High soil nitrogen levels and insect herbivory suppress tall thistle (Cirsium altissimum) rosette survival

Background/Question/Methods

The effect of herbivores on plant performance is often highly variable and the causes of this variability are poorly explained for many herbivore-host plant interactions.  Many of the proposed hypotheses center on plant resource availability, yet there is also evidence that herbivore diversity may modify individual species effects.  To address interactions between soil resources and herbivore diversity on plant performance, we initiated a nitrogen addition X herbivore exclusion experiment that manipulates access by insects and aboveground mammals in restored tallgrass prairie in south-central Kansas.  Here we address whether 1) nitrogen availability limits tall thistle rosette annual survival rate and probability of bolting (producing a reproductive stalk), 2) herbivory by insects and aboveground mammals limits rosette survival and bolting probabilities, and 3) there are interactive effects between nitrogen availability and herbivore taxonomic groups on tall thistle rosette annual survival and bolting probabilities.

Results/Conclusions

Tall thistle rosette survival from May 2010 to May 2011 was reduced by 23% (F_1,85 = 7.81, p = 0.006) where nitrogen was added relative to ambient nitrogen.  Ambient levels of insect herbivory produced a 25% reduction (F_1,85 = 5.95, p = 0.017) in rosette survival as compared to plots treated with insecticide. There was a marginally significant interaction between nitrogen addition and insect herbivory on tall thistle rosette survival (F_1,85 = 3.71, p = 0.057) with a higher suppressive effect of insects in unfertilized than fertilized plots.  The proportion of plants that were rosettes in 2010 that bolted in 2011 was positively related to rosette diameter in May 2010 (F_1,85 = 7.46, p = 0.008), but was unaffected by any of the experimental treatments or their interactions.  Therefore, while our preliminary analyses suggest that soil resource availability influences the magnitude of insect herbivores’ effects on tall thistle juvenile performance, this effect did not depend on the presence of an additional taxonomic group (aboveground vertebrates).