Elena Cubera

Effect of single Quercus ilex trees upon spatial and seasonal changes in soil water content in dehesas of central western Spain

The spatial and temporal evolution of soil water content (θ) in Quercus ilex dehesas has been investigated to determine how trees modify the soil water dynamics and the nature of tree-grass interactions in terms of soil water use in these ecosystems. Soil physical parameters and θ were measured at different distances from the tree trunk (2–30 m) in the upper 300 cm of soil. θ was measured monthly by TDR during 2002–2005. Tree water potential was determined during the summers of 2004 and 2005. At deeper soil layers, mean θ values were higher beyond than beneath tree canopy during dry periods. θ depletion beyond tree canopy continued even in summer, when herbaceous plants dried up, suggesting that trees uptake water from the whole inter-tree space. Results have shown a high dependence of trees on deep water reserves throughout late spring and summer, which helps to avoid competition for water with herbaceous vegetation.RésuméL’objectif de ce travail a été de déterminer les effets de chênes verts (Quercus ilex L.) isolés sur la teneur en eau du sol (θ) et la nature des interactions arbre-strate herbacée sous climat semi-aride, en terme d’utilisation de l’eau du sol dans ces écosystèmes. Les paramètres physiques du sol et θ ont été mesurés jusqu’ à 300 cm de profondeur et à différentes distances (2 à 30 m) autour des arbres. θ a été mesurée par TDR, mensuellement de 2002 à 2005 dans quatre dehesas. Le potentiel hydrique des arbres a été mesuré durant les étés 2004 et 2005. Essentiellement en profondeur et en été, les valeurs moyennes de θ furent plus élevées au-delà de la canopée que sous les arbres. La diminution de θ au-delà de la canopée des arbres a continué à diminuer encore en été lorsque les plantes herbacées étaient sèches, suggérant un prélèvement d’eau par les arbres. Nos résultats suggèrent alors que les arbres peuvent utiliser de l’eau localisée loin deux même à des distances de 20 m. et qu’ils sont très dépendants des réserves d’eau en profondeur (100–300 cm) pendant la fin du printemps et en été, ce qui contribue à diminuer la concurrence pour l’eau entre arbres et strate herbacée.

Root system of Quercus suber L. seedlings in response to herbaceous competition and different watering and fertilisation regimes

Different management practices are commonly applied to increase pasture yield of Mediterranean open woodlands, but the consequences of increasing competition for soil resources with these practices on tree recruitment are still unknown. In a greenhouse study, belowground competition of Quercus suber L. seedlings growing together with natural (OakNP) or improved pasture (OakIP) was evaluated, and their root systems compared with those of seedlings growing in bare soil (OakBS). Two watering levels and two regimes of P2O5 fertilisation were also tested. Because of competition, the OakIP seedlings had their fine root mass density, coarse root length, and shoot mass reduced by up to 40, 36, and 39%, respectively, when compared to OakNP seedlings. OakNP and OakBS seedlings showed similar average root density parameters and shoot mass values, indicating that Q. suber seedlings growing with natural pasture is a low competitive system. High availability of water and phosphorus did not mitigate the strength of competition between herbaceous plants and oak seedlings, and favoured the pasture to the detriment of the trees. Our findings suggest that P2O5 fertilisation and irrigation practices performed to improve herbaceous productivity will negatively influence recruitment of Q. suber seedlings.