Displacement of native plant species by non-native species is generally thought to increase with increasing productivity. In some ecosystems, however, disturbances and productivity are positively correlated. Negative correlations between non-native and native species therefore could be caused in part by differences in how these species respond to disturbances. I used path analysis to investigate the possible impact of a non-native grass, Microstegium vimineum, on native plant communities in some young and mature hardwood forests on floodplains and terraces in north Mississippi. I established 125 3.14-m² circular plots along flooding and tree-canopy disturbance gradients within patches of varying density of Microstegium.

Results/Conclusions

Both Microstegium production and plant species richness were correlated with canopy gap fraction and flooding disturbance, with the most productive patches of Microstegium and the lowest richness occurring in sandy soils in periodically flooded depressions and canopy gaps. The best path model included direct negative effects of flooding and gaps on density-adjusted species richness (r = -0.53 and -0.20, respectively) and a direct positive effect of canopy gaps on Microstegium (r = 0.52). There was evidence of a direct negative effect of the most abundant species on Microstegium in mature stands (r = -0.31) but not in young stands. The negative effect in mature stands was attributable to a negative partial correlation between Microstegium and Chasmanthium laxum. The effect of Microstegium on density-adjusted species richness was weak overall (r = -0.03), but Microstegium appeared to have a significant negative effect on species richness in young stands (r = -0.21). Microstegium production was not correlated with the abundance of rare species and was positively associated with species indicative of disturbances and wetlands. Results suggest that Microstegium negatively affects some resident species in young stands, especially in canopy gaps, but its effect is small compared to the effect of flooding. In mature stands, negative correlations between Microstegium and species richness appear to result largely from differences in how these species respond directly to disturbances that produce canopy gaps.