Trees contribute to a number of important ecosystem services through their influence on soil properties and functions, including water purification and regulation. Forest soils tend to have a higher saturated hydraulic conductivity (Ks) than soils under other vegetation, so the establishment of streamside tree buffer zones and strategically planted woodland is increasingly being promoted to reduce or intercept surface runoff from agricultural and urban environments that may contribute to diffuse pollution and flooding. Despite this, our understanding of how trees affect soil hydraulic properties is still extremely limited. The main aims of the work outlined in this thesis were to compare the effect of a broadleaf (sycamore) and a conifer (Scots pine) tree on Ks to determine if this differs between species type, to examine the effect of land use on Ks under forest cover and to investigate the key underlying mechanisms involved. Results showed significantly higher Ks under undisturbed mature Scots pine forest compared with sycamore forest, indicating a species effect. Forest soils also had significantly higher Ks than pasture grazed by sheep; however, when sheep were grazed under forest cover, there was no significant difference between forest and pasture, suggesting that the effect of land use masked the effect of trees on Ks. An investigation of soil flow pathways and pore characteristics showed that significantly higher Ks corresponded with significantly higher soil macroporosity and macropore flow. A mesocosm experiment, undertaken to investigate the influence of tree roots and associated soil macrofauna on Ks, was found to be of insufficient duration for significant changes to occur; however non-significant higher Ks observed under Scots pine compared with sycamore were consistent with the study of more mature trees. The results of a glasshouse experiment investigating the influence of leaf litter on Ks showed that leaf litter had a negative effect and litter type had more of an effect than litter quantity.