Heterogeneity of roots and resources during biological invasion of native prairie

Background/Question/Methods High resource variability has been posited as a precondition for biological invasion, suggesting that invasion might reduce heterogeneity. We tested this by measuring the spatial heterogeneity of resources and belowground production in native Montana prairie and adjacent sites invaded by Agropyron cristatum (crested wheatgrass), an introduced perennial. Both vegetation types were also subject to fully-crossed mowing and fertilization treatments to investigate the possible impacts of grazing and landscape position on invasion dynamics. We examined root and resource heterogeneity along 40-cm long transects that ran downwards at 45° from the soil surface. Root data were obtained from minirhizotrons from late-March to mid-August of 2007. Resource data were collected twice during the peak of the growing season in early summer. Results/Conclusions Root length heterogeneity beneath native prairie (CV = 152%) was significantly higher than beneath A. cristatum (CV = 114%). Thus, invasion reduced root spatial heterogeneity because roots of the invasive A. cristatum occupy soil volume more evenly than those of native grasses. Similarly, invasion significantly reduced light heterogeneity measured above the rhizotrons (native prairie: CV = 24%; A. cristatum: CV = 16%). In contrast, June soil moisture heterogeneity was significantly greater beneath A. cristatum, which may be a result of A. cristatum being physically active and C4 dominated native prairie being less active, rather than root heterogeneity. The heterogeneity of soil available nitrogen was not influenced by invasion. Mowing and fertilization had mostly non-significant effects on heterogeneity. The results are consistent with the hypothesis that invasion reduces the variability of the occupation of both above- and belowground space, and suggest that species identity (invader vs. native) is important relative to other factors (grazing and fertility).