Impact of soil nitrogen and tree canopy cover on Microstegium vimineum in a restored urban Piedmont stream

Background/Question/Methods

Microstegium vimineum (MIVI) is a shade-tolerant exotic C-4 grass found in Eastern North America that successfully establishes in restored riparian ecosystems due to altered habitat and nutrient content.  As a dominant species, MIVI can lower diversity of plant communities.  Previous research has found that in the absence of competition, it responds positively to nitrogen but in competition with native plants, nitrogen does not affect MIVI.  East Jesters Creek located in Morrow, Georgia is a low-order restored urban Piedmont stream.  It is nestled between a preserve and an encompassing urban region.  Plant communities present at East Jesters Creek consist of re-vegetated riparian, volunteer road-side, non-native, and upland forest species.  Our objective was to determine the factors that impact MIVI growth in a restored urban Piedmont riparian ecosystem.  MIVI cover and tree canopy cover were estimated in 8, 10x10 m² sites in riparian zones.  Soil nitrogen, phosphorous, potassium, organic matter, pH, and compaction data were collected in 12, 10x10 m² sites in riparian zones.  Spearman’s rank-order test was used to determine if MIVI, tree canopy cover, and soil nitrogen were correlated.  Wilcoxon signed-rank tests were used to determine if the selected biotic and abiotic factors differed between preserve and urban riparian zones. 

Results/Conclusions

Our case study results revealed that MIVI cover is positively correlated with soil nitrogen content and negatively correlated with tree canopy cover, a finding consistent with other studies.  Soil abiotic factors and tree canopy cover did not vary between preserve and urban riparian zones (p>0.05), but MIVI cover was significantly higher in the urban riparian zone of East Jesters Creek (p<0.05).  Correlation results found a moderately positive association between MIVI cover and soil nitrogen (R=0.41) and a moderately negative association between MIVI cover and tree canopy cover (R=-0.71).  Our results from the East Jesters Creek study suggest that MIVI could lower diversity scores in restored Piedmont riparian zones where soil nitrogen is high and tree canopy cover is not fully developed. Additionally, high MIVI cover in restored urban riparian ecosystems could be used as an indicator of potentially high soil nitrogen and a lack of adequate canopy cover due to stand age or low tree density.  The success of MIVI in urban riparian ecosystems may be temporary as planted trees grow and form the mature canopies typical of Piedmont forests.