THE IMPORTANCE OF DEEP SOIL LAYERS TO SUPPLY WATER TO AGRO-FORESTRY SYSTEMS: A CASE STUDY OF A MATURE CHESTNUT ORCHARD IN NORTHERN PORTUGAL

Traditional soil management practices in chestnut orchards of Northern Portugal involve several annual surface tillages, being cited as main reason the weed control for water conservation. Over the last ten years, several studies have been carried out related to soil moisture, leaf water potential, soil quality indicators, orchard productivity and health, and different soil management practices. The obtained results questioned the utility of surface tillage and led to a recent study, developed during the dry seasons of 2003 and 2004, where the role of deep soil layers in providing a water supply to the trees was evaluated. Soil moisture values at different depths down to -100 cm were compared with water content obtained at 1.5 MPa pressure (wilting point), on soil samples of the same layers. To assess the water uptake by trees, leaf water potential (w) was measured during the same period. The following observations must be emphasized: (i) at mid July the soil moisture at -30 cm soil depth is lower than the water content at 1.5 MPa, whereas at -75 cm soil depth the values maintain higher than that reference value during all the summer period, emphasizing the importance of deep layers on the water supply in agro-forestry systems, especially in those with a Mediterranean type climate; (ii) no significant differences were found in predawn leaf water potential values among the tested treatments and those were mostly near -0.6 MPa, showing favourable vegetation conditions, meaning the existence of enough soil water independently of the management practice.