recruitment, population densities of the long-lived mature trees would be mainly affected by mortality. Population densities were the same, so any differences in water stress clearly did not affect tree mortality at this site.

Our data do not support the hypothesis that wash water is particularly significant to the survival of established O. tesota trees. It may be that substantial wash water is critical to the successful recruitment of new seedlings of this species as reported by Stromberg et al. (1991; 2007). Our study area provides an excellent location for a long-term study to address that hypothesis.

Our data suggest that the long-term survival of mature trees rely on ground water access. This raises significant concern regarding the future use of groundwater at the Desert Sunlight Solar Farm Project, currently under construction approximately 3 miles (4.8 km) from the study site (WorleyParsons, in litt; Bureau of Land Management and Ironwood Consulting, Inc., 2010). Use of this water could lower the static ground-water levels below the depth that the various desert phreatophytes can reach. Monitoring the water status of these trees may not reveal significant problems for the persistence of the population until it is too late to save the trees that rely on this water source for their existence. Further, it would preclude the successful establishment of any future seedlings of these phreatophytic species, thus ensuring their extinction in this area.

Although this study contributes to our understanding of the ecology of O. tesota, many open questions about this species' response to environmental conditions remain. Multiyear data on reproduction and growth of these trees would be beneficial to understanding how they respond to the berms. However, time constraints for this project did not allow for this. Therefore, further research should be conducted on this tree that plays such a prominent role in the Sonoran Desert landscape.

Thanks to B. Lewis from the BLM Colorado Office for providing a copy of his and T. Morris' report. Thanks to J. Stromberg of Arizona State University for copies of her work associated with desert phreatophytes. Also, thanks to P. Summers from BLM for all his assistance in getting information on static water levels at Corn Springs. To N. Cervin and E. Hendricks, thank you for your countless weekends helping to collect data in the freezing cold and the melting hot summers. Finally, thanks to C. Leon for the copious amount of support through this process.

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Submitted 11 August 2015. Acceptance recommended by Associate Editor, James Moore, 18 February 2016.

Dawn Hendricks,* H. Jochen Schenk, C. Eugene Jones

Department of Biological Science, California State University Fullerton, 800 North State College Boulevard, Fullerton, CA 92831

* Correspondent: dhendricks@fullerton.edu
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Author:Hendricks, Dawn; Schenk, H. Jochen; Jones, C. Eugene
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