A landscape-level natural experiment was used to determine the belowground causes and consequences (potential “legacy effects”) of invasions by the exotic shrub Rhamnus catharticaL. (European buckthorn; hereafter buckthorn) into woodlands. The long periods over which these invasions occur limits the utility of short-term, manipulative experiments for this investigation. Ninety-seven buckthorn-invaded points located throughout Chicago-area woodlands that varied in invasion severity were each paired to an adjacent buckthorn-free point. Invasion severity was quantified as an invaded point’s score along the major axis of a PCO ordination constructed from measures of buckthorn basal area, percent cover, height, sapling density, and invasion age (3-62 yr) made at that point (PCO1 = 63.1% and PCO2 = 20.0% of total variation). Soil parameters hypothesized to promote and/or respond to buckthorn invasions (moisture, pH, total carbon, total nitrogen, inorganic nitrogen, calcium, leaf-litter mass, and exotic earthworm biomass) were measured at each point. The difference in values for these parameters between each invaded and uninvaded point of all point pairs was calculated (ΔRVs). The intercept and slope of the relationships between ΔRV and invasion severity values was then used to determine if differences between invaded and uninvaded points reflected pre-invaded conditions and/or buckthorn-induced changes.
Results/Conclusions
The framework of this study (sampling design and statistical modeling of ΔRVs) proved useful in detecting the belowground causes and consequences of buckthorn invasions. The relationships of the following ΔRVs: moisture, pH, total carbon, total nitrogen, ammonium-N, and calcium to invasion severity all had intercepts greater than 0 and slopes equal to 0. These relationships revealed that these RVs were higher at the invaded points than at the uninvaded points prior to invasion and did not change in response to the invasion. Buckthorn is, therefore, more likely to invade where these RVs are elevated. The relationships of both soil-nitrate levels and leaf-litter mass to invasion severity revealed no pre-existing differences in these RVs between invaded and uninvaded points (intercept = 0) and that both of these RVs decreased as invasion severity increased (slope < 0). Decreased soil-nitrate levels and leaf-litter mass are, therefore, buckthorn-induced changes, i.e., potential “legacy effects.” Contrary to prior hypotheses, earthworms neither promoted (intercept = 0) nor responded to buckthorn invasions (slope = 0).