Can livestock exclusion affect understory plant community structure? An experimental study in the dry Chaco forest, Argentina

TRIGO, CAROLINA B.; VILLAGRA, PABLO E.; COWPER COLES, PATRICIO; MARÁS, GUSTAVO A.; ANDRADE-DÍAZ, MARÍA S.; NÚÑEZ-REGUEIRO, MAURICIO M.; DERLINDATI, ENRIQUE J.; TÁLAMO, ANDRÉS

FOREST ECOLOGY AND MANAGEMENT

ELSEVIER SCIENCE BV

Año: 2020 vol. 463

0378-1127

Livestock grazing can be a problem for forest conservation because it can generate heterogeneous and unpredictablechanges in plant communities. Understanding these changes is important for generating managementstrategies that are compatible with long-term conservation of threatened forests. Livestock exclusion is auseful experimental approach used to evaluate grazing effects. However, the evidence showing the effects ofgrazing on forests is mixed and little in know about the responses of different plant life forms, especially in dryforests. In this study, we evaluated the effects of a 7?8 year of livestock exclusion experiment on understory plantcommunity structure in the dry Chaco forest (Argentina). We categorized understory plant life forms as shrubs,succulents (Cactaceae family+Bromelia hieronymi), and herbs (forbs, grasses and vines). Then, we compared theplant community structure (richness, diversity, density and cover) and understory structure (soil hardness, baresoil and vegetation vertical and horizontal structure) between five excluded plots and five grazed plots, in apaired design. We found that livestock exclusion lead to an increase in grass species richness and grass cover aswell as an increase in lower understory biomass (0?0.5 m) and a decrease in percentage of bare soil. On excludedplots, dominant herbs were Setaria nicorae (grass), Trichloris crinita (grass), and Justicia squarrosa (forb). Grassspecies that were recorded exclusively on excluded plots were Gouinia latifolia, T. crinita, and Pappophorummucronulatum, all forage species preferred by livestock. In contrast, on grazed plots, the dominant species wasStenandrium dulce (forb), a species with resistance strategies to grazing. As for the other variables, we did not findstrong differences between excluded and grazed plots. Livestock grazing did not modify the ensemble structureof shrubs and succulents nor did it change the horizontal vegetation structure or soil hardness. Our evidencesuggests that the assemblage composed by shrubs and succulents seems to be tolerant to livestock grazing, andthat the grass assemblage has the ability to quickly recover when grazing stops. Finally, the effectiveness ofexclusion as a management tool will depend on which attribute of the plant community to be conserved orrecovered. In dry Chaco forests after many years of grazing at moderate stocking rates, livestock exclusion couldhelp recover grass cover, generate opportunities for the establishment of certain grass species that are sensitiveto grazing, and increase ground cover.