Patterns of floodplain forest mortality and recruitment along the Upper Mississippi and Illinois Rivers: associations with forest fragmentation and flood inundation

Different rates of floodplain forest recruitment and mortality can reveal important changes in ecosystem processes that drive forest dynamics, resulting in net changes in forest cover, thereby influencing a wide range of river habitat and morphological characteristics. We evaluated characteristics of forest change areas in the Upper Mississippi River System. An overlay technique was used to map patches of forest loss, gain, and persistence between 2010 and 2020 in relation to a series of explanatory variables. We quantified a net decline in forest cover ranging from 3.2 to 16.8% in the uppermost five study reaches, and a net increase in forest cover ranging from 0.5 to 4.6% in the southernmost three reaches. Patches of forest loss and persistence were similarly tall (> 15 m), dense (> 90% cover), silver maple (Acer saccharinum) dominated forests, whereas forest gain patches were short (< 15 m), less dense (< 66% cover) and more likely to be dominated by willow (Salix) species. Both forest loss and gain patches were smaller than forest persistence patches and were typically found in areas with low neighborhood forest density (< 50% forested 10 ha neighborhood). Areas that experienced more than three flood events per growing season, more than 100 consecutive days of inundation during a single flood event, and more than 60 mean total days of inundation per growing season from 2011 to 2020 showed a net loss of forest cover in all study reaches. In contrast, net increases in forest cover were restricted to areas that experienced less than a single flood event per growing season, less than 40 consecutive days of inundation during a single flood event and less than 30 mean total days of inundation per growing season from 2011 to 2020. Forest mortality along these river reaches is associated with forest fragmentation and an increasingly wetter hydrological regime.