Direct and indirect effects constrain shrub encroachment in semi-arid Patagonian steppes

CIPRIOTTI, P.A.; AGUIAR, M.R.

APPLIED VEGETATION SCIENCE

WILEY-BLACKWELL PUBLISHING, INC

Lugar: Londres; Año: 2011 vol. 15 p. 35 - 47

1402-2001

Question: What are the long-term effects of grazing exclusion on the population structureand dynamics of, and interactions among, three dominant shrub species?Location: Grass?shrub Patagonian steppe, Chubut, Argentina.Methods: Permanent plots were established in grazed paddocks and paddocks excludedfrom grazing in representative Patagonian rangelands. Shrub abundance, population sizestructure, short- (two, 3-year periods) and long-term (matrix models) population dynamics,and neighborhood interactions of three native and co-dominant shrub species (Mulinumspinosum, Senecio filaginoides, and Adesmia volckmanni) were measured and analyzedusing different statistical approaches.Results: Total density of shrubs was 74% higher in paddocks excluded from grazing, duemainly to increases in Mulinum (80%) and Senecio (68%) species. However, differences insize structure between ungrazed and grazed paddocks were only detected in Mulinum.Demographic rates differed between shrub species, time periods and grazing conditions. Inparticular, recruitment in the short term (especially in wet years) and population growth ratein the long term (λ) were higher in paddocks excluded from grazing only in Mulinumpopulations. Senecio populations showed a marginal increase in recruitment and mortalityindependent of the grazing condition in the wet and dry period. Grazing exclusion modifiedthe balance of neighborhood interactions among the three shrub species. In grazingexclusion paddocks, there was a balance between positive and negative inter?specificinteractions, while in grazed paddocks there were more negative intra? and inter? specificinteractions, resulting in a net negative balance of neighborhood interactions.Conclusions: Our understanding of woody encroachment in arid rangelands can beinformed through evaluation of direct and indirect effects of grazing exclusion on theabundance and demography of dominant woody species. In Patagonian arid steppes, theoccurrence of woody encroachment in rangelands excluded from grazing can be explainedby altered responses in plant?animal and plant?plant interactions among shrub species.