Utilizing Multiple Approaches to Understand the Ecology of Rhamnus cathartica L. Invasion and Management

Exotic shrubs are becoming an increasingly dominant component of plant communities throughout the temperate regions of North America; and thus a concern for management. I studied these invasions and their management using three different investigative approaches. In Chapter 1, I used a landscape-scale natural experiment in conjunction with statistical modeling to determine if the belowground differences between woodlands with and without the exotic shrub Rhamnus cathartica (European buckthorn; hereafter buckthorn) reflect pre-invaded conditions or buckthorn-induced changes. I found that the higher levels of soil moisture, pH, total C, total N, NH4+-N, and Ca2+ observed in buckthorn-invaded woodlands pre-date and likely promote buckthorn invasion. I also found that buckthorn invades areas with higher rates of leaf-litter decomposition, but buckthorn then further accelerates decomposition and causes spring NO3--N to initially increase and then later decrease as invasions progress. In Chapter 2, I used a manipulative field experiment to determine if amending soils with buckthorn mulch can limit buckthorn reinvasion. I found that the mechanical disturbance of tilling mulch into the soil, and not the actual mulch, greatly reduced reinvasion by killing small buckthorn individuals that were overlooked during initial removal. I also found that recruitment of new buckthorn individuals rapidly declined overtime, suggesting that buckthorn seeds are short-lived. Therefore, repeated annual follow-up control of overlooked and newly recruiting buckthorn individuals may deplete buckthorn’s remnant seedbank, causing more prolonged reductions in reinvasion than what are typically observed. In Chapter 3, I used an individual-based model to investigate invasions by exotic shrubs into light-limited woodlands. I found that rates of spread were highly sensitive to moderate variation in reproductive age and fecundity, and that canopy gaps facilitate spread by affecting these life-history traits, but not by affecting dispersal. The changes in reproductive age and fecundity that increased rates of spread produced non-parallel changes in the following invasion characteristics: the proportion of invasions reproducing, degree of clumping, and invasional lag. Model outcomes were consistent regardless of where invasions started. From these model outcomes, I determined empirical investigations that will likely improve our understanding of exotic-shrub invasions, and management strategies that may limit their spread.