Maritime Pine, Its Biological and Silvicultural Traits for the Basis of Natural Resources: An Overview

Abstract

Maritime pine (Pinus pinaster Aiton) is a forest tree species with a high representation in southwestern European countries, in particular Portugal, Spain, and France. The species traits and their flexibility and plasticity are of importance both for timber and to the sustainability of the forest systems. Extensive research has been made on the maritime pine systems and productions. The aim of this study is to review the state-of-the art on the knowledge of the species, their forest systems, and their productions, to identify vulnerabilities and to summarize tools to help its management. The specific objectives of this review are: i) characterizing maritime pine, its distribution, genetic material and provenances, the biotic and abiotic disturbances, the diversity and sustainability of its forest systems; (ii) its management, encompassing the silvicultural systems and practices; (iii) to list existing growth models, simulators and decision support systems; and (iv) present information on wood technology, including sylvotechnology, wood properties, and their use.

1. Introduction

Maritime pine (Pinus pinaster Aiton) is a conifer with a large area of distribution and of particular value, namely in terms of provisioning, regulating, and supporting ecosystem services. In Europe, its main distribution occurs in the Southwest Atlantic region (Portugal, Spain, and France), and to a lesser extent in other regions of Mediterranean influence. It has also been successfully introduced to other continents. One major benefit of the maritime pine forests, inherently associated with its expansion, is wood production and the supply of timber. The species plasticity and rusticity associated with its many functions, from production to protection, is linked to its wood quality and yields, make it a specie of primordial importance in several countries. Currently, it is prone to a suite of abiotic and biotic disturbances (e.g., fire, drought, pests, and diseases), which can act simultaneously or not. The forest system and production sustainability have to be thought holistically, with the selection of the better-suited management systems and sites to promote optimized yields and wood quality. The aim of this review is to provide information on the state of the technical knowledge of maritime pine and its forest systems. The objectives are fourfold: (i) distribution and ecology of maritime pine (Section 2); (ii) silviculture (Section 3); (iii) models, simulators, and decision support systems (Section 4), and (iv) wood technology (Section 5).

2. Distribution and ecology of P. pinaster

Maritime pine (P. pinaster Aiton) is an evergreen conifer species belonging to Pinaceae and Pinus genera. It is a plastic specie characterized by its fast growth, shade intolerance, and being rustic ([1, 2] and references therein). Its area of distribution ranges from Portugal to Greece and from Morocco to Tunisia, whether as continuous ancient or recent areas (Figure 1, [3]). The specie is reported as native in France, Italy, Spain, Morocco, and Portugal [4]. It can be found outside its natural range in Australia, New Zealand, South Africa, Chile, Argentina or Uruguay [5], Turkey, the Balkans, United Kingdom, and Belgium (Figure 1, [3]). Its distribution is probably associated with the species traits’ plasticity and wood quality. The specie’s prolific seed production, wind-dispersed seed, and rapid growth rate, support the qualification of the species as an aggressive colonizer in some of the countries where it was introduced [4].

Figure 1.

Area of distribution of maritime pine (source: Caudullo et al. [3]).

Maritime pine develops for a range of mean annual temperature between 13 and 15°C, and 8 to 10° C in the colder months, mean annual precipitation larger than 800 mm (100 mm in the dry season), altitudes up to 800 m. It has low sensitivity to autumn and winter frosts, but high to spring ones, and has a high sensitivity to snow. It prefers soils of light texture, with good drainage and with a depth larger than 30 cm, where root systems develop better but do not tolerate, calcareous, saline, hydromorphic, and compacted soils. Its ability to grow in shallow and nutrient-poor sites is due to not being very demanding regarding mineral nutrition and by establishing ectomycorrhizal associations that improve its ability to uptake nutrients in soils with pH less or equal to 5 [1, 2, 6, 7, 8]. The root system consists of superficial roots, which ensure the stability of the tree and support the fine roots, responsible for the absorption of water and nutrients, and deep roots, which ensure the attachment to the soil and the tree’s access to water from deeper groundwater levels [6]. It provides good anchorage regardless of soil water content, except when in full saturation in sandy soils [9]. Nonetheless, the lower the nutrients’ availability the lower the potential growth of the trees [10, 11]. It reaches 30–40 m in height [12], its longevity is between 80 and 300 years [1, 2] and it is shade intolerant [2]. It resists well the summer water deficits, characteristic of the Mediterranean region, as due to the high sensitivity of the stomata to water deficit it is able to maintain tissue hydration at adequate levels [13, 14]. Its imminently pioneering character is notorious in the success of its use in the fixation of coastal dunes formed by sands poor in organic matter, minerals, and water retention capacity [2].

In France, maritime pine occupies an area of 1015 thousand hectares, with the Landes having the largest monospecific area. While it represents 5% of the metropolitan French forested area, it is the most harvested species with 6.7 Mm³/year of removals [15]. It is also widely distributed in northwest Spain, in the Autonomous Communities of Galicia and Asturias and the province of León, and is the most important coniferous tree species in terms of both surface cover, with an area of 433,754 ha, and wood production [16, 17] with a volume harvested in 2017 of 3.4 Mm³ [16]. In Portugal mainland, its distribution extends along a coastal strip of low altitude from North to South as well as in the inner North and Central regions, up to an altitude of 700–900 m mainly under Atlantic climatic influence, and mostly in the Southwest to North aspects. It is the most represented conifer species in northern and central Portugal, occupying an area of 713.3 thousand hectares and a growing stock of 67 Mm³ [18]. Wood availability is estimated at 1.8 Mm³, in 2018, with a consumption of 4.2 Mm³ [19]. Typical stands are shown in Figures 2–4.

