El establecimiento espontáneo de árboles en la Pampa Un enfoque experimental
Facelli, J. M.; León, R. J. C.
The absence of trees in the natural pampean vegetation has raised a prolonged controversy. Many authors have discussed the theme arguing different hypotheses. However, none of them has been tested experimentally. The soundest hypothesis supports the idea that woody vegetation is prevented because of grass competition for water and soil compactness. This paper reports experimental results on the dynamics of a spontaneously established elm (Ulmus pumila L.) population in a pampean successional grassland, in order to assess the validity of this hypothesis. The main results are: a) The establishment rate decreased during the successional time, probably due to grass cover increase and to soil compactness. b) A higher number of seedlings gets established on mechanically removed soil than in the community dominated by grasses. Intermediate values were recorded in areas where vegetation was removed by mowing without disturbing the soil. Seedling mortality was high in all treatments. c) The remotion (by hand mowing) of herbaceous vegetation enhanced the growth of young elms in late spring and early summer. It is concluded that grasses interfere intensively with establishment processes of tree populations. Soil compactness and competition for water may prevent successful seedling establishment in mature grassland communities. Only when grasslands are heavily disturbed (e.g. by human activities) exotic tree invasion becomes possible in some pampean grasslands.