Nature Note: Why Don’t Trees Drown During Spring Floods?
Trees that are inundated seasonally have to find a way to keep their roots oxygenated to survive. Photo by Mark Bowie.
Spring flooding presents a challenge to the trees living along rivers. They risk root death due to water -logged soils and don’t yet have their leaves to produce oxygen to pump down to the roots. Furthermore, lack of oxygen in the soil mobilizes iron and manganese. While these nutrients are needed by plants, in high concentrations they can become toxic.
When trees are exposed to floods they develop specialized tissues for obtaining and storing oxygen. These tissues are retained from year to year, increasing the likelihood that the tree can survive subsequent events. Examples include lenticels in the bark through which oxygen passes, porous gas-conducting cells called aerenchyma, and specialized above-ground roots. The tissues can be efficient enough that the roots begin to export oxygen to the surrounding soil, limiting the mobilization of iron and manganese. Scientists have blocked lenticels in mangrove trees and the oxygen content of submerged roots has dropped from 15%-18% down to 2% or less within just two days.
Another strategy wetland plants might take is to limit metabolism or switch to metabolic pathways that don’t require oxygen. Limiting metabolism can mean shutting down leaf production or photosynthesis until after the flood subsides. For this reason, floods during the growing season can be more damaging than early spring floods before leaf-out. Alternative metabolic pathways are a short-term strategy as they are less efficient and often lead to the build-up of toxic by-products. Humans do the same thing. The muscle soreness that results from exercise comes from a build-up of lactic acid when our muscles don’t get sufficient oxygen.
Typical floodplain forest trees in our region include silver maple and green ash with black willow and American elm also occurring frequently.
