Resin Production in Pinus: A Review of the Relevant Influencing Factors and Silvicultural Practices

Abstract

Resin is a renewable non-timber forest product that is used as a raw material in a wide range of goods, thereby holding significant socioeconomic importance and relevance across multiple industrial sectors. This study aims to provide a comprehensive review of the main factors influencing natural resin production in Pinus stands, as well as to address the effects of these factors on tree growth dynamics and resin yield optimization. Among these factors, dendrometric characteristics, environmental conditions, and silvicultural practices, such as thinning, pruning, and prescribed burning, are particularly relevant. However, the scientific literature presents conflicting results regarding the influence of these factors on resin yield, as well as the impacts of resin tapping on tree growth and wood quality. These divergences highlight the complexity of the process and reinforce the need for further studies to clarify the interactions between silvicultural practices in Pinus stands and resin production. Understanding these practices is essential for the development and implementation of efficient silvicultural models aimed at optimizing resin tapping that are properly tailored to the specific conditions of each site. In this context, the development of management models that integrate both timber and resin production is fundamental for simulating management scenarios, generating yield forecasts, and supporting decision-making processes. It is worth noting that management models focused on maximizing resin production may differ from conventional approaches intended for pulpwood or sawtimber production. Nevertheless, integrating resin tapping with timber harvesting holds significant potential to increase the profitability of forest operations.

1. Introduction

Pine resin, in addition to its ecological relevance as a key defense mechanism against biotic and abiotic disturbances, stands out as a renewable natural resource and a valuable non-timber forest product [1,2,3]. This viscous plant exudate is composed of two main fractions: turpentine, the volatile liquid component, and rosin, the solid non-volatile component [4,5]. The chemical compounds derived from resin have been widely used across a broad range of industrial applications and sectors, including the manufacture of paints, insecticides, pharmaceuticals, cosmetics, perfumes, adhesives, disinfectants, footwear, biofuels, and food products, among others [2,6,7,8].

Resin tapping constitutes an activity of high socioeconomic and environmental value, primarily due to the provision of ecosystem services. It ensures the supply of a natural and renewable raw material that is widely used in a broad range of industries as an alternative to fossil-based resources [9]. Resin tapping promotes rural employment, contributing to population retention in the countryside and mitigating land abandonment [10], strengthens territorial monitoring, thereby reducing wildfire risk [11], and supports the provision of key regulating services, such as carbon sequestration [12,13].

Resin tapping represents a strategic activity within the framework of the bioeconomy, and decarbonization, as resin, its primary product, has the potential to replace petroleum-derived compounds across various industrial sectors [14]. This product is composed of a complex mixture of terpenes, carbon-rich molecules, the biosynthesis of which depends on the fixation of atmospheric carbon [15]. Accordingly, it is assumed that resin extraction directly influences the carbon balance in Pinus plantations, making resin production a relevant component in carbon allocation and sequestration processes [13,16]. Although resin yield is subject to the physiological variability of trees and the impacts of climate change [17], there is evidence that, in moderately fertile soils, increased atmospheric CO₂ concentrations may enhance resin flow [18]. In this context, resin tapping tends to function as an increasingly efficient carbon sink, particularly in light of the intensification of climate change [12,18].

Although resin production and accumulation contribute to carbon storage, sequestration rates may vary between planted and natural forests. Comparative studies addressing carbon sequestration rates in both types of forests when managed for resin extraction are still scarce. Tsaktsira et al. [16], for example, observed that commercial plantations of Pinus halepensis exhibited atmospheric carbon sequestration rates that were at least eleven times higher than those recorded in natural stands of the same species. This difference can be attributed to the greater structural homogeneity of planted pine stands, which favors light interception and resource utilization [19]. Furthermore, the implementation of silvicultural practices tends to enhance growth and increase biomass accumulation, and, consequently, carbon storage [20,21,22]. Another relevant factor is the developmental stage, as younger stands generally exhibit a higher capacity for carbon uptake compared to older forests [23]. These factors influence both biomass growth and resin production.

Several species of the genus Pinus can be used for resin production, with the choice of species being primarily determined by their adaptability, productivity, and geographical distribution [24,25]. However, despite the diversity of potential species, global resin production is mainly derived from four pine species [26]. Pinus elliottii is widely used for resin extraction in several countries, including China [25,27,28,29], Brazil [24,30], and the United States [31]. The use of this species can be attributed to its high adaptability, elevated productivity, low resin crystallization rate, and high turpentine content [28]. Pinus massoniana stands out as the most economically important resin-producing species in China, the leading country in global pine resin production and export [32]. This species exhibits a wide distribution, rapid growth, and high resin yield [32]. Pinus merkusii is one of the most important timber species and the primary one to be used commercially for resin extraction in Indonesia [33,34,35]. In addition to its high resin yield, it stands out as one of the most widely used species in reforestation programs and soil erosion control [36]. It is a species that is tolerant of heat and drought, and is highly light-demanding [37,38]. Pinus yunnanensis, a species with high resin content exploited for resin production in China [39,40], is fire-adapted, tolerant of dry and infertile soils, has strong natural regeneration capacity, is light-demanding, and is also used in traditional medicine [41,42].

