The Effects of Drought and Shade on Bottomland Hardwood Regeneration

Floodplain forests support a high diversity of tree species adapted to regenerate under fluctuating water and light availability. Regeneration regulates species composition, and shade and flood tolerance influence the likelihood of regeneration. Regeneration failure of shade-intolerant and flood-tolerant tree species commonly occurs in southcentral and southeastern floodplain forests of the United States, also known as bottomland hardwoods (BLHs). In many BLHs reduced flooding has resulted in a dryer floodplain. These changes in flooding are linked to recent shifts in species composition. The mechanisms controlling regeneration in BLHs and these composition shifts are poorly understood. In a controlled germination experiment, I osmotically induced water stress in seeds of five BLH tree species. I found that germination decreased with decreasing water availability in all species except for overcup oak. Desiccation sensitive acorns of oaks germinated across a wider and lower range of water potentials but reached maximum germination slower than desiccation resistance seeds of green ash and sugarberry. Morphology of over 300 seedlings covering 11 tree species was examined in the field, and a subset of five species were grown in a greenhouse under a factorial combination of shade and reduced water availability. In both experiments, 21 plant traits were examined to determine the variation in biomass allocation among species and the impacts of shade and water availability on morphology. Differences in morphology and treatment response coincided with species life history. Shade-intolerant species possessed flood-tolerant and drought-sensitive hydraulic architecture and allocated less biomass towards vertical growth and more towards structural carbon in response to shade. In contrast, shade-tolerant, flood-intolerant species possessed root and stem architecture that was more efficient at water exploration, cavitation resistant, and in response to shade, allocated biomass towards efficient vertical growth. Shade-tolerant, flood-intolerant species, such as sugarberry and green ash, possessed seed and seedling traits that aid in regenerating within shaded drier floodplains, while shade-intolerant, flood-tolerant species, such as overcup oak, possessed traits that hinder their regeneration. These opposing regeneration strategies rely on conflicting hydrology to be effective. Shade-intolerant, flood-tolerant species benefit from regular flood disturbance removing competition while shade-tolerant, flood-intolerant species benefit from a drier less-connected floodplain