Long-term dynamics of plant species richness in response to fertilization: What are the effects of colonization, dominance, and species functional traits?

Background/Question/Methods

Although fertilization generally causes declines in plant species richness, the timing and magnitude of these responses can vary.  We examined data from a 23-year ongoing fertilization experiment in a successional abandoned agricultural field at the Kellogg Biological Station LTER, where species richness did not decline until 14 years into the experiment.  We explored two mechanisms that might account for this delayed response and how / if this was related to the introduction of fire as a management tool in this system.  The two mechanisms evaluated were: 1) changes in colonization and interactions with fertilization treatments and / or prescribed burning; and 2) temporal changes in the abundance of species with specific functional traits that may have led to changes in dominance-diversity relationships.  To examine colonization dynamics, we compared changes in colonization over time in the control and fertilized treatments and if this was related to the onset of fire management.  We also evaluated how changes in the abundance of species in different functional group (i.e. different clonal growth forms and heights) varied over time, and if this was related to the divergence in species richness in the treatments.

Results/Conclusions

We found that fertilization generally did not affect the variation in colonization over time.  However, the introduction of prescribed burning to control woody invaders in the ninth year of the experiment may have reduced colonization in fertilized plots.  We also found that over time, fertilization increased the biomass of the functional group consisting of tall species with runner clonality (fertilization increased average tall-runner biomass from 249 [unfertilized] to 572 [fertilized] grams per square meter in the most recent decade of the experiment and from 138 to 333 grams per square meter in the previous decade).  Forbs in the genus Solidago, not grasses, accounted for over 70% of the biomass of this functional group.  Fertilization did not significantly increase the biomass of any other height-clonality functional groups.  When examining the relationship between tall-runner biomass and species richness, we found a significant, quadratic relationship (r² = 0.157) between tall-runner biomass and species richness. This suggests that when tall-runner biomass is low, it has little effect on species richness, but as tall-runner biomass begins to exceed around 300 grams per square meter, this functional group leads to a decline in species richness.  The negative impact of fertilization on species richness appears to be mediated by decreasing the abundance of rare, non-dominant species.