Category: Environment

Trekking out to my research sites near North Carolina’s Alligator River National Wildlife Refuge, I slog through knee-deep water on a section of trail that is completely submerged. Permanent flooding has become commonplace on this low-lying peninsula, nestled behind North Carolina’s Outer Banks. The trees growing in the water are small and stunted. Many are dead.

Throughout coastal North Carolina, evidence of forest die-off is everywhere. Nearly every roadside ditch I pass while driving around the region is lined with dead or dying trees.

As an ecologist studying wetland response to sea level rise, I know this flooding is evidence that climate change is altering landscapes along the Atlantic coast. It’s emblematic of environmental changes that also threaten wildlife, ecosystems, and local farms and forestry businesses.

Like all living organisms, trees die. But what is happening here is not normal. Large patches of trees are dying simultaneously, and saplings aren’t growing to take their place. And it’s not just a local issue: Seawater is raising salt levels in coastal woodlands along the entire Atlantic Coastal Plain, from Maine to Florida. Huge swaths of contiguous forest are dying. They’re now known in the scientific community as “ghost forests.”

The insidious role of salt

Sea level rise driven by climate change is making wetlands wetter in many parts of the world. It’s also making them saltier.

In 2016 I began working in a forested North Carolina wetland to study the effect of salt on its plants and soils. Every couple of months, I suit up in heavy rubber waders and a mesh shirt for protection from biting insects, and haul over 100 pounds of salt and other equipment out along the flooded trail to my research site. We are salting an area about the size of a tennis court, seeking to mimic the effects of sea level rise.

After two years of effort, the salt didn’t seem to be affecting the plants or soil processes that we were monitoring. I realized that instead of waiting around for our experimental salt to slowly kill these trees, the question I needed to answer was how many trees had already died, and how much more wetland area was vulnerable. To find answers, I had to go to sites where the trees were already dead.

Rising seas are inundating North Carolina’s coast, and saltwater is seeping into wetland soils. Salts move through groundwater during phases when freshwater is depleted, such as during droughts. Saltwater also moves through canals and ditches, penetrating inland with help from wind and high tides. Dead trees with pale trunks, devoid of leaves and limbs, are a telltale sign of high salt levels in the soil. A 2019 report called them “wooden tombstones.”

As the trees die, more salt-tolerant shrubs and grasses move in to take their place. In a newly published study that I coauthored with Emily Bernhardt and Justin Wright at Duke University and Xi Yang at the University of Virginia, we show that in North Carolina this shift has been dramatic.

The state’s coastal region has suffered a rapid and widespread loss of forest, with cascading impacts on wildlife, including the endangered red wolf and red-cockaded woodpecker. Wetland forests sequester and store large quantities of carbon, so forest die-offs also contribute to further climate change.

Assessing ghost forests from space

To understand where and how quickly these forests are changing, I needed a bird’s-eye perspective. This perspective comes from satellites like NASA’s Earth Observing System, which are important sources of scientific and environmental data.

Since 1972, Landsat satellites, jointly operated by NASA and the U.S. Geological Survey, have captured continuous images of Earth’s land surface that reveal both natural and human-induced change. We used Landsat images to quantify changes in coastal vegetation since 1984 and referenced high-resolution Google Earth images to spot ghost forests. Computer analysis helped identify similar patches of dead trees across the entire landscape.

The results were shocking. We found that more than 10% of forested wetland within the Alligator River National Wildlife Refuge was lost over the past 35 years. This is federally protected land, with no other human activity that could be killing off the forest.

Rapid sea level rise seems to be outpacing the ability of these forests to adapt to wetter, saltier conditions. Extreme weather events, fueled by climate change, are causing further damage from heavy storms, more frequent hurricanes and drought.

We found that the largest annual loss of forest cover within our study area occurred in 2012, following a period of extreme drought, forest fires and storm surges from Hurricane Irene in August 2011. This triple whammy seemed to have been a tipping point that caused mass tree die-offs across the region.

Should scientists fight the transition or assist it?

As global sea levels continue to rise, coastal woodlands from the Gulf of Mexico to the Chesapeake Bay and elsewhere around the world could also suffer major losses from saltwater intrusion. Many people in the conservation community are rethinking land management approaches and exploring more adaptive strategies, such as facilitating forests’ inevitable transition into salt marshes or other coastal landscapes.

For example, in North Carolina the Nature Conservancy is carrying out some adaptive management approaches, such as creating “living shorelines” made from plants, sand and rock to provide natural buffering from storm surges.

A more radical approach would be to introduce marsh plants that are salt-tolerant in threatened zones. This strategy is controversial because it goes against the desire to try to preserve ecosystems exactly as they are.

