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Maritime pine occupies a distinctive position among European resin-producing species, and the Portuguese findings fit within a broader pattern of research on how tapping systems interact with tree physiology and environment. Resin flow in pines is fundamentally a defensive response: oleoresin is synthesized in epithelial cells lining resin ducts and stored under pressure in a network of vertical and horizontal ducts, so that when a wound severs these ducts, resin exudes and seals the injury. Stimulant pastes exploit this physiology by inducing stress responses—sulfuric acid causes localized tissue necrosis that prolongs resin flow, while ethephon releases ethylene, a plant hormone that triggers defensive metabolism and can sustain secretion over longer periods. The observation that a sulfuric acid paste performed best under the traditional open-face method, while an ethephon-containing paste excelled under the closed method, is consistent with the different wound dynamics involved: open wounds lose moisture and resin volatiles rapidly, whereas the enclosed channel of the closed system maintains a microenvironment in which ethylene-driven stimulation can act more persistently.

The comparable performance of bio-based pastes under the closed method carries particular significance for the sector. Conventional stimulant pastes rely on sulfuric acid, which poses risks to workers, can degrade wood tissue around the tapping face, and raises environmental concerns regarding bark and soil exposure. Bio-based alternatives, typically formulated from plant-derived or other renewable constituents, reduce these hazards while maintaining yield when paired with a tapping system that conserves the stimulant and the resin. Because the closed method also reduces impurities and evaporation losses of turpentine—the volatile fraction that constitutes a valuable co-product—the combination of closed tapping and bio-based stimulation emerges as a technically coherent pathway toward more sustainable resin harvesting, even if per-tree yields remain somewhat lower than those of the traditional system.

The site-dependent effect of reopening frequency illustrates a recurring theme in resin research: the same silvicultural decision can produce divergent outcomes across climatic gradients. At Nazaré, a coastal site presumably with milder, more humid conditions, reopening wounds every 21 days maintained higher yields, plausibly because frequent reactivation of the wound surface sustains resin flow under conditions where the tree has sufficient water and carbon resources to support repeated secretory effort. At the less productive inland sites, extending the interval did not reduce production, implying that resin exudation there is limited less by wound freshness than by overall tree vigor or water availability. For forest managers, this suggests that reopening frequency is a tunable variable: in moisture-limited stands, less frequent reopening can cut labor costs—one of the largest expense items in resin harvesting—without sacrificing yield, whereas in more productive coastal stands, tighter schedules may be justified economically.

The finding that trees with diameter at breast height below 20 cm are not economically viable for tapping aligns with long-standing practice guidelines in Iberian resin forestry. Smaller trees possess fewer and narrower resin ducts, thinner sapwood, and less stored carbohydrate reserves, all of which constrain the volume of oleoresin that can be mobilized in response to wounding. Moreover, wounding young trees carries silvicultural costs: tapping faces create entry points for fungi and insects, and extensive wounding before trees reach commercial maturity can compromise stem quality and future timber value. Establishing a minimum diameter threshold therefore serves both an economic function—concentrating labor on trees that return adequate yields—and a biological one, protecting the productive potential of the stand over successive tapping cycles.

The absence of a significant effect of prescribed burning on resin production is noteworthy given the growing use of prescribed fire as a fuel-reduction tool in Mediterranean pine forests. Resin tapping and fire management are often viewed as potentially conflicting activities, since tapping faces and accumulated resin can be vulnerable to fire damage, and burned bark may suffer additional injury. The reported result suggests that, under the conditions studied, controlled low-intensity burning did not measurably suppress resin yield, which opens the possibility of integrating resin harvesting into fire-adapted management regimes. This integration has broader implications: resin tapping gives landowners a recurring revenue stream that depends on maintaining live, vigorous stands, which in turn incentivizes fuel management and stand tending. Economic studies in the Iberian Peninsula have previously identified resin tapping as a lever for wildfire risk reduction precisely because it keeps active management in forests that might otherwise be abandoned.

From a carbon accounting perspective, the study’s framing of resin as additional carbon storage deserves elaboration. The non-volatile fraction of pine resin—rosin, composed largely of abietane and pimarane diterpenes—is incorporated into products such as adhesives, coatings, inks, and sizing agents that can persist for years or decades. When this biogenic carbon replaces petrochemical feedstocks in those applications, it represents both substitution of fossil carbon and temporary storage of atmospheric carbon in durable goods. The volatile fraction, turpentine, is itself a bio-based solvent and chemical feedstock. Quantifying the climate benefit of resin harvesting therefore requires considering the full life cycle: the carbon fixed in resin products, the fossil emissions avoided through substitution, and the effects of tapping on tree growth and stand carbon stocks. Research on tapped versus untapped stands indicates that moderate tapping intensities generally have limited impact on radial growth, though this depends on wound size, number of faces, and site quality.

