COS 204-3 - Root responses of congeneric tree species to waterlogging

Abstract: Urban trees provide many environmental benefits, but often face challenging growing conditions. One of these challenges is waterlogged soils caused by soil compaction and poor drainage. These issues are likely to be exacerbated by the projected increase in flooding events due to climate change. Root responses to waterlogging mediate how trees respond to waterlogging stress, yet are not well understood. Our goal was to identify how root responses to waterlogging influence tree tolerance levels and possible recovery strategies. We assessed the effects of a two-week waterlogging period on saplings using two congeneric contrasts of species previously identified as tolerant and intolerant based solely on aboveground responses. Sugar maple and saucer magnolia are reported to be intolerant to waterlogging, whereas silver maple and star magnolia are reported to have high or moderate tolerance, respectively. Saplings were grown in pots constructed with windows that allowed us to monitor root production, mortality, and periodic sampling of root tissues for analysis of peroxidase activity and lipid oxidative damage. Additional aboveground measurements of stem diameter, leaf count, photosynthesis rate, and stomatal conductance were taken to assess tree performance.

High root mortality, reduced photosynthesis rates, and reduced stomatal conductance were recorded for waterlogged individuals in all species except silver maple. No significant difference was found between peroxidase activity in maple species, making peroxidase scavenging an unlikely strategy explaining silver maple waterlogging tolerance, though other scavengers may still be involved and could account for silver maple’s tolerance levels. Sugar maples were the only species that showed significant increases in lipid oxidative damage, leaf loss, and reduced stem growth post-waterlogging. Unlike other species, Sugar maples did not recover post-waterlogging, possibly due to an excess of stress at the root level or insufficient energy reserves to enable post-stress recovery. Future research should investigate other free-radical scavengers that may play a role in tolerant species mitigation of oxidative damage (e.g., ascorbic acid, glutathione, catalase, and superoxide dismutase). Questions regarding how timing and length of waterlogging impact the natural cycles of tree nonstructural carbohydrate storage and use should also be addressed, as the ability to catabolize energy reserves to support new flushes of fine root growth post-waterlogging may be an important strategy for some tolerant species. Better understanding of different strategies used by trees to cope with waterlogging stress will help inform urban tree selection in repeatedly flooded and/or completely inundated soils.