Role of a remnant floodplain forest in uptaking urban stormwater runoff

Background/Question/Methods

It is well recognized that urbanization increases surface water runoff, through a reduction in pervious surface area. In the absence of human alterations, wetlands, floodplain forests and other bottomland ecosystems are able to capture and process excess water. Where storm runoff is increased by urbanization, the capacity of vegetated areas to mitigate water quantity may be taxed, negatively impacting the ecosystem services as well as ecosystem function. We examined the role of a natural floodplain forest in mitigating quantity of urban runoff, focusing on the following questions: what is the maximum capacity of this floodplain forest to capture and uptake stormwater? What are the impacts of altered flood regimes (quantity and timing) on physiology of bottomland trees? These questions were examined in the Oliver’s Woods Ecological Laboratory in Norman, OK. This is a typical central Oklahoma bottomland forest, dominated by green ash (Fraxinus pennsylvanica) and American elm (Ulmus americana), both of which are considered moderately tolerant of flooding. We continuously measured soil moisture and tree level transpiration, as well as stem growth and leaf carbohydrates (recent photosynthates) in regularly flooded and non-flooded areas of the forest, to determine the effect of flooding on productivity and survival of flooded trees.

Results/Conclusions

We found that maximum canopy transpiration rates can be quite high, upwards of 4 mm per day, suggesting the potential for substantial mitigation of stormwater. However, there may be temporal mismatches between timing of maximum transpiration and flood occurrence. Despite the increased flood intensity, mature green ash trees do not appear to be negatively impacted, having higher growth rates than in the non-flooded plots. This is likely due to the presence of higher soil moisture year round, including during periods in which precipitation is infrequent. This work contributes to better understanding of the services provided by floodplain and riparian forests within urban areas and will allow for more informed land use decisions. Understanding the capacity of natural systems to mitigate stormwaters will also encourage the development of alternative approaches to storm water mitigation, where necessary to alleviate detrimental effects on natural ecosystems and obtain the level of services desired.