Discussion
Our results demonstrate positive effects of soil microbiome on several plant performance traits for most of the evaluated populations ofT. officinale . However, the soil microbiome appeared to be particularly relevant for T. officinale plants in the introduced range, as indicated by its higher impact on plant performance in the introduce versus native distribution range. Additionally, it played a more significant role under stressful conditions, as indicated by its higher influence on plant performance at the extremes of the latitudinal gradient within the introduced range.
Taraxacum officinale is considered a serious invasive species in their introduced range due its great environmental tolerance, higher plasticity and/or ecotypic differentiation (Molina-Montenegro et al. 2012b, 2014). In fact, T. officinale plants from Chilean populations have shown to possess higher performance than plants from European populations (Molina-Montenegro et al. 2010, 2011), and to have negative impacts on local plant communities (Muñoz and Cavieres 2008). Although previous studies have shown the T. officinale display several traits related to higher invasive capacity (Pysek and Richardson 2007), none have evaluated whether the soil microbiome can modulate -at least in part- such traits. For example, accumulating evidence supports that soil microbiome exerts positive effects on several fitness-related traits in weeds and invasive species (Trognitz et al. 2016). In addition, soil microbiome has been suggested to increase growth rate, improve defense against native herbivores and enhance the competitive ability of invasive species (Matos et al. 2019, Kalske et al. 2022). In agreement with these previous findings, we showed that different traits in T. officinale plants from most of the evaluated populations were positively affected by soil microbiome. Since the disruption of the soil microbiome was more severe for the introduced plants, it suggests a pivotal role for the soil microbiome in the invasion success in the introduced range.
The level of environmental stress can determine the relative importance of soil microbiome as a driver of plant performance. In our study, the soil microbiome had the most positive impact on plants from the northern and southern populations, with plants from the central populations showing a lesser effect. Depending on the range of environments they cover, long latitudinal gradients often exhibit higher levels of abiotic stress at their extremes (De Frenne et al. 2013). Therefore, the pattern we observed could be due to an increased dependence on beneficial biotic interactions as environmental stress increases, as stated by the stress gradient hypothesis (sensu Betness and Callaway 1994). In our study, the primary factor contributing to stress differs between the northern and southern extremes of the latitudinal gradient. In northern Chile, precipitations are very scarce and temperatures high, while in the south, temperatures can be extremely low, often dropping below freezing in winter (Di Castri and Hajek 1976). Although soil microorganisms are likely to play a crucial role in assisting plants to cope with water scarcity and high temperatures in the north while with low temperatures in the south (Acuña‐Rodríguez et al. 2020, Liu et al. 2020), our results showed that the plant performance variables were more sensitive to environmental conditions in the northern positions of the latitudinal gradient and to disruptions in the soil microorganism community. Therefore, soil microorganisms appear to be crucial for the invasive T. officinale plants to thrive with the dry and warm environmental conditions.
The soil microbiome can play a critical role in mediating interactions between native and invasive plant species in a variety of ways. Since plants recruit the associated soil microbiome from the surrounding soil, invasive plants could be generalists but able to create a positive rhizosphere environment, diversifying phenotypes, and functions (Coats and Rumpho 2014). It is not surprising then, that several hypotheses that attempt to explain success of plant invasion point out to processes that occur (or can occur) in the rhizosphere microbiome (enemy release hypothesis; accumulation of local pathogens, enhanced mutualist hypothesis, and plant–soil feedback) (see e.g. , Eppinga et al. 2006, Callaway et al. 2008, Blumenthal et al. 2009). Thus, the biological interactions that result from plants and microorganisms, either bacteria or fungi, in the rhizosphere soil (whether mutualistic or pathogenic) are crucial in determining the invasive capacity of plant species worldwide (Traveset and Richardson 2014, Dickie et al. 2017). In our study, it is evident that the invasion success of T. officinale plants in the introduced range could be mediated by a net positive interaction with soil microorganisms. However, the evidence for specificity in the effect of soil microorganisms and plants from the native and introduced range is limited. Rather, our results suggest that the success of T. officinale is linked to an intrinsic capacity to recruit microorganisms that exert positive phenotypic effects associated to increased tolerance on a variety of environmental conditions.