IFEVA   02662
INSTITUTO DE INVESTIGACIONES FISIOLOGICAS Y ECOLOGICAS VINCULADAS A LA AGRICULTURA
Unidad Ejecutora - UE
congresos y reuniones científicas
Título:
Effects of grazing on temperate grasslands: opal phytoliths evidence and relevance for palaeoclimatic reconstructions
Autor/es:
DEL PUERTO, LAURA; PIÑEIRO, GERVASIO; INDA, HUGO; GARCÍA RODRÍGUEZ, F
Lugar:
Mar del Plata
Reunión:
Congreso; 7th International Meeting on Phytolith Research y el 4º Encuentro de Investigaciones Fitolíticas del Cono Sur; 2008
Institución organizadora:
Asociación Paleontológica Argentina y Asociación Argentina de Sedimentología
Resumen:
About 87% of the Uruguayan territory is occupied by natural grasslands, which represent the basis of the national agro-economic development. These grass-dominated ecosystems form – with the Pampas in Argentina and the Campos in southern Brazil – the Rio de la Plata grasslands, one of the largest natural temperate and subhumid grasslands of the world. The present structure and function of these grasslands derive from the interaction between regional climate, soil properties and the disturbance regime (grazing, fire, exotics species invasions), that was drastically altered by European’s settlements 500 years ago.  Previous work in paired grazed and ungrazed fenced plots (where livestock was excluded for periods of 4 to 30 years), showed that grazing removal induces changes in community structure, ecosystem functioning and productivity of Uruguayan grasslands. Among other effects, grazing promoted increases in plant diversity and the replacement of cool-season tussock grasses by warm-season prostrate grasses. Using a similar approach, we sampled soils up to one meter depth in five pairs of grazed and ungrazed plots in Uruguay and Argentina. At these sites livestock was excluded from the ungrazed plots for the last 10 to 30 years. Our objectives were: 1) to test the sensibility of phytolith analyses to evaluate changes in plant species composition occurred after grazing removal in decadal timescales, 2) to describe potential changes in plant species assemblages occurred after livestock introduction 5 centuries ago and 3) to obtain a pre-disturbance grassland record that is climatically adjustable. At all sites, C3 grass phytoliths contents were graeter in the topsoil (first 5 cm) of ungrazed plots compared to the grazed plots, mirroring observed increases in C3 grasses at ungrazed plots. Below 5 cm depth, phytolith assemblages were similar between each grazed and ungrazed paired plots, thus revealing a homogeneous pre-exclosure species composition. Soil samples from 5 to 30 cm depth revealed that C3 grasses decreased and C4 grasses increased during the last period of time (C3 grass phytoliths increased from 5 to 30 cm depth), while soil samples from 30 to 100 cm depth show a stable species composition across time at each site. A 14C analysis of a soil sample obtain from 30 to 50 cm was dated in ~2500 yr. These results suggest that species composition where relatively constant during soil formation and that livestock introduction 500 years ago could have decreased C3 grasses (and increased C4 grasses). In agreement, grazing showed the same effect on community composition in short-term periods (5 to 30 years) at grazed – ungrazed plots. Finally, phytolith assemblages from the topsoil profiles (0-5 cm) of ungrazed sites were related with meteorological and geographical variables. A linear correlation between C3/C4 phytoliths and temperature was established, and implications for paleoenvironmental reconstruction are discussed.