Oak regeneration failure is a wide-spread concern, yet there is little consensus on the mechanisms driving oak decline. In the Chicago region, the relative abundance of oaks, especially red oak (Quercus rubra), has declined over the last several decades with sugar maple (Acer saccharum) often replacing oaks. Concurrently, forests in this region have been experiencing relatively high levels of nitrogen (N) deposition. Previous research has shown that chronic N deposition contributes to red oak regeneration failure in these Midwestern forest communities; N deposition negatively affects seedling biomass of red oak, but not sugar maple. However, the mechanisms driving this pattern are unclear. Furthermore, as oaks show a clear preference for ammonium (NH4+) over nitrate (NO3-) due to the high energetic costs of assimilating NO3-, these effects may be enhanced when N deposition is nitrate-rich. In order to identify the mechanisms driving the observed N-induced declines in red oak seedling growth, we established a greenhouse experiment to evaluate the effects increasing rates of experimental N deposition (0, 6.25, 12.5, 25, or 50 kg N ha-1 yr-1) and increasing proportion of N as NO3- vs. NH4+ (0, 25, 50, 75, or 100% NO3-) on red oak and sugar maple growth and physiology.
Results/Conclusions
Rather than having a negative effect on oaks, N addition had a positive effect on sugar maples. Over the course of the 2.5 year experiment, red oaks and sugar maples both exhibited plastic growth responses to N addition, but increased biomass allocation to diameter or height varied between species. In particular, as experimental N deposition rate increased, changes in sugar maple diameter also increased (P=0.061; R2=0.80) while red oak showed no diameter responses to N addition (P=0.386). In contrast, relative increases in red oak height were positively correlated with increased experimental N deposition rate (P<0.001; R2=0.85) while sugar maple height change was not altered by N addition (P=0.791). However, regardless of experimental treatment, sugar maple height increases were more than five times greater than oak height increases (P<0.001). As such, N-induced increases in oak height were not enough to compensate for inherently faster height growth of sugar maples. These results suggest that declines in oak growth and regeneration under field conditions may be driven by competitive interactions with sugar maples – sugar maples may exhibit greater plasticity to changes in N availability and be inherently better competitors for light and other resources in N-rich environments than oaks.