Water table decline alters growth and survival of Salix gooddingii and Tamarix chinensis seedlings
Introduction
Riparian ecosystems in the American Southwest compose less than 3% of the landscape area (Naiman and Decamps, 1997), however, they are highly valued for wildlife habitat, biodiversity, recreation, flood attenuation, and water quality (Carothers, 1977, Patten, 1998). Despite their importance, demands for water and subsequent development projects, including dams and diversions, have contributed to a decline of southwestern riparian forests (Stromberg, 1993, Busch and Smith, 1995, Stromberg et al., 1996, Patten, 1998). Dam operations alter the hydrology of rivers, often reducing sediment loads and increasing erosive power causing greater channel incision and water table decline (Reily and Johnson, 1982). The abrupt downstream decline of alluvial water tables caused by dams and diversions has been proposed as a mechanism of riparian forest decline (Rood and Heinze-Milne, 1989, Rood and Mahoney, 1990, Mahoney and Rood, 1991, Mahoney and Rood, 1992). Also, flood attenuation and altered timing of peak flows by dams and diversions can limit regeneration by downstream native trees (Fenner et al., 1985, Bradley and Smith, 1986, Rood and Heinze-Milne, 1989, Rood and Mahoney, 1990, Mahoney and Rood, 1998) and promote the establishment of exotics that are more competitive than natives under drier conditions (Smith et al., 1991, Busch and Smith, 1995, Cleverly et al., 1997, Smith et al., 1998). Chronic regeneration failures by native riparian trees can shift community composition towards dominance by exotic and more xeric species (Friedman et al., 1995).
The germination requirements of many western riparian trees are well known (Stromberg, 1993, Braatne et al., 1996, Mahoney and Rood, 1998). High spring flows typically scour competing vegetation and deposit sediment, creating safe sites for germination. Most native riparian trees are adapted to this disturbance regime and produce seeds prolifically in synchrony with flood water recession in early spring (Stromberg, 1993). The presence of significant numbers of seedlings every year suggest that seed availability is not limiting to establishment, although many of these seedlings do not survive the first year (Stromberg, 1993, Scott et al., 1997, Mahoney and Rood, 1998). The primary causes of mortality are typically water stress caused by rapid water table declines (Mahoney and Rood, 1991, Stromberg, 1993, Mahoney and Rood, 1998) and scouring by late summer and fall floods (Stromberg, 1993). A key to seedling establishment is rapid root growth as the water table declines (Mahoney and Rood, 1991, Mahoney and Rood, 1992). Often the rate of water table decline on dam-regulated rivers is very rapid, which can cause extensive water stress and mortality in new seedling populations (Mahoney and Rood, 1998). Populus seedlings are known to have root growth rates of 0.6–1.3 cm/day, resulting in 72–162 cm of root growth by the end of their first season (Fenner et al., 1984, Mahoney and Rood, 1998), however, it has been reported that Populus seedlings can survive ground water decline rates of 2–4 cm/day (Mahoney and Rood, 1991, Mahoney and Rood, 1992, Segelquist et al., 1993). These findings suggest that some riparian tree seedlings can use water from the capillary fringe above the alluvial water table (Mahoney and Rood, 1998). The thickness of the capillary fringe can range from less than 10 cm in coarse cobbles to greater than 100 cm in fine silts (Mahoney and Rood, 1998).
The ’recruitment box’ conceptual model of Populus seedlings based on these ideas combines root growth rates with rates of water table and capillary fringe declines to estimate successful establishment sites (Mahoney and Rood, 1998). Utilizing a 2.5 cm/day rate of water table decline combined with Populus root growth rates and the thickness of the capillary fringe, Mahoney and Rood (1998) determined that for many Populus species, successful establishment would occur at elevations 0.6–2 m above the late summer low flow stream stage. The model was used successfully on the Truckee River in Nevada to promote Populus regeneration by regulating rates of water table decline in a controlled flood. This model should have applications in other semi-arid riparian areas around the world, and may be an important tool in conservation and restoration of riparian habitats. However, the model will need calibration for different species and specific stream conditions (Mahoney and Rood, 1998, Shafroth et al., 1998). The estimated survivable rate of ground water decline of 2.5 cm/day used by Mahoney and Rood (1998) may not be applicable in southwestern riparian ecosystems, which are typically much hotter and drier than the systems reviewed in their study. In addition, Salix gooddingii Ball has been shown to be more sensitive to changes in ground water availability than the co-occurring Populus fremontii S. Wats. (Stromberg, 1993) and grows well in saturated soils (Stromberg, 1997). In contrast, Tamarix chinensis Lour. has been reported to be more drought tolerant than native species (Cleverly et al., 1997, Smith et al., 1998) and is able to persist indefinitely in drier soils (Everitt, 1980).
