Influence of forest stand age on soil water repellency and hydraulic conductivity in the Mediterranean environment

The hydrological response of forest soil in the Mediterranean environment is characterised by high runoff and erosion rates, mainly due to low infiltration and high repellency of soils. However, little literature exists about the effects of forest ages on soil water repellency (SWR) and hydraulic conductivity (SHC). This study evaluates these hydrological parameters in five Pinus nigra Arn ssp. Salzmannii stands of different ages in Central-Eastern Spain; one of these stands, unmanaged, was chosen as reference system. SWR (measured in terms of water drop penetration time, WDPT) and SHC as well as the main physico-chemical properties and surface characteristics of soils were surveyed in forty-five plots. Water infiltration was higher in the older stands (including the older and unmanaged forest) and lower (by over 60%) in the more recent pine forests. Four of the studied stands did not show water repellency; only the more recent plantation showed a slight SWR. The differences in SHC among the forest ages were mainly driven by the organic matter (OM) and nutrient contents of the soils as well as by the bulk density and quantity of dead wood. SWR was similar among the plots (despite significantly differences in WDPTs), although having variable OM contents. Considering these differences in soil properties, SHC and SWR were simply predicted for each forest stand using on dbRDA models and the following soil properties: (i) OM and total nitrogen contents of soil (for SHC and SWR); (ii) dead wood and bulk density (for SHC); and (iii) clay content and the percentage of bare soil (for SWR). Overall, this study has showed that, when a new forest stand is planted, decreases in water infiltration, with subsequent increases in runoff generation capacity) of the soils, can be expected. Conversely, no water repellency is likely to affect new pine plantations.