It is also worth highlighting that Pinus caribaea, which is cultivated in tropical regions, offers high resin yield, rapid growth, and potential use in erosion control and reforestation [43,44,45]. Pinus pinaster, which is widely distributed across the Mediterranean Basin and along the Atlantic coast of Portugal, Spain, and France, is the main species used for resin extraction in these regions [46,47,48,49,50]. It is a fast-growing species [51] that has adapted to a wide range of environmental conditions [52], from humid coastal areas with mild temperatures to inland regions with hot and harsh climates [53,54]. Pinus sylvestris, with a wide distribution area and commonly found in Europe and Asia, is characterized by good adaptability, drought tolerance, and resilience in poor soils. It is frequently employed in land rehabilitation projects and is exploited for resin extraction [55,56,57,58]. Finally, Pinus oocarpa is a highly adaptable species that is the leading resin-producing pine in Mexico [59,60,61].

Resin production is influenced by multiple factors, including the tree’s own genetic and dendrometric characteristics, as well as external factors such as edaphoclimatic conditions and the silvicultural practices that are implemented in stands designated for resin tapping [3,27,47,62,63,64,65,66].

Silviculture consists of applying techniques to control the establishment, composition, structure, and growth of forest stands [67]. It aims to ensure the long-term continuity of ecological functions and productivity, while meeting landowner objectives in an economically and ecologically appropriate manner [68]. The use of silvicultural practices in forest management can provide a broader range of ecosystem benefits and services, such as guiding stand development or modifying the type of goods or services produced, which would not otherwise be achieved without intervention [69]. Conversely, the absence of silvicultural actions can lead to high stand density, which directly increases competition among the trees for light, water, and nutrients [70].

Understanding how factors related to silvicultural practices, dendrometric characteristics, and environmental conditions influence resin production, as well as identifying the most favorable conditions to maximize yield, represents a strategic tool for the efficient management of Pinus stands. However, significant knowledge gaps remain regarding the impacts of these factors on resin yield, for which the literature presents divergent results. The same issue applies to our understanding of the effects of resin tapping on tree growth, particularly in terms of diameter, height, and volume. Clarifying how these factors affect yield is essential for the development of management strategies aimed at maximizing production. Furthermore, there is a notable scarcity of silvicultural models specifically designed for stands that are managed for resin extraction, which underscores the need for consistent research in this field. Therefore, this study aims to provide a comprehensive review of the main factors influencing natural resin production in Pinus stands, as well as to address the effects of these factors on tree growth dynamics and resin yield optimization.

2. Methodology

This study was developed through a systematic literature review based on international databases, namely, Web of Science, Scopus, and Google Scholar. The majority of the identified references were drawn from peer-reviewed scientific articles that were indexed in international journals, covering the period from 1969 to 2025. In addition, non-indexed documents such as theses, technical reports, and forestry legislation relevant to the topic were also considered. Notably, more than 70% of the references were published after 2010, ensuring an updated perspective, while earlier works were included when they represented classical contributions or when more recent information was scarce.

The selection of bibliographic references followed a systematic procedure aimed at gathering the most relevant scientific contributions on resin tapping in Pinus species. The inclusion criteria prioritized, in descending order, peer-reviewed scientific articles, technical or scientific books, book chapters, and doctoral or master’s theses.

Across all databases, an initial pool of approximately 740 articles was identified. Following a more detailed screening of abstracts and conclusions, 350 studies were retained, while those not aligned with the objectives of this review were excluded. After full-text reading, 188 articles that met the predefined criteria were selected. The selected studies were subsequently classified into thematic sections that are consistent with the structure of this review, read in full, interpreted, and compared to assess their complementarities and divergences before being integrated into the manuscript.

Keywords, used individually or in combination, included: Pinus spp., resin yield, resin tapping, resin production, forest management, dendrometric characteristics, DBH, height, tree growth, crown management, environmental factors, silvicultural practices, pruning, thinning, and prescribed burning.

3. Factors Affecting Resin Productivity

3.1. Tree-Related Factors

3.1.1. Dendrometric Characteristics of the Tree

Each tree exhibits particular traits regarding its resin productivity and capacity to adapt to environmental conditions. This variability in resin yield among pine trees is associated with a range of factors, including dendrometric characteristics, environmental conditions, and silvicultural practices [71,72]. Among the dendrometric traits, diameter at breast height (DBH), total height, and tree age are the most frequently reported characteristics in the literature.

In this context, López-Álvarez et al. [73] proposed a dynamic model derived from the Bertalanffy–Richards growth equation to estimate the accumulated annual resin yield per P. pinaster tree during the tapping season. This model incorporates variables such as DBH, total tree height, tapping method, and the time elapsed since the beginning of the tapping campaign. The authors emphasized the potential of this model as a practical and effective decision-support tool for forest management in stands dedicated to resin production. Specifically, it allows for the prediction and planning of individual resin yields, the optimization of tapping campaign duration, and the maximization of annual income by integrating resin production with tree growth dynamics. Complementarily, Gómez-García et al. [74] modeled the distribution of resin production in P. pinaster stands in Spain using the Weibull probability function. Their findings also revealed a high degree of variability in resin yield among trees, both between and within plots. This variability was attributed to dendrometric and silvicultural factors, the tapping method employed, and the local climatic conditions.

