Individually, herbivory and interspecific competition have been shown to strongly reduce plant performance.  However, whether these factors combine in a multiplicative, synergistic or inhibitory manner to affect plant performance remains poorly understood.  Improved understanding of the combined effects of herbivory and competition could help in designing control strategies for noxious weeds.  We conducted a study from May - October 2007 that examined the combined effects of herbivory and neighboring plant competition on a native thistle of the Great Plains, Cirsium altissimum (tall thistle).  Effects of herbivores were manipulated by excluding insect herbivores with insecticide and rodent herbivores with cages.  Neighboring plant competition was manipulated by clipping all plants within 40 cm around focal tall thistle rosettes.   

Results/Conclusions

Preliminary data analyses indicate that, as expected, clipping neighboring vegetation led to significantly greater rosette growth measured as the proportional increase in number of leaves per rosette (F1,48 = 16.31, P=0.0002).  Interestingly, clipping the neighboring vegetation significantly reduced the proportion of leaves per rosette that suffered herbivore damage (F1,78 = 8.48, P=0.0047).  The finding that the magnitude of herbivore damage increased with neighboring plant presence suggests that the combined effects of herbivores and neighboring plants on tall thistle rosette growth may be greater than predicted from their individual effects.  The data indicates that maintaining extensive cover by neighboring grasses may be an effective way to reduce performance of rosette-forming weedy plants.