By Michelle Sutton
Hurricane Katrina (August 2005) and Hurricane Sandy (October 2012) savaged tree populations with high winds and flooding. Katrina pushed huge amounts of brackish water on land and lingered for 30 days or more, while Hurricane Sandy brought one storm surge of salt water that retreated with the same day’s tides. Here we look at some of the impacts of hurricane-related flooding on trees in New Orleans and greater southern Louisiana and in Long Island and New York City.
What are the major reasons flooding is so punishing for trees? Dr. Kamran Abdollahi, professor of forest ecophysiology in the urban forestry program at Southern University in Baton Rouge, La., explained that flooding fills soil pores, denying tree roots access to the oxygen they need for respiration and water and nutrient uptake. “In the urban environment where soils are already compacted by human activities, flooding exacerbates compaction and its negative effects,” said Abdollahi. “Flooding can also negatively affect root anchoring and tree stability.”
Long Island and New York City
Arborist Joel Greifenberger is the owner of Valley Tree and Landscape in Long Beach, Long Island. Valley has planted more than 25,000 trees for New York City (NYC) in more than 25 years. Greifenberger said that on Long Beach, Hurricane Sandy brought several feet of salt water on land, “bay to ocean,” for about 12 hours. That brief flooding event left dramatic damage to the region’s trees, with some surprising victims.
The biggest shock was how poorly Long Island’s many London plane trees fared. They were long thought to be flood and salt tolerant and had been widely advocated for seaside use. Greifenberger said the damage manifested in stages; the following spring, an average of a third of the canopy was affected.
“It wasn’t typical dieback in that it didn’t affect the whole crown,” he said. “A large section of the crown on one side would not break bud, while the rest leafed out normally.” However, over the 2013 growing season, the trees continued to show signs of decline, until by 2014 their bark started to peel off and the trees died.
Last summer, Long Beach removed more than 1,400 dead trees. “My guess is that 85 percent or more of them were London planes,” said Greifenberger. “We used to have our streets lined with allées of them, like American elms back in the day. This was a huge blow to our city.” The city will avoid planting monocultures in the future, no matter how flood-tolerant any one tree species is thought to be.
Arborvitaes in his area were instantly killed by the floods, as were blue atlas cedars. “I had a job where I’d planted 200 blue atlas cedars at 4-inch caliper,” said Greifenberger. “They were at 7-inch caliper when the storm came; they were dead within weeks. That was heartbreaking.” Leland cypresses also were quick to die. Tulip trees never put leaf on again. Every single Japanese maple died. Pine trees and magnolias were a mixed bag. Across species, mortality was high for newly planted/younger trees.
In the “happy surprises” column, Greifenberger said, “Blue spruces, for one, never looked better; no one expected that. Junipers and red cedars did fine, as did holly trees. The Kwanzan cherry trees did ok if they’d been in the ground at least three years. Honeylocusts and pears did ok, and Norway maples and zelkovas did well.”
New York City lost 10,926 trees to storm damage from Sandy and shared with Long Island the experience of significant die-off of London plane trees. NYC Parks and Recreation prepared a report after Sandy related to flooding. With regard to London planes, more than 1,500 failed to leaf out at all the following season, and more than 2,500 leafed out 50 percent at best, with further decline anticipated.
Part of that report asks, “How will we change what we plant because of Hurricane Sandy?”
NYC Parks and Recreation Director of Street Tree Planting Matthew Stephens said, “Sandy highlighted that we, as urban forest managers, must continue to be vigilant in our efforts to specify trees that will be resilient to not only a diverse array of urban factors, but also changing environmental factors. For example, going forward, trees we choose to plant within the advisory flood zone for a one percent storm must be tolerant of both coastal conditions, as well as inundation.” To give you an idea of how important this consideration is, 14 percent of NYC streets fall within the advisory flood zone.
Integrating what was learned from Sandy’s particular toll, Stephens and colleagues identified a list of 75 tree species and cultivars that the City considers worthy of use in the advisory flood zone for a one percent storm. “Every new tree planted within the flood zone will now be chosen from the refined palette to ensure our trees are long-lived and resilient,” said Stephens. Those trees include species and cultivars of hedge and red maple, birch, hackberry, hawthorn, coffeetree, elm, oak, blackgum and zelkova.