But if forests are dying anyway, having a salt marsh is a far better outcome than allowing a wetland to be reduced to open water. While open water isn’t inherently bad, it does not provide the many ecological benefits that a salt marsh affords. Proactive management may prolong the lifespan of coastal wetlands, enabling them to continue storing carbon, providing habitat, enhancing water quality and protecting productive farm and forest land in coastal regions.

Every two weeks, Dawn Upton floods her lawn. She treks into her back yard, twists open two valves big as dinner plates, and within minutes is ankle-deep in water.

“You have to have irrigation boots, girl,” she says during a video tour of her property in Mesa, a suburb of Phoenix, Arizona. She flips her camera to reveal green grass, then tilts her phone skyward at four towering palm trees. As she walks, she pans across pecan, pomegranate, and citrus trees – lemon, orange, a grapefruit sapling. A tortoise, between 80 and 100lbs, lumbers toward her, chewing. “There’s Simba,” Upton says. “Hey buddy! What is that, Simba? You can’t eat it.” She pats him affectionately on the head.

This lush half-acre is Upton and her husband’s oasis, fed by flood irrigation in the heart of the Sonoran desert.

Upton is among a handful of homeowners – by one accounting, just 1% – of metro Phoenix’s 4.4 million people to receive flood irrigation. The Salt River Project, the area’s largest supplier of such water, delivered almost 60,000 acre-feet of water to that small number of residents in 2019, or 7.5% of the water it delivered that year to all customers combined.

In that same year, the Salt River Project sent 36,003 acre-feet to Phoenix-area schools, parks, golf courses and churches (and 63,500 acre-feet to farmers – another story entirely) to irrigate trees and turf.

To provide scale for that type of usage: one acre-foot of water can sustain three Phoenix-area families for a year. The entire city of Chandler, Arizona, population 261,000, uses 60,000 acre-feet of water annually.

The water, untreated, is cheap. To flood her yard twice a week, except in winter, Upton pays $100 a year to Salt River Project and $350 to her subdivision’s irrigation water delivery district, a special taxation district she helped start. If it were city water, “we could never afford this,” Upton says. “It’d be over $600 a month.”

The total amount of raw water to irrigate lawns and trees in private homes, parks, and schools has changed little in the last 36 years. Some people deem the practice a harmless anomaly. Others defiantly defend it against a backdrop of conservation messaging and intensive planning for climate change, drought, and future water scarcity.

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Phoenix has a deep history of environmental injustice. Low-income communities and communities of color suffer disproportionately from Phoenix’s extreme heat, a problem compounded by water access and affordability.

No one appears to have studied how flood irrigation correlates with wealth or race. Research indicates white, wealthier people are more likely to live in grassier, shadier neighborhoods. In one study from 2008, local researchers found that during one heat wave, the temperature discrepancy between a wealthier neighborhood and a poorer one in Phoenix hit 13.5F. Trees and grass accounted for the difference.

Whiter, wealthier people were more likely to have more vegetation, and in turn, cooler climates, the authors found. That study did not examine how greener areas were watered, but any irrigation has costs. “Affluent people ‘buy’ more favorable microclimates,” the researchers concluded.

Cynthia Campbell, water resources advisor for the city of Phoenix, says she understands why wealthy neighborhoods might still have flood irrigation while poorer ones don’t, even if both have legal rights to the water: high-income families can afford to spend hundreds of dollars on water delivery, pipeline repairs, and irrigation-district taxes. For lower-income ones, that kind of spending might not be possible.

Sarah Porter, director of the Kyl Center for Water Policy at Arizona State University (ASU) Morrison Institute, sees “two Phoenixes”. One is water-rich, the other water-poor, she says.

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Step away from Phoenix’s suburban sprawl into what little natural desert now remains, and you’ll find yourself in a dense ecosystem. Saguaro cacti stretch 40ft high, and barbed cholla stand alongside outstretched ocotillo plants. In the spring, after winter rains, a faint sage green carpets the desert floor.

Only in recent decades has the landscaping of metro Phoenix begun to resemble the surrounding desert. In 1978, grass dominated 80% of home landscapes. By 2014, that had dwindled to less than 15%, as the popularity of desert landscaping, or xeriscaping, grew. The area receives on average 8in of rainfall a year.

Urban flood irrigation reflects an essential tension in the management of Arizona’s water. On one hand, city and state leaders seek to reassure everyone that the region has sufficient water. Arizona’s economy has long been tied to growth, and leaders don’t want to scare off would-be residents with warnings of scarcity, like the fact that groundwater is over-pumped, that the state is exploring options to desalinate ocean water or brackish groundwater, or that Arizona will likely soon take mandatory cuts to its share of Colorado River water.

On the other hand, cities in this bastion of Sunbelt conservatism encourage residents to use water “wisely”.

