Competition and herbivory in co-invasive, non-native plants of the suburban/exurban forest herb layer

Background/Question/Methods

Suburban/exurban forests are often invaded by multiple, non-native plant species. When these plants become dominant, their competitive interactions with each other and their interactions with herbivores may become the main drivers of community dynamics, including the ultimate competitive success of the different co-invasive species. Most research, however, focuses on their competition with native species, and it often is assumed that invasive plants have escaped significant herbivory. First, we investigated whether three co-invasive species of the deciduous forest herb layer did indeed escape herbivory and if they differed, using garlic mustard (Alliaria petiolata, ALPE), Japanese stilt-grass (Microstegium vimineum, MIVI), and Norway maple (Acer platanoides, ACPL). We planted 360 focal seedlings of each into individual, randomized, cleared plots across three New Jersey forests. Each seedling either grew alone, with two neighbor seedlings of one of the other non-native species, or with two native sugar maple seedlings. We harvested survivors, scanned the leaves digitally, and quantified herbivory. Second, we tested the effects of competition and herbivory on ALPE and MIVI growth in the greenhouse. We planted seedlings in a response surface design crossed with clipping of ALPE, MIVI, ALPE+MIVI, or neither, and measured root and shoot dry mass after three months.   

Results/Conclusions

In the field experiment, herbivory was similar for ALPE (biennial herb), MIVI (annual grass), and ACPL (tree seedling), with 15% missing leaf area (putatively due to insects and molluscs), indicating that these invasive species have not escaped herbivory. Neighbors (competition) had no effect on the herbivory outcomes for any of the species. In the greenhouse, we experimented with deer-like herbivory, because we have observed it on ALPE and MIVI in suburban/exurban forests with high deer pressure. Clipping influenced the outcome of competition. Without clipping, the species with lower intraspecific competition outgrew the other, except at the highest density, when MIVI always outgrew ALPE. When  one species was clipped, it was outgrown by the other species at all densities and ALPE:MIVI ratios. When both were clipped, ALPE outgrew MIVI in all cases, because clipping was much more detrimental to MIVI than ALPE. The implications for natural communities are that ALPE should be able to invade MIVI stands and vice versa at lower densities, but when the invasion(s) result in high densities, MIVI should dominate – unless deer herbivory on these species is a factor. In that case, the relative deer impact on the two species will determine their competitive outcome.