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- Materials and methods
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Understanding the ecological effects of water management is key to sustainable development world-wide (Postel 2000; Jackson et al. 2001). Semi-arid and arid environments are particularly vulnerable to water-related land-use practices and are currently threatened by rapid population and socio-economic changes. A clear challenge to future development and management of water resources is abating tension between human and ecosystem requirements for fresh water (Postel, Daily & Ehrlich 1996; Steen 1998). In many locations, surface-water runoff is already utilized, and cities and agricultural centres have turned to groundwater aquifers to augment supply during drought (National Research Council 1993). The practice of recharging shallow aquifers during wet years, and then pumping these stored resources during dry years, has benefits over water storage in surface reservoirs because of lower evaporation rates. However, cycles of recharge and pumping result in increased fluctuation of the water table, which has ecological consequences for plant communities that are composed of groundwater-dependent (phreatophytic) species. Furthermore, pumping has led to the drying of springs and seeps in many regions, and this trend is likely to continue as water resources become further allocated to increasingly large urban centres.
In basins and valleys of the intermountain western USA, some zones between wetland and upland vegetation have been described as grass-dominated meadows (West & Young 2000). Plant species occupying these meadows are sometimes characterized as facultative wetland (Reed 1988), i.e. they are intermediate between obligate wetland species and species occurring only in upland (precipitation-dependent) zones. Facultative species generally benefit from shallow groundwater but can exist where groundwater is inaccessible. The relationships between plant species composition, distribution and cover, and groundwater depth, can be complex; nevertheless, clear patterns have been identified (Stromberg, Tiller & Richter 1996). An obvious pattern is related to rooting depth. Herbaceous meadow species with shallow roots thrive in regions of shallow groundwater (Allen-Diaz 1991; Castelli, Chambers & Tausch 2000). Roots of facultative phreatophytic shrubs reach groundwater at greater depths (Sorenson, Dileanis & Branson 1991; Groeneveld & Or 1994; Stromberg, Tiller & Richter 1996).
Sustainable water management of vegetation occupying zones with intermediate groundwater depth requires knowledge of the degree to which groundwater change affects plant cover or floristic composition. It also requires knowledge of how plant cover responds to water inputs directly from precipitation. To study these responses, simultaneous measurements are required for vegetation cover and the hydrologic parameters: groundwater depth and precipitation. Planning and foresight are necessary to ensure these measurements occur in the correct locations to capture the vegetation response to groundwater decline from pumping and, in the past, few water diversion projects included monitoring of biotic resources. Fortunately, quantitative remote-sensing methods can be used to determine the percentage plant cover from images acquired over the past two decades (Elmore et al. 2000). Remote measurements of plant cover have been shown to provide useful information for a variety of applications (Okin et al. 2001; Rogan, Franklin & Roberts 2002). Total plant cover from remote sensing provides an integrated valuation of the extent to which vegetation cover is altered by environmental factors such as depth to groundwater, precipitation, grazing and disturbance (Dube & Pickup 2001; Asner, Borghi & Ojeda 2003; Elmore, Mustard & Manning 2003). When used in conjunction with detailed field data, remote measurements of plant cover also highlight the regional significance of changes observed at smaller scales.
This study analysed the vegetation response within a plant community termed ‘alkali meadow’, which exists in scattered locations throughout the Great Basin and Range but is restricted to zones of shallow groundwater. In eastern California, USA, alkali meadow is an important plant community for conservation because (i) plant cover is dominated by facultative wetland species, (ii) it is essential habitat for numerous rare and endangered species, and (iii) groundwater extraction for water export is practised or proposed throughout much of the region. In previous work we used remote-sensing technologies to identify and classify regions of Owens Valley, California, where vegetation was affected by groundwater pumping (Elmore, Mustard & Manning 2003). With the present study we focused on plots of alkali meadow vegetation for which measurements of groundwater depth through a 16-year period were available. The data set included periods of drought and groundwater pumping that were sufficiently decoupled to allow separate analyses (Elmore, Mustard & Manning 2003). Our objectives were to: (i) understand the absolute and relative importance of groundwater and precipitation in influencing plant cover within alkali meadow vegetation; (ii) identify a maximum effective rooting depth that is generally characteristic of alkali meadow vegetation; and (iii) determine if a general model of plant cover response to groundwater decline is applicable across plots, or if plot characteristics, such as the proportion of herbaceous and woody plants, also influence vegetation cover response.