The minimum DBH required to commence resin tapping in Pinus species is regulated according to the legislation of each country. In leading resin-producing countries such as China and Brazil, commercial tapping of P. massoniana and P. elliottii generally begins when trees attain a DBH of at least 20 cm and 15 cm, respectively [30,75]. In the European context, such as in Portugal and Spain, P. pinaster trees must have a minimum DBH of 20 cm and 25 cm, respectively [76,77].

Previous studies on the influence of tree dendrometric characteristics on resin production have identified a significant positive correlation between DBH and yield potential, showing that trees with larger diameters tend to produce more resin [30,47,62,63,66].

This positive correlation between DBH and resin yield can be explained by the fact that larger trees have a greater number of resin ducts in the xylem [78,79] and possess higher photosynthetic capacity, which generally gives them greater ability in terms of resin biosynthesis due to their more developed vascular systems [62], allowing them to allocate more resources to resin production and accumulation [80]. Additionally, Garcia-Forner et al. [47] suggested that larger trees with wider crowns have a greater ability to absorb and store carbon, which contributes to increased resin reserves for rapid exudation in response to mechanical injury.

Garcia-Forner et al. [47], as well as Almeida [81], reported a moderate positive correlation (r = 0.57 in both cases) between the DBH and resin yield of P. pinaster in Portugal. In contrast, Caglayan [82] and Zas et al. [64] observed weaker correlations (r = 0.33 and r = 0.14 to 0.18, respectively), suggesting that DBH alone is not sufficient to fully explain the complexities of resin production. Although all studies were conducted on the same species, the divergences observed in the results regarding the relationship between DBH growth and resin yield may be attributed to variability in tree dendrometric traits (including DBH and age), local climatic conditions (particularly temperature), the different methods employed for resin extraction (microtapping or conventional tapping systems), and the period during which extraction was carried out.

Ribas et al. [83], in studying the influence of dendrometric growth on resin production in P. elliottii, found that trees with a DBH under 22 cm yielded more resin during the first tapping campaign. However, in trees with a DBH over 22 cm, the second campaign had higher yields. Regardless of DBH class, the third campaign always resulted in the lowest production. The authors concluded that DBH class was strongly correlated with resin yield, with an estimated influence of about 34%. Conversely, Gurgel Filho and Gurgel Garrido [84] estimated this influence at only 11%.

Total tree height is another dendrometric variable for which the relationship with resin production has been studied. Taller trees may have better access to light; therefore, they may photosynthesize more, thereby acquiring more carbon for resin production [48]. However, the literature presents divergent findings regarding this correlation, both within and across Pinus species. While most studies indicate a positive correlation, the strength of that correlation varies from weak [64], to moderate [62,85]. This inconsistency was particularly notable in research on P. pinaster in Spain; Rodríguez-García et al. [62] found a moderate positive correlation (r = 0.46), while Vázquez-González et al. [3] reported a negative correlation (r = −0.10). These contrasting findings highlight the need for caution when using a single dendrometric variable to explain resin production potential.

In assessing the possible influence of tree age on P. pinaster resin yield in Spain, Zas et al. [71] identified age as the most important determinant, exerting a strong and consistent influence on resin flow and yield. Trees aged 30–40 years produced nearly two to three times more resin than those aged 20–30 and 10–20 years, respectively. Based on these results, the authors recommended only initiating resin tapping in stands older than 30 years. They also noted that older trees typically have more growth rings and, therefore, a greater number of resin canals than younger trees. Moura et al. [48] concluded that, to maximize co-production of resin and timber in P. pinaster in Portugal, resin extraction should be carried out in stands over 40 years old, managed under controlled density conditions to reduce competition.

As trees age, they tend to increase their photosynthetic capacity and their carbohydrate reserves, which can influence the induction of terpene biosynthesis [86,87]. Since resin is composed of a complex mixture of terpenes, chemical defense compounds, and high carbon content [1,88], its induction and accumulation are also associated with the biosynthesis of these compounds [89,90,91]. Kim et al. [92], in their evaluation of total emission rates and the composition of monoterpene compounds in young and mature Pinus koraiensis trees, observed significant age-related variation, with older trees exhibiting the highest emission rates.

3.1.2. Crown Management

The structure of the tree crown, including height, diameter, volume, surface area, and shape, is of great importance for tree and stand growth, productivity, and forest dynamics [93,94,95]. Crown architecture is a key determinant of forest productivity and nutrient cycling [96], as it directly influences the leaf surface area that is available for light interception, gas exchange, and water flow via photosynthesis and evapotranspiration [97], which are all essential processes for tree growth. Larger crowns increase carbon uptake due to the expanded leaf area and greater light interception [98]. Tree growth depends on the amount of light absorbed by the leaves, the efficiency with which it is converted into biomass, and the allocation of photosynthates to different tissues [93].

