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.