There is considerable evidence that alkali meadow vegetation in Owens Valley is phreatophytic (Sorenson, Dileanis & Branson 1991; Manning 1997; Steinwand, Harrington & Or 2006) but a specific understanding of plant cover response to water table changes has been elusive. Because the depth to which plants can acquire groundwater is limited (Jackson et al. 1996; Jackson et al. 1999; Sperry et al. 2002), we hypothesized that a response threshold exists, exhibited by the depth-to-water (DTW) beyond which plant cover ceases to respond to fluctuations in groundwater depth. We view the identification of this threshold DTW to be critical to the sustainable management of alkali meadow vegetation.
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- Materials and methods
- Supporting Information
In the model of cover that included DTW and WYP, 37% of the variation in cover could be explained by DTW (Table 1 and Fig. 2). Cover was high when DTW was shallow and low when DTW was deep (P < 0·001). Variation in precipitation did not show a statistically significant correlation with cover (P = 0·07) in this model, nor was there a significant interaction between precipitation and DTW (P = 0·18) (Table 1).
Table 1. Model results analysing effects of depth to groundwater (DTW) and water year precipitation (WYP) on vegetation cover in 47 plots, 1986–2001
|Parameters (n = 705, R2 = 0·37)||SS*||Estimate||P|
|Intercept|| || 2·00 ± 0·03||< 0·001|
|log DTW||20·3||−0·97 ± 0·05||< 0·001|
|WYP|| 0·2||0·002 ± 0·001|| 0·07|
|log DTW × WYP|| 0·1||0·009 ± 0·007|| 0·18|
The significance of the DTW and precipitation effects was dependent on the range of DTW at the modelled sites. Models that included data from sites where DTW was shallower than c. 2·5 m exhibited a significant effect from DTW, and precipitation was not a significant effect (Fig. 3). When the data were restricted to plots and times where DTW was deeper than c. 2·5 m, cover was significantly correlated with precipitation (P < 0·05) but was unaffected by further fluctuations in DTW (P > 0·05).
Figure 3. Results of sequential multiple regression trials for models using all data, with groundwater (DTW) below a designated depth shown on the x-axis. Significance levels (P-values) for the precipitation and DTW estimates are plotted on the y-axis. When DTW was ≥ 2·5 m, DTW no longer had a significant effect on cover and the effect of precipitation became significant.
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Including plot identity as an explanatory variable in the regression model greatly increased the variation explained by the model (r2 = 0·80; Table 2). Plots differed in mean cover beyond what could be explained by DTW or WYP, therefore plots with approximately the same DTW exhibited different cover values (P < 0·001; Table 2). There were also different relationships among plots between cover and DTW (P < 0·001; Table 2). In some plots, there was little influence of DTW on cover, while for other plots cover declined strongly with increasing DTW (Fig. 4, inset). This result led us to investigate whether a plot feature that was simple to measure could account for the contribution of plot identity to the overall model explanatory power. The variation among plots in the relationship between DTW and cover could not be explained by maximum groundwater depth (P = 0·30), total plant cover in 2003 (P = 0·13) or shrub cover in 2003 (P = 0·34) (data not shown). Among the parameters we tested, the best predictor of total plant cover sensitivity to DTW was the 2003 perennial herbaceous cover. Plots with a greater cover of the more shallowly rooted perennial herbaceous plants were more sensitive to fluctuations in DTW (r2 = 0·63, P= 0·001; Fig. 4).
