The effects of whole tree harvesting on site hydrology and soil structure at Beddgelert Forest, N. Wales, UK
The ITE biogeochemistry group monitoring solute movement at Beddgelert Forest provided an opportunity to study the hydrology of a steep section of hillslope in a high rainfall environment. The aim of the experiment was to characterise and compare the hillslope hydrologies of one forested and one whole tree harvested site. Particular attention was paid to the influence of trees both directly on soil water pathways and indirectly on soil characteristics. Atmospheric inputs were monitored for the slope and individual plots for one year. In spite of high rainfall volumes, the slope was not waterlogged indicating a soil with high conductivity. However, frequent macropore flow was not observed at the site. Tensiometer results showed that the mineral soil remained unsaturated. Therefore, a type of preferential flow dominates at both sites. A one dimensional modelling approach to soil water movement confirmed that mesopores within the soil could conduct a large volume of water rapidly. Modelling demonstrated the importance of soil structure especially a large pore size distribution. Analyses of active soil water pathways based on tensiometer results were inconclusive. Downslope moisture gradient combined with high conductivity suggested that large quantities of water could be transmitted. Similarly, the well structured surface soil and marked horizon development also indicate lateral flow may be dominant. The study showed that saturated hydraulic conductivity was highly variable at both the forest and whole tree harvested sites. Analysis of semi-variograms indicated that most of the variance occurred at a sampling distance of 50 cm (i.e. individual tree roots and slate fragments were causing variations in Kg). Investigations of soil structure found more vertical cracks in the forest soil compared to the whole tree harvested site. At the more detailed ped scale, fractal dimensions of both sites were similar. Based on these results combined with temporal moisture content data the research has demonstrated that first, the hydrologicai regime at both sites were similar. Second, the direct impact of trees was limited. Third, the large cracks at the forest site were not significant. The major result of this research was that at both sites vertical flow in the Ah/Ea horizon dominates and a significant amount of water moved laterally within the Eag, Bs and C horizons. This result has major implications for the solute chemistry and movement of acid deposition in that soil water born solutes will tend to enter water courses more rapidly than if vertical flow predominated.