The return of greenery to the urban landscape in the aftermath of such catastrophic hurricanes as Katrina, Harvey, Irma or Maria may not seem like a pressing issue. But its impact on flood-prone cities can be substantial.

It happened after Katrina, and could likely reoccur after the most recent spate of vicious hurricanes. The raft of problems resulting from abandoned and overgrown properties since Katrina could plague other cities recently hit by vicious hurricanes. City leaders must keep invasive trees from overtaking these vacated lots, a scenario that complicates recovery and causes harm, according to new research.

“Once [invasive trees] become established, they become more expensive to remediate, and they become seed sources for the continued spread of these plants, which produce very few benefits for people, and cause no small numbers of problems and concerns for residents,” said Joshua Lewis, a research professor at Tulane University’s ByWater Institute and lead author of a new study in the journal Ecosphere that examines vegetation in post-Katrina New Orleans.

Abandoned property in aftermath of Hurricane Katrina. Source: Pixabay

Invasive species took over deserted lands. “People used abandoned and newly forested lands to dump garbage and hide illegal activities, sometimes even violent crimes,” Lewis said. By lending cover to criminals, overgrown plants pose a threat to personal safety, especially at night and especially for women, the study said. To be sure, numerous factors contribute, but “the environment itself plays an important role for enabling these activities,” Lewis added.

Overgrown spaces also threaten health and property. Used tires dumped illegally in discarded lots collect water, providing the perfect breeding ground for disease-carrying mosquitoes. Unmanaged tree roots may disturb infrastructure or building foundations, and damaged or diseased trees may fall, endangering people and nearby buildings. Additionally, the unpleasant sight of wild vegetation can hurt property values, according to the paper.

The researchers, who studied plant life across New Orleans since 2005, when Katrina struck, also found that demographic factors such as wealth, race, housing, and land abandonment most accurately predicted the extent to which invasive species dominated the neighborhoods. That is, areas with large numbers of African-American residents, high rates of land abandonment, and relatively low median household incomes saw more invasive species creep in than some wealthier areas that endured fiercer winds and more intense floods during Hurricane Katrina.

Areas with larger numbers of white residents, higher incomes, and less land abandonment had larger native trees, like the Southern Live Oak, an “iconic and culturally resonant species,” regarded as an emblem of the Old South, Lewis said. “A whole range of neighborhoods and demographics were hit by Katrina. They don’t all have equal access to private capital and the ability to navigate housing recovery programs, and that is what we see driving the type of vegetation emerging on these abandoned properties.”

Live Southern Oak tree. Source: Pixabay

Because invasive vegetation often provides little in the way of ecosystem benefits for residents — and in some cases represents more of an environmental hazard than an asset — it represents an issue of environmental justice, “where asymmetries in public and private investment in landscape management result in unequal benefits and hazards between neighborhoods and social groups,” the study said.

“This depresses real estate values, makes it more difficult for residents wishing to return to their neighborhoods to get financing for repairs or new construction, dissuades businesses from investing in important retail services like grocery stores and so forth,” Lewis said.

Turning abandoned properties into green spaces can produce “benefits in terms of flood water absorption, recreational space, and ecologically appropriate habitat,” Lewis added. “However, simply proposing these initiatives immediately after a flood or hurricane also presents problems.”

In the case of New Orleans, a plan developed by the Urban Land Institute less than a year after Katrina “drew outrage from New Orleanians, as it proposed the wholesale abandonment of large swaths of the city, the erasure of entire urban neighborhoods, and their replacement with parks, green space and green infrastructure like rain gardens,” Lewis said.

The Urban Land Institute plan for rebuilding New Orleans. Source: Urban Land Institute

“While some of these ideas have merit and should be pursued when appropriate, such an immediate and far-ranging proposal makes assumptions about which neighborhoods could, or should, be redeveloped,” he added. “This plan was never implemented, and for good reason. But instead, we were left with a fairly unplanned and uncoordinated recovery for several years in the storm’s aftermath.”

