Influence of individual oak (Quercus robur) trees on saturated hydraulic conductivity

The influence of single trees on saturated hydraulic conductivity (Ks)was investigated for six isolated oak trees (Quercus robur) growing on a Dystric Gleysol in an area of parkland in northwest England. The Ks was measured within the A soil horizon over a 0.10–0.25 m depth using a borehole permeameter. A dataset of 119 Ks values was obtained and comprises of 55 values from around 1 oak tree at distances of 1–13 m from the trunk, 45 tests around 5 other oak trees, and 19 tests in open grassland. For the intensively sampled tree, Wilcoxon rank sum tests showed a significant difference (p < 0.05) between the median Ks at 3, 5, 7 and 11 mfrom the trunk and that in the surrounding grassland. At 3 m from this tree, the median and geometric mean Ks were a factor of 2.3 and 3.4, respectively, larger than those of the open grassland. Further, the geometric mean Ks decreased at a rate of -4 x 10-7 m s-1 m-1 from 1 to 9 m from the trunk, though it increased at 11 m, before declining again. A similar pattern in geometric mean Ks was observed in the 45 values around the five other oak trees. A literature review of the potential positive and negative effects of trees on Ks was used to provide tentative explanations for the observed patterns and to highlight the new data needed to support more robust interpretations