Mitigation of Global Climate Change through Genetic Improvement of Resin Production from Resinous Pines: The Case of Pinus halepensis in Greece

Abstract

Carbon sequestration by forests and storage in biomass has gained great interest globally in climate change mitigation. Resinous pine forests act as ideal carbon sinks because, in addition to capturing atmospheric CO₂ for biomass production, they produce resin (resin concentration in C: 77.17% w/w), contributing further to the mitigation of the greenhouse effect. Greece until the 1970s was considered one of the main resin-producing countries of Europe, due to the quantity and quality of resin products collected from natural populations mainly of Pinus halepensis Mill. Previous and current research has shown that resin production is a genetically controlled trait (h² > 0.70) that exhibits great variability among trees (resin 0.5–33.0 kg per tree and year). The above led to the genetic selection of P. halepensis genotypes with constant over time high resin yields (≥20 kg per tree and year) and consequently greater atmospheric CO₂ sequestration for more effective counteracting climate change but also for economic reasons for the benefit of resin producers. These high-yielding genotypes were cloned through grafting on P. brutia rootstocks and became potential trees for establishing commercial pine plantations. Thus, one hectare of commercial plantation of 500 P. halepensis trees, with a resin yield of 20 kg per tree, is expected to sequester 28.31 tn CO₂ per year (instead of 2.82 tn of CO₂ per year of a natural stand of 400 P. halepensis trees based on a resin yield of 2.5 kg per tree), at the productive age of 25 years. In this case, commercial plantations with improved genotypes of P. halepensis have great potential not only in mitigating the concentration of CO₂ in the atmosphere, but also in restoring degraded marginal areas and arid soils, and at the same time can offer social and economic benefits to the local communities.

1. Introduction

Climate change refers to shifts in temperature and weather patterns from the standard average on our planet for a long period of time [1]. Currently, it is the most pressing problem humanity has to deal with, requiring radical and decisive measures as its constantly growing impacts are touching every possible aspect of people’s lives. Among the main causes of climate change are human activities such as deforestation, biomass burning, wetland drainage, soil cultivation, and fossil fuel use that have intensified since the beginning of the industrial revolution [2,3]. The above contributes to the gradual increase in the concentration of greenhouse gases (GHGs) in the atmosphere, especially in carbon dioxide, methane, and nitrous oxide, which enhance the natural greenhouse effect and the increase in temperature [4].

The emissions of CO₂ in the atmosphere were within constant bounds for at least 400,000 years, until 1950, and turned out to be directly related to temperature [4,5,6]. The concentration of CO₂ has increased by 31% from 280 ppm in 1850 to 380 ppm in 2005, and is still rising by 1.7 ppm or 0.46% yearly, reaching 419 ppm in January of 2023 [6]. Methane (CH₄) and nitrous oxide (N₂O) concentrations have also a steady increase over the same period [6,7,8,9]. Emissions of increased amounts of CO₂ result in climate change that includes: (a) global warming, i.e., atmospheric temperature has risen by 1.18 °C since the end of the 19th century, with the ten warmest years recorded after 2005, indicating a continuous increase in temperature, (b) sea level rise by 15–23 cm during the 20th century, and (c) an increase in the incidence of extreme weather events and catastrophic wildfires along with significant changes in ecosystems [8,9].

Major concerns about the adverse effects of global climate change have led to the need for restrictive measures [10]. In addition to mitigating greenhouse gas emissions (mainly CO₂), the effective management of additional forest plantations in order to absorb carbon and convert it into more stable forms, is one of the few low-cost and energy alternatives [3,11,12].

The forest as an ecosystem, directly or indirectly affects natural resources (wood, water, soil, fauna), climatic factors, air pollution, plant biodiversity, recreation, and together with the oceans is the basic regulating mechanism of the global climate and the composition of the atmospheric air. Forests still retain uniqueness in social, ecologic, and economic systems worldwide, not only because of their big size but also due to their longevity and land coverage of up to 30% (3.4 billion ha), while 1/3 of global coal resources are stored in forests’ biomass [13]. The reduction of CO₂ emissions through forests has already been monitored [2,14]. For example, one hectare of beech (Fagus sp.) forest absorbs 4 tn CO₂ and produces 2.5 tn O₂ annually [15].

The scope of the work is to highlight the importance of resin produced yearly by pine trees in sequestering CO₂ in a sustainable global carbon pool aiming at mitigating the increase in the concentration of CO₂ in the atmosphere. This could be achieved by further increasing the resin produced through the genetic selection of highly efficient pine genotypes, which is discussed in this paper.

