Trees and forests as infrastructure
Recent widespread flooding in northwest Iowa has had devastating impacts on a range of critical infrastructure, and the same may be said about localized flooding events that occur annually across our state and throughout the Midwest. When we think of infrastructure necessary for societal function, we immediately think of buildings, roads, the power grid, etc. Surprising to many, however, is the fact that trees and forests are also key pieces of infrastructure – especially when it comes to water quality and flood mitigation services. Vigorous, well-managed forests on the landscape, especially those that border our rivers and streams, provide disproportionately large water quality and quantity services in comparison to their relatively small landscape footprint.
When floodwaters overtop streambanks and access forested floodplains and streamside (riparian) areas, flow velocity slows greatly in part due to drag created by rigid stems (Photo 1) – delaying flood flow delivery to downstream communities. As floodwaters recede, significant volumes may be stored in oxbows and other floodplain depressional features – again, reducing/delaying the amount of water immediately transferred to downstream communities. Through windthrow (exposure of root masses as trees lean from high winds), trees encourage “pit and mound” micro-topographical features areas that increase the floodplain “storage sponge”. In addition, these floodplain storage opportunities allow floodwaters to infiltrate the soil and recharge shallow groundwater. Although trees and forests may have impressive flood mitigation power, potential does exist for floods to damage these critical pieces of infrastructure – especially floods that are “out of season” (floods in the growing season are particularly damaging), of heightened intensity, and of extended duration.
Photo 1: Inundated floodplain forest along the Des Moines River, early spring 2019. Note the lower water velocity within the forest (foreground) compared to the main channel (background). Dormant season floods have relatively low negative impact on trees.
How flooding impacts trees and forests
First, it should be noted that our “historic annual flood regime” is highly beneficial to streamside and floodplain forests. Nutrient cycling, energy transfer, and aquatic and terrestrial wildlife habitat all rely on a “healthy” annual connection between the river and its floodplain. In addition, regeneration of bottomland species such as cottonwood and sycamore greatly depend on flooding to supply fresh sediment required for seed germination. The regeneration cycles of these species have evolved with annual floods – syncing the dispersal of their seed with timing of sediment deposition from spring floods. Altered flood regimes, as we’ve experienced over the past few decades (e.g., flooding later in the year, floods of extended duration) may have detrimental impacts to trees and forests, however. To understand potential detrimental impacts, it’s important to understand a bit about tree growth and physiology, and the timing and degree of flooding.
Tree roots require adequate pore space in soil (space between soil particles) to allow for root elongation, oxygen availability, and water holding capacity. When these pores become saturated (filled with water) for extended periods of time, oxygen becomes depleted and trees begin to decline. This is especially important in the upper few feet of soil, as that is where the majority of tree roots exist (in contrast to every tree root image that I see on t-shirts, in which roots are equally spread between shallow and deep, in an upside-down fan shape). The time that trees can tolerate saturated soils depends on species, and may range from days to months. This extent is often regarded as species “flood tolerance”. So, flood duration matters, with the concept being applicable to over-watered landscape trees and urban trees in soils with limited pore space (e.g., clay soils). Young trees are even more susceptible to flooding, as most species can only survive a limited period when floodwaters overtop canopies – inhibiting gas exchange between leaves and the atmosphere. The last forms of damage are physical and chemical. Floods deliver wood, debris, and who-knows-what chemicals, which can cause physical and physiological harm. It is interesting to note that many floodplain species (e.g., silver maple) have adapted a multi-stem form and rapid growth rate, perhaps to better sustain and “bounce-back” from debris impact.
Recovery and resilience
Visual assessment of woodlands and trees should be performed once floodwaters recede as well as over the next few growing seasons. Inspect the overall structure of trees for wounds and breakage from flood debris. Assess the root zone adjacent to the trunk and under the dripline for erosion and scour. Examine canopies for discoloration, chlorosis (yellowing), defoliation, general dieback, leaf growth anomalies (reduced size), and early leaf drop. Again, it’s critical to assess trees and woodlands for a few growing seasons following flooding to note rapid recovery or extended decline. Trees impacted by flooding are stressed, which means they are more vulnerable to secondary pests and pathogens such as wood borers and fungal cankers – so record instances of these as well.
The key to recovery and resilience is vigorous trees and woodlands. Enhancing tree vigor through practices such as mulching, pruning, and providing adequate water during dry periods will help trees recover faster post flood. In woodlands, vigor may be enhanced by appropriate thinning, removal of invasive species, and reduction of uncontrolled livestock grazing. Above all, vigor is maximized when species are matched to site conditions! The range of flood tolerance for a variety of tree species commonly found in Iowa’s woodlands and urban/landscape areas may be found in the ISU Extension publication Understanding the Effects of Flooding on Trees (SUL 1). Professional foresters (e.g., Iowa DNR District Foresters, private consultants) are available to assist with pre-flood management and post-flood recovery in woodlands. Certified Arborists are available to assist with flood issues in urban and landscape situations.
