Fragmented forest patches typically experience multiple anthropogenic global change drivers that can synergistically impact plant community composition and ecosystem function. Importantly, the relative effects of multiple drivers can change through time, creating ambiguity in how to assess the independent impacts of each driver. Here, we explore two ubiquitous global change drivers—habitat disturbance and invasion by nonnative species—in a fragmented temperate deciduous forest in Connecticut, USA. We demonstrate that invader impacts can be amplified, dampened, negated or reversed when co-occurring with two common forest disturbances: logging and invasive plant removal. We present results from two studies focused on the plant community and ecosystem impacts of a well-known forest invader, Microstegium vimineum (Japanese stiltgrass). First, we measured ecosystem properties that relate to carbon and nitrogen cycling in invaded and uninvaded patches in a chronosequence of forest stands that that varied in their time-since-logging. Second, within a single forest stand, we monitor the plant community response to the intentional removal of M. vimineum. Using our findings, we discuss a means of differentiating between disturbance-related and invasion-related impacts using multiple types of control plots and present a framework to help explain the net effects of disturbance and invasion through time.
Results/Conclusions
Using our framework for disentangling overlapping effects of invasion and co-occurring disturbances through time, we found that accounting for the independent-effects of disturbance allowed us to disentangle the effects of each driver. For example, in our study on the impacts of logging and invasion, we found that that during its period of peak impact, logging amplified the positive effect of M. vimineum on soil microbial biomass by 24%, reduced the invader’s positive effect on soil water holding capacity by 5%, negated the invader’s positive effect on the particulate organic matter carbon pool (from a 9% increase to no significant effect), and reversed the direction of the invader’s impact on net nitrogen mineralization rate from a 51% increase to a 52% decrease. Additionally, the influence of a co-occurring disturbance on the invader’s impacts was dynamic through time. The results from our site therefore demonstrate that failure to account for the impacts of a co-occurring disturbance, as well as the time since disturbance, can result in flawed inference about the nature of an invader’s effects.