Anne Huth (UA HWR), Nancy Grimm, and David Lewis (ASU -Bio) measuring vertical hydraulic gradient in the San Pedro at the Grayhawk Ranch site

J. Stromberg (ASU)

The goals of this task area are to understand the effects of human activities, such as groundwater pumping and global climate change, on the structure and functioning of riparian ecosystems.

Science Questions

• How do structure, composition and diversity of riparian vegetation vary across a) longitudinal river gradients of site elevation, hydrology, channel geomorphology, and fire disturbance and b) lateral floodplain gradients of flood frequency and resource availability?



• What are the hydrologic thresholds for establishment and maintenance of valued riparian plant species and associations?



• How might the San Pedro riparian vegetation change in response to human-caused changes in stream flow and groundwater regimes and climate?

Approach

We are addressing these questions by conducting an opportunistic (natural) experiment. Our network of 28 sites distributed in the Lower and Upper Basins of the San Pedro River span gradients of site hydrology (groundwater depth, surface flow frequency), elevation and climate (rainfall, temperature), channel geomorphology, and fire history. For each site, we have data on woody and herbaceous vegetation structure and a suite of physical variables.

Key results

• We described the range of hydrologic conditions, with respect to surface flow frequency and ground water depth, over which Fremont cottonwood (Populus fremontii), Goodding willow (Salix gooddingii), and saltcedar (Tamarix ramosissima) grow along the San Pedro River in southeastern Arizona.



• We identified hydrologic thresholds above which the native cottonwood-willow trees are dominant over the more xerophytic, non-native saltcedar shrubs.



• We described changes in forest biomass structure across hydrologic gradients of stream flow frequency and depth to groundwater.



• We identified hydrologic thresholds for maintenance of riverine marshland associations.



• We described patterns of change in herbaceous species abundance, composition, and diversity across longitudinal gradients of elevation, floodplain width, and site hydrology.



• We began data analysis to describe effects of fire on riparian vegetation structure.



• We began development of a riparian vegetation change model.

One major research finding and its implication is:

Along many rivers of the southwestern United States, riparian pioneer tree communities have shifted from native Fremont cottonwood-Goodding willow (Populus fremontii-Salix gooddingii) forests to woodlands and shrublands of the exotic tree/shrub saltcedar (Tamarix ramosissima). Because saltcedar woodlands often have different ecological functions than cottonwood-willow forests, this vegetation change has generated management concern. A handful of environmental factors are believed to contribute to this shift in species composition. For example, changes in flood disturbance patterns can influence species establishment, with saltcedar often becoming abundant on dammed, flood-suppressed rivers. Our results from the free-flowing San Pedro River highlight the role of other hydrologic factors on species distribution patterns. Across longitudinal river gradients, cottonwood and willow abundance (measured as canopy cover, vegetation volume, basal area, and stem density) increased at sites with shallow groundwater and higher flow frequencies, whereas saltcedar abundance decreased. Fremont cottonwood and Goodding willow maintained dominance at sites with flow frequencies greater than 78-80% and groundwater depths less than 3.3-2.8 meters (Figure 3-3).

These research results relate directly to one of the major goals of TA3, which is to understand how riparian systems are affected by changes in water quantity due to groundwater pumping or conjunctive stream-water management. These results are significant in that they provide river managers with hydrologic threshold values that can serve as management goals, highlighting the importance of focusing on underlying site conditions and of restoring the hydrologic conditions under which native trees are more competitive. With this information river managers can set target goals and identify trigger points for management actions. Our efforts to disentangle effects of site hydrology from other key environmental factors that influence riparian vegetation structure also are of potential value to river managers. Our results have theoretical implications regarding the interplay of flood disturbance and water resource availability on riparian plant species diversity patterns.

Plans

The next logical step is to test the robustness of the relationships we have described between riparian vegetation attributes and site hydrology, site elevation and floodplain geomorphology, by extending our analysis to other low gradient, alluvial rivers in the Sonoran Desert region. Our purpose is to determine whether the particular stressor-response relationships that characterize the San Pedro River riparian ecosystem are typical of riparian systems throughout the region. Alternatively, it is possible that 1) different quantitative relationships link the same suite of indicator attributes to the imposed hydrologic and climatic stressors, or 2) different biotic attributes serve as the key indicators of riparian ecosystem response to changes in water availability. We will evaluate these possibilities by developing quantitative stressor-response relationships for riparian ecosystems on two additional rivers (one dammed and one undammed).

This research will provide a foundation for expanding the applicability of our biohydrology models to the regional scale. Ultimately, our goal is to develop general riparian vegetation models that predict riparian vegetation structure and diversity from key environmental factors. Our project has potential to link with studies of economic valuation of rivers as flyways for birds, given that bird habitat is linked with vegetation structure, which is in turn dependent on river hydrology. Our studies also can integrate with those of ground and surface water modeling of rivers. As hydrology (MODFLOW) models predict hydrologic change, our linked biohydrology models can then predict vegetation change.