Untangling the ecosystem impacts of non-native plants: insights from a Microstegium vimineum invasion along a logged chronosequence
Isolating the ecosystem impacts of plant invasions poses significant methodological challenges. For one, accurate information on the duration of a plant invasion is often lacking. Increasing evidence shows that the magnitude of ecosystem impacts can vary dramatically based on the time since invasion; however, few studies have actually captured how impacts of plant invaders change over time. Secondly, plant invasions often coincide with other types of human disturbance (such as logging), complicating efforts to isolate the ecosystem impacts of the invader alone. In order to address these two compounding issues, we studied the ecosystem effects of the invasive grass species Microstegium vimineum in both logged and unlogged stands, along a logged chronosequence in a northeastern deciduous forest (North Branford, CT). In this region, Microstegium vimineum typically invades and establishes in a forest stand immediately post-logging. We measured soil nitrogen and carbon cycling in paired invaded/uninvaded plots at sites logged 2,5,9,12 and 15 years ago. We compared these values to a nearby unlogged and uninvaded control stand. We asked: (1) How do different disturbance types, alone and together, influence carbon and nitrogen cycling? and (2) Does the magnitude of ecosystem impacts vary based on the time since disturbance?
Ecosystem processes were altered both by Microstegium vimineum and by logging, and these ecosystem impacts exhibited non-linear changes through time. We found strong differences in carbon and nitrogen cycling between paired invaded/uninvaded plots across all logged sites. Microstegium vimineum-invaded plots had both elevated rates of net nitrification (t=4.2, p<0.01) and total nitrogen mineralization (t-2.9, p<0.01) relative to uninvaded, logged plots. Similarly, invaded plots had higher rates of carbon mineralization (t=2.5, p<0.05), and greater total soil organic carbon (SOC; t=2.44, p<0.05). These ecosystem impacts exhibited clear temporal differences. Using a linear mixed model approach, we observed a significant time-since-logging by treatment (invaded versus uninvaded) interaction for the rate of carbon mineralization (t=-2.53, p<0.05), net nitrification (t=2.30, p<0.05), and for both the faster-cycling particulate organic carbon (POC t=4.82, p<0.001) and total SOC (t=4.90, p<0.001) pools. Effect sizes of ecosystem impacts were non-linear through time, generally at their lowest at 2 and 15 years post-logging, and trending towards a maximum at 9-12 years post-logging. Results indicate that ecosystem impacts of Microstegium vimineum in northeastern deciduous forests should be evaluated based on the time since disturbance and within the context of other disturbance regimes.