Plasticity of second-year Nyssa aquatica seedlings under simulated light competition

Abstract

Nyssa aquatica, a common wetland species found in floodplains of the southeastern U.S., often forms dense, monospecific stands and is considered shade-intolerant to moderately shade-tolerant. Its regeneration typically occurs when canopy gaps and favorable hydrologic conditions allow seed germination and seedling establishment. Altered hydrology and drier conditions in southeastern floodplains have facilitated encroachment by other bottomland species, which may intensify competition for light and stunt regeneration of N. aquatica. However, there is little empirical evidence of N. aquatica seedling performance in shade. We developed a semi-controlled study to determine whether N. aquatica seedlings are phenotypically plastic and if specific functional traits that influence fitness (i.e., growth rates) are negatively impacted under increasing shade (0%, 60%, 70%, and 90%). Measured parameters of potted second-year seedlings included growth rate, number of leaves, leaf area, internodal distance, and leaf thickness. We hypothesized that seedlings would exhibit plasticity across all measured parameters and that growth rates would decrease with increasing shade levels. Seedlings exhibited plasticity with all parameters except leaves per stem. Radial and height growth peaked under 60% shade, while radial growth was lowest in 90% shade and height growth was lowest in full-sun. These results suggest that N. aquatica seedlings are highly plastic and perform optimally under moderate shade (particularly 60%). A full-sun environment may impose physiological stress, though higher soil moisture in field sites may mitigate the negative response we observed. Given the species’ reduced radial and height growth in 90% shade, it is likely their competitive ability and fitness are reduced in similar field conditions. Interactions between shade tolerance and hydrologic stress (e.g., flooding or drought) may play a more critical role, highlighting the need for further research into these complex ecological dynamics.