Effects of the Invasive Shrub Rhamnus davurica on Secondary Succession in Central Virginia

Abstract:

The introduction of novel, invasive plant species can pose ecological problems for native communities. The prevalence of these invasive species have increased over the last century, due to ease of international movement. Rhamnus davurica (Dahurian buckthorn) is an invasive shrub found in dense thickets within regenerating systems following abandonment of agriculture. The high abundance of R. davurica in successional fields at the Blandy Experimental Farm in northwestern Virginia appears to have arrested the process of succession. Using a combination of field and greenhouse experiments, we attempt to determine if the presence of R. davurica significantly alters the course of secondary succession at Blandy Experimental Farm, and with what potential mechanisms. We collected species composition data to determine the abundance of buckthorn (none, low, medium, or high) in each of twenty randomly selected early successional plots. We analyzed various ecosystem properties and processes, including soil temperature, soil moisture, below-canopy light levels, soil respiration, net nitrogen mineralization and nitrification, and litter decomposition. Possible allelopathic effects of buckthorn are assessed in the greenhouse experiment. Results from our first growing season indicated significantly lower soil temperatures (means of 19.3 °C vs. 20.2 °C), higher volumetric soil moisture (means of 14.3 % vs. 12.0 %), and higher soil respiration (means of 1.20 mmol CO₂ m^-2 s^-1 vs. 0.97 mmol CO₂ m^-2 s^-1) in high buckthorn plots vs all other plots. High buckthorn plots had the least below-canopy light (~30 mmol photons m^-2 s^-1 photosynthetic photon flux density), whereas plots without buckthorn had ~200 mmol photons m^-2 s^-1 at peak daily light levels. Our species composition data revealed a positive association between abundance of buckthorn and the abundances of other invasive species. The mechanisms R. davurica utilizes to invade and establish in early successional areas are not widely known. This study aims to determine the characteristics of Dahurian buckthorn that allow it to be successful in creating dense monospecific thickets that exhibit little to no regeneration of native species. Once we know more about Dahurian buckthorn, we will be able to make effective decisions about future land-use and management practices.