Crown development is significantly influenced by site conditions and stand density [99]. As trees grow, competition between crowns may occur, with each tree striving to optimize its surrounding space and reduce excessive shading [100]. Competition can be defined as a reduction in tree performance caused by neighboring individuals that compete for light and nutrients through their own crowns [101], ultimately affecting tree growth and survival [102].

Understanding the relationship between crown size and other dendrometric variables, such as diameter and total height, can contribute to more effective silvicultural strategies for resin production. Some earlier studies suggest that resin yield is closely related to the ratio between crown size and total tree height. However, no recent studies were found addressing these specific relationships.

Clements [103] observed that, to achieve good resin yields in P. elliottii and Pinus palustris, the crown must occupy at least one-third of the tree’s total height. He also found that trees with crowns equal to or greater than 50% of their height had high resin yields, while those with 35% had moderate yields, and those with 25% had low yields. Moreover, trees with crown ratios of between 40% and 55% maintained first-year yield levels over three consecutive tapping seasons, whereas trees with ratios of below 40% showed decreasing yields over time. According to the author, the crown ratio has a greater impact on multi-year resin yield than on first-year production. A 10% increase in crown ratio was estimated to increase resin production by 6 to 7 tonnes per 10,000 tapped trees.

Baena et al. [104] reported that vigorous P. elliottii trees with crowns occupying 50% or more of their total height tend to produce higher quantities of resin and sustain that yield over several years. Trees with a crown ratio of about 35% show moderate production that does not remain consistent, while those with crowns below 35% exhibit low and declining yields. Thus, to optimize resin production, these authors recommend initiating resin tapping only in trees where the crown occupies more than 40% of the total height. They argue that the high yield that is achieved in the second year, following the exudation stimulus of the first year, would be maintained in subsequent tapping campaigns.

Schopmeyer and Larson [105] found that both DBH and crown ratio had a highly significant effect on resin yield in P. elliottii and P. palustris. For trees with DBHs between 22.8 and 35.6 cm, a 10% increase in crown ratio could raise resin yield by approximately 7497.4 kg per campaign of 10,000 trees. For crown ratios of between 20% and 50%, a 2.54 cm increase in DBH could result in an additional 5327.1 kg per 10,000 tapped trees.

In addition to the crown-to-height ratio, another relevant aspect to consider in resin production is the ratio between the sun-exposed crown length and the shaded crown portion. Understanding how light exposure and shading affect crown function could be essential for optimizing resin yield. However, no specific thresholds for light-exposed crown proportion were found in the literature regarding P. pinaster.

It is well established that sunlight reaching the crown drives photosynthesis and carbon assimilation [106], processes that directly affect pine resin production, since resin is composed mainly of hydrocarbon molecules, the synthesis of which requires high photosynthetic resource input [107]. Therefore, trees with greater light exposure tend to exhibit higher photosynthetic rates and, possibly, greater carbon allocation toward resin production. This was observed by Wei et al. [108], who found that high-resin-yielding clones of P. massoniana had significantly higher photosynthetic capacity than low-yielding clones. In contrast, Gaylord et al. [109] noted that the highest resin flow in Pínus ponderosa occurred under low photosynthesis and higher water stress conditions. It is important to note, however, that excessive solar radiation can induce water stress due to increased evapotranspiration [110], which may substantially reduce canopy conductance and leaf area, thereby diminishing both photosynthesis [111] and carbon assimilation [112].

Silvicultural practices such as thinning and pruning are important tools for modifying crown structure, improving light conditions, and providing an optimal light environment for the species, while also reducing resource competition and enhancing growth [113,114].

3.2. Factors External to the Tree

3.2.1. Environmental Factors

Resin production can vary in response to different environmental conditions, which may exert a direct influence on photosynthetic activity and, consequently, alter both resin flow and yield. This process depends on carbon assimilation and allocation [17]. The assimilation rate is also affected by environmental factors during the tree’s growth, such as temperature, irradiance, carbon availability, and water and nutrient levels [115]. Increases in radiation and temperature promote photosynthetic processes and result in higher resin yields [116,117].

Temperature and humidity are particularly influential factors in resin production [64,118]. Several studies have confirmed this effect; for example, Gaylord et al. [109], Rodríguez-García et al. [119], Neis et al. [120], Zas et al. [64], and Caglayan et al. [66] reported strong and significant positive correlations between resin yield and temperature. This relationship can be explained by the effect of temperature on physiological processes, such as photosynthesis, which are directly involved in the biosynthesis and secretion of resin. Moreover, high temperatures can enhance metabolic activity, reduce viscosity, and increase the resin’s fluidity, thereby favoring its exudation and, consequently, the tree’s yield [47,66].

According to Gurgel and Faria [118], lower temperatures make resin less fluid, promote crust formation due to air humidity, and can hinder resin flow through resin ducts. Caglayan et al. [66] observed that the highest resin yields in Pinus brutia occurred at temperatures between 23 °C and 27 °C. In turn, Will et al. [121] found that temperatures between 20 °C and 25 °C were optimal for resin flow in P. taeda and P. elliottii.

