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Tree stability is of great importance not only in forestry, but also in other contexts, such as dam and levee integrity. Extensive damage to New Orleans after levee failures during Hurricane Katrina in 2005 prompted widespread reevaluation of effects of trees and treefalls on the integrity of levees. We undertook a static winching study of western cottonwood (Populus fremontii Wats.) and valley oak (Quercus lobata Nee) trees on levees in the California Central Valley. We hypothesized that Quercus would be stronger; and that the size-stability relationship for both species would be non-linear. We further hypothesized that critical turning moments would decrease with greater soil moisture and with greater sheltering from neighbouring trees. As expected, the critical turning moments increased with tree size and that relationship was non-linear. Size-controlled critical turning moments of Quercus were much greater than Populus, but large inter-site variation made the difference non-significant. While the critical turning moments were not significantly influenced by differences in soil moisture, soil bulk density or soil texture for these sites, significant variation among sites suggest that history or unmeasured components of the environment can exert a strong influence on tree stability. Mirroring trends seen in natural wind disturbances of forests, larger trees were more likely to uproot, while smaller trees exhibited a more even mix of trunk breakage and uprooting. Pulled trees with greater abundance of very close neighbours showed no differences in critical turning moments compared with trees with no nearby (i.e. within 5 m) neighbours. The largest trees had critical turning moments exceeding 1 million newton-meters, demonstrating that large healthy trees in unrestrictive soil conditions can be extremely stable. While the maximum turning moments reported here are above those reported in the literature, when corrected for size, the trees of this study were in the upper range but not beyond turning moments previously reported, suggesting that opengrown trees may not have critical turning moments that greatly exceed those of forest and plantation trees.

An evaluation of the stability of Quercus lobata and Populus fremontii on river levees assessed using static winching tests