Complementary effects of soil compaction and mowing on ecosystem functioning mimic effects of large grazers

Background/Question/Methods

Large grazers have a major impact on their habitat, strongly affecting above- and below ground compartments of the food web. However, the exact mechanisms via which they exert such a strong influence are poorly understood. Classically, studies have focused on the aboveground impacts these large grazers have on the ecosystem, for instance by mimicking biomass removal. Recently, studies have stressed the additional importance of the effects on below ground processes. Especially on fine-textured clay soils, effects of large, soil compacting grazers can be strong, affecting soil aeration, which is an important determinant for both vegetation and soil fauna.

We conducted a full factorial experiment to separate effects of trampling and biomass removal on a salt-marsh in Western Europe that was dominated by the late successional tall grass Elytrigia atherica.  To mimic trampling effects by large herbivores on soil structure, we compacted the soil using a rammer in spring, and to mimic biomass removal, we kept a monthly mowing regime throughout the growing season of 2011. We compared our treatment to a cow-grazed reference site in the dry period (June) and the wet period (September).

Results/Conclusions

Results from the mowing + compaction treatment were most comparable to the grazed control, indicating that our experiment was successful. The cover of E. atherica was strongly reduced, in favor of grazing tolerant grasses and annuals. Soil fauna response was highly season-dependent: under dry conditions no effect of trampling was found, but under wet conditions, species richness and abundance declined sharply, equaling the compacted treatments to levels of the grazed marsh. Furthermore, we found a clear tendency towards smaller body size in soil fauna, especially in the lower soil stratum (5-10 cm depth) in September.

Mowing alone did not lead to reduced soil aeration and compaction alone did not lead to changes in vegetation composition. Our results therefore show that large grazers exert important effects on the plant and animal communities via changes soil parameters (belowground effects) as well as via changes in light competition (aboveground effects).

We therefore conclude that effects of large grazers can only be understood when taking both effects on the aboveground and the belowground biotics and abiotics into account. Especially in these wet, fine-textured soils, their impact via belowground processes on vegetation and soil fauna may be more important than aboveground effects.