Nutrient influences on the life history states of an annual plant community

Background/Question/Methods

Nutrient deposition has been shown to decrease species diversity while simultaneously increasing aboveground biomass.  One mechanism for nutrient-driven diversity loss occurs at the transitions between propagule and adult life stages. Because of allocation and stoichiometric tradeoffs, we expect life history stages (seeds, seedlings, and adult plants) to respond differently to nutrient addition. We used a simple annual system with modified population demography methods as a model to fully elucidate how nutrients influence multiple stages of a plant community.  In an experiment that applied factorial nitrogen, phosphorus and potassium additions to an existing Iowa grassland community, we quantified three plant life history states and their transitions, among annuals. We analyzed these three states (reproductive adults, seed production, and seedling establishment) and the transitions between these states using a stage-structured model framework.

Results/Conclusions

Nutrient additions influenced both the states and transitions in our stage-structured model of our annual community.  Nitrogen tended to have a positive influence on these states, while potassium additions decreased state values.  Potassium plots decreased the number of reproductive stems per plot (p=0.05), while nitrogen increased the number of seeds relative to the other treatments (p<0.05).  Nutrients had no effect on the number of germinating seedlings between the plots.  Transitions between states showed a different pattern.  The number of stems transitioning to seeds increased with nitrogen additions (p<0.005), while the number of seeds transitioning to seedlings decreased with nitrogen (p<0.05) but increased with higher order nitrogen additions, namely the nitrogen:potassium combination (p=0.05).  Nutrient additions had no effect on the transition of seedlings to reproductive adults.  These results suggest that while nutrient additions may increase annual community propagule pressure, other processes such as litter buildup, or light limitation decrease the probability that this community of seeds will transition to seedlings.  Nutrient-driven loss of diversity may be due in part to different sensitivities of different life history changes which together act to destabilize species coexistence.