Seed bank study
We established a greenhouse experiment to test the hypothesis that high diversity of native plants would decrease the likelihood of invasion by non-native plants. Previous studies provide mixed support for this hypothesis. The patterns we observed in our field study suggested there was a negative relationship between the diversity of native plants and the abundance of buckthorn but whether native plant diversity is driving the relationship or whether common buckthorn invasion is responsible for the lower diversity of native plants is not clear. As a result, it is hard to predict whether native plant diversity will be an important factor in determining the invasibility of Minnesota’s forests. The potential confounding effects of other environmental factors such as soil nutrients and propagule pressure also make it difficult to directly answer this question based on the field surveys alone. We also wanted to account for the effects of other environmental variables such as light levels and leaf litter depth that our field study suggested were important in determining invasibility. The best way to combine these goals was a controlled greenhouse experiment that held constant environmental conditions while varying plant diversity, light, and litter directly. This dual approach that used manipulative experiments to test hypotheses based on patterns seen in the field is a robust way to investigate potential mechanisms of invasibility. We used native plant species that were abundant in our field surveys and also decided that the effects of earthworms should be part of the experiment since they are a ubiquitous presence in Minnesota’s forests and have been shown to have profound effects on native plant diversity.
Our greenhouse experiment included 264 pots with different combinations of native species. We also manipulated light levels and leaf litter and added earthworms to a half of the microcosms in each treatment. Each pot had an established community of native plants before we added seeds of invasive species (common buckthorn, barberry, garlic mustard, and dandelion) so we could directly test the effects of our treatments on the success of the invasives. After nine weeks of monitoring the experimental microcosms, we harvested all biomass of the native and invasive species.
Creating native communities
As we found in the field study, common buckthorn was the most successful invader. Barberry and dandelion germinated in some of the pots whereas garlic mustard did not germinate in any experimental pots even though we followed published protocols for pre-germination scarification treatment. After the experiment concluded our analyses indicated higher native diversity did lead to lower invasive biomass. Also, invasive earthworms had a significant effect on invasive biomass so that pots with earthworms had more invasive plant biomass regardless of the native species diversity.
Furthermore, when worms were absent invasive species biomass was lower in high light due to seed desiccation. This was an unexpected result since most previous studies suggest that buckthorn grows best in high light. Across the litter treatments, invasive plants had lower biomass in high litter as we expected based on field observation and previous studies. However, when worms were present in pots, the effects were reversed and there was high germination of invasive in the pots with more litter. Overall, forest invaders experienced higher germination in the presence of earthworms and these belowground invaders appear to be an important variable determining a site’s invasibility.
Invasives often germinated on earthworm castings
We also ran a seedbank study in the greenhouse to investigate propagule pressure, i.e., the number of seeds of a given species released into an area. This is an important component of invasion potential and something our field study suggested was an important predictor buckthorn abundance in our study sites. Estimating the difference between actual propagule pressure, i.e., the numbers of seeds in the seedbank, and realized abundance of an invasive species, i.e., the actual number of stems of the invasive species in a given area, is a way to estimate whether environmental characteristics of a site are suppressing invasion. To investigate whether characteristics of Minnesota’s deciduous forest play a role in suppressing invasion by buckthorn we collected soil from 12 forest stands across a low to high gradient of buckthorn abundance. We exposed this soil to ‘ideal conditions’ in the green house (high light, adequate moisture, and low competition) to germinate as many invasive species as possible from the seed bank.
Counting germinants from seedbank
The seedbank greenhouse study concluded in July 2012. Far fewer buckthorn seeds germinated from the collected soils than expected so we did not have sufficient statistical power to analyze these results. Due to the spatial variability of buckthorn seeds in the soil, future studies would need to collect far more soil to determine a representative available seedbank.