The invasion and displacement of native vegetation by Rhamnus cathartica: Toward a mechanistic understanding

Background/Question/Methods

Rhamnus cathartica (common buckthorn) has invaded many eastern deciduous forests of the US, including those in Wisconsin. Usually forming dense thickets, buckthorn appears to exclude native woody vegetation by reducing light to levels below which natives can regenerate. Alternatively, buckthorn may exclude native vegetation by below-ground competition or allelopathy. Using a combination of experimental approaches during two growing seasons, we tested the degree to which buckthorn excludes the regeneration of woody plants and by what mechanism. In one experiment, we tested whether buckthorn did indeed exclude native vegetation and whether this effect varied among species. To accomplish this, we followed woody species in 12 paired plots; for each pair, we removed existing buckthorn from one plot but allowed it to remain in the other. In a second experiment, we tested whether buckthorn prevents native vegetation from establishing via above- or below-ground effects. To test for below-ground competition from this invasive, we followed native plants in buckthorn thickets in which the buckthorn was tied back to reduce above-ground competition from it. We compared seedling establishment and growth from this treatment to those of seedlings in paired plots in which we removed buckthorn but simulated above-ground competition using shade frames.

Results/Conclusions

Preliminary results show that removing buckthorn significantly increased native seedling recruitment (Wilcoxon Signed Ranks Test, p=0.011), with even higher recruitment during the second growing season (p=0.004). Buckthorn seedling recruitment, however, was not significantly different between treatments (p=0.077 in season 1; p=0.153 in season 2). Mortality for both native and buckthorn seedlings significantly decreased with adult buckthorn removal (p=0.001 for natives; p=0.034 for buckthorn). Mortality rates for buckthorn seedlings in the removal plots were much lower than for native seedlings in the same treatment. This suggests buckthorn seedlings may be more vigorous than natives, thus complicating restoration efforts. These findings support the hypothesis that buckthorn is excluding native species and its own seedlings, but to a lesser extent. In the second experiment testing above- vs. below-ground mechanisms, native species recruitment was significantly higher in the tie-back treatments (p=0.029), with buckthorn recruitment only marginally greater (p=0.100). This finding suggests that buckthorn chiefly utilizes above-ground means to compete; however, below-ground mechanisms contributed more than we had anticipated in this study. The outcome of this research will aid in buckthorn control efforts and the development of restoration strategies.