MECHANICAL RESISTANCE BY AN ECTORGANIC SOIL LAGER ON ROOT DEVELOPMENTOF SEEDLING PINUS-SYLVESTRIS

We investigated early root development of Pinus sylvestris seedlings i n relation to bulk density and natural particle layering in an ectorga nic soil layer from a bracken (Pteridium aquilinum) stand. Responses i n root development to two levels of bulk density (0.07 and 0.15 g/cm(3 )) in mixed bracken substrate were compared with effects in peat of si milar bulk densities, and in sand of three different bulk densities (0 .37, 0.52, and 0.67 g/cm(3)). The effect on root growth of the natural horizontal layering of the organic particles was examined by comparin g intact with mixed ectorganic bracken soil profiles of similar bulk d ensities (resp. 0.09 and 0.07 g/cm(3)). Root length growth was signifi cantly reduced in the organic and sandy substrates of high bulk densit y. Root diameter was not affected by bulk density in the organic subst rate, but increased with higher bulk density in sand. Preservation of horizontal layering in the intact ectorganic profile significantly red uced root length compared with mixed substrate of similar bulk density . Roots growing in high bulk density, and intact, organic substrate sh owed increased twisting, which resulted in a smaller depth reached by the root relative to total root length produced. In sand, root twistin g did not change with increased bulk density. It is suggested that roo ts growing through organic substrate follow a path of least resistance . This implies that organic particle size and orientation are more imp ortant in determining root development than bulk density. This study p oints out that the natural layering of organic particles presents anot her constraint on the establishment of plant species in sites with a w ell-developed ectorganic soil layer. Disturbance of this layer may the refore enhance establishment of seedlings by reducing the mechanical r esistance of the ectorganic soil profile to developing seedling roots.