Contrasting responses of soil nematode communities to native and non-native woody plant expansion

Legacy of pine invasion disrupts plant-soil feedback of native and non-native grasses

Plant-soil feedback (PSF) is a standing concept helping to explain plant species coexistence. Accumulating evidence suggests that PSF depends on the environmental context. One particularly impactful disruption of the environmental context is the invasion of alien species into a community. While PSF between native and non-native species has been linked to invasion success, the effect of an invader on PSF between resident plants has received little attention. We explored how Pinus contorta (lodgepole pine) invasion impacts PSF between four resident grasses (two native and two non-native species). We grew the plants in live soil to condition species-specific soil communities, followed by a response phase where the plants were grown in conspecific and heterospecific soil. Both phases were performed with soil from uninvaded, invaded, or controlled invasion areas. Pairwise PSF between the species in uninvaded soil was negative and became neutral to positive in invaded soil. This effect persisted even after P. contorta removal. Our results suggest a destabilizing effect of P. contorta invasion on the coexistence of the resident grasses, benefitting non-natives over natives. This work exemplifies an overlooked mechanism by which an invader can alter interactions between resident species and adds a novel dimension to the context dependence of PSF.

Nematodes as Drivers of Plant Performance in Natural Systems

Nematodes form an important part of soil biodiversity as the most abundant and functionally diverse animals affecting plant performance. Most studies on plant-nematode interactions are focused on agriculture, while plant-nematode interactions in nature are less known. Here we highlight that nematodes can contribute to vegetation dynamics through direct negative effects on plants, and indirect positive effects through top-down predation on plant-associated organisms. Global change alters these interactions, of which better understanding is rapidly needed to better predict functional consequences. By expanding the knowledge of plant-nematode interactions in natural systems, an increase in basic understanding of key ecological topics such as plant-soil interactions and plant invasion dynamics will be obtained, while also increasing the insights and potential biotic repertoire to be applicable in sustainable plant management.

Invasive plants exert disproportionately negative allelopathic effects on the growth and physiology of the earthworm Eisenia fetida

Exotic invasive plants possess the capacity to disrupt and extirpate populations of native species. Native plants' increased sensitivity to invaders' allelochemicals is a mechanism by which this can occur. However, it is not clear whether and how the allelopathic effects of invasive plants affect members of the soil faunal community - particularly the important functional guild of earthworms. We used the model earthworm Eisenia fetida to investigate the responses to extracts from the widely invasive Asterids (Ageratina adenophora, Bidens pilosa, Erigeron annuus) and closely-related native species in a greenhouse experiment. We observed declines in body mass and respiration, and increases in oxidative and DNA damage biomarkers in the native earthworm E. fetida when grown under root and leaf extracts from these invasive plants. These effects were concentration-dependent, and worm growth and physiology was most negatively affected under the highest concentrations of leaf extracts. Most importantly, extracts from invasive plants caused significantly more negative effects on E. fetida than did extracts from native plant species, indicating allelopathy from invasive plants may inhibit earthworm physiological functioning. These results expand the domain of the novel weapons hypothesis to the earthworm guild and demonstrate the utility of E. fetida as a bioindicator for plant allelochemicals.

Community- and trophic-level responses of soil nematodes to removal of a non-native tree at different stages of invasion

Success of invasive non-native plant species management is usually measured as changes in the abundance of the invasive plant species or native plant species following invader management, but more complex trophic responses to invader removal are often ignored or assumed. Moreover, the effects of invader removal at different stages of the invasion process is rarely evaluated, despite a growing recognition that invader impacts are density or stage-dependent. Therefore, the effectiveness of invasive species management for restoring community structure and function across trophic levels remains poorly understood. We determined how soil nematode diversity and community composition respond to removal of the globally invasive tree species Pinus contorta at different stages of invasion by reanalysing and expanding an earlier study including uninvaded vegetation (seedlings removed continuously), early invader removal (saplings removed), late removal (trees removed), and no removal (invaded). These treatments allowed us to evaluate the stage-dependent belowground trophic responses to biological invasion and removal. We found that invaded plots had half the nematode taxa richness compared to uninvaded plots, and that tree invasion altered the overall composition of the nematode community. Differences in nematode community composition between uninvaded nematode communities and those under the tree removal strategy tended to dilute higher up the food chain, whereas the composition of uninvaded vs. sapling removal strategies did not differ significantly. Conversely, the composition of invaded compared to uninvaded nematode communities differed across all trophic levels, altering the community structure and function. Specifically, invaded communities were structurally simplified compared to uninvaded communities, and had a higher proportion of short life cycle nematodes, characteristic of disturbed environments. We demonstrate that a shift in management strategies for a globally invasive tree species from removing trees to earlier removal of saplings is needed for maintaining the composition and structure of soil nematode communities to resemble uninvaded conditions.