A before and after in the upper basin of the Pirón River. Sierra de Guadarrama, Segovia.

 

The Pirón River originates in the Sierra de Guadarrama mountain range at the Fuente del Mojón spring, at the foot of the Malagosto mountain pass, in the province of Segovia, and flows into the Cega River in the Valladolid municipality of Cogeces de Íscar. It passes through the municipalities of Santo Domingo de Pirón and Basardilla, where it is dammed in its mountain section at the Aprisqueras Reservoir. The upper basin that feeds the reservoir is made up of the public utility mountains No. 253 “Aprisqueras”, No. 254 “Majalperro” and the “Arroyo de las Corzas” mountain, which was reforested by the forestry administration.

The Scots pine (Pinus sylvestris) forests, which once dominated these mountain landscapes, are less resilient to grazing, uncontrolled logging, or fire, unlike the Pyrenean oak (Quercus pyrenaica) woodlands, which have a high capacity for stump and root regrowth. From the Celtiberian settlements to modern times, livestock farming, primarily sheep and goats, was the main form of subsistence for the inhabitants of these slopes. This continuous activity over time progressively transformed the landscape, to the detriment of the mid- and high-mountain pine forests.

In the 1964 photo, you can see the result of this ancestral pastoral culture, which is reflected in numerous place names of different sites and infrastructures, such as drovers' roads and livestock trails, shearing stations, ranches, washing places, fulling mills, sheepfolds, and countless constructions and trades mainly linked to sheep farming. The decline of the traditional agricultural system and the massive migration from the countryside to the city led to a drastic decrease in sheep farming. In the photo, you can observe houses that accommodated workers, and we also see cattle farming, dedicated to milk production and maintaining the draft oxen used in the reforestation process. We can also locate the buildings of the forest nursery in the photo, where plants were produced. The reforestation works would last about 20 years.

In the lower left quadrant of the photo, you can see the state of Mount "Mata Pirón," a degraded coppice of Pyrenean oak (Quercus pyrenaica) that has survived to this day due to the value of its firewood and hunting, combined with its ownership type: a mountain of changing ownership that served as a recreational area for the royal houses that have succeeded each other throughout the history of Spain. First belonging to the Community of City and Land of Segovia and then to the Crown Estate, it passed into private hands in the second half of the 19th century.

In the photo of the current state, we can observe how these oak woodlands have been evolving favorably due to a beneficial silviculture, based on thinning, to guide it towards a structural high forest. The reforestation was carried out with the native Scots pine (Pinus sylvestris) and began in the late 1950s. The objective was a hydrological-forestry restoration financed by the State and aimed at providing supplementary employment to workers who still lived from the traditional agricultural system.

The result is assessable from many points of view. The installation and permanence over time of a tree canopy generates a positive change in soil conditions. In addition to buffering the effect of unfavorable surface processes, such as erosion or rapid mineralization of organic matter, the dynamics of their root systems increase the edaphic volume and improve their physical and chemical properties, as a result of the deep incorporation of organic matter, which translates into more diverse and resilient ecological conditions.

The small Aprisqueras reservoir basin, which barely exceeds eleven hundred hectares and encompasses these forest systems, plays an important and positive effect on the hydrological cycle. Currently, the forests of this basin have about 170,000 m³ with bark (w.b.) of wood with a current annual growth of 6,800 m³ w.b./year and approximately 40,000 m³ w.b. have been extracted during the last 20 years from this sustainable, renewable, and biodegradable natural resource with hardly any consumption of external inputs, through improvement and stability cuts, adjusting the density to the season, species, age, structure, and thickness, achieving a greater and more vigorous growth of the trees. In addition, its products and derivatives have an important substitution effect for non-renewable raw materials, with high energy inputs, a high ecological and carbon footprint, and products from highly polluting fossil fuels, helping the decarbonization process.

In this small territory, 234,000 tons of CO₂ equivalent are currently accumulated with an annual growth of 9,350 tons of CO₂, not counting soil organic matter. They maintain a livestock population of about 300 livestock units for six months and have become very diverse and rich in mushrooms, with the genus Boletus and Lactarius, among others, being highly appreciated as edible species. It is part of the SG-50.005 mycological reserve and is a hunting reserve with species such as the Iberian ibex, roe deer, and wild boar. A fishing reserve is also established.

Considering biological diversity, the upper Pirón basin harbors habitats and species of community interest, boasts more than 600 plant species, and iconic animal species such as the Spanish imperial eagle (Aquila adalberti), the golden eagle (Aquila chrysaetos), the Eurasian black vulture (Aegypius monachus), and the Iberian wolf (Canis lupus signatus). These forest systems are multifunctional, as we have seen previously, and provide important ecosystem services: hydrological regulation, carbon sequestration, obtaining renewable natural products, landscape, culture, leisure, recreation, and hosting plant and animal biodiversity. And they continue to provide them, which has been accredited by their inclusion in the Sierra Norte de Guadarrama Natural Park and the Sierra de Guadarrama National Park.

 

Old images from the State Forest Heritage, deposited in the Segovia Provincial Historical Archive and in the Segovia territorial archive of the Junta de Castilla y León.

The following people collaborated in the realization of this article:

• Francisco Javier Plaza Martín. Forestry Engineer of the Territorial Environmental Service. Segovia. Junta de Castilla y León.
• Mario Lozano Enguita. Forestry Engineer of the Territorial Environmental Service. Segovia. Junta de Castilla y León.
• Celso J. Coco Megía. Forestry Engineer in the Red Estatal de Montes Públicos (State Network of Public Forests), REMP.

 

(versión en español)