Quercus ilex forests are influenced by annual variations in water table, soil water deficit and fine root loss caused by Phytophthora cinnamomi

Itishypothesised thatmajor reductions intree vitality are related tomarked changes insoilwater content, extremely wet winters followed by dry summers, and the presence of pathogenic organisms which take advantage of this situation. This study helps clarify the role of annual variations in water table, soil water content and fine root abundance in the decline of Quercus ilex forests, with special focus on trees growing in Phytophthora cinnamomi-infested soils. Conducted in western Spain, the study included 5 Q. ilex dehesa stands in which tree water status and soil water dynamic were compared in declining and non-declining trees, and 96 additional stands in which fine root abundance and pathogen assessment were compared in declining and non-declining trees. Declining trees showed significantly lower values than non-declining trees for leaf water potential and stomatal conductance. The period of waterlogging (2 months maximum, fluctuating from −0.5 to −4.5m), the soil water content values observed in summer (significantly higher in declining trees) and the similar presence of Pythium spiculum in declining and non-declining trees are not sufficient in themselves to explain tree health status. However, fine root density was 16.2% lower in declining than non-declining trees and 42% lower in P. cinnamomi-infected than non-infected trees. Root damage caused by P. cinnamomi in combination with periods of saturated soils favourable for the pathogen but unfavourable for the tree, coupled with small-scale differences in soil water-holding capacity, explain the symptoms and water status of declining trees. The combination of root damage and water stress explained above-ground symptoms of declining trees and will probably determine tree survival.