Data from: 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, i.e., 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 analyzed 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.