Cattle raising in abandoned crop-field patches within a forest matrix: ecological consequences of land-use change

ASTRADA, ELIZABETH; RESCIA, ALEJANDRO; QUINTANA, RUBEN DARIO

Rosario

Congreso; IX International Rangeland Congress; 2011

Organizing Committee of the IX IRC 2011

Introduction. In the Argentinean Chaco region, the abandonment of agriculture is usually followed by cattle roaming freely between former crop fields and the diverse surrounding native ecosystems. This work analyses the effect of different dispersal agents on the establishment of woody vegetation in new grazing lands. The objective is to identify the main agent that contributes to the spreading of woody species that are most effective in colonization. Particular emphasis has been placed on the role of cattle as a dispersal agent for woody legumes from adjacent ecosystems.   Materials and Methods.Thestudy was carried out at 9 grazing sites, each of which consisted in an abandoned crop plot and neighbouring forests ecosystems. The richness and basal area of woody species were recorded in 244 sampling units (5 x 5 m). A cluster analysis based on the basal area was performed for the forest samples to identify the communities neighbouring the abandoned crop plots. Woody species were grouped according to their dispersal type as anemochorous, barochorous, epizoochorous and endozoochorous (through birds and cattle). Afterdata angular transformation, one and two–way ANOVAs was used to compare the proportion of richness and basal area of each dispersal type among and within the four considered communities.   Relative abundance of woody species in new grazing areas (abandoned crop plots) and adjacent forest. Results. Thirty-two woody species were identified in samples from the three forest communities (mixed forest with Schinopsis balansae and S. quebracho-colorado, Prosopis alba and P. ruscifolia forest -“Algarrobal-vinalar”- and P. ruscifolia forest -“Vinalar”-) and the new grazing sites (abandoned crop plots). From the 32 species, 30 occurred in the adjacent forests (27, 21 and 18 species, respectively, for the above-mentioned communities) and only 16 in the abandoned crop plots. The two-way ANOVA showed significant differences in the relative richness and abundance of the species with different dispersal types (p < 0.05), for all the considered communities. The endozoochoricspeciesdispersed by cattle had the highest abundance. The one-way ANOVAs showed no significant differences (p > 0.05) between the new grazing areas and Algarrobal-vinalar and Vinalar forests for any of the analysed dispersal types (see Figure). The cattle may cause the acceleration of lignification and transformation of grazing patches into Algarrobal-vinalar and Vinalar forests due to both seed transport and the creation of favourable microenvironmental conditions for seed germination. The role of cattle is reinforced by the failure of anemochorous species to settle in the abandoned crop plots, even when they were surrounded by a community, like the mixed forest, in which such species are important. In fact, their abundance was lower in the abandoned crop plots than in these forests. The birds and other mechanisms, which are rare among woody species, were unlikely to have any effect on the colonisation of the abandoned crop plots.   Conclusions.The results evidence that cattle is particularly efficient in contributing to the colonisation of abandoned crop plots, having a strong effect on the transformation of grazing areas into environments with lignified plant species. On this basis, appropriate livestock management aiming at maintaining a mosaicofdifferentland-use patches should consider rotation systems that include periods without livestock grazing in environments dominated by herbaceous species.