Forest restoration: assessing the role of priority effects as a mechanism for resistance to plant invasion

Background/Question/Methods

Sufficient native species recruitment is essential for maintaining healthy forest ecosystems. However, many forested lands are embedded in human-modified landscapes where disturbances, fragmentation, and introduced species are common stressors on native plants. Some introduced species may become invasive and prevent natural recruitment of native species. Frequently management relies on their removal, but the potential lack of native propagules could hinder the system's recovery. In disturbed landscapes, native seed pools are diminished, while invasive seeds are frequently abundant. If priority effects are driving plant invasions after vegetation removal, promoting early arrival of natives could curtail the risk of re-invasion. Here, we present the results of a multiyear field experiment that asks whether plant success during restoration is determined by the order of species arrival in temperate forests, and if the strength of priority effects is affected by the abiotic environment. Following the invasive species removal, we carried out a seed addition of native forbs experiment. We tested the role of priority effects under two treatments of invasive plant removal, single and repeated clipping. And to better disentangle priority effects from other processes affecting native regeneration, we also carried out a soil amendment treatment, straw added vs. control.

Results/Conclusions

In both naturally regenerating and seeded plots, native regeneration during the growing season was strongly associated with the amount of native cover early in the season. These results underscore the importance of advanced regeneration in the recovery of native vegetation after invasive plant removal. We did not find any effect of the seeding treatment, indicating that lack of propagules is not the sole cause of low native regeneration. Still, the positive effect of seeding increased with repeated clipping. Our experiment also showed that lack of native regeneration could, in part, be due to negative soil legacy effects from invasive plants. However, straw addition did not amend these effects, and under repeated clipping, the effect of straw on natural regeneration was negative. Our results demonstrate that management practices of invasive species removal need to be carefully assessed a priory by also investigating potential responses of the native community. In our particular experiment, we documented the importance of advanced native regeneration as the priority effect most effective to ensure restoration, while seeding may require repeated removal of invasive plants and amendment of harmful soil legacies.