Pulse disturbances in forest invasion: The importance of interactions between propagule pressure and disturbance characteristics

Background/Question/Methods

Disturbance is commonly implicated in the invasion of exotic plants and extensive empirical evidence supports the disturbance-invasion relationship.  However, studies of this relationship typically quantify the effects of disturbance on invasion as a dichotomous impact in which disturbance is either present or absent. There is growing awareness that this approach is overly simplistic and that variation in key disturbance characteristics can alter the outcome and confound predictions.  For instance, studies have begun to document the important impacts of varying disturbance severity and frequency. However, few studies have addressed the role of disturbance duration and timing or considered the influence of interactions between these disturbance characteristics and propagule pressure in determining exotic plant abundance and community invasibility.  

In this study, we quantified the impact of the short-term, transient disturbance caused by gypsy moth (Lymantria dispar) canopy defoliation on the invasion of three exotic plant species: Alliaria petiolata, Berberis thunbergii, and Microstegium vimineum. Gypsy moth defoliation events are often seen as a nuisance but with little long term ecological impact due to the rapid rate of recovery following disturbance. We investigated the importance of this pulse disturbance and its interaction with propagule pressure in contributing to long-term patterns of exotic plant invasion.

Results/Conclusions

The forest canopy recovered rapidly following gypsy moth defoliation and the invasibility of the forests returned to previous levels by the following growing season. However, evidence of the influence of gypsy moth disturbance was still evident in the exotic plant population several years later.

Our results show that the duration of disturbance impacts was an important predictor of changes in abundance for only one of the studied plant species (A. petiolata). Similarly, models of the role of disturbance severity in determining changes in the extent of invasion were not well supported for any species. However, the interactions between these disturbance characteristics and propagule pressure consistently resulted in highly supported models for all three plant species. A study of this disturbance that did not quantify the role of propagule pressure would have erroneously concluded that gypsy moth defoliation did not have a measurable impact on the studied exotic plant species.

Our results suggest that even short-term, pulse disturbances can have significant and long lasting impacts on the extent of exotic plant invasion and further highlight the need to quantify propagule pressure and it’s interaction with disturbance characteristics in order to understand the complex outcomes for exotic plant abundance and community invasibility.