Invasive plant species are a major threat to biodiversity. They out-compete native plant species and disrupt native mutualisms, leading to local extinctions and posing a threat to endangered plants and animals. Predicting which species will become invasive, however, has proven to be difficult. The role of plant phenology (that is, timing of growth and flowering) may be essential to understanding the success of invasive plant species, and it requires more study.  We designed a target-neighbor field study to address questions about phenology and competition, specifically:  (1) Can complementary growth and flowering phenology of a native community resist invasion? (2) What are the underlying mechanisms responsible for this outcome?  This is a factorial experiment with three native neighbor species treatments: an early-flowering functional group, a late-flowering functional group, and a mixed functional group, consisting of both early and late native species.  There are also three target treatments: an early-flowering invasive (Hesperis matronalis), a late flowering invasive (Nepeta cataria), and treatments where no invasive is planted.  We predict that when the target invasive has the same phenology as its native neighbors, that invasive will experience more intense competition. 

Results/Conclusions

Preliminary results show that the phenological treatments have a significant effect on numerous vegetative and fitness traits including above-ground biomass, growth form, flowering schedule (including date of first flowering and  flower production ), fruit number, and the ratio between fruit mass and vegetative biomass. The early invasive, H. matronalis was most affected in terms of biomass by those native neighbors that had similar early phenologies, supporting our prediction.  However, our prediction was not observed in the late invasive, N. cataria.  Competitive effects were found to be greater on biomass of the late invasive N. cataria  than the early invasive H. matronalis, when examined with and without competitors present. The early-flowering native neighbors showed the greatest overall biomass, followed by the mixed natives and then late native neighbors. Interestingly, the opposite pattern was seen for the height of those same treatments, with early neighbors the shortest and late neighbors the tallest.  We also found that the height and length of flowering of the natives were affected by the invasive treatments, but biomass, volume, date of first flowering, and total flower number produced were not. These results indicate the importance of timing of growth and flowering in investigations of community assembly and invasive processes.