Assessing oak replacement in dehesa system through in-plot analysis


Abstract: Dehesa is the most extended agrosilvopastoral system in south Spain and Holm oak the main tree species. Continuous grazing combined with other practices such as cultivation and burning, is associated with failures in oak recruitment and hence, in the long term, with loss of tree canopy cover. The aim of this paper is to analyze the evolution of holm oak density in dehesas in the last decades through in-plot dynamic analysis. Using aerial photographs from 1977 and 2011, we have compared tree canopy cover and density in 200 plots of 1 ha within 16 dehesa-farms of Andalucía (South of Spain). Overall, tree density has reached similar values for both dates, 32 trees/ha, but we found significant differences between regions and farms. In areas with a denser tree canopy cover in 1977, holm oak density has decreased significantly whilst it increased in those with previous low tree canopy. On the other hand, tree canopy cover has experienced an increase in this period, from 29% to 34%. This increase was also observable at regional and farm levels. The widespread increase in coverage has been mainly due to the growth of existing trees and, secondly, to the incorporation of new trees. The annual rate of tree loss is 0.23 trees/ha. These results indicate that tree density of Andalusian dehesas has remained constant over the last 40 years through a tradeoff driven by farmers between tree loss and recruitment.

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