Effects of the invasive leafy spurge (Euphorbia esula L.) on plant community structure are altered by management history

Invasive species threaten biodiversity and ecosystem functioning, often causing changes in plant community composition and, thus, the functional traits of that community. Quantifying changes in traits can help us understand invasive species impacts on communities; however, both the invader and the plant community may be responding to the same environmental drivers. In North America, leafy spurge (Euphorbia esula L.) is a problematic invader that reduces plant diversity and forage production for livestock. Its documented effects on plant communities differ amongst studies, however, potentially due to differences in productivity or land management. To identify the potential effects of leafy spurge on plant communities, we quantified leafy spurge abundance, plant species richness, forage production, functional group composition and community weighted mean traits, intensively at a single site and extensively across ten sites differing in management. We then tested how leafy spurge abundance related to these variables as a function of site management activities. Leafy spurge abundance was consistently associated with fewer plant species, reduced forage production and more invasive grass. Community-weighted specific root length also consistently increased with leafy spurge abundance, suggesting that belowground competition may be important in determining co-existence with leafy spurge. Other changes were dependent on management. Native forbs were excluded as leafy spurge became more abundant, but only in grazed sites as these species were already absent from ungrazed sites. Taller plants better persisted in dense leafy spurge patches, but only in grazed sites, consistent with either facilitation of taller species via associational defences or competitive exclusion of shorter species in ungrazed sites and dense leafy spurge patches. These results show that, despite some emergent properties of invasion, management context can alter invasion impacts by causing changes in the plant community and its interactions with the invader.

Identifying thresholds in the impacts of an invasive groundcover on native vegetation

Impacts of invasive species are often difficult to quantify, meaning that many invaders are prioritised for management without robust, contextual evidence of impact. Most impact studies for invasive plants compare heavily invaded with non-invaded sites, revealing little about abundance–impact relationships. We examined effects of increasing cover and volume of the non-native herbaceous groundcover Tradescantia fluminensis on a temperate rainforest community of southern Australia. We hypothesised that there would be critical thresholds in T. fluminensis abundance, below which the native plant community would not be significantly impacted, but above which the community’s condition would degrade markedly. We modelled the abundance–impact relationship from 83 plots that varied in T. fluminensis abundance and landscape context and found the responses of almost all native plant indicators to invasion were non-linear. Native species richness, abundance and diversity exhibited negative exponential relationships with increasing T. fluminensis volume, but negative threshold relationships with increasing T. fluminensis cover. In the latter case, all metrics were relatively stable until cover reached between 20 and 30%, after which each decreased linearly, with a 50% decline occurring at 75–80% invader cover. Few growth forms (notably shrubs and climbers) exhibited such thresholds, with most exhibiting negative exponential relationships. Tradescantia fluminensis biomass increased dramatically at > 80% cover, with few native species able to persist at such high levels of invasion. Landscape context had almost no influence on native communities, or the abundance–impact relationships between T. fluminensis and the plant community metrics. Our results suggest that the diversity of native rainforest community can be maintained where T. fluminensis is present at moderate-to-low cover levels.

Integrated analysis of mRNA-seq and miRNA-seq reveals the advantage of polyploid Solidago canadensis in sexual reproduction

BACKGROUND

The invasion of Solidago canadensis probably related to polyploidy, which may promotes its potential of sexual reproductive. S. canadensis as an invasive species which rapidly widespread through yield huge numbers of seed, but the mechanism remains unknown. To better understand the advantages of sexual reproduction in hexaploid S. canadensis, transcriptome and small RNA sequencing of diploid and hexaploid cytotypes in flower bud and fruit development stages were performed in this study.

RESULTS

The transcriptome analysis showed that in the flower bud stage, 29 DEGs were MADS-box related genes with 14 up-regulated and 15 down-regulated in hexaploid S. canadensis; 12 SPL genes were detected differentially expressed with 5 up-regulated and 7 down-regulated. In the fruit development stage, 26 MADS-box related genes with 20 up-regulated and 6 down-regulated in hexaploid S. canadensis; 5 SPL genes were all up-regulated; 28 seed storage protein related genes with 18 were up-regulated and 10 down-regulated. The weighted gene co-expression network analysis (WGCNA) identified 19 modules which consisted of co-expressed DEGs with functions such as sexual reproduction, secondary metabolism and transcription factors. Furthermore, we discovered 326 miRNAs with 67 known miRNAs and 259 novel miRNAs. Some of miRNAs, such as miR156, miR156a and miR156f, which target the sexual reproduction related genes.

CONCLUSION

Our study provides a global view of the advantages of sexual reproduction in hexaploid S. canadensis based on the molecular mechanisms, which may promote hexaploid S. canadensis owing higher yield and fruit quality in the process of sexual reproduction and higher germination rate of seeds, and finally conductive to diffusion, faster propagation process and enhanced invasiveness.

Evidence for the primacy of living root inputs, not root or shoot litter, in forming soil organic carbon

Soil organic carbon (SOC) is primarily formed from plant inputs, but the relative carbon (C) contributions from living root inputs (i.e. rhizodeposits) vs litter inputs (i.e. root + shoot litter) are poorly understood. Recent theory suggests that living root inputs exert a disproportionate influence on SOC formation, but few field studies have explicitly tested this by separately tracking living root vs litter inputs as they move through the soil food web and into distinct SOC pools. We used a manipulative field experiment with an annual C₄ grass in a forest understory to differentially track its living root vs litter inputs into the soil and to assess net SOC formation over multiple years. We show that living root inputs are 2-13 times more efficient than litter inputs in forming both slow-cycling, mineral-associated SOC as well as fast-cycling, particulate organic C. Furthermore, we demonstrate that living root inputs are more efficiently anabolized by the soil microbial community en route to the mineral-associated SOC pool (dubbed 'the in vivo microbial turnover pathway'). Overall, our findings provide support for the primacy of living root inputs in forming SOC. However, we also highlight the possibility of nonadditive effects of living root and litter inputs, which may deplete SOC pools despite greater SOC formation rates.