Discussion
Pre-extraction processes of surface mining and subsequent reclamation activities provide efficient means of de-constructing and re-constructing a new landscape. However, belowground conditions resulting from these current best management practices (i.e., compacted soils) produce challenging circumstances for establishing and sustaining a new diverse plant community (Bohrer et al. 2017a). Furthermore, the presence and establishment of exotic species, like KBG, add additional stresses to fostering a desirable plant community (Bohrer et al. 2017b). In an attempt to address these various challenges associated with a reclaimed landscape, we explored the impacts of alternative reclamation practices on a newly reclaimed grassland. We found that plant composition significantly differed between 2019 and 2020, and two out of three of the treatment effects (i.e., seed mix and ripping) had a significant effect on the plant community composition, regardless of year. Additionally, penetration resistance and soil moisture readings are showing early trends that suggests a distinction between treatments.
Distinct plant community assemblages are beginning to form in response to seeding mixtures, ripping techniques, and their interaction over time. TSR and G/F treatments frequently shared plant community primary drivers across all functional groups, in 2019. These treatments were commonly associated with native perennials, C3 grasses, and C3 forbs. This trend is likely attributed in large part to the seed mix, but also the improved growing conditions for vegetation provided by ripping from the topsoil horizon (Ashby 1997; Bauman et al. 2014; Fields-Johnson et al. 2014). C3 grasses occurred more often with G/F and TSR treatments, yet planted C3 grass species (i.e., NAVI (Nassella viridula) and PASM (Pascopyrum smithii )) were commonly found on all treatments in both years. The USDA Natural Resources Conservation Service recommended these native species for revegetating disturbed/reclaimed landscapes which likely contributes to their high proportion use in the seed mix and them being well represented.
Generally, shorter-lived species (i.e., annuals and short-lived perennials) occurred in G and SSR treatments in 2019. This representation of short-lived species is typical of recently disturbed/reclaimed landscapes (Foster & Tilman 2000; Alday et al. 2011) and may explain some of the increases of species richness found among G and SSR treatments experienced from 2019 to 2020. However, some of the G/F and TSR treatments also experienced increases in species richness between 2019 and 2020 without a heavy association with short-lived species. Fluctuation of species richness in the early stages of ecological recovery is a commonly documented occurrence in previous grassland restoration work (Sluis 2002; Middleton et al. 2010). Changes in species richness between 2019 and 2020 suggests that all treatments, but especially G and SSR treatments, may experience more pronounced shifts in species composition over time.
By the second sampling season interactions between TSR and G/F treatments and G and SSR treatments became increasingly more noticeable. However, community assemblages between seeding mixtures were found to be less distinct from one another which expresses the dynamics of recently reclaimed grasslands. For example, while intentionally seeded C3 forbs occurred more frequently in G/F and TSF treatments, C3 forbs were a primary driver of the vegetation community for G and SSR treatments. Expression of those short-lived species found on G and SSR treatments in 2019 and the retention of longer lived C3 forb species found in G/F and TSR treatments may have prompted this shift in assemblages between seeding mixtures. Additionally, C4 grasses and C4 forbs (i.e., SATR (Salsola tragus ) and BASC (Bassia scoparia ); annual invasive species) became prominent drivers of species composition in 2020. The C4 grasses appear to more readily occupy G/F and TSR treatments, but C4 forbs seemingly occupy all treatments. C4 grass prominence is not wholly uncommon in restored grasslands (Camill et al. 2004), but the broad occurrence of C4 forbs across all treatments may contribute to the increased similarity between seeding mixtures’ community assemblages. Early predominance of seeded species, especially the forbs and C4 grasses, indicates that G/F and TSF treatments are promoting conditions for a desirable plant community, with the exception of KBG. Yet, the interaction between G/F and TSR and G and SSR treatments, along with the overlapping community composition that exists among these different main effects may indicate that these communities are becoming increasingly more similar over time.
One notable trend not observed on any of our treatments was the strong influence of exotic species. This is especially notable given previous research efforts attributed uneven surfaces created by ripping/disking/tilling to increased weed production via seed capture (Redente, E.F. & Hargis 1985). Treatments that experienced TSR (i.e., surface level disturbance) had a greater association with natives in 2019 and 2020. Strong establishment of native species from the seed mixtures and/or seed bank may be inhibiting the initial establishment of exotic species, preventing exotic species to be dominate drivers of species composition. However, chances of invasion by aggressive non-native species can, and often do, increase over time.
Early findings suggest, when used together, TSR and G/F mixtures could aid in improving the growing conditions, especially for native, perennial, C3 grasses and intentionally seeded C3 forbs species. However, our results found that those treatments actively reclaimed with combinations of alternative reclamation practices were not significantly different to standard practices (i.e., the reference site). Thus, early trends indicate that standard practices are providing relatively similar conditions to those created by alternative reclamation practices. It must be noted though that while TSR and G/F were found to have the lowest PR values, average PR values at all depths for these treatments fell between 4.25-6.13 MPa, which is well above the 2 MPa that restrict root penetration (Benjamin et al. 2003). Additionally, the application of ripping occurred only once in this study and settling and dispersion of soil particles due to rain events often result in re-compaction of previously ripped/tilled areas (Busscher et al. 2002). For this reason, it is important that PR readings continue to be taken to determine if the effects of these alternative reclamation practices will persist or change over time.
Variability between reclamation combinations was most notable at the 30 and 40 cm depth intervals which is most likely the depth where ripping activities, from the 56 cm ripping shanks, were most impactful for those TSR treatments. Generally, at these depths TSR treatments had the lowest volumetric soil moisture compared to those that received SSR. Decreased soil moisture on TSR treatments at these depths indicates greater infiltration and dispersion of water is occurring likely as a function of the mechanical creation of macropores (Hangen et al. 2002). Comparatively, SSR which experienced no surface-level ripping activities, had greater volumetric soil moisture possibly attributing to the disproportionate amount of micropores that developed from compacted conditions that developed during pre and post mining activities. Given smaller pores tend to hold water more tightly, perhaps the micropores contributed to a greater retention of water. While surface-level manipulations, or the lack of, seems to be the simplest explanation for these trends, variation of volumetric soil moisture between treatments was also associated, to varying degrees, with the seed mixtures. Treatments with combinations of TSR and G/F consistently showed significant differences from SSR and G plus mulch treatments, as with PR and community composition. However, the TSR treatments are still relatively compacted, and without species specific root data it is hard to determine how much the individual species are influencing the soil moisture at the 30 and 40 cm depth intervals.