What it all means

A three-day flood barely bothered most of the 16 tree species in this study, but a ten-day flood separated them sharply. Whether a tree bounced back afterwards mattered as much as whether it coped during the flood itself. And the usual measurements of tree traits could not predict which species would struggle. This page sums up what the study found and what it means for planting trees in flood-prone city sites.

Five takeaways

Short floods are usually manageable — only hop hornbeam was clearly hurt by three days. Its vitality (how healthy a tree looks, based on wilting and leaf discolouration) dropped and it lost leaves, while every other species kept a performance score above 0.75. Wild cherry's photosynthesis also dipped, but its overall score did not fall into the worst group.
Long floods hit hard — and hit unevenly. After ten days of flooding, net photosynthesis (the rate at which leaves turn light into food) fell in 10 of the 16 species. By the end of the 16-day recovery period, six species still showed reduced vitality. The worst-hit species lost far more ground than the rest.
Recovery is the make-or-break. Tulip tree, hop hornbeam and wild cherry stayed damaged even after the recovery period, with values below 0.5 on the species performance index (a single score summarising how a species performed compared with its control, where 1 means no effect). By contrast, Norway maple, silver maple, sweetgum and small-leaved lime regained photosynthesis during recovery. City floods are often short and repeated, so recovery ability is a key selection criterion.
Standard traits are poor crystal balls. The team measured 12 commonly used traits — including wood density, leaf size, leaf mass per area (how much leaf there is per unit of area), and root-to-shoot ratio (how much root mass a tree has for each unit of shoot). These showed only weak links to flooding sensitivity, and a principal component analysis (a method that looks for groupings among many traits at once) found no clear groupings. Trait tables alone should not decide species selection.
Old tolerance classifications were only partly right. Some species labelled sensitive coped well: silver birch, European hornbeam, black tupelo, Swedish whitebeam, wild service tree and small-leaved lime. The two species labelled moderately tolerant, silver maple and sweetgum, were among the least affected. But several species labelled sensitive did struggle: hop hornbeam, tulip tree, wild cherry, common hackberry, kobus magnolia and service tree of Fontainebleau.

The single most important message: for flood-prone city sites, ask how quickly a species recovers from flooding, not only how well it survives the flood itself.

What this means for city planting

Choose for recovery, not just survival. A tree that bounces back can keep providing shade and stormwater benefits season after season.
Expect repeated short floods, not one long soak. Urban flooding is often brief and frequent, which is exactly the situation this study tested.
Diversify species. A mix spreads risk: if one species suffers, others can still keep the urban canopy working.
Combine measurements with local experience and site conditions. Data help, but nothing replaces watching how trees actually perform in a particular place.
Be cautious with trait-based lists. Since the 12 common traits did not predict flooding sensitivity, treat them as one input among many, not as the final word.

Honest limits

What this study can and cannot tell you:

The trees were young and grew in pots outdoors at a commercial nursery in south-west Sweden during one summer (2023).
Flooding was simulated by submerging the pots in water. This mimics short urban floods, but not every real-world situation.
Mature trees may react differently — tolerance can change with age, and available rooting space matters.
Other city stresses, such as drought, heat, salt and compacted soil, were not tested together with flooding, yet in real streets these stresses interact.
The findings apply to these 16 broadleaf species, not to all trees.

Still open

Some traits are tied directly to roots and oxygen and were not measured in this study:

Aerenchyma: Spongy air channels inside roots and stems that can carry oxygen down to flooded roots.
Adventitious roots: New roots that sprout from a stem or trunk, above the normal root system.
Lenticels: Small pores in the bark that let gases pass between the tree and the air.

These features may predict flooding responses better than the traits measured here. The authors also call for more research on mature trees and on repeated floods, since those are the real conditions city trees face.

The full study is open access and free to read: doi:10.1016/j.ufug.2026.129674. To go deeper into this website, meet the 16 species on the The 16 trees page, or see how each measurement turned out on the Results explained page.