Facilitation of Alliaria petiolata invasion through favorable soil microenvironments in a primarily pine forest stand

Background/Question/Methods

Alliaria petiolata (garlic mustard) is an invasive non-native herb that has become a prolific weed in many deciduous North American forests. While its invasibility of deciduous systems is well documented, the plant’s success in dense pine plantations on highly sandy soil is less understood. We investigated garlic mustard presence at Sand Ridge State Forest in central Illinois.  Some of the pine plantations in this forest are interspersed with planted and volunteer Robinia pseudoacacia L. (black locust). The original hypothesis was that the dinitrogen-fixing bacteria associated with R. pseudoacacia and the litter produced from the leaf matter likely adds nutrients to a generally low carbon and nitrogen soil matrix. The resulting soil microenvironments would promote increased vegetation in forest understory through increased soil fertility. With an extensive nitrogen infrastructure in the pine forest stand mixed with mature and juvenile R. pseudoacacia, the traditional competitive mechanisms of A. petiolata as rapid-growth alien could facilitate its invasion. Plant survey data indicated increased A. petiolata density in pine stands mixed with R. pseudoacacia. Plots were studied in pine stands and mixed pine/R. pseudoacacia sites so that soil chemistry and biology could be examined with and without garlic mustard and with and without R. pseudoacacia. 

Results/Conclusions

Results to date suggest that soils with garlic mustard infestations exhibit higher rates of soil respiration, lower fungal:bacterial ratios, greater bacterial numbers, greater numbers of N fixing bacteria, higher rates of mineralization and nitrification, and lower soil C:N ratios.  As R. pseudoacacia. is also present on this site it is likely that garlic mustard is either attracted to soils with high nitrogen and bacterial components or that garlic mustard alters soil chemical and microbiological parameters.  Research is planned to distinguish between these two alternate hypotheses.  Understanding these plant-soil interactions will strengthen restoration and conservation management practices.