COS 182-6 - From the bottom up: First year growth of five floodplain tree species in a controlled moisture regime experiment to assess cause behind shifting floodplain forest communities

Abstract: Florida's Apalachicola River has experienced significant alterations to its hydrologic regime since the 1950s. Water extraction, dam construction, and navigational dredging projects have exacerbated drought conditions in the floodplain with extended periods of low flow. A suspected reaction has been observed reductions in densities of floodplain forest trees since the 1970s with the most significant declines affecting low elevation swamp species such as tupelo (Nyssa spp.) and ash (Fraxinus spp.). Observations suggest that areas historically dominated by highly flood tolerant swamp species are now being infiltrated by less flood tolerant bottomland species such as oaks (Quercus spp.) and hickories (Carya spp.). We aimed to test two main hypotheses; first that mixed bottomland hardwood species are more resilient to dry soil conditions than swamp species in the first year of growth, and second that higher soil moisture improves growth and survival of swamp species while inhibiting growth and survival of bottomland hardwood species. Here we present observations from a controlled experiment focusing on the early life stage of 3 highly flood tolerant tree species (Nyssa aquatica, Nyssa Ogeche, Fraxinus caroliniana) and two moderately flood tolerant tree species (Quercus lyrata and Carya aquatica) subjected to 12 different moisture regimes over their first growing season (May - November 2022) in a complete randomized block design. Seventy-five hundred seeds collected from the floodplain were sown in a full-sun environment and placed in treatments with various durations of moisture deficiency, partial saturation, or complete inundation. We compared germination, growth, and mortality at three intervals over the growing season. Results indicated that drier conditions did not negatively impact growth of swamp species but did cause high mortality in water hickories. Additionally, higher soil moisture did not limit germination success of bottomland hardwood species, but rather delayed timing of germination. Swamp species also did not show higher growth in moist conditions compared to dry conditions. Overcup oak seedlings showed the fastest height growth in their first 5 weeks compared to other species. These results suggest that dry conditions may not be the direct cause of declining swamp species density but highlights the possibility that competing bottomland species are now able to invade swamp environments, fill in the midstory canopy, and compete for sunlight with highly shade intolerant swamp species such as tupelo. Additional studies of canopy dynamics and competition for light resources will improve our understanding of forces behind shifting forest composition in this floodplain.