Assessing the mechanisms and impacts of shrub invasion in forests: A meta-analysis

The encroachment of invasive shrubs in forest understories can have detrimental effects on native plant recruitment. As a result, removal of invasive species is a common practice although long-lasting success is rare. In order to effectively conserve and manage invaded forests, it is crucial to understand the mechanisms that drive shrub invasion, that is, high propagule pressure, low native resistance and exploitation of empty niches. To gain a deeper understanding of the invasion process in forest ecosystems we conducted a meta-analysis of the work done in this topic. We collected data on invasive species and native community performance, and on the abiotic conditions of forest understories under low and high levels of shrub invasion. We analysed data from 124 articles that yielded 377 unique observations. Our results revealed that while invader performance did not vary by the mechanism of invasion, the impact on the native community was significantly detrimental when invasion occurred via low biotic resistance, and only marginally significant via propagule pressure. Invasive species performance was associated with increases in light availability, but not with other resources (soil water or nutrients). When assessing impact on native performance as a function of invasive performance, results were again only significant under the low biotic resistance mechanism. Lastly, impacts were stronger when invasion took place by a single invader. Synthesis and applications. Taken together, these results suggest that restoration efforts should focus on (i) increasing the presence of strong native competitors or functionally diverse native communities, (ii) decreasing sources of invasive shrub propagules while keeping the canopies closed when invasion occurs via high propagule pressure, (iii) avoiding management techniques that degrade or diminish canopy cover and (iv) prioritizing management of forest understories dominated by particularly impactful invasive shrubs.