Abstract

A typical upland soil catena afforested with Sitka spruce (Picea sitchensis (Bong.) Carr.) was chosen to examine the localised effect of trees on soil permeability. A borehole permeameter was used to measure soil permeability at 0.2 m and approximately 2 m distance from the stem of 20 trees at a fixed measurement depth of 0.25 to 0.45 m. In the case of the 'near-tree' measurements, this corresponded to soil beneath the main root plate of each conifer. Two principal elements of the soil catena: the ferric podzol of the mid-slope and histosol soil of the foot-slope were investigated.

The preliminary data set shows that within the ferric podzol element, the permeability of the soil beneath individual conifers was a factor of 5.4 less than that of the adjacent soil. In contrast, within the histosol sub-tree permeabilities could not be distinguished from those of soil 2 m away from each tree. The decrease in sub-tree permeability within the podzol may be caused by sensitivity of the Bs1 horizon to consolidation by tree weight or by enhanced illuviation resulting from changes in local soil chemistry. The histosol may be less sensitive to such processes. The results of a consolidation test applied to the Rawls and Brakensiek (1989) model of soil permeability supported the possible role of consolidation in the reduction of soil permeability beneath conifers in podzolic soil. Additional data on soil bulk density, porosity and texture are required to corroborate either the consolidation or illuviation hypotheses.

As the Bs1 horizon of ferric podzol soil is typically slowly permeable, a further decline may (i) restrict root development and thus increase windthrow hazard, and (ii) increase the lateral flow of water within podzolic Eag horizons and thus affect stream acidification. Deep ploughing of a site prior to afforestation may mitigate such impacts.