The Functioning, Management and Persistence of Dehesas

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Abstract

Dehesas are the most widespread agroforestry systems in Europe, where they cover 3.1 million hectares. They are multipurpose open woodlands, mostly created by clearing the natural forests, where livestock rearing, cereal cropping, cork and firewood harvesting, and hunting are combined. In dehesas, trees can be seen as “ecosystem engineers”, as they allow the maintenance of grass production in poor soils under a semiarid climate. We summarize the most outstanding results on both the effect of trees on the production and quality of the understorey (crop and native grasses) and also on the consequences of reduced tree density for the physiological condition and production of trees. The ecological basis of tree-understorey interactions is explained based on spatial distribution and use of above and belowground resources. Dehesas have been considered habitats to be preserved because they maintain a high biological diversity including several globally endangered animal species. They are considered an example of sustainable land use, although their conservation has been threatened in the last few decades. Excessive tree cutting, including complete elimination in some cases, has taken place as a consequence of increased mechanisation and stocking rates. This has caused a lack of natural regeneration and tree death in over-aged stands. We make a critical analysis of the ecological stability and sustainability of the system following four different approaches related to current problems: (i) historical evolution of the dehesa range, (ii) soil degradation and erosion, (iii) plot and farm-level factors precluding tree regeneration, and (iv) economic profitability of the dehesas. From these analyses, we derive a number of recommendations for dehesa management aimed at ensuring both its multifunctional role and its sustainability. The critical role of the shrub understorey for the ecological function, nutritional contribution and biodiversity is emphasized.