Soil heterogeneity increases plant diversity after 20 years of manipulation during grassland restoration

Abstract

The “environmental heterogeneity hypothesis” predicts that variability in resources promotes species coexistence, but few experiments support this hypothesis in plant communities. A previous 15‐yr test of this hypothesis in a prairie restoration experiment demonstrated a weak effect of manipulated soil resource heterogeneity on plant diversity. This response was attributed to a transient increase in richness following a post‐restoration supplemental propagule addition, occasionally higher diversity under nutrient enrichment, and reduced cover of a dominant species in a subset of soil treatments. Here, we report community dynamics under continuous propagule addition in the same experiment, corresponding to 16–20 yr of restoration, in response to altered availability and heterogeneity of soil resources. We also quantified traits of newly added species to determine if heterogeneity increases the amount and variety of niches available for new species to exploit. The heterogeneous treatment contained a factorial combination of altered nutrient availability and soil depth; control plots had no manipulations. Total diversity and richness were higher in the heterogeneous treatment during this 5‐yr study due to higher cover, diversity, and richness of previously established forbs, particularly in the N‐enriched subplots. All new species added to the experiment exhibited unique trait spaces, but there was no evidence that heterogeneous plots contained a greater variety of new species representing a wider range of trait spaces relative to the control treatment. The richness and cover of new species was higher in N‐enriched soil, but the magnitude of this response was small. Communities assembling under long‐term N addition were dominated by different species among subplots receiving added N, leading to greater dispersion of communities among the heterogeneous relative to control plots. Contrary to the deterministic mechanism by which heterogeneity was expected to increase diversity (greater variability in resources for new species to exploit), higher diversity in the heterogeneous plots resulted from destabilization of formerly grass‐dominated communities in N‐enriched subplots. While we do not advocate increasing available soil N at large scales, we conclude that the positive effect of environmental heterogeneity on diversity can take decades to materialize and depend on development of stochastic processes in communities with strong establishment limitation.

Ecosystem restoration through aerial seeding: Interacting plant–soil microbiome effects on soil multifunctionality

“…During restoration, the relationship between components (such as plants, soil microorganisms, and soil multifunctionality) and restoration time will vary, as will the response of individual components to restoration time. Most studies in the terrestrial environment have shown that with ecological restoration, plants (Baer et al, 2019; Liu, Liang, et al, 2020), soil microorganisms (Yan et al, 2019; Zhong et al, 2019), and soil multifunctionality (Yan et al, 2019; Zhong et al, 2019) will gradually restore, with a positive linear relationship. However, in some studies of soil microorganisms, it has been found that the response to ecological restoration can become saturated (i.e., the diversity of soil microorganisms does not increase after a certain number of years) (Klopf et al, 2017) and lagging can occur (i.e., soil microorganisms do not respond immediately to restoration, but have a time interval before recovery begins) (Boeddinghaus et al, 2019; Strickland et al, 2017).…”

Revising the trait‐based filtering framework to include interacting filters: Lessons from grassland restoration

“…Others have used fire, grazing, litter removal or direct manipulations to alter abiotic filters such as light, water and soil nutrient availability (e.g. Baer et al., 2020; Funk et al., 2015; Hulvey & Aigner, 2014). Studies addressing the biotic filter have focused on grazing and competition with invasive species (e.g.…”

“…Work from tallgrass prairie suggests that the strength of nitrogen [N] availability (an abiotic filter) can vary over time. Specifically, high N availability decreased species diversity in the first few years after restoration, but increased diversity 15 years following restoration as long‐term fertilization increased the cover of forb species relative to dominant grass species (Baer et al., 2020). Recent work has also shown that the importance of the species pool size varies across spatial scales (Catano et al., 2021).…”

State changes: insights from the U.S. Long Term Ecological Research Network

“…Second, we need to restore tallgrass prairie from woody states and agricultural conditions. KNZ data demonstrate that restoration from these alternate land cover states can be labor-intensive (Larson et al 2019), will take many years to accomplish, and that restoration goals need to consider the effects of dispersal limitation, habitat heterogeneity, and interannual climate variability on restoring biodiversity (Baer et al 2016(Baer et al , 2019. Long-term ecological research has been, and will continue to be, invaluable for identifying appropriate ecological disturbance regimes required for long-term conservation of tallgrass prairie.…”