It is widely accepted that prairie species are adapted to nitrogen-poor conditions; however, population trajectories of species along nutrients gradient are not well-understood. Reverse fertilization may promote successful prairie restorations in human-altered systems. We conducted experiments to quantify demographic responses of tallgrass prairie species (prairie-natives) compared to old-field successional species (prairie-invasives) in response to manipulated levels of soil nutrients.

We present reults from analyses of a congeneric pair of “native” and “invasive” forbs, Solidago rigida and Solidago canadensis, which were transplanted in monospecific stands in an experiment with a factorial combination of 3 N and 2 Ca levels during May 2005. Nitrogen was reduced by applying 6kg m^-2 sawdust, and was increased by application of NH₄NO₃ at 50 g N m^-2. Calcium was increased using CaSO₄ at 30 g m^-2. Growth, mortality and fecundity were censused during the 2005, 2006 and 2007 growing seasons. Seed germination rates were measured in subplots receiving identical soil manipulations as the monitored plots.

Matrix projection models were constructed to determine population growth rate under manipulated soil conditions. Sensitivity and elasticity analyses were conducted to evaluate the contributions of growth or reproduction to population growth, and to evaluate trade-offs in growth or reproduction in response to experimental treatments.

Results/Conclusions

Preliminary results show that individual growth measures are not consistently positively correlated with population growth. Plant biomass of the non-nitrophilic “native” S. rigida consistently increased with nitrogen addition, but population growth was greatest under nitrogen poor conditions. Elasticity analyses show that population growth is more responsive to initial survival of juveniles than to juvenile growth. Seed production and seed germination are also greatest for S. rigida under low nutrient conditions. Although individual growth responses of the “invasive” S. canadensis, this nitrophilic species outperformed its congener at all nutrient levels, seed production and germination rates were not as responsive to soil nutrients. This result suggests that S. canadensis may not be able to displace S. rigida under low nutrient conditions. A Life Table Response Experiment will verify the significance of growth/fecundity trade-offs in response to soil nutrients in maximizing population growth for these contrasting species.