Regarding the relationship between relative humidity and resin yield, studies report conflicting results, some finding a positive correlation [17], and others a negative one [116,117,119]. Moura et al. [17] observed that both the tree growth and resin yield of P. pinaster and Pinus pinea increased with temperature, with growth being more sensitive to soil and air humidity than resin production. Rissanen et al. [122] found that diurnal changes in stem resin exudation were positively correlated with temperature and transpiration rate.

Precipitation shows similar variability in its effects on resin yield, with some studies indicating positive correlations [66,123] and others negative ones [64,120,124]. Brito et al. [125], in their analysis of resin yield and quality in various Pinus species in relation to precipitation and temperature, concluded that the highest resin production values generally occurred during months with lower precipitation and when average temperatures ranged between 18 °C and 20 °C, with yields decreasing when temperatures fell below or exceeded these limits.

3.2.2. Resin Tapping Season

During a resin tapping campaign, yield fluctuates across the seasons, with distinct periods of higher and lower productivity. As mentioned previously, various factors influence resin production, although not all of them can be controlled. However, some can be adjusted, such as the choice of the optimal season for tapping.

Resin extraction should occur during a period when local climatic conditions are most favorable to resin flow, as these conditions will largely determine the duration of the productive season [104]. Resin exudation is regulated by daily variations in light incidence, which modulate the overall physiological activity of plants [104]. According to García-Méijome et al. [126], the geographic location of pine stands and specific climate characteristics should be considered before initiating tapping, since resin yield also depends on site conditions.

Resin yield is affected by the tapping season [120]. In P. pinaster stands in Portugal and Spain, the highest resin production period typically occurs during spring and summer, when temperatures and solar radiation levels are higher and both humidity and precipitation are lower [17,116,126]. According to Touza et al. [127], cold and humid periods should be avoided. During colder seasons, tree metabolism slows down, resulting in reduced resin production and transport [104].

Rodríguez-García et al. [116] suggested that in adverse climatic scenarios, where resource optimization is a priority, a shorter tapping period that is limited to the hottest months (June to September) could be advisable, with only minimal losses in resin yield. Yi et al. [29] found a significant seasonal effect on resin yield in P. elliottii, with the highest exudation occurring in summer, ranging from 5.81 to 7.02 mL per 4 h, and the lowest in winter, ranging from 2.11 to 2.32 mL per 4 h, coinciding with peak temperatures.

Clements [103] recommended suspending resin tapping when the average daily temperature drops below 18 °C, as resin will not flow through the resin duct system in economically viable amounts under such conditions, regardless of the extraction method used. Resin output is considered profitable only when the temperature reaches or exceeds 18 °C for at least 30% of the interval between bark incisions. For example, a 14-day interval would require a duration of 101 h at 18 °C.

Despite climatic conditions, the interruption of resin tapping is essential to provide a rest period for the tree [104]. During this interval, which corresponds to the autumn/winter seasons, it is recommended that trees should not be tapped [104]. This interruption may contribute to the restoration of the tree’s physiological functions [128].

3.3. Silvicultural Practices Used in Stand Management That May Influence Resin Production

3.3.1. Thinning

The stand conditions of pine trees are critical for resin extraction and production. In forest management, stand density control is considered a key parameter as it directly influences the structure, growth, and development of forest stands according to specific production objectives [129], since density affects DBH, total height, crown diameter, and length, all of which have a direct impact on resin yield. Among the most common silvicultural practices used to regulate stand density, thinning stands out as a fundamental tool of the process.

Thinning is employed to reduce stand density through the removal of suppressed or overtopped trees [130], providing greater spacing for the remaining trees, which are then better aligned with production and yield goals. This practice enhances the conditions for tree growth and improves the availability of site resources such as light, water, and nutrients [5,131,132,133]. By reducing intra-specific competition, thinning promotes increased growth rates and improves both the quality and quantity of timber, ultimately raising its market value [134,135]. Additionally, thinning can serve as a forest management strategy for climate change mitigation and adaptation [136,137], while also improving drought resistance and contributing to the restoration of stand structure [138,139].

Thinning is characterized by its type, intensity, and frequency. The main thinning types applied in even-aged pure P. pinaster stands include: (i) low thinning, which removes trees with smaller crowns, poorly formed stems, or dead individuals, this practice being particularly suitable for shade-intolerant species and the most frequently used method; (ii) systematic thinning, wherein trees are removed according to a fixed pattern, based on their spatial position in the stand—such as whole rows or strips; and (iii) selective thinning, which involves selecting future crop trees across all social classes and removing their competitors [140,141].

Thinning intensity, interval, and frequency should be guided by the stand’s development rather than by a rigid schedule [142]. To determine thinning intensity, that is, the number of trees to be removed, the Wilson Spacing Factor is most commonly used, which provides a relative density index for the stand [143]. The thinning regime to be adopted depends on the stand’s age and quality, rotation length, market demand, and, most importantly, the site’s characteristics and management objectives [144].

