Evolution of the tree cover (Quercuspyrenaica Willd and Quercus ilex subspecies ballota (Desf. Samp)) in a dehesa over the last 100 years

1. During the first 60 years, the decrease in the numbers of both the holm oak and the oak was due to tree felling. The first 6 years were devoted to tree felling and cut clearing in order to clear dense forest with a view to transforming it into pasture and farmland; during the next 54 years the cut trees were used as a source of heat for the home and of wood for construction and the making of farming tools and diverse utensils. 2. Tree felling has gradually declined over the last 40 years (ít has almost disappeared) as the use of wood has been replaced by gas and gas-oil for fuel and iron and cement for construction. However, the mortality rate has increased. The deaths are mainly the result of the effects of the xylophagous insects (its effects are not usually lethal to trees under 100 yrs. old) ando ther as yet undetermined causes, perhapso f drought. 3. In stands 2 and 3 (N orientation) agriculture benefitted the old oak, which deveioped on the fringes and rocky outcrops and immediately invaded the territory as soon as the cultivation of cereals was stopped. A similar phenomenon occurs int he SW orientation witht he holm oak. 4. Autochthonous livestock (mainly cattle) on an extensive free range basis impedes the recovery of oak density, eliminating almost all oft he youngest ones in the underbrush and shrub phases, whent heir diameter in the first meter of height is less than 9 cm. It does not eliminate the holm oaks, but greatly retards their growth. When young holm oaks are taller than Zm, they can aiso be broken by domestic livestock if they do not reach a diameter of 6 cm in the first meter. 5. The end result is a rapid and dramatic decrease in tree-cover.