Table 2. Model results including plot identity for plots exhibiting greater than twofold variation in log DTW
|Parameters (n = 200, R2 = 0·80)||SS*||Estimate||P|
|Intercept|| || 1·76 ± 0·06||< 0·001|
|Plot||2·47|| ||< 0·001|
|log DTW||1·28||−0·71 ± 0·08||< 0·001|
|log DTW × plot||0·97|| ||< 0·001|
|WYP||0·31||0·008 ± 0·002||< 0·001|
|WYP × plot||0·40|| || 0·037|
|log DTW × WYP ||0·03|| 0·02 ± 0·01|| 0·171|
|log DTW × WYP × plot||0·26|| || 0·244|
Figure 4. The slope of the groundwater (DTW) vs. vegetation cover regression for selected plots (see text) plotted against the total cover of perennial herbaceous species (including grasses) measured in the plot in 2003. Inset: vegetation response to DTW for two plots with the highest and lowest cover of herbaceous perennials (HP), demonstrating the differences in response for these plots.
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- Materials and methods
- Supporting Information
The 16-year data set of precipitation, DTW and remote-sensing derived vegetation cover showed that vegetation cover was highly responsive to variation in groundwater depth (Table 1 and Fig. 2). Vegetation cover is most sensitive to groundwater change initially, when groundwater is close to the surface, and sensitivity tapers off as DTW increases (Fig. 5). This indicates that cover response to declining groundwater is largest at the initiation of groundwater pumping. One explanation for this response is that vegetation is adapted to groundwater fluctuation within the range established previously (Shafroth, Stromberg & Patten 2002). When groundwater declines below this range, water availability decreases for some individuals or species and, as a result, total vegetation cover declines.
Figure 5. A graphical presentation of the results of the general model, the coefficients of which are presented in Table 1. (a) Model results showing the vegetation cover relationship to DTW, with standard error. (b) The impact of changing groundwater depth modelled as the difference in vegetation cover for two groundwater values separated by 0·2, 0·4 or 0·8 m DTW.
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Owens Valley meadow cover response patterns are controlled by effective rooting depth. For the 47 plots used in this study, the average maximum effective rooting depth was located at 2·5 m. This estimation was based on three observations: (i) the range of DTW in 1986 prior to pumping in most areas was within the range of 0–3·5 m, and 66% of the plot DTW data was in the range 0·75–2·5 m (Fig. 1); (ii) vegetation cover was correlated with groundwater fluctuation only when modelled data included sites where DTW was within 2·5 m of the surface (Fig. 3); and (iii) vegetation cover was reduced and significantly correlated with precipitation when DTW was below 2·5 m (Figs 2 and 3).
The plot data revealed only a weak plant cover response to precipitation across the entire range of groundwater depths (P = 0·07; Table 1). This result indicates that of the two water input sources, groundwater exerts a stronger influence on cover in Owens Valley meadows. Similarly, other studies have shown that groundwater availability in arid regions affects plant distribution patterns across the landscape (Allen-Diaz 1991; Stromberg, Tiller & Richter 1996; Castelli, Chambers & Tausch 2000). Our inability to detect a strong precipitation influence on meadow cover underscores some important characteristics of meadow systems in desert regions. First, within our data and within these systems in general, when plants have access to groundwater the roots of dominant Owens Valley meadow species potentially have access to water through as much as 2·5 m of saturated soil. This large volume of water is available to plants throughout the entire growing season. Precipitation, in contrast, averages 13 cm, rarely exceeds 30 cm (Fig. 1) and partly evaporates before being absorbed by plants or soil. Therefore precipitation is a sparse and unreliable water source relative to groundwater.
Undoubtedly, Owens Valley meadow plants absorb precipitation water, but the contribution of precipitation to the characteristic we measured, late summer total vegetation cover, is negligible where groundwater occurs within the root zone. Research on species in similar systems and habitats has shown precipitation to stimulate physiological activity but not contribute measurably to canopy growth (Snyder & Donovan 2004). Others have shown plants using groundwater and precipitation (Torres et al. 2002), plants switching from groundwater to precipitation (Chimner & Cooper 2004) and different species at the same site using different water sources (Yepez et al. 2003). We also recognize the role of late summer annual plants in increasing the response of vegetation cover to annual precipitation (Elmore, Mustard & Manning 2003). Finally, precipitation has other roles, such as mobilizing soil nutrients (Burke et al. 1998) and temporarily washing salts from the soil surface (Guler & Thyne 2004). Any or all of these mechanisms could be responsible for the observed (albeit weak, P = 0·07; Table 1) relationship between plant cover and precipitation.