Stand density influences tree competition for nutrients and other resources, thereby affecting both growth and resin yield. Hood et al. [145] reported that thinning treatments significantly increased tree growth and resin canal size. Similarly, Rodríguez-García et al. [62], when comparing resin yield between two stands of the same age that were under identical climatic conditions but with differing soil characteristics and densities (206 trees/ha versus 108 trees/ha), found that the lower-density stand produced more resin (3.21 kg/tree/year) and consisted of more vigorous trees with greater DBH and a higher percentage of live crown.

Thus, thinning appears to be a fundamental technique for developing silvicultural models aimed at managing P. pinaster stands for timber production or for combined timber–resin production. In the Mediterranean Basin, resin tapping in pine stands is generally conducted alongside timber harvesting [11], representing a complementary economic activity to wood production [31].

Although one of the main concerns associated with resin tapping is the potential negative correlation between resin extraction and timber quality, some studies indicate that resin-tapped trees produce denser wood [46], yielding improved mechanical properties [32,63] and greater deformation and elasticity [63]. However, other studies have found no significant effects [116,146]. According to Miina et al. [147], revenues from resin production may exceed the economic losses from any reductions in timber yield or quality. Moreover, Moura et al. [48] advocate for the coproduction of resin and timber in P. pinaster stands that are over 40 years old, emphasizing that the impact of resin tapping on tree growth is negligible.

Sandim et al. [148], when evaluating different simulated forest management models for P. pinaster stands in Portugal (with a final harvest at 45 years), observed that the scenario involving four thinning operations, starting at age 16 and recurring every 8 years, allowed resin tapping to begin as early as 25 years of age, while the absence of thinning operations delayed tapping until age 30 (

Table 1. Measurement metrics of the forest stand and a description of the scenarios used for the forest evolution simulation.

	Scenario 1	Scenario 2	Scenario 3	Scenario 4	Scenario 5
Age (years)	16	16	16	16	16
Number of thinnings	0	1	2	3	4
1° Thinning (years)	-	16	16	16	16
2° Thinning (years)	-	-	31	26	24
3° Thinning (years)	-	-	-	36	32
4° Thinning (years)	-	-	-	-	40

). Furthermore, when assessing combined timber and resin production, the most economically favorable scenario, which was based on net present value (NPV), involved only three thinning interventions, with the first at age 16 and subsequent interventions every 10 years.

Reinforcing the importance of silvicultural practices in forest management, as well as in the productivity and economic viability of resin tapping, Lima et al. [149] found that by reducing stand density, interventions such as brush clearing and thinning promoted increased diameter growth in pine trees. However, after resin tapping began in the 11th year, a reduction in diameter, height, and volume growth was observed. When analyzing the economic viability of resin tapping across six simulated scenarios, the authors reported that those scenarios including resin extraction yielded higher profits, ranging from a 42.3% to a 62.7% increase, compared to scenarios without resin tapping. The most economically advantageous scenario, based on the highest net present value (NPV), benefit–cost ratio, and internal rate of return (IRR), was the one with the highest stand density (1180 trees/ha), the largest tapped area, and an average resin yield of 3 kg per tree. Moreover, although resin tapping directly affected tree growth in terms of diameter, volume, and height compared to untapped trees, it proved to be a feasible and economically beneficial practice (

Table 2. Characterization of P. elliottii stands and the different scenarios simulated for economic analysis, with and without resin tapping.

Characteristics	Area 1		Area 2
	Scenario A	Scenario B	Scenario A	Scenario B	Scenario C	Scenario D
Brush clearing (year)	4–5	4–5	4	4	4	4
Thinning (year)	10	10	4	4	4	4
trees/hectare	800	800	1180	1180	1180	1180
Management	Untapped	tapped	Untapped	tapped	tapped	tapped
Resin yield (kg/tree)	-	3.0	-	3.0	2.5	2.0

Kg = kilograms.

).

The use of silvicultural models that include combined timber and resin production for P. pinaster stands allows forest managers to simulate and forecast outputs under different management scenarios, thereby supporting informed decision-making. In Portugal, however, existing silvicultural models for P. pinaster are focused exclusively on timber production [150].

The Forest Services and Promotion Center of Castilla y León [151] proposed a silvicultural model aimed at the joint production of timber and resin for P. pinaster stands in Segovia, Spain (

Table 3. Silvicultural model for timber and resin production in P. pinaster stands in Segovia.

Age (Years)	Operation	Initial Density (Trees/ha)	Target Density (Trees/ha)
0–15	Thinning + pruning	variable (>1000)	< 800
15–25	Thinning + pruning	< 800	450–500
25–35	Low thinning	450–500	150–200
60–70	Regeneration cut	150–200	70–100
70–90	Regeneration cut	70–100	20–50
90–100	Final harvest	20–50	1–3

). This model also includes pruning during the first 25 years as a recommended silvicultural practice. It advises three thinning operations, two intermediate cuts, and a final harvest between the ages of 90 and 100.

Martínez et al. [152] proposed a silvicultural model for Galicia targeting saw timber and resin yield from P. pinaster stands, considering two site quality classes: high quality (Site Index 20) and medium-to-low quality (Site Index 14). The model assumes final harvesting at 40 and 45 years, respectively, with resin tapping beginning three years prior to the final cut (

Table 4. Silvicultural model for timber and resin production in P. pinaster stands in Galicia, sorted according to site quality classes.

