Co‐occurring nonnative woody shrubs have additive and non‐additive soil legacies

Abstract

To maximize limited conservation funds and prioritize management projects that are likely to succeed, accurate assessment of invasive nonnative species impacts is essential. A common challenge to prioritization is a limited knowledge of the difference between the impacts of a single nonnative species compared to the impacts of nonnative species when they co-occur, and in particular predicting when impacts of co-occurring nonnative species will be non-additive. Understanding non-additivity is important for management decisions because the management of only one co-occurring invader will not necessarily lead to a predictable reduction in the impact or growth of the other nonnative plant. Nonnative plants are frequently associated with changes in soil biotic and abiotic characteristics, which lead to plant-soil interactions that influence the performance of other species grown in those soils. Whether co-occurring nonnative plants alter soil properties additively or non-additively relative to their effects on soils when they grow in monoculture is rarely addressed. We use a greenhouse plant-soil feedback experiment to test for non-additive soil impacts of two common invasive nonnative woody shrubs, Lonicera maackii and Ligustrum sinense, in deciduous forests of the southeastern United States. We measured the performance of each nonnative shrub, a native herbaceous community, and a nonnative woody vine in soils conditioned by each shrub singly or together in polyculture. Soils conditioned by both nonnative shrubs had non-additive impacts on native and nonnative performance. Root mass of the native herbaceous community was 1.5 times lower and the root mass of the nonnative L. sinense was 1.8 times higher in soils conditioned by both L. maackii and L. sinense than expected based upon growth in soils conditioned by either shrub singly. This result indicates that when these two nonnative shrubs co-occur, their influence on soils disproportionally favors persistence of the nonnative L. sinense relative to this native herbaceous community, and could provide an explanation of why native species abundance is frequently depressed in these communities. Additionally, the difference between native and nonnative performance demonstrates that invasive impact studies focusing on the impact only of single species can be insufficient for determining the impact of co-occurring invasive plant species.

Responses of the soil fungal communities to the co‐invasion of two invasive species with different cover classes

“…Multiple invasive species may coexist in the same natural ecological system (Kuebbing et al . ,b, ; Wang et al . ).…”

“…Invasive species can trigger pronounced changes in structure and function of native ecosystems (Kuebbing et al . , ; Wang et al . ).…”

“…More importantly, the successful establishment of one invasive species could create a microenvironment in the invaded ecosystems that might increase the possibility of successful colonisation of subsequent invaders (Flory & Bauer ; Kuebbing et al . , ). Co‐invasion of two or more invasive species is therefore likely in some invaded ecosystems (Kuebbing et al .…”

“…Co‐invasion of two or more invasive species is therefore likely in some invaded ecosystems (Kuebbing et al . ,b, ; Kuebbing & Nuñez ; Wang et al . ).…”

The ecological effects of Chinese privet (Ligustrum sinense) invasion: a synthesis

“…Alteration of soil nutrient cycles and nutrient availability can change competitive interactions among plant species (e.g., Aerts and Berendse 1988), but this has not been directly studied in relation to L. sinense. Changes in soil microbial and fungal communities have been suggested as another mechanism through which L. sinense influences native and nonnative plant communities (Greipsson and DiTommaso 2006;Kuebbing et al 2014Kuebbing et al , 2015Kuebbing et al , 2016, and some of these changes are likely associated and interact with changes in soil nutrient dynamics (Deyn et al 2004;Ehrenfeld 2003), which in turn are likely related to changes in litter decomposition. The complexity of the biogeochemical process as it relates to these changes makes it difficult to parse out simple cause-effect relationships.…”