Grazing-induced effects on soil properties modify plant competitive interactions in semi-natural mountain grasslands

Abstract

Plant-soil feedbacks are widely recognized as playing a significant role in structuring plant communities through their effects on plant-plant interactions. However, the question of whether plant-soil feedbacks can be indirectly driven by other ecological agents, such as large herbivores, which are known to strongly modify plant community structure and soil properties, remains poorly explored. We tested in a glasshouse experiment how changes in soil properties resulting from long-term sheep grazing affect competitive interactions (intra- and inter-specific) of two graminoid species: Nardus stricta, which is typically abundant under high sheep grazing pressure in British mountain grasslands; and Eriophorum vaginatum, whose abundance is typically diminished under grazing. Both species were grown in monocultures and mixtures at different densities in soils taken from adjacent grazed and ungrazed mountain grassland in the Yorkshire Dales, northern England. Nardus stricta performed better (shoot and root biomass) when grown in grazing-conditioned soil, independent of whether or not it grew under inter-specific competition. Eriophorum vaginatum also grew better when planted in soil from the grazed site, but this occurred only when it did not experience inter-specific competition with N. stricta. This indicates that plant-soil feedback for E. vaginatum is dependent on the presence of an inter-specific competitor. A yield density model showed that indirect effects of grazing increased the intensity of intra-specific competition in both species in comparison with ungrazed-conditioned soil. However, indirect effects of grazing on the intensity of inter-specific competition were species-specific favouring N. stricta. We explain these asymmetric grazing-induced effects on competition on the basis of traits of the superior competitor and grazing effects on soil nutrients. Finally, we discuss the relevance of our findings for plant community dynamics in grazed, semi-natural grasslands.

Inter-Specific Competition, but Not Different Soil Microbial Communities, Affects N Chemical Forms Uptake by Competing Graminoids of Upland Grasslands

“…Exclusion of sheep grazing has led to dominance of the dwarf-shrub Calluna vulgaris (L.) Hull, and the graminoids Deschampsia caespitosa (L.) Beauv. and E. vaginatum , and it has reduced soil N availability, soil microbial activity, soil microbial biomass N, and the ratio of IN to ON in comparison with the adjacent continuously-grazed grassland [21], [23]. We collected soil from 5–7 sampling points on each of the grazed and ungrazed areas, and made a composite sample.…”

“…On the other hand, E. vaginatum seeds (also from plants collected in our field sites and propagated in the glasshouse) were planted 1 week after N. stricta in order to avoid large differences in plant size. Since competition between these graminoids occurs even at low densities [21], we used 2 plant individuals per microcosm in a substitution design. Thus, previously grazed- or ungrazed-inoculated microcosms were assigned randomly to the following plant competition treatments: 1) N. stricta intra-specific competition; 2) E. vaginatum intra-specific competition; and 3) inter-specific competition with one individual of each species per microcosm.…”

“…Moreover, grazing-induced changes in soils can then feedback to influence plant species performance and competition [16]–[20]. For instance, Medina-Roldán et al [21] found that grazing-induced increases in soil microbial activity and soil N availability in temperate acid grasslands increased the competitive ability of Nardus stricta L. relative to Eriophorum vaginatum L. (nomenclature follows [22]), which might partly explain the dominance of the former species in grazed grassland. However, whether grazing impacts on soil mediate plant uptake of different chemical forms of N (ON and IN) remains unexplored.…”

“…We tested three hypotheses. First, we hypothesized that the more metabolically-active soil microbial community of grazed grasslands [21], [23] would facilitate N uptake in both plant species, although the increase would be larger for N. stricta due to its greater competitive ability. Second, we hypothesized that E. vaginatum would take up more glycine than N. stricta , since the former is known to grow in soils with higher proportion of ON relative to IN [23].…”

Crabs Mediate Interactions between Native and Invasive Salt Marsh Plants: A Mesocosm Study

“…Crooks [13] reviewed the ecosystem-level consequences of abiotic resource modification (or “ecosystem engineering”) by invasive species. However, the role of native soil modifiers in mediating plant interactions remains largely unknown, although a recent study reported that grazers might mediate competition between two graminoid species via altering soil nutrient status [14].…”