Operation	Site Index 20			Site Index 14
	Age (Years)	Density (Trees/ha)	Production	Age (Years)	Density (Trees/ha)	Production
First thinning	15	350	32 m3/ha	20	350	30 m3/ha
Resin tapping before second thinning	22–25	350	2433 kg/ha	27–30	350	2268 kg/ha
Second thinning	25	350	166 m3/ha	30	350	121 m3/ha
Resin tapping before final harvest	37–40	400	4971 kg/ha	42–45	400	4578 kg/ha
Final harvest	40	400	534 m3 /ha	45	400	345 m3 /ha

).

The authors observed that, for site quality 20, the total loss in timber production due to resin tapping would be 19 m³/ha, representing 2.5% of the total timber yield, whereas for site quality 14, only 10.3 m³/ha would be lost, corresponding to 2%. In contrast, the combined production of timber and resin would result in a total resin yield of 7405 kg/ha in the higher-quality stand and 6846 kg/ha in the lower-quality stand. Furthermore, under a conservative estimate, assuming a resin price of EUR 1/kg and stumpage prices of EUR 15, EUR 25, and EUR 32 per m³ for the first thinning, second thinning, and final harvest, respectively, the financial return was projected to increase by 30%.

As observed in Table 1, Table 3 and Table 4, management models consistently recommend an early first thinning, occurring at approximately 15–16 years of age. This practice facilitates the earlier attainment of target diameters and biomass, thereby optimizing stand productivity and mitigating the decline in forest development rate caused by inter-tree competition [148]. The models also indicate that at least two thinning procedures should be performed at regular intervals. In this context, region-specific mathematical models and simulators are valuable tools for determining the optimal number of thinnings for diverse forest conditions [153]. While most models suggest a final felling age of between 40 and 45 years, it is crucial to recognize that this long rotation length may negatively affect the cash flow and viability of smaller-scale forestry projects. Consequently, forest managers should prioritize the pursuit of industrial products that enable the implementation of shorter harvesting cycles.

The difference between the previously mentioned silvicultural models highlights the importance of adopting silvicultural practices and management regimes that are tailored to the specific characteristics of each site.

3.3.2. Pruning

Pruning is another important silvicultural practice for forest management that influences tree growth. It involves the deliberate removal of branches, thereby restricting knots and related defects to a central knotty core [154,155]. In this way, it increases the volume of clear wood and improves the quality of timber products [155,156]. Although it is a high-cost intervention, it is recommended that pruning should preferably be carried out early in the rotation, shortly after crown closure, while the branches are still small and alive [156,157,158]. To reduce costs, it is advisable to prune only those trees that will remain until the final harvest [156].

Although there are no studies directly correlating pruning with resin production, it is possible to establish an indirect relationship through the influence of pruning on crown structure and tree growth, considering the effects of these factors on resin yield. From the perspective of crown architecture, pruning reduces the leaf area and, consequently, decreases the amount of light intercepted by the canopy, thereby constraining photosynthetic activity and carbon assimilation, which are key processes directly related to the biosynthesis and production of resin [159,160,161]. Thus, severe pruning may reduce resin output. Conversely, low-intensity pruning can enhance light-use efficiency and increase photosynthetic rates in the upper canopy by removing the less efficient foliage from the lower crown [162].

In addition, pruning is often associated with the dendrometric traits of the tree, which, in turn, exert a direct influence on resin yield (see Section 3.1). Resin flow shows a strong relationship with tree growth dynamics [64,82]. Liu et al. [163], when evaluating the relationships between growth traits, morphological characteristics, and resin yield in P. massoniana, found that all traits exhibited significant genetic correlations with resin yield, with the number of live branches showing the strongest correlation (r = 0.99), followed by DBH (r = 0.73).

However, findings regarding the effects of pruning on these traits are inconsistent. Some studies have reported a significant reduction in diameter growth [164,165], possibly due to the decreased production of photoassimilates resulting from reduced leaf area [162]. In contrast, other authors observed an increase in diameter growth following pruning [166,167], while Moreno-Fernández et al. [168] found no significant effect. This divergence among studies may be explained by differences in the species being investigated (P. pinaster, Pinus radiata, P. brutia, Pinus patula, and P. massoniana), as well as variations in pruning intensity and stand age. These findings highlight the need for a better understanding of the effects of different pruning intensities and frequencies on the growth of various pine species, in order to establish more appropriate regimes and to avoid severe interventions that may compromise tree development. Furthermore, they suggest that the relationship between pruning, growth, and resin production may not be linear, as it is also influenced by both genetic and environmental factors.

Conversely, branch removal may cause mechanical damage to stem tissues, triggering defense responses in the tree that induce the formation of traumatic resin ducts [169,170,171]. This process may influence the resin yield of the tree.

3.3.3. Prescribed Burning

Prescribed burning is a silvicultural practice that can be employed in the management of Mediterranean forest ecosystems with the objective of reducing fuel loads and the horizontal and vertical continuity of forest fuels, thereby modifying fire behavior, fire severity, and the risk of large wildfires, while minimizing their intensity and extent [172].