The methodological approach of combining analysis of variance for treatment comparisons with Spearman’s rank correlations for dendrometric and climatic associations reflects the practical realities of field experiments in forestry. Resin yield data are typically non-normal, bounded at zero, and strongly influenced by individual tree variation, making rank-based correlation a robust choice for exploring relationships between yield and variables such as tree diameter, crown dimensions, temperature, and precipitation. Dendrometric predictors—particularly diameter, which integrates tree age, vigor, and sapwood area—have repeatedly emerged as the strongest single predictors of resin yield in maritime pine and related species, which is why minimum diameter rules feature prominently in tapping regulations across resin-producing countries.

The three study sites span a gradient of productivity within Portugal’s maritime pine estate, a resource that covers substantial areas of the country and has historically underpinned one of the world’s notable resin industries. Portuguese resin production, centered largely in the central and northern regions, supplies rosin and turpentine to international markets and supports rural employment in regions with limited economic alternatives. The sector has faced pressures from stand aging, fire losses, and competition from petroleum-derived substitutes, making productivity gains and cost reductions strategically important. Findings that longer reopening intervals can maintain yields at less productive sites speak directly to the labor-cost problem, since reopening and resin collection are the most labor-intensive operations in the traditional system.

The closed tapping method, whose qualitative advantages the study confirms, represents the direction of travel in modern resin technology. By enclosing the tapping channel, the system protects the wound from rain, dust, insects, and microbial contamination, yielding cleaner resin with lower water content—a benefit for downstream processing, since impurities increase refining costs and reduce the value of crude resin. Reduced evaporative loss of turpentine preserves the volatile fraction, which is often the more valuable component per unit mass. Shorter operating time per tree and improved working conditions also address the sector’s chronic labor shortage, as resin tapping is physically demanding and increasingly performed by an aging workforce. Mechanization efforts in Spain and Portugal have focused on chain-sinkers and automated paste application, and the closed method is compatible with these innovations.

More broadly, the study contributes to a growing literature on non-wood forest products as components of the forest bioeconomy. Resin is one of several forest-derived biochemicals—alongside tannins, essential oils, and bark extracts—attracting renewed interest as industries seek renewable alternatives to fossil-based adhesives, fragrances, and polymers. Maritime pine is particularly well suited to this role because of its high resin yield, its extensive planted area in southwestern Europe, and the existence of established processing chains. Optimizing the intersection of tapping method, stimulant chemistry, reopening schedule, stand structure, and site climate, as this research attempts, is the kind of integrated analysis needed to translate biological potential into economically viable and environmentally responsible production systems.

For practitioners, the practical synthesis is that no single prescription fits all stands. Where trees are large and sites productive, the traditional method with sulfuric acid paste maximizes yield; where labor is scarce, environmental certification matters, or resin quality commands a premium, the closed method with bio-based or ethephon-containing pastes offers a competitive alternative despite lower gross yields. Reopening schedules should be calibrated to local climate, with longer intervals viable in drier or less productive areas. Prescribed burning can proceed without measurable yield penalty under appropriate conditions, and tapping should be restricted to trees above the 20 cm diameter threshold. These evidence-based adjustments, grounded in the Portuguese trials, provide a template that can be adapted to other maritime pine regions and, with appropriate local validation, to other resin-producing pine species facing similar management trade-offs.

Subject of Research: …

Article Title: Factors influencing resin production in maritime pine (Pinus pinaster Ait.): climate, stand structure and resin tapping methods

Article References: Lopes, D. A., Sandim, A. S. A., Fernandes, C. A. M. G., Louzada, J. L. P. C., Seixas, N., Farias, I. S. L., Silvestre, A. J. D., & Silva, M. E. C. M. (2026). Factors influencing resin production in maritime pine (Pinus pinaster Ait.): climate, stand structure and resin tapping methods. Frontiers in Forests and Global Change, 9, Article 1980945. https://doi.org/10.3389/ffgc.2026.1980945

Image Credits: AI Generated

DOI: 10.3389/ffgc.2026.1980945

Keywords: Factors, influencing, resin, production, maritime, pine, Pinus, pinaster, climate, stand, structure, tapping