Soil rock fragment is stronger driver of spatio-temporal soil water dynamics and efficiency of water use than cultural management in holm oak plantations

Water is the main component in the conservation and functioning of Mediterranean ecosystems. There is a trade-off between plant water use during establishment and soil water conservation. Rock fragments and cultural management are two key factors affecting soil water dynamics on forest establishment. In this work, we studied the interactions of different cultural management techniques (tillage, herbicide and mulching) with soil rock fragment content (soil gravel) and their effects on the spatio-temporal water dynamics and efficiency of water use in Mediterranean forest plantations. Soil water content (SWC, l/m³), soil water depletion rate (k) and water use efficiency (WUE, μmol CO₂/mol H₂O) were monitored along a two-year period in a holm oak reforestation. The results show that SWC and its spatio-temporal dynamics were defined by a significant interaction between the cultural management techniques and the rock fragment content. Cultural management techniques had weak effects on SWC when compared to the control treatment, although significant differences were found. The soil rock fragment content accounted for most of the variability in SWC; and soils with a content below 5% had higher water contents. The cultural treatments showed some influence on water behaviour under SWC more limited conditions, that is, when rock fragment contents was higher than 15% and/or in summer periods. Finally, SWC was also a limiting factor for the WUE of holm oak seedlings, being the WUE values higher when the soil water supply was more restricted. The maximum WUE was reached at a rock fragment content in the soil profile of 17%, combined with mulch treatment, during dry summer conditions. Thus, the interaction between the rock fragment content and cultural management is a key element in trade-offs between water conservation and plant water-use objectives in Mediterranean forest restoration strategies.