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).