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Issue link: https://read.dmtmag.com/i/648999
24 City Trees 24 City Trees Flooding from the Mighty Mo (Missouri River) had major impacts from June through August/early September of 2011. In the Omaha area, the River swelled to over 36 feet/11 m deep (only 4 feet/1.2 m shy of the record set in 1952) and tripled in speed to nearly 12 miles per hour (19 km/h), eroding soil and washing debris downstream. The Garrison Dam in North Dakota doubled its previous water release record, sending 152,000 cu feet (4304 cu m) per second rushing downstream. That's equivalent to over 6000 average-sized refrigerators' worth of water every second. It is amazing that any trees remained standing after the reservoirs had drained back down to normal levels. The flooding began the month after I began working as a Nebraska Forester, and I knew it was an opportunity to observe nature in action and share what I learned. The Bob Kerry Footbridge, which arches over the Missouri River, remained open to the public during the event. This gave me a front row seat from which to photograph and document flooding response from different tree species. The force of the flooding deposited up to 2 feet (0.6 m) of sediment around some trees while leaving the buttress roots of others completely exposed at a comparable depth. Mature cottonwoods heavily eroded by the swollen river were observed to have two or three root systems at various depths—a response to past sedimentation events. Most cottonwood trees that died during the flooding fell over while still green due to unbalanced canopies and rotten smaller roots. One assessment I made for After Katrina, tens of thousands of native southern magno- lia in southern Louisiana region were wiped out within a month. My colleagues and I studied the surviving tree populations. Wherever flood water from Katrina stood for more than five or six days, the southern magnolias died. 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 stand- ing water. It is challenging to find species to use that are both wind and flood tolerant. I 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." Because it's a tree with such cultural importance to us in the Southeast, not replanting it at all would be a mistake. —Dr. Hallie Dozier, Professor of Forestry at Louisiana State University, New Orleans Much of Texas, and especially the Dallas/Ft. Worth region, received record amounts of rainfall in spring of 2015. Flooding was severe for many areas for a prolonged period of time. All regional lakes were overfilled and many were not releasing water until downstream flooding subsided. This was especially true locally for the Trinity River system. Under these circumstances many forested areas were flooded for over three months from April/May into July and even early August. I had always assumed that prolonged inundation of a tree's root system during the growing season would result in sure mortality. So far, that has not been the case; however, the full extent of damage is yet to be seen. Thus far, river bottom species such as boxelder (A. negundo), sycamore (P. occidentalis), and cottonwood (Populus deltoides) have done the best. Cedar elm (U. crassifolia) have fared a little less well and some oaks species have suffered the most— Shumard (Q. schumardii) and Texas red oaks (Q. buckleyi) and especially post oak (Q. stellata) and blackjack oak (Q. marilandi- ca) seem to have been hit the hardest. However for all species, the damage is hit-or-miss. There are trees of the same species growing right next to each other that were flooded for three months and one will be dead while the other looks just fine. Of course who knows what the future holds for these trees. I suspect just as with any other major stress to a tree, the resulting possible problems could take years to surface. For now, though, I've gained an appreciation for the way that certain trees seem to be able to handle severe flooding better than I'd expected. —Courtney Blevins, Fort Worth Regional Forester, Texas A&M Forest Service Advancing Urban Forestry Post-Katrina Dr. Hallie Dozier (Louisiana State University) and Dr. Kamran Abdollahi (Southern University) found abundant evidence that those 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 the ones compromised by poor site conditions like 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. Dr. Abdollahi says 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 keep- ing 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. Abdollahi says that there has also been a growing focus on the need for arborists to be appropriately educat- ed and licensed. Powerful flooding can erode soil from underneath concrete slabs and wipe out whole sections of paved road, like this one in Bellevue, Nebraska. Photo by Graham Herbst