1 | Introduction
Plants make species-specific changes to the biotic and abiotic conditions of their near-soil environment which can affect the fitness of future occupants (Bever et al. 1997; Bezemer et al.2006; Gundale and Kardol 2021). This phenomenon, deemed plant-soil feedback, can have a large influence on competitive interactions, community composition and function (van der Putten et al. 2013; Lekberg et al. 2018; Crawford et al. 2019). The strength and direction of a feedback is the product of several interacting mechanisms including soil-nutrient availability, the presence of pathogenic natural enemies and beneficial mutualists, and the effects of secondary chemicals (i.e. allelochemicals) that are exuded from plants (Bennett and Klironomos 2019).
Woody plant encroachment into grasslands is a global phenomenon that alters ecosystem function (Eldridge et al. 2011; Naito and Cairns 2011). The conversion of grasslands to woodlands can decrease biodiversity, change ecosystem structure and function, reduce productivity for livestock, alter water resource availability, and change the carbon balance (Barger et al. 2011; Ratajczak et al. 2012; Anadón et al. 2014; Acharya et al. 2018). Managing for encroaching species is difficult because the influence of factors differs between study species and systems (Tomiolo and Ward 2018). Fire suppression and livestock grazing are land-management practices that are frequently cited as the primary drivers of woody plant encroachment (Briggs et al. 2005; Van Auken 2009). The global trend of climate change, specifically increased temperature, nutrient deposition, and elevated CO2 levels, may also explain continental-scale patterns of woody species expansion (Devineet al. 2017). An additional factor that may promote encroachment is plant-soil feedback, which is a mechanism that can promote the establishment of woody species and reinforce the dominance of a woody state (Peters et al. 2020).
In North America, woody encroachment is occurring in the deserts and rangelands of the west, the savannas of the south, and the grasslands of the Great Plains region (Van Auken 2000; Ratajczak et al. 2012). Tree cover in rangelands of the western United States has increased by as much as 50% in the last 30 years, resulting in ~$5 billion in lost revenue (Morford et al. 2021). Encroachment in the Great Plains region of the United States is particularly concerning, with invading woody shrubs (e.g. Cornus drummondii ) and trees (e.g. Juniperus virginiana ) replacing grassland plant communities at a rate of up to 1.7 % per year (Barger et al. 2011).
Understanding how successful woody encroachers establish and spread is critical to being able to manage them effectively and efficiently. It is of particular importance to understand mechanisms that provide an advantage to species in their expanded range and to quantify the strength of that advantage. This paper explores plant-soil feedback as a potential mechanism that has facilitated the movement of eastern redcedar (J. virginiana ) from its historical range into the prairies of the Great Plains and into disturbed areas within their current ranges. Eastern redcedar (hereafter redcedar) is the most common, widely distributed conifer that is native to eastern North America (Fowells, 1965; Ward, 2020).
Redcedar tolerates a wide variety of climatic conditions including temperature extremes and drought. Redcedar is considered a long-lived, early seral species that can be dominant in a forest or woodland habitat until later seral species establish (Lawson, 1990; Briggs et al., 2002). Historically, populations persisted where there was reduced threat of fire, such as on rocky outcrops or barrens (Guyette et al. 2002; Briggs et al. 2002). Several mechanisms have been proposed that explain why redcedar is a successful encroacher. In tallgrass prairies there is strong evidence for the interaction of extended fire regimes and livestock grazing intensity being determinants of redcedar expansion (Briggs et al. 2005). The transition from grassland to woodlands in the Great Plains is largely attributed to land-management practices that have greatly extended fire-return intervals beyond their pre-European settlement levels (Briggs et al. 2005; Bielskiet al. 2021; Fogarty et al. 2021). There is also some evidence that the C3 photosynthetic pathway may provide an advantage to redcedar trees under elevated CO2conditions over many of the warm-season C4 grasses that co-occur in its range (Iverson et al. 2008; Huntley and Baxter 2013).