How dependent are cork oak (Quercus suber L.) woodlands on groundwater? A case study in southwestern Portugal

Abstract

Cork oak (Quercus suber L.) woodlands in the Mediterranean basin may cope with drought continuing to grow during the dry summer period. This growth pattern relies on the presence of extensive and deep root systems and on their access to groundwater. When the water table level is not within the reach of roots or decreases below a certain level following periods of drought, tree growth and survival might be affected. So far, little is known about the sensitivity and resilience of cork oak to both the rising and lowering of the water table over a long-term period, e.g., during one cork production cycle (9–10years). In this study, we determined whether changes in groundwater levels influence cork growth in two cork oak woodlands located at the Tagus alluvium aquifer in southwestern Portugal. The selected areas have contrasting groundwater levels and an integrated framework was developed combining: (a) a standardized annual cork growth index series for tree (cork) growth assessment; (b) a monthly Standardized Precipitation Evapotranspiration Index (SPEI) for drought periods assessment and; (c) a Geographical Information System (GIS)-based model for mapping the mean water table depth in the dry period. Results showed a decrease of cork growth index (up to 40%) immediately following a severe drought, which lowered the water table in the dry period. Moreover, in the site with deeper groundwater levels the reduction of cork growth index was higher. The mean water table depth (in dry period) of −2.5m was found to be a trigger value for cork growth. Our findings suggest that, under shallow water table conditions groundwater mitigates the seasonal fluctuations of precipitation and trees may use groundwater to decouple cork growth from the annual precipitation regime. We conclude that, under changing Mediterranean environments, with expected increase in drought events, the decrease of groundwater levels may severely affect cork (and tree) growth and ultimately tree survival. The management and conservation of cork oak woodlands should therefore rely on adequate policies for the sustainability of groundwater resources.