Riparian Responses to Climate Change at the Santa Clara River

The value of the SCRC for conservation of biodiversity and natural resource sustainability is based on a long-standing partnering of research and conservation communities, even before the current SCRC took its present form. In 2006 the Santa Clara River Trustee Council supported research by UC Santa Barbara and UCLA biologists on the impacts and control of invasive Arundo donax (giant reed). That project also included plans to develop a University of California research station on our River to focus on conservation science, including how climate change may alter riparian ecosystems. A new initiative is now being developed that addresses not so much the potential impacts of climate change, but how we might adapt to changes that may already be happening.

The Santa Clara River watershed encompasses a broad range of climate conditions, from the mild, foggy coastal zone to Mojave Desert-like conditions in the upper watershed in Los Angeles County, and foundational riparian species such as cottonwoods are found throughout that range. To survive across that climate gradient, these trees must either be tolerant of an extreme range of conditions, or more likely possess genetic differences that allow local forms to thrive within a narrower envelope of environmental conditions.

Now we must wonder how warming, drying conditions will alter these relationships, and to address this question we are partnering with a team led by Dr. Tom Whitham at Northern Arizona University that is growing cottonwoods and other plants from many locations in ‘common gardens’ across the Southwest. The program is called SEGA, or Southwest Experimental Garden Array, and it allows researchers to compare physiological traits and their genetic basis that enable trees to thrive under stressful condition. In our changing environment, the objective is to recommend how to implement ‘assisted migration’ of genetic forms of the same tree species to other locations where they may better tolerate the environments of the future, thus avoiding the catastrophic loss of entire riparian ecosystems and the associated large number of endangered wildlife species.

We are currently reviewing locations in the SCR watershed where SEGA study sites can be set up, likely three to represent coastal, inland and intermediate conditions. The first  garden will be established in an ‘intermediate’ zone at the Sespe Cienega near Fillmore where we are already doing Arundo control. After multiple years of Arundo treatment we will then carry out riparian restoration with assistance from local students and community volunteers, but the SEGA study will provide key information on what types of native trees will be most successful for the larger restoration effort.

This project has important implications for regional water management planning in that the objective is to promote native riparian flora that can survive with lower water availability and warmer temperatures, while retaining the ecosystem services that riparian systems provide for wildlife and other valued functions. Riparian management can thus address the dual concerns of increasing water demand for economic purposes and decreasing supply owing to climate change. This is an increasingly important question for managers, as promoting social behaviors to reduce the threat of climate warming and drying is desirable, but cannot replace functional measures to mitigate the inevitable changes that are coming, and already affecting our ecosphere. 

Tom Dudley, Marine Science Inst., UC Santa Barbara; tdudley@msi.ucsb.edu