Hikers space themselves out along the narrow trail winding through the floodplain forest above the bottom lands of the Minnesota Valley National Wildlife Refuge. It’s late March 2020 and the social distancing and stay-at-home directives required by the COVID-19 pandemic have just been enforced, but the outdoors is open to everyone.
Usually I have the forest to myself when I visit to observe ice-out and photograph the landscape’s resurgence from winter. This compelling landscape is not a traditionally picturesque one. Split downed trees, bleached from winter burn or charcoal-black if they’ve been under water, tangle with brushy shrubs and a dusting of last-year’s dessicated leaves.
In 2018, I started a new series about the floodplains and river terraces in the Refuge. Located along the Minnesota River south of the International Airport and east of the Mall of America, the Refuge is a predominantly floodplain-bottom lands environment notable for migrating avian visitors that use the nearby Mississippi River flyway for navigation and habitat.
[Black and white images are Bass Ponds Trail, Spring 2019 and color photographs are from the Bluff Trail on Long Meadow Lake and along the Minnesota River main channel, March-April 2020.]
AccuWeather pops up on my iPhone unbidden (there’s a strong cell phone signal here because the Refuge edges the metro) and notifies me about a flood warning for the Mississippi River. High water also will soon invade the Refuge. Words like “flood” and “forest fire” conjure negative responses because of the associations with the damage they may cause. But ecologically, both are necessary and restorative.
Floodplain forests are dynamic systems that rely on annual disturbance for their health; on seasonal pulses of water, organic matter and nutrients. New habitat is created after each flooding event. Receding floodwaters expose rich mineral-soil with about twice the organic matter as upland forest soils, and most floodplain trees and herbaceous plants are adept colonizers, capable of rapid growth.
Floodplain trees including soft maples and cottonwoods have various adaptations for supplying oxygen to tissues below the water or to roots in saturated soils, and for avoiding damage when dormant with low respiration rates in inactive tissues. Flooding from heavy rains while the plants are actively growing proves most destructive. Many may perish due to anoxia; their submerged roots suffocate due to lack of oxygen. All trees survive better when dormant, and middle-aged trees are more resistant than younger or older trees.
While spring floods are common events, recently they are intensified by global climate patterns that bring unpredictable large rainstorms. Also, over the past 100-150 years, nitrates from human activities, especially the burning of fossil fuels and industrial fertilizers, have been carried into floodplain forests by runoff and groundwater.
Because organic matter has run off from uplands or been carried in by flood waters, these lowland forests have twice the amount of organic matter than most forests. The bulk of it is processed by invertebrates and other decomposers in a single season! The tangled mass of decaying branches and leaves in my photographs is well on its way to becoming soil.
Floodwaters move around this organic material and deposit it in places that previously had none. Instead of cycling within the community, the organic matter mineralized in floodplain forests may well have been produced elsewhere, or end up elsewhere.
Floodplain communities are also “nitrogen sinks” because they immobilize nitrogen or return it to the atmosphere.
Some floodplain trees sequester large amounts of nitrogen, often far more than needed for growth and survival. If these trees remain intact after death, either submerged in the river (where they can remain for hundreds of years) or buried in sediments (for up to thousands of years) the nitrogen taken up by the living tree is effectively immobilized. This immobilization lessens the effects of nitrates as pollutants downstream and ultimately in oceans (such as the Gulf of Mexico).
Perhaps this is why the trails are busy today. The floodplain forest, in all its beautiful, ugly and complicated glory, is a dynamic and resilient system and we are here to observe and witness its changes.