This practice consists of the controlled application of fire for forest stand management under favorable weather and fire behavior conditions [173]. Additionally, prescribed burning can promote the regeneration of fire-adapted species [174,175], such as P. pinaster, which produces serotinous cones that release seeds following fire events [176]. It may also serve to restore habitats, control pests and diseases, improve pasture quality [177], and reduce carbon emissions from wildfires [178,179].

Several studies analyzing the effects of prescribed burning on P. pinaster stand structure have reached different conclusions. Botelho et al. [180] found that fire may enhance tree growth, while Jiménez et al. [181] concluded that only trees subjected to the most severe treatment affecting both the stem and crown exhibited a significant reduction in radial growth, despite a marked increase in sap flow density. In contrast, low-intensity fires, provided that they did not cause damage to surface roots, had no effect on the trees.

The application of prescribed burning in P. pinaster stands has also been investigated for its potential influence on resin production. Some studies suggest that low-intensity fires may increase resin flow by inducing the development of resin canals [182,183,184] and reducing resin viscosity [185]. However, other research has shown no significant effect of fire on resin yield [186].

Perrakis et al. [187], in a four-year study monitoring the effects of prescribed spring and autumn burns conducted in 2002 on the resin flow of P. ponderosa, found that, prior to burning, there was no significant difference in resin flow between the autumn burn treatment specimens and the control (unburned) specimens. However, after the intervention, trees subjected to both spring and autumn burns exhibited significantly higher resin flow than the control group. Although both spring- and autumn-treated trees peaked in resin production in the second year after treatment (2004), the differences between years were not statistically significant. The results indicated that even four years after the burn treatments, resin flow remained higher in burned trees than in unburned ones.

Prasetya et al. [183], when studying the effect of different forest fire types on resin productivity in P. merkusii trees in Indonesia, found that all fire types, comprising litter and shrub fires, bole fires, and crown fires, had a significant effect on resin production. Bole and crown fires yielded the highest average resin productivity, at 24.54 g/tree/day and 21.698 g/tree/day, respectively, while the control group had the lowest productivity at 12.78 g/tree/day. Moreover, between the sixth and eighth tappings, resin production declined across all fire types, which the authors attributed to increased rainfall at the study site. This led to lower temperatures and light intensity, which, in turn, reduced resin productivity.

Rodríguez-García et al. [186], when evaluating the effect of low-intensity prescribed burning on the numbers and areas of resin ducts and resin production in P. pinaster stands in Spain (2015–2016), applied four treatments: burning and tapping, burning only, tapping only, and a control treatment (no burning and no tapping). They found that, although resin production did not differ significantly between burned and unburned trees, certain anatomical traits associated with resin secretion were influenced by prescribed burning. The numbers and areas of resin ducts were greater in tapped trees than in untapped ones, regardless of whether burning was applied. However, burned and tapped trees exhibited values that were nearly twice those observed in trees that were only tapped in 2016, suggesting that the combined effects of fire and tapping promoted the development of defensive structures. Additionally, the absence of anatomical differences between trees in the control group and those subjected only to burning indicates that the burns were not severe enough to trigger a defense response, at least at the anatomical level.

In a follow-up study, Madrigal et al. [188], when conducting a similar study in the same area but over a four-year period (2018–2021), found that although resin production varied over the study period, no significant differences in resin yield were detected within the same year between stands subjected to both prescribed burning and tapping and those subjected to tapping alone.

4. Conclusions

Silvicultural practices such as thinning, pruning, and the use of prescribed burning have significant impacts on the development of Pinus stands and the production of forest resources. Understanding the silvicultural practices applied in the management of Pinus stands is essential for developing and implementing efficient silvicultural models aimed at optimizing resin tapping while still being adapted to the specific conditions of each site. The optimization of these practices can enhance both stand productivity and resilience, contributing to effective and sustainable forest management.

Several factors influence resin production. However, the inconsistency of findings in the scientific literature regarding the effects of these factors on resin yield, as well as the impacts of resin tapping on tree growth and wood quality, highlights the high level of variability involved in this process. These aspects underscore the need for further research to deepen our understanding of the relationships between silvicultural management practices in Pinus stands and resin production.

The integration of resin tapping with timber harvesting has the potential to make forest exploitation more profitable.

The advancement of knowledge on the key factors determining resin yield is essential for improving the management of this activity. Based on the remaining knowledge gaps, several directions for future research can be highlighted: (i) the establishment of reference values for dendrometric parameters specific to each species, such as DBH range, height, and optimal age for the onset of resin tapping; (ii) the standardization of silvicultural practices to optimize production, including the definition of thinning intensity and frequency, as well as pruning regimes; (iii) in-depth studies on the impact of prescribed burning on resin yield, considering both its applicability and potential effects; (iv) the development of silvicultural models specifically designed for stands intended for resin production, taking into account the main factors influencing yield and site productivity; (v) assessment of the long-term effects of combined resin and timber production; (vi) interactions with climate change and carbon sequestration; (vii) a comparison of sustainability indicators according to forest origin: natural regeneration versus plantation forests; (viii) genetic improvement and the selection of high-yielding species.