2. Pinus Species and Their Valuable Byproduct “Oleoresin”

Gymnosperms such as species of the genus Pinus have the ability to grow in a wide range of environmental conditions, even in quite severe and extreme climatic and soil conditions. Pine forests operate as reservoirs of atmospheric CO₂ (natural carbon sinks) and contribute to the mitigation of the greenhouse effect [3]. Certainly, a percentage of the stored carbon is distributed to the roots and soil but it is very small compared to the amount of carbon the tree uses to produce wood [16]. Apart from woody biomass, resinous pine forests are able to produce large amounts of resin (pine oleoresin), contributing significantly to further storage of CO₂ (Figure 1). Thus, the advantages of creating resinous pine plantations derive from the simple and low-cost cultivation requirements, along with the high yields in biomass and resin. Resin is produced from various pine species around the world (e.g., P. elliottii, P. taeda, P. pinaster and P. palustris in the USA, P. maritima, P. halepensis, P. pinea and P. brutia in Europe, etc.) [17,18,19]. According to Cunningham [19], the global resin production in 2010 reached approximately 1,100,000 tn. The main providers were China, Brazil, and Indonesia.

The chemical composition of resin is a mixture of volatile (turpentine) and non-volatile terpenoids (rosin). The basic structural element of all terpenes is isoprene (C₅H₈). Polymerization of isoprene units constitutes the various terpene classes. Thus, due to its chemical structure (presence of isoprenes), the resin is considered a natural polymer (mixture of hydrocarbons) (Figure 2) [17,21].

Terpenes are considered the largest chemical group of byproducts on our planet (more than 40,000 different metabolites) and pine terpenes in particular are still the basis of the oldest and largest chemical industry with renewable raw materials [3,22]. They could be used in improving the quality of liquid fuels and also in biofuels [23,24,25,26], in the stored food industry against Listeria monocytogenes [3] and Salmonela enderica [27,28], in a number of pharmaceuticals [29,30,31,32,33,34,35] and environmentally friendly products such as insecticides and repellents [36]. Also, some terpenes exhibit anti-tumoral action [37,38,39], while others have antimicrobial properties [40].

On the other hand, oleoresin is an incriminating factor in summer wildfires at pine forests. Due to its flammability, it could be a potential fuel material. The combustion of the resin is no different from other hydrocarbons currently used to produce energy according to the formula: CxHy + (x + y/4) O₂ → xCO₂ + y/2H₂O + Energy.

3. Production of Oleoresin from P. halepensis Trees

Aleppo pine (P. halepensis) is an evergreen coniferous tree of the Pinaceae family. It is characterized by its high adaptive capacity, as it grows on very different soils, even arid, dry, and shallow. It is a drought-resistant species that withstand dry periods of up to 6 months, but has difficulty adapting to cold climates and low temperatures. It is also a well-adapted species to forest fires, due to the highly competitive ability of its young trees against other types of understorey vegetation and because its cones are produced annually but remain closed for a long time. In addition, it is a very important forest species, ecologically, economically, and aesthetically [15,41].

Aleppo pine is the predominant pine species of resin production in many Mediterranean countries, as it produces significant quantities of resin [15,17]. The resin is a secondary product of pines and generally of all conifers, formed in the process of evolution as a defense mechanism of these species against biotic factors (e.g., pathogens, insects, etc.). It flows through the resin canals and is usually collected by wounding the bark (resin harvesting, tapping). Overall, P. halepensis as a resinous pine is a typical multipurpose tree and in addition, its forests act as “ideal carbon sinks” [3,20].

P. halepensis is a tree widely distributed all over the Mediterranean basin (over 3,000,000 ha) [41]. In Greece, its range extends from Chalkidiki, in the north, to Peloponnese, in the south and certain islands of the Aegean and Ionian Seas (370,000 ha, with a mean annual growth in wood of approximately 2.5 m³/ha) [42]. Greece until the 1970s was the main oleoresin producer country of Europe, due to the quantity and superior quality of resin products from Aleppo pine natural populations with an average annual production of approximately 2.5 kg per tree (Figure 1) [43]. Other pine species used for resin production were P. brutia and P. nigra, with P. halepensis however being the most productive of the three species [17,20].

Considering the wood density of 550 kg/m³ [43], which is equivalent to the carbon content of approximately 50% by weight of wood, it turns out that one hectare of P. halepensis forest sequesters 0.69 tn C/year (or 2.53 tn CO₂/year, based on the molecular weight of 44 for CO₂) for wood biomass production alone (

Table 1. Carbon (C) and CO₂ sequestration rates for the various types of management of P. halepensis trees.

Management Type	Sequestration of C (tn of C/ha/year)	Sequestration of CO2 (tn of CO2/ha/year)
Wood biomass production from natural stands of P. halepensis (density 400 trees/ha)	0.69 (0.55 * × 2.5 m3/ha ** × 0.5 ***)	2.53 (0.69 × 44 ÷ 12)
Resin production from natural stands of P. halepensis (2.5 kg resin/tree) (density 400 trees/ha)	0.77 (1 tn/ha × 0.7717 ****)	2.82 (0.77 × 44 ÷ 12)
Resin production from commercial plantations of genetically improved P. halepensis (20 kg resin/tree) (density 500 trees/ha)	7.72 (10 tn/ha × 0.7717)	28.31 (7.72 × 44 ÷ 12)

* Rate of dry weight of wood (w/v), ** Annual growth of wood per hectare, *** Content of wood in C (w/w), **** Content rate of C (0.7717) in resin (w/w).

