Utilizing Multiple Approaches to Understand the Ecology of Rhamnus cathartica L. Invasion and Management
Iannone III, Basil V.
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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.