New Orleans and Greater Southern Louisiana
Arborist John Benton’s company, Bayou Tree Service, works primarily in New Orleans and Baton Route, La. Bayou is full-service, but has specialized in historic tree preservation — especially of live oaks — since 1980. His crews were able to get into New Orleans starting four days after Katrina and worked many months, in extreme conditions, on tree clearing and hazard mitigation.
“Everyone was predicting the demise of the live oaks,” said Benton. “Because, although we know they are very tolerant of abuse, no one had observed these trees after they’d been inundated for 30 days or more. We were all worried. But the flooded live oaks did just fine.”
Benton pointed out that because so many other species blew over or declined during and after the flood, the live oaks had little competition when it came time to rebound. The few other tree species that did well include palms, baldcypress, and slash pine.
After the flood waters receded, “lawns were completely brown and looked desert-like,” Benton said. “Most of the ornamental shrubs and herbaceous plants were dead brown, and then here were these live oaks with dark, dark green and healthy leaves, looking like they had survived nuclear fallout.” It only enhanced Benton’s deep reverence for this incredibly forgiving species with which he has worked so closely for 35 years.
By contrast, tens of thousands of native southern magnolia in the region were wiped out within a month. Dr. Hallie Dozier, professor of forestry at Louisiana State University in New Orleans, has studied the surviving tree populations.
“Wherever flood water from Katrina stood for more than five or six days, the southern magnolias died,” she said. “I would say that in some spots where water lingered even fewer days than that, the trees were still lost within the next month or year. Southern magnolias are very wind tolerant, but unfortunately very sensitive to standing water. It is challenging to find species to use that are both wind and flood tolerant.”
Recently, Dozier had a conversation with the New Orleans city forester about the wisdom of replanting southern magnolias. “I said, ‘Well yes, you should, but not in such big swaths and not in low-lying areas.’ But because it’s a tree with such cultural importance to us in the Southeast, not replanting it at all would be a mistake.”
One lesson arborist John Benton took from the mass southern magnolia mortality is that “native” species aren’t necessarily better prepared for extreme stress. It’s all about whether the tree has been properly matched to the site. So southern magnolias, for instance, like to be on high, dry, mineral soils. Their being native is not reason alone to use them, at least not in low-lying areas. Meanwhile, crape myrtles, originally from Asia, weathered the flooding well.
Savannah hollies and other hybrid hollies took a beating after the flooding; Benton was quite surprised to see that in mature, tall hedges of hollies, every single one died. Hollies grow natively in coastal and swampy areas, yet they were devastated by the flood waters. Benton noticed more insects in the wake of the flooding, in particular, scale insects. “Of course, scale was most likely to hit trees that were already in poor health,” he added.
Drs. Dozier and Abdollahi found abundant evidence that urban trees that had been categorized in poor condition by inventories prior to Katrina were the most likely to be wiped out by the storm effects. The trees that were hardest hit were those compromised by poor site conditions such as soil compaction, root disruption, or inadequate rooting volume. This just makes common sense, but the fact that it was borne out so dramatically, with so much tree damage to deal with, has yielded a silver lining: more rapid advancement of arboricultural sophistication and urban forest appreciation in the region.
Abdollahi said that post-Katrina educational programs for arborists and city leaders have intensified in southern Louisiana. City planners have been eager to learn more about how to better design infrastructure to make planting sites more hospitable for tree roots. The necessity of keeping tree inventories, hazard assessments, and maintenance up to date has received a lot of attention. And of course, the message of “Right Tree, Right Place” has been driven home. According to Abdollahi, there has also been a growing focus on the need for arborists to be appropriately educated and licensed.
“This disaster gave my colleagues and me a lot of opportunities to talk to people who previously didn’t want to invest in trees,” said Abdollahi. “They thought trees should be just be there and provide benefits. I explain how trees are like any valuable infrastructure; they require maintenance in order to deliver the benefits we expect.”
The devastation of Katrina to cherished urban trees drove home this message.
Michelle Sutton (michellejudysutton.com) is a horticulturist, writer, and editor.
Photo by Mark Moran, NOAA Aviation Weather Center, public domain.