Drought and Forest Decline in the Iberian Peninsula: A Simple Explanation for a Complex Phenomenom?

Abstract

Different episodes of forest decline have been reported in several areas of the Northern Hemisphere during the 20th century. The explanation of this process included anthropogenic (air pollution), biotic (pathogens) and climatic factors. Among the climatic factors, the effects of acute or chronic droughts have been the most common explanation for the massive dieback observed. In the Iberian Peninsula, besides the pathogenic explanation which gives a paramount relevance to the fungus Phytophtora cinnamomi in many situations, the role of an increment in aridity (chronic) or the consequences of severe droughts (acute) have been empirically supported. The evident synchronism between forest decline and abnormally adverse climatic conditions, in addition to the reversibility of the process when precipitation intensity increased, are two major arguments for supporting the climatic involvement in the phenomenon. Nevertheless, the two most affected species in the episodes of oak decline in the Iberian Peninsula were Quercus ilex and Q. suber, which are representatives of the so called Mediterranean woody flora and which have been considered as more drought resistant than the temperate oak species that co-occur in this territory. The climatic complexity ofthe Iberian Peninsula, the many mechanisms for coping with water stress that have been described within the sclerophyllous Mediterranean flora and the effects of human management partially explain this paradox. Other forest species have also been affected by massive decline. Pinus sylvestris and Abies alba, which have their southern distribution limit in the mountain ranges of the Iberian Peninsula, are two examples of this situation. Both cases, besides the inmediate effects of some climatic perturbations, need the incorporation of different predisposing factors – mainly historical aspects to obtain a complete image of the process.