The battle for suburbia: Japanese stilt-grass vs. garlic mustard – and deer
Fragmented, suburban forests are essential for biodiversity and ecosystem services, and they offer many human communities a connection to nature. However, these forests are challenged by the dual problem of overabundant deer and invasion by multiple, potentially interacting non-native plant species. Two dominant herb layer invaders in eastern forests are garlic mustard (Alliaria petiolata) and Japanese stilt-grass (Microstegium vimineum). We have established a well-replicated factorial field experiment with all combinations of staged, novel invasions of one, both, or neither species, and exclusion or presence of deer (with fences), across six suburban New Jersey forests (224 4x4 m plots). These forests differ in ambient deer pressure, but all are in an area with about 20 deer/km2. We have measured invasion success since the seed additions in November 2012, in order to test whether these species invade as ‘passengers’ on the ecosystem change caused by overabundant deer, and to determine if they facilitate each other’s invasion, setting the stage for a possible ‘invasional meltdown’ scenario.
Initial recruitment of the two species among and within forests was highly variable. We added equal numbers of seeds to each addition plot, but recruitment of Alliaria in the first summer ranged from 0-188 plants per plot, and 13-605 for Microstegium; it clearly dominated the initial invasion phase. Fewer Alliaria plants recruited in forests with more leaf litter, which explained 68% of the variation, but it did not explain Microstegium recruitment. By the Spring 2014 census, Microstegium percent cover averaged 7% in the three forests with lower deer pressure (measured as percent foliage cover in the shrub layer) and 25% in those with higher deer pressure. This was driven by much higher cover in the two forests that experienced multiple tree falls from Hurricane Sandy in Fall 2012, creating large canopy gaps. However, there was no relationship on a per-plot level between sunlight and Microstegium cover. Alliaria cover averaged only about 1.5% cover, in all forests. The only strong influence on each other’s cover was that Alliaria cover was significantly lower in plots where Microstegium also was added, but only in the forests with higher deer pressure, which also had the higher Microstegium cover. These early-stage results lend support to the hypothesis, for Microstegium only, that invasion of suburban forests follows a passenger model of ecosystem change. It also points to a hierarchical model of competition between co-occurring invasive species, rather than facilitation.