Avoiding ordinances limiting individual water use, they instead have water-conservation websites, literature on optimizing watering systems, rebate and incentive programs, and workshops encouraging residents to consider low-water desert landscaping and low-flow toilets. They don’t demand that residents use less water, while other US cities do.

The average resident in metro Phoenix, where water prices are among the lowest in the nation, consumed more than 115 gallons of water per day in 2018. That’s down from about 135 in 2005, but still well above the average resident of Tucson (less than 85 gallons), which never had comparable access to water. The 2015 US average consumption was 83 gallons per day.

Large-scale changes to urban flood irrigation would come at a cost – converting to drip or sprinklers is expensive. A common claim is that trees raised on flood irrigation will die without it, but some evidence suggests otherwise. Two studies have found that mature citrus and olive trees, both of which are present in Phoenix, can respond positively when converted from flood to sprinkler, drip, or other irrigation, although they take a few years to adapt.

Meanwhile, Phoenix has an urban heat island problem – concrete and asphalt absorb the sun’s radiation during the day and release it at night, worsening every year – which flood irrigated trees and grass may assuage. Average night-time temperatures today are nearly 9F hotter than they were 50 years ago. Last year, Phoenix broke records with the number of days over 100F; its deadly summer heat can exceed 120F.

Advocates of flood irrigation also invoke the desire to maintain the historic character of a neighborhood, Phoenix’s “oasis” feel, and, inevitably, the property values commensurate with both.

Wendy Wonderley moved to Arizona in 1984. Before retiring to Indiana in January, she lived in several Phoenix neighborhoods with flood irrigation, calling them “very pleasant to live in”. She frames flood irrigation as a question of values.

“If you’re going to live here in this desert,” Wonderley explains, “Then you’re going to have to alter some of the attributes of it.”

“If Phoenix as a society decides, ‘Nope, we really want desert vegetation everywhere,’ then that’s that, but it would be a real shame,” she says.

“It’s just not popular to regulate people’s water use,” says Kelli Larson, a professor of geography at ASU who has studied landscape choices and municipal ordinances. In a 2017 paper she co-authored, she found that despite campaigns to urge people to switch to desert landscaping, preferences for lush, grassy landscapes still exist, especially among long-term residents. Many want more grass than they have.

In another study published last year, Larson and her co-authors examined municipal landscaping ordinances in metropolitan areas across the US, including Phoenix. They found that 68% of the two dozen municipalities in greater Phoenix stress water conservation, compared to 87% of municipalities in the Los Angeles area, and that a greater proportion of municipalities in the water-rich Minneapolis-St Paul area (68%) restrict landscape irrigation than do those around Phoenix (56%).

Pending water scarcity in Phoenix, Larson adds, is less about whether it will be available, and more about to whom. “It’s going to become institutionally limited, economically limited, way before it’s going to become actual physical scarcity,” she says.

Tempe, southeast of Phoenix, still operates flood irrigation for about 900 residents and 16 city parks. It recovers just half those costs from users. On a video call, Braden Kay, Tempe’s director of sustainability, explains the city’s support for flood irrigation partly as a need to maintain large trees for cooling. Terry Piekarz, Tempe’s municipal utilities director, who is also on the call, nods. “We’re not saying, ‘Don’t [use water],’” he says, echoing other city officials interviewed for this article. “We’re saying, ‘Use water wisely.’”

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Under Arizona’s labyrinthine water laws, transferring water from one place to another is difficult. All water from the Salt River Project must be used on Salt River Project lands – designated a century ago by legal decree. Similar legal and jurisdictional challenges affect Arizona’s share of the Colorado River and its groundwater. Although the effects of ongoing drought and climate change, including less surface water, are prompting calls for greater flexibility in managing water, options remain limited.

For example, if Porter were to decline her own home’s allocation of flood irrigation water – which she has a “complex relationship” with – it can’t be sent to, say, water-stressed North Phoenix, which isn’t legally entitled to it. Instead, when she declines her water, it goes to the municipality where she lives, Phoenix, for treatment and distribution to Salt River Project territory.

Someday, Porter dreams, that could change.

Her wish is that people like her, who have irrigation rights, could one day forgo their own allocation and instead opt into alternative water uses, like watering trees or rehabilitating dry riverbeds.

She has also spoken with Salt River Project about the possibility of installing a tank in her yard to collect flood irrigation water to use in a less water-intensive sprinkler system – an option that is legal but expensive.

As for Dawn Upton, she says she’s aware of living in a desert – she turns off the faucet when brushing her teeth – and sometimes questions whether everything that made flood irrigation possible should’ve been done at such a massive scale at all. But, that can’t be undone. Plus, she adds, “when you look at this lawn, I don’t think I want it any other way”.