Size structure and regeneration of Spanish holm oak Quercus ilex forests and dehesas: effects of agroforestry use on their long-term sustainability

Abstract

Most evergreen oak forests growing in the flat areas of the southwestern Iberian Peninsula have been gradually transformed into a unique kind of pastoral woodland, the Spanish dehesas and Portuguese montados, by means of an agroforestry use. The opening of dehesas by clearing and ploughing closed forest to obtain savannah-like landscapes with a density of some 40 trees per ha started in the early Middle Ages, although most current dehesas were created during the last half of the 19th century and the beginning of the 20th century. Dehesas cover at present an estimated 3.1 million ha and support a high biological diversity. In this paper we compare the current age structure of holm oak Quercus ilex L. dehesas and forests in order to ascertain whether the agroforestry use is hampering the natural regeneration of tree populations and hence the long-term sustainability of dehesas. We characterise the size structure of both mature trees and minor stems growing in forests and dehesas in two intensively studied plots, and then we develop indicators for population structure at wider spatial scale (10 dehesa and 10 forest replicates). Size structure could be reliably used as an indicator of age structure as neither site quality nor intraspecific competition could account for the variation in tree size observed neither within nor among the studied holm oak populations. The age structure of mature trees in forests closely fitted a negative exponential distribution, thus indicating a continuous input of individuals to the youngest age class (67.4% of mature trees on average for the 10 replicates). Contrary to this, in dehesa replicates the age distribution of mature trees was symmetrical and leptokurtic due to the disproportionate importance (78.0%) of central age classes (30–40 cm in diameter at breast height (DBH)). Apart from showing that the regeneration failure of dehesas is due to their agroforestry use, we suggest that this problem is inherent to their system of exploitation, i.e. it operates ever since each dehesa farm starts to be formed out from forests, although it has been exacerbated by the recent intensification of the agroforestry use of dehesas.