Spatial and temporal change in the population dynamics of four wetland tree species in response to climate change

Sarah McCarthy-Neumann, Biology, Alma College, Alma, MI, Angus Pritchard and Jonathan P. Evans, Biology, University of the South, Sewanee, TN, Jennifer M. Cartwright, Lower Mississippi-Gulf Water Science Center, U.S. Geological Survey, Nashville, TN

Background/Question/Methods
Regeneration and survival of bottomland hardwoods are affected by environmental variables related to the hydrologic regime. Climate change, specifically alterations to precipitation patterns may have outsized effects on these forests. In southern, middle TN, precipitation has increased by 15% since 1960. The goal of our research was to assess the evidence whether this change in precipitation patterns resulted in shorter growing seasons and novel recruitment failure in common canopy trees for a forest wetland. In 2001 and 2018, the density of overcup oak, sweetgum, willow oak, and river birch seedlings, saplings and adults were mapped within 2.3 ha of a seasonally flooded karst depression. Growing season calculated for each 2 x 2 m subplots for each year from 2001-2018.
Results/Conclusions
Overall, the growing season in the deep was ~41 days longer on average than in shallow areas between 2001 and 2018. Sapling and small adults are restricted to shallow areas and their abundance has declined substantially. Overcup oak and sweetgum individuals that recruited into the adult life history stage were repelled from the deep zone. Overcup oak experienced lower mortality (11%) relative to willow oak (54%) and river birch (51%), and sweetgum mortality (26%) was lower than willow oak over the 17-year study. For overcup oak and river birch probability of mortality was correlated with the growing season experienced by the adult due to its elevation in the pond, whereas there was no influence of growing season on sweetgum and willow oak mortality. However, for overcup oak the relationship between mortality and growing season depended on size class with small adults (5.00-14.99 cm DBH) experiencing a decrease in predicted probability of mortality as length of growing season increased and large adults ( > 40 cm DBH) experiencing the reverse. River birch's probability of adult mortality, regardless of size class, declined for individuals located in areas of the pond with longer growing season. Overcup oak and sweetgum adults had greater basal area increment in shallow relative to the intermediate and deep regions of the pond. Results of hydrologic modeling, based on rainfall and temperature records covering 1854-2018, show ponding durations after 1970 considerably longer than historical norms, across ponding-depth classes. Our results strongly suggest that climate change is a driving factor suppressing tree regeneration since 1970 in this seasonally flooded karst depression. To our knowledge, this is the first reported forest community type that may go 'extinct' due to climate change.