In the Sonoran desert, low elevation (<1250 m) riparian forests were traditionally dominated by P. fremontii and S. gooddingii (Stromberg, 1993). However, in response to water development, many of these native populations have been invaded and often replaced by the exotic, T. chinensis, (Christensen, 1962, Stromberg, 1993, Brock, 1994, Busch and Smith, 1995). Whereas field observational studies have provided insight into the factors that regulate establishment of these southwestern riparian species (Stromberg, 1993, Stromberg, 1997), more information is needed on the effects of different rates of water table decline on seedling establishment and growth. In this study, we utilized rhizopods (Mahoney and Rood, 1991, Mahoney and Rood, 1992) to investigate the effects of differing rates of ground water decline on growth and survival of one important, native riparian tree species, S. gooddingii, and one invasive exotic, T. chinensis.
Section snippets
Materials and methods
Seeds were collected on 16 May 1998 from natural populations of both S. gooddingii and T. chinensis along the Hassayampa and Bill Williams Rivers in Arizona. Seeds were frozen and stored in the dark in sealed jars containing silica gel desiccant for 7 months until they were germinated.
Rhizopods (see Mahoney and Rood, 1991, Mahoney and Rood, 1992 for construction details) were constructed in a greenhouse to simulate alluvial water table decline. Each rhizopod consisted of 15 growth tubes (pods)
Results
There were distinct differences in volumetric soil moisture content (θv) at depths of 0–30 cm in the four treatments throughout the course of the water manipulations (Fig. 1). In the 1 cm/day treatment the capillary fringe persisted in the surface soil for 22 days (Day 47) after the water table started declining, after which the surface soil dried more rapidly. In the 2 cm/day treatment this rapid drying phase began after 8 days (Day 33), and had begun by the first measurement (Day 28) in the 4
Discussion
It is evident from the results of our experiment that the exotic, T. chinensis, is more tolerant of ground water declines than the native, S. gooddingii. Tamarix is known to be a facultative phreatophyte (Turner, 1974), that is able to use water from the alluvial water table and associated capillary fringe, but that is also able to extract water and survive indefinitely in unsaturated soils (Everitt, 1980, Busch and Smith, 1995). This attribute was demonstrated by the high survival of Tamarix
Acknowledgements
We would like to thank Brad Blake, manager of the greenhouse complex at Northern Arizona University, for helping to care for our seedlings. We would also like to thank Drs. Thomas Kolb and Stephen Hart and two anonymous reviewers for helpful comments on earlier drafts of this manuscript. This research was supported by funds from the Ecological Restoration Program at NAU and an US EPA STAR Fellowship awarded to J.L. Horton.
References (30)
- et al.
Response of a hybrid poplar to water table decline in different substrates
For. Ecol. Manage.
(1992) - Braatne, J.H., Rood, S.B., Heilman, P.E., 1996. Life history, ecology, and conservation of riparian cottonwoods in...
- et al.
Plains cottonwood recruitment and survival on a prairie meandering river floodplain, Milk River, southern Alberta and northern Montana
Can. J. Bot
(1986) - Brock, J.H., 1994. Tamarix (salt cedar), an invasive exotic woody plant in arid and semi-arid riparian habitats of...
- et al.
Mechanisms associated with decline of woody species in riparian ecosystems of the southwestern United States
Ecol. Monogr.
(1995) - Carothers, S.W., 1977. Importance, preservation, and management of riparian habitats: an overview. In: Importance,...
The rate of naturalization of Tamarix in Utah
Am. Midl.Nat.
(1962)- et al.
Invasive capacity of Tamarix ramosissima in a Mojave Desert floodplain, the role of drought
Oecologia.
(1997) Ecology of saltcedar- a plea for research
Environ. Geol.
(1980)- et al.
Observations on seeds and seedlings of Fremont cottonwood
Desert Plants
(1984)
Effects of regulated water flows on regeneration of Fremont cottonwood
J. Range Manage.
Restoration of riparian forests using irrigation, artificial disturbance, and natural seedfall
Environ. Manage.
A device for studying the influence of declining water table on poplar growth and survival
Tree Physiol.
Streamflow requirements for cottonwood seedling recruitment- an integrative model
Wetlands
The ecology of interfaces: riparian zones
Annu. Rev. Ecol. Syst.
Cited by (134)
Plant-groundwater interactions in drylands: A review of current research and future perspectives
2023, Agricultural and Forest MeteorologySalix regeneration in fluvial landscapes: Empirical findings based on a systematic review
2023, Ecological EngineeringModeling revealed the effect of root dynamics on the water adaptability of phreatophytes
2022, Agricultural and Forest MeteorologyRiparian Vegetation and the Fluvial Environment: A Biogeographic Perspective
2022, Treatise on Geomorphology