Background

Rivers and streams of the American Southwest have been heavily altered by human activity, resulting in significant changes to disturbance regimes. Riparian vegetation in aridland floodplain systems is critically important as foraging, migrating, and breeding habitat to birds and other animal species. To conserve riparian ecosystems and organisms, understanding how plants and animals are affected by disturbance processes and multiple stressors is critical.

Research

We studied how wild fire, invasive nonnative trees, and climate shape the structure of southwestern riparian forests and the birds inhabiting them. To determine how changes in disturbance regimes will affect riparian habitat along Southwestern floodplain river systems, we measured the response of native and nonnative plants to wildfire, modeled populations of native trees under past and future conditions, and summarized future streamflow projections for streams throughout the region. We compared results with data from the upper Gila River, where a more-natural flooding regime maintains native vegetation.  We also assessed vulnerability of species to climate change.

In areas where surface flows and groundwater dynamics have been altered, wildfire increases mortality rates of trees, such as Fremont cottonwood, and encourages the spread of saltcedar and other nonnative shrubs, reducing nesting opportunities for birds that require large trees. Reduction in stream flow volume and changes in timing of peak flows are projected to occur in response to climate warming, limiting the ability of native vegetation to cope with wildfire and nonnative species.

Because climate change projections vary among different streams, we are assessing vulnerability at the landscape scale to identify areas where protection from fire and other stressors are most needed.

Key Findings

• Alteration of disturbance regimes has increased the frequency of wildfire disturbance, which may favor nonnative plants over native trees.
• Cottonwoods and other native trees can recover from fire and compete against nonnative species if the natural hydrological regime is maintained.
• Climate change is likely to further alter flood and drought regimes, but projections vary among southwestern streams.
• Riparian-nesting birds rely on cottonwoods and other native trees for foraging and nesting sites and are vulnerable to climate change effects and loss of habitat from other disturbances.