Scars on 300-year-old pines from resin extraction. Dendrochronology shows tapping slowed growth by 40% but trees survived centuries of harvesting, a sustainable practice abandoned in the 1980s.
The bleeding trees of Guadarrama
In the pine forests of Madrid's Sierra de Guadarrama, many older trees bear long, vertical scars. These are not signs of disease or damage, but relics of a centuries-old industry. For generations, workers known as resineros harvested resin from Maritime pines (Pinus pinaster) and Scots pines (Pinus sylvestris). The practice supported the rural economy, particularly from the mid-19th century until the 1980s.
The traditional extraction technique, known as the "American method" in Spain since the 1950s, involved stripping a section of bark and making periodic incisions, or grooves, into the trunk. A worker would first shave the rough outer bark, then make a 12-centimeter wide wound. A sticky and fragrant sap would bleed from this wound and drip down a metal gutter into a collection pot. Every 12 to 18 days during the tapping season, from March to November, the resinero would return to make a new groove just above the last, moving progressively up the tree.
This industry provided the raw materials for turpentine, rosin, varnishes, and glues. The work was physically demanding and tied to the seasons, but it sustained many rural communities. The practice declined sharply in the 1980s when Spain's production was overtaken by cheaper resin imported from China and the rise of synthetic petroleum-based substitutes.
A record written in the rings
The history of resin tapping left a record inside the trees themselves. Scientists use dendrochronology, the study of tree rings, to analyze the long-term effects of this practice. By comparing the ring widths of tapped and untapped trees in the same forest, researchers can measure the physiological impact of the harvesting.
Studies on Pinus pinaster in central Spain show that the growth of tapped trees was significantly lower than that of untapped trees, with growth reductions estimated between 25% and 56%. The repeated wounding forces the tree to allocate resources to defense and sap production, diverting energy away from wood production, which results in narrower annual rings. Despite this reduction in growth, the trees show resilience. Many of the scarred pines in the Sierra de Guadarrama are over 100 years old, and some have survived centuries of tapping.
Dendrochronological dating of the scars inside tree cross-sections has helped reconstruct the history of forest management, revealing an intensive period of resin extraction between the 1920s and 1960s. These studies also found that tapped and untapped trees exhibit a similar response to climatic variations, meaning the scarred trees still provide reliable data for climate reconstructions. Today, with a renewed interest in renewable bioproducts, resin tapping is seeing a small-scale revival in Spain.
💡Fun Facts
Pine resin is an ingredient in many everyday products, including cosmetics, food additives, tires, and even chewing gum.
The presence of resin tappers in the forest is credited with reducing the risk of wildfires, as they maintain the undergrowth and act as lookouts.
Spain has one of the largest forested areas in Europe, after Sweden and Finland, with an estimated 18 million hectares of woodland.
Historically, resin and tar were essential for shipbuilding, used to waterproof the hulls of wooden vessels.
The Sierra de Guadarrama National Park is accessible 24/7, but visitor centers have specific hours, typically 9:00 AM to 3:00 PM.
Admission
Free
Accessibility
The park has numerous trails of varying difficulty. Trails through pine forests like the La Barranca Valley Route (8.9 km loop) are on established paths but can be uneven and rocky.