In Mediterranean ecosystems, special attention needs to be paid to forest–water relationships due to water
scarcity. In this context, Adaptive Forest Management (AFM) has the objective to establish how forest resources have to
be managed with regards to the efficient use of water, which needs maintaining healthy soil properties even after
disturbance. The main objective of this investigation was to understand the effect of one of the AFM methods, namely
forest thinning, on soil hydraulic properties. At this aim, soil hydraulic characterization was performed on two
contiguous Mediterranean oak forest plots, one of them thinned to reduce the forest density from 861 to 414 tree per ha.
Three years after the intervention, thinning had not affected soil water permeability of the studied plots. Both ponding
and tension infiltration runs yielded not significantly different saturated, Ks, and unsaturated, K–20, hydraulic conductivity
values at the thinned and control plots. Therefore, thinning had no an adverse effect on vertical water fluxes at the soil
surface. Mean Ks values estimated with the ponded ring infiltrometer were two orders of magnitude higher than K–20
values estimated with the minidisk infiltrometer, revealing probably soil structure with macropores and fractures . The
input of hydrophobic organic matter, as a consequence of the addition of plant residues after the thinning treatment,
resulted in slight differences in terms of both water drop penetration time, WDPT, and the index of water repellency, R,
between thinned and control plots. Soil water repellency only affected unsaturated soil hydraulic conductivity
measurements. Moreover, K–20 values showed a negative correlation with both WDPT and R, whereas Ks values did not,
revealing that the soil hydrophobic behavior has no impact on saturated hydraulic conductivity.
The primary purpose of this work was to assess the persistence of water repellency in the surface horizon of coarse-textured soils under natural Quercus robur ecosystems, and Pinus pinaster and Eucalyptus globulus plantations, in the northwest of the Iberian Peninsula. Water repellency was determined by applying the water drop penetration test (WDPT) to soil samples collected from variable depths (0–40 cm). Measurements were made on field-moist samples obtained at the end of the dry period and on samples dried at 25ºC in the air. All soils exhibited very high (severe to extreme) water repellency in the topmost soil layer (0–5 cm) but no significant differences among the three plant species studied. Extreme persistence was observed down to 20 cm in the soils under eucalyptus and down to 10 cm in those under pine. The soils under oak were those exhibiting the highest variability in water repellency and the greatest decrease in it with increasing depth (especially in relation to soils under eucalyptus).
Water repellency exhibited significant positive correlation with the C content and C/N ratio of the soils. Soil water repellency was similar in the air-dried samples and field-moist samples.