For their study, Lewis and colleagues focused on eight neighborhoods representing a cross-section of demographics and flooding severity to learn how plant life responded after the hurricane hit. They recorded the age and size of trees and other plants, and the composition of species at sample sites. They analyzed aerial photographs and vegetation surveys. They conducted interviews and collected census data to determine the influence of the city’s physical and social geography on regrowth.

“We looked at images taken in October 2005, only a month or so after Katrina struck, and only a few weeks after the city was finally pumped dry,” Lewis explained. “There were clear indications that trees, shrubs and grasses had been defoliated and killed throughout the city, but most especially in places that suffered prolonged flooding. We then analyzed an additional set of images, from 2013, eight years after Katrina. This enabled us to have a window into how vegetation cover rebounded in the years after the storm.”

Immediate aftermath of Hurricane Katrina in the Lower 9th Ward (top), Gentilly (bottom left), and St. Bernard Parish (bottom right). Note defoliation of trees and shrubs. Source: Ecosphere

In some neighborhoods, there was little vegetation disturbance, and “eight years later we saw similar extents of vegetation cover as we saw before Katrina,” he said. “In others, however, and most especially in areas that had dealt with demolitions and land abandonment, we saw that vegetation cover had expanded dramatically.”

What was growing in these areas? To find out, the scientists established 180 vegetation survey plots throughout the study neighborhoods, then went out to take a look. They identified and measured trees, shrubs and grasses.

White, wealthier areas with stable vegetation during the storm “had more native hardwood trees, ornamental plants, and commercial turf grasses,” eight years later, he said. On the other hand, lower-income, non-white areas with significant changes from the storm “were characterized by opportunistic, non-native, and invasive trees and shrubs, and herbaceous plants called forbs, rather than grasses,” he said. “We saw that tree diameters were also smaller in these neighborhoods, indicating that many of these trees had grown in after Katrina.”

In addition to examining patterns of vegetation growth, the study also rejected popular media depictions of “nature reclaiming the city,” Lewis said, pointing to one article that described the city’s Lower 9th Ward as “jungleland.” The area “remains an important urban neighborhood with thousands of residents,” he said.

Study neighborhoods and context in New Orleans, Louisiana. Green circles indicate vegetation inventory locations. Source: Ecosphere

These depictions have prompted criticism by residents who are battling social stigmas that discourage repopulation and redevelopment, the study said. “According to one neighborhood activist who has devoted years to implementing alternative land uses on abandoned properties in the Lower 9th Ward, for “every vegetation-covered lot, there is a story, and much of the story is hardworking people trying to make the historic Lower 9th Ward whole again,” the study said.

“Our interviews with residents and public officials showed that vegetation maintenance on abandoned lands was a public policy problem, coming down to difficult funding decisions, legal hurdles and sensitivity over property access,” Lewis said. “Portrayals of this process as a purely natural one, of nature winning some sort of battle, ignores these details.” He pointed out that property that has grown wild, but is privately owned, presents a legal problem for public agencies that may not have permission to enter and remove the overgrowth.

“After several years, local authorities began gaining control of properties that were bought out by the state of Louisiana, and began mowing and maintaining them,” Lewis said. “However, there were thousands of other properties that languished as residents struggled to navigate housing recovery finance programs, were displaced far from the city, were elderly, or in some cases didn’t have formal legal title to the land.” State-managed lawn-scapes were interspersed with privately owned properties with limited or sometimes no management of vegetation at all. “We ended up with this bizarre mosaic of lawns and forests,” Lewis said.

City, state and federal agencies “were not adequately prepared to deal with the complex and politically intense planning process in the aftermath of the storm,” he said. Plans for “rain gardens and green infrastructure were rolled out in such an insensitive fashion that they were understandably widely derided and rejected by many residents.”

It has taken officials more than a decade to find funding for these approaches and to begin a public dialogue about overgrowth and urban flooding, Lewis said. He added that storm-prone cities like New Orleans should develop plans “ahead of time that avoid placing hard-hit neighborhoods at continued risk from the public health and safety problems generated by these rapidly emergent forests.”

Marlene Cimons writes for Nexus Media, a syndicated newswire covering climate, energy, policy, art and culture.