California Water Use Threatens Biodiversity in the Long Term

The state of California contains more biodiversity than the rest of the U.S. and Canada put together, but that biodiversity has long been put at risk by human water use.

The diversion of water from the San Francisco Bay Delta, for example, is one of the forces famously driving the delta smelt to extinction. Now, a new study published in Proceedings of the National Academy of Sciences this month shows another counterintuitive way in which human water use in California is putting its unique riverside woodlands at risk. 

By diverting water in ways it would not otherwise flow, human management is providing some stream-side, or riparian, ecosystems with excess water that gives them a short-term boost, but undermines their long-term sustainability. 

“Across California, many river ecosystems are effectively being irrigated by water management decisions,” lead study author Melissa Rohde, who is a Ph.D. candidate at the State University of New York College of Environmental Science and Forestry (CUNY-ESF) and a scientist with the Nature Conservancy of California, explains to Treehugger in an email. “This is resulting in a ‘live fast, die young’ phenomenon.”

Live Fast, Die Young

So what exactly does this mean? 

Native species in California have adapted to a Mediterranean climate that alternates between a rainy season in the winter and spring and a dry season in the summer, an ESF press release explained. Typically, riverside trees like willows, cottonwoods, and oaks would rely on groundwater during the dry months. 

However, Rohde and her team looked at five years of data showing groundwater, streamflow, and satellite imagery of vegetation greenness from 2015 to 2020. This led to a surprising discovery. Many of the tree strands in the drier parts of the state, where natural water flow had been most altered by humans, stayed greener for longer and were less dependent on groundwater, as a Cardiff University press release explained. This meant the human re-routing of water, whether redirected rivers, irrigation canals, or wastewater discharge, was giving these ecosystems an artificial boost. 

“The riparian forests are not being harmed by the extra water,” study co-author Dr. Michael Singer, from Cardiff University’s School of Earth and Environmental Sciences, tells Treehugger in an email. “Quite the opposite. They are thriving.”

At least for now. The threat, Rohde explains, is to the longer-term survival and regeneration of these ecosystems. The artificial water boost puts that at risk for several key reasons.

1. Too Much Stability: The consistency of human-directed waterways disrupts the natural process by which trees use floodplains to release and disperse their seeds. This means the watered tree strands thrive momentarily but do not generate new saplings. 
2. Too Much Competition: The traditional dry periods in the summer helped native trees outcompete invasive species, which are equally boosted by the extra water.
3. Too Much Growth: The fast growth fueled by the extra water actually means the trees grow in less dense forests, making them more vulnerable to drought, disease, and death.

“The issue is that riparian ecosystems have a lot of value ecologically and to society, and this may soon be lost for many miles along rivers and streams in California because these forests will not be replaced when they die,” Singer explains. 

Why Does This Matter?

This “live fast, die young” phenomenon is occurring in a larger context of biodiversity loss and climate change and has the potential to make both problems worse. 

Most of the impacted woodlands noted by the study are in the agricultural hub of California’s Central Valley, according to both press releases. This region lost 95% of its floodplain woodlands in the influx of human settlement beginning with the Gold Rush of the 1850s. That makes the few woodlands that do survive important havens for endangered and threatened species like salmon, steelhead, riparian brush rabbit, least bells vireo, and willow flycatcher, Rohde tells Treehugger. If the woodlands can not replenish themselves, the species they host are at greater risk. 

Further, the phenomenon has the potential to interact with California’s intertwined struggle with drought, wildfires, and climate change. 

“Climate change could accentuate the issue because increasingly common water shortages would support additional diversion of water for human consumption and agriculture,” Singer says. “This may create conditions for 'live faster, die younger' in these fragile ecosystems.” 

Further, if the woodlands do not replenish themselves, this could worsen the climate crisis by depriving the state of one crucial means of carbon storage.

“[O]nly live trees can sequester carbon from the atmosphere,” Singer adds, “So untimely death of these trees will be unfavorable for the carbon budget.”

Finally, the situation could increase wildfire risk. Fires tend to travel quickly upstream, Singer explains, so if these trees die and are not replaced, they could ease that momentum. Further, Rohde notes, one of the non-native species that also thrives on the excess water—arundo—burns hotter than native plants. This risk would increase if the depletion of groundwater due to drought kills off trees like willows and cottonwoods, but leaves the weeds to thrive. 

Groundwater Dependent Ecosystems

For Rohde, protecting these unique riverside woodlands goes hand in hand with sustainably managing California’s groundwater. The riparian woodlands are an example of a groundwater-dependent ecosystem (GDE). 

“These ecosystems rely on groundwater in California’s semi-arid climate, especially during dry summers and periods of drought,” Nature-Conservancy-led partnership the Groundwater Resource Hub explained. “GDEs provide important benefits to California including habitat for animals, water supply, water purification, flood mitigation, erosion control, recreational opportunities and general enjoyment of California’s natural landscape.”

To this end, Rohde and her Nature Conservancy colleagues rely on the Sustainable Groundwater Management Act. This act, which was passed by the California legislature in 2014, empowers groundwater sustainability agencies to make decisions about groundwater use in their area based on economic, social, and environmental concerns. As part of this work, they are supposed to investigate all GDEs in their area and make decisions consistent with their protection. 

Beyond California, Rohde and Singer’s research is part of a broader, $2.5 million collaboration between SUNY ESF, the University of Cardiff and the University of California, Santa Barbara to understand the signs of water stress on arid riverside ecosystems in both France and the U.S. Southwest in the context of climate change and increased human water demand. 

“We hope to develop a set of what we call 'water stress indicators (WSIs)', developed by multiple methods,” Singer explains. “These WSIs may provide land and water managers with [a] window into critical states in riparian ecosystems, even providing early warnings of ecosystem collapse.”