). This estimation is close to that of Padilla et al. [44], who have mentioned that the captured amount of CO₂ of a P. halepensis plantation in Israel (density of 360 trees/ha) is 0.99 tn C per hectare and year. Furthermore, the total amount of CO₂ sequestration by P. halepensis of 5.35 tn per hectare and year, resulting from the annual increase of 2.53 tn in wood production and 2.82 tn from the resin yield of one hectare of natural stand (Table 1), is higher than the amount of CO₂ captured by a typical broad-leaved forest (e.g., Fagus sp., 4 tn CO₂ per hectare and year) for the annual increase in wood production [15].

Chemical analysis of P. halepensis resin resulted in: C 77.17%, H 9.83%, Ν 0.07%, O and S 12.93% [21]. The average yield of Greek P. halepensis is 1.7–3.3 kg per tree per year [43], but some elite genotypes located in the region of Chalkidiki Peninsula and the Euboea Island reached remarkably high annual yields of resin ranging from 10 to 33 kg/tree/year [45,46,47]. Applying the same calculations (as for the wood biomass), it was estimated that: one hectare of P. halepensis forest, of the same plantation density (400 trees/ha), which produces 2.5 kg of resin per tree per year, accumulates approximately 0.77 tn of carbon per year (Table 1). This amount is almost equal to that of carbon absorbed for wood biomass production.

4. Use of Highly Resin-Yielding Genotypes of P. halepensis to Mitigate the Greenhouse Effect

Genetic improvement strategies [13,48] that include the appropriate selection and reproduction of high-yielding genotypes, both for wood and resin, seem to be an efficient measure to mitigate climate change. To fulfill this purpose, the accurate evaluation of elite phenotypes in two or more growing seasons and their clonal propagation are useful tools [47,49,50,51,52,53,54,55,56] as depicted in Figure 3.

The “resin production” and the “oleoresin chemical components” traits have been proved, by numerous studies, to be mainly genetically controlled (heredity h² = 0.42–0.90) for North American resinous pines [18,57,58,59,60,61,62,63,64]. Concerning P. halepensis, the heritability of the “resin production” trait was found to be very high (h² = 0.75–0.85) [65], showing, also, high variability in resin production between individuals (0.5–33 kg/tree) [45]. The above means that the creation of commercial plantations with seeds is also very effective. In addition, the productivity of resin remains constant over time for each tree, implying that the selected high-performance phenotypes are also superior resin-producing genotypes [45,61,66]. Creating then “super-resinous” commercial plantations, by genetic improvement and cloning methods (Figure 4), could substantially contribute to the storage of atmospheric carbon by following two essential steps, firstly by selecting elite genotypes based on constant (over time) high-resin-yielding phenotypes [66,67], and secondly by cloning these elite genotypes and establishing commercial plantations [50,68,69]. Specifically, for resin production, it is proposed the selection of super-resinous trees (resin yield at least 20 kg/tree/year), according to their constant productivity over time and their cloning by the grafting technique [46,47]. In this case, the density of commercial plantations is suggested to be of 500 trees/ha (plant spacing 4 × 5 m). Plantations of high-performance Aleppo pine clones with a rotation time of 60 years are expected to reach their resin-productive period in their 25th year of age and will be capable to absorb 7.72 tn of carbon/ha/year (or 28.31 tn of CO₂/ha/year), i.e., eleven times more compared to the amount of carbon absorbed for the production of wood biomass of 0.69 tn C/ha/year (or 2.53 tn CO₂/ha/year) (Table 1).

5. Conclusions

Natural stands of resinous pines such as P. halepensis, besides the biomass production process by utilizing CO₂, further contribute to atmospheric CO₂ sequestration due to resin production and this contribution increases even more in the case of high-yielding genotypes. Furthermore, commercial plantations with genetically improved genotypes of P. halepensis for high resin production sequestrate at least eleven times more atmospheric CO₂ than natural stands, contributing greatly to counteracting the greenhouse effect and thus mitigating climate change on our planet on a sustainable basis. In addition, pine resin (oleoresin) is a renewable product with multiple applications at national and global levels, especially in innovative technological fields of green chemistry and not only, while its exploitation will have multiple positive impacts on the social and circular economy. The added value that the resin acquires with its new applications will drastically reduce the risk of wildfires since local communities, in order to protect their income, are very likely to act as natural custodians of the commercial forest plantations of P. halepensis in their regions. We feel that this carbon-sequestration activity is in line with the Kyoto Protocol under the United Nations Framework Convention on Climate Change, which encourages afforestation, that is, the conversion of non-forested land to forest.