Nutritional status of Quercus suber populations under contrasting tree dieback

Abstract

Summary ‘Los Alcornocales’ Natural Park (southwest Spain), one of the most important Mediterranean cork oak (Quercus suber) forests in Europe, has experienced a substantial tree dieback in the past decades. We hypothesize that areas experiencing high eutrophication should exhibit higher leaf nitrogen (N) concentrations and lower N resorption efficiency and proficiency indices than areas under lower environmental stress, and this could be related with forest functioning and tree mortality rates. To test this hypothesis, we collected leaf and soil samples from four plots in the southern part of the park, near a polluted industrial area, and four plots in the northern part of the park, far from this pollution source. N and phosphorus (P) in green and senescent leaves and soil NH4 1 -N, NO3 2 -N and PO4 3− -P were analysed. Foliar N and the leaf N:P ratio were significantly higher in the southern plots (high-mortality plots) than in the northern plots. The resorption efficiency was significantly higher for P than for N in all southern and northern plots. The values of resorption proficiency indicated incomplete resorption for N and complete resorption for P in all plots. Our results suggest that oak populations showing high-mortality rates may be associated with tree N and P imbalance.