Terrestrial vegetation depends on precipitation consistently across biomes, and sensitivity to precipitation change is largest when mean annual precipitation is lowest (Huxman et al. 2004). In contrast, Owens Valley meadow exhibits stable high plant cover in a highly variable, low-rainfall environment, suggesting this ecosystem has more in common with wetlands than upland systems. Large volumes of mountain snowmelt maintain a stable shallow groundwater aquifer that buffers plant communities from the effects of drought (Danskin 1998; Elmore, Mustard & Manning 2003). Similar to other intermountain settings (Schulze et al. 1996), the availability of these groundwater resources has allowed for the establishment and survival of plants that are not dependent on precipitation within these localized regions. Throughout arid regions, vegetation dependent on groundwater resources might be more widespread than previously estimated if plant assemblages functionally similar to Owens Valley meadows are included.
Cover response to declining groundwater availability can be explained mechanistically. When groundwater begins to decline beneath alkali meadow vegetation, the most shallowly rooted plants (grasses and forbs) are the first to respond. As individual plants and plants in more sensitive microhabitats (e.g. on elevated locations or in more coarse-textured soils; Sperry & Hacke 2002) lose contact with the groundwater table, total plant cover declines as a result of mortality of those groundwater-disconnected plants. In locations where there is a high total cover of shallow-rooted plants, the decrease in total cover is more rapid. Where cover of deeper-rooted species (shrubs) is greater, the response to groundwater decline can be slower, but nevertheless the response generally follows a logarithmic decline. As groundwater declines below the rooting depth of most plants, precipitation becomes the primary water source for plants remaining on the site, and cover switches from responding to groundwater declines to responding to precipitation fluctuations. The weak response between precipitation and cover at deep DTW suggests that perennial meadow vegetation is not well adapted to switching to precipitation in these conditions. However, where seeds are available, annual plants can respond to precipitation, particularly during the first wet years following drought (Elmore, Mustard & Manning 2003).
These results show that alkali meadow attains high plant cover only when groundwater is well within the root zone of the perennial grasses and shrubs. The data set was limited in that the remote-sensing data did not indicate whether plant community floristic composition is maintained through a period of groundwater decline and recharge. However, evidence from Owens Valley suggests that periods of low groundwater aid the recruitment success of deeper-rooted shrub species and annuals that are not phreatophytic (Elmore, Mustard & Manning 2003). If this trend were to continue and prove to be a robust feature of the Owens Valley landscape, it would have the effect of decoupling vegetation cover from changes in the shallow groundwater aquifer. Such a change in system functioning (a change in the maximum rooting depth or a switch to precipitation dependence) would represent a threshold response to an alternative state. Research in a variety of systems has highlighted the fact that alternative states can be stable, result in the degradation of ecosystem services, and can be very costly to manage for the return of the previous ecosystem structure and functioning (May 1977; Scheffer et al. 2001; van de Koppel et al. 2002).
The vegetation response model and remote-sensing methods described here can be used to guide management of groundwater levels and vegetation cover within Owens Valley meadows, and perhaps management of other plant communities that are dominated by facultative wetland species. If sustained plant community composition and resistance to drought (i.e. the capability of utilizing groundwater as a buffer from drought) is a management objective, then groundwater must remain within the root zone of these plants. Management options (decisions on whether to pump groundwater or to allow natural recharge) are available when groundwater is within approximately 2·5 m of the surface. Below this depth, precipitation dynamics dominate and further changes in DTW have no effect on plant cover. Remote-sensing data can play an important role when interpreting vegetation changes in areas where DTW has not been monitored. Its primary strengths are to identify the regional extent of vegetation change measured at field sites, and to highlight areas that over time become sensitive to precipitation variability. This might assist managers in determining where further pumping should be avoided. Through a combination of remote-sensing data analysis, targeted field monitoring and informed groundwater management, it might be possible to balance the water needs of humans with environmental resources.