4 | Discussion
The growth of woody species is limited by above- and belowground competition during early stages of establishment in grasslands (Bush and van Auken 1990; Ward 2020). Identifying mechanisms that could promote survivorship and growth of woody species during their seedling stage is critical to understanding how they encroach into grasslands (Van Auken 2000). We observed a negative plant-soil feedback that suppressed the height and biomass of grasses grown in soil conditioned by redcedar for two of the four species used in this experiment. This suggests plant-soil feedback may facilitate the establishment of redcedar in its encroaching range depending on the local plant community at the site of establishment, much has it has done for other species combinations (Aldorfová et al. 2020).
In our experiment, grass growth in live and sterilized away (redcedar) soil was reduced when compared to growth in live and sterilized home soils for the C3 grasses B. inermis and P. smithii . Plants frequently experience strong negative feedback when growing in live home soils due to accumulation of specialized predators (Bever 1994; Petermann et al. 2008; Lekberg et al. 2018). Therefore, the observed suppression of grass growth in redcedar-conditioned soils relative to home soils is noteworthy and may represent a key factor in redcedar expansion into grasslands. Negative feedbacks from dissimilar heterospecific species on target species can be derived from either an antimicrobial effect of soil biota in the conditioned soil (Haichar et al. 2014) or from the production of allelochemicals that negatively affect the growth of the target plant directly or by inhibiting the establishment of beneficial soil microbial communities (Mommer et al. 2008; Bennett and Klironomos 2019). In this experiment, we observed the inhibition of phytometer growth in sterilized away soils, which may be indicative that redcedar exudes an allelochemical into its near-soil environment. We are uncertain why C3 species showed negative feedbacks and not C4 species. A possible explanation is that the C3 redcedar has novel weapons against these two species (Callaway and Ridenour 2004; Orians and Ward 2010). The Eurasian origins of B. inermis that now occupies the entire contiguous United States and the recent switch to dominance of P. smithii in parts of the Great Plains during the Dust Bowl could indicate that these species have had relatively limited exposure to any secondary chemicals produced by redcedar (Weaver 1942; Knapp et al. 2020). Another possibility is that because redcedar is a C3 plant, it produces a stronger negative feedback with other C3plants. Further study of more C3 grass species will be needed to determine if this is a causal relationship or a coincidence.
The modification of the soil environment by allelopathic woody plants is an important process that can create a positive feedback for their encroachment (Eldridge et al. 2011; Caracciolo et al.2016). Researchers have explored the possibility of allelopathy in several North American Juniperus species with mixed results (Schott and Pieper 1985; Norman and Anderson 2003). Past investigations of redcedar allelopathy have focused on germination rates of prairie plants. For example, Corbett and Lashley (2017) found redcedar litter additions did not negatively affect germination of test species. However, Stipe and Bragg (1989) noted suppression of germination for a different pool of test species grown in soil collected from a redcedar stand. Our findings take this research one step further by demonstrating the suppression of plant performance following successful germination. Taken together, the ability of redcedar to reduce the germination rate of grasses and suppress their growth following establishment may be a key factor in its successful encroachment of prairies.
Our experimental results show a negative feedback for grasses grown in soil conditioned by redcedar, but interpretation of these results must also consider the myriad factors that influence plant-plant interactions in the field. Our study examined growth of individuals in a greenhouse, using potting mix and sand as soil substrates, and comparing live inoculations of conditioned soil with those that had been sterilized under heat and pressure. The strength of plant-soil feedbacks measured in artificial conditions have been found to be inflated relative to those observed in field conditions (Kulmatiski and Kardol 2008). Confounding factors that could change the relative strength of feedback in field conditions include the near-neighborhood community composition and competitive interactions. For example, we observed strong suppression of individuals of B. inermis and P. smithiigrown in live and sterilized redcedar soils. In field conditions, individuals of B. inermis and P. smithii could be expected to grow in patches where they have many conspecific neighbors (Fink and Wilson 2011; Ott and Hartnett 2015). In the prairies of the Great Plains, B. inermis has been shown to have positive conspecific plant-soil feedback that can exclude heterospecific plants (Vinton and Goergen 2006). Additionally, when B. inermis occurs at high density, it has been shown to be a strong competitor with redcedar seedlings (Hamati et al. 2021). In mixed-grass prairies, P. smithii invests heavily in spreading its resources through rhizomes that aid in ensuring plant survival in changing conditions (Ott and Hartnett 2015). Taken in this context, it is unlikely that the allelopathic effect of redcedar seedlings could fully displace B. inermis or P. smithii in a dense monoculture. However, if the suppressive effect of redcedar is sufficiently large to allow redcedar individuals to establish and survive long enough to overtop their competitors, then plant-soil feedbacks could be an important factor in the spread of the redcedars. Inherently, this effect will only apply to near neighbors that overlap in the rooting zone of redcedars (i.e. over a short distance). Further studies are needed to determine the strength of this effect in field conditions, the size of the area of impact around trees, the longevity of the effect in the soil, and how the strength of suppression changes with tree size or age.