Figure 2.

A mature stand of Pinus pinaster (Mata Nacional de Leiria, Portugal).

Figure 3.

Natural regeneration of Pinus pinaster after clearcutting (Mata Nacional de Leiria, Portugal).

Figure 4.

Adult stand of Pinus pinaster (Vale do Tâmega, Portugal).

The importance of maritime pine is not confined to its area, but it is also related to its economic returns and goods and services its stands and forests provide. Maritime pine major products (wood and resin) have a wide variety of uses, involving a complex forestry-industrial sector and integrating, in addition to the set associated with the transformation of wood, a range of enterprises processing non-woody forest raw materials, with emphasis on resinous products. Its contribution to the national economies is relevant. For example, in Portugal, this sector has 8516 companies and is responsible for 57,843 employees (representing 88% of industrial companies and 81% of employment in the Forestry Sector) and generate 1225 € million of Gross Value Added, €4348 million of Turnover, and €1876 million of exports (3.1% of national exports of goods) [19, 20].

In Portugal, it is the main wood-producing species for general purposes, which, in addition to a medium wood density, combines good strength characteristics and easy working. According to [20], of the 4.5 million m³ of P. pinaster wood consumed in 2019 in Portugal, 1.82 million m³ corresponded to timber wood, 1.07 million m³ to pellets, 0.68 million m³ to wood panels, 0.56 million m³ for pulp and paper, 0.20 million m³ for biomass, and 0.15 million m³ for poles, pilings, posts, and sleepers. In addition, it has also to be highlighted the production of resin extracted from this species, which in the last 8 years has ranged from 6000 to 8000 t per year [20].

The importance of this sector goes far beyond the purely economic aspects, as its stands are essential for the populations life quality, with a direct impact on the quality of air, soil, and water and, in general terms, in the surrounding ecosystem. For example, P. pinaster forests constitute the largest carbon reservoir in the Portuguese forest (90.3 Gg CO₂) and also the most carbon stored per hectare (119.4 t CO₂/ha) [20].

Maritime pine stands, due to its low crown cover, result of its shade intolerance, enable the development of an herbaceous and shrub understorey. This understory encompasses a suite of species resulting in moderate to high species richness. Also, it serves as shelter and reproduction spots for several bird, mammal, and reptile species [21]. Diversity is also enhanced by the different stand structures, from pure even-aged to mixed uneven-aged [22, 23, 24].

The sustainability of the pine stands and their productions are dependent on their resilience to disturbances, which include type, intensity, and frequency. Silvicultural practices are disturbances of low intensity and high frequency, with the aim of promoting growth. In general, its effects promote the system sustainability. Inversely, high intensity and low-frequency disturbances, such as fires or storms, may endanger the system sustainability [25]. Maritime pine stands are prone to fires, especially when a well-developed understory promotes the continuity of the vertical profile of the stand. The effects of forest fires on forest stands in general, and on maritime pine in particular, are twofold: the destruction of the stand and effects on soil. The resilience of the stand is linked to the regeneration which in turn is associated with the intensity of destruction (total or partial), type of regeneration (sexual or asexual reproduction), and the availability of seeds (whether in the soil or in the tree crowns). Maritime pine regenerates by seed (it is not able to sprout) and as long as seed is available, stand regeneration occurs [26]. It is well known the effect of vegetation on soil conservation and reduction of erosion risk, which is especially relevant in climates subject to high-intensity rainfalls, such as the Mediterranean climate. Also, vegetation, especially the arboreal, gives a primordial contribution to the maintenance and improvement of the soil’s physical, chemical, and biological properties, thus contributing to maintain and improve site quality ([27] and references therein). Maritime pine stands are frequently in sites of low quality, many times in steep slopes areas with high-intensity rainfalls [28, 29]. Thus, its sustainability can be enhanced by disturbances of low intensity and high frequency, such as the silvicultural practices (thinning and pruning) that can prevent those of high intensity and low frequency, such as fires. Maritime pine is also vulnerable to wind damage [30]. Extreme wind events associated with severe extratropical cyclones (storms) have caused extensive damage in Europe. In France, Nouvelle-Aquitaine region, the damage of Martin and Klaus storms affected predominately maritime pine (37 million m³), which correspond to 15% and 32% of the maritime pine standing volume in the region in the former and latter storm, respectively [31]. The uprooting of trees and stem breakages have been reported for the species in Portugal [32, 33, 34], which may result from soil characteristics and individual tree social status, and the critical turning point at the base of the stem was correlated to tree size and particularly to stem weight or volume [35].

Among the biotic agents affecting the species, the pine processionary moth, Thaumetopoea pityocampa (Lepidoptera, Thaumetopoeidae) is referred to as the most serious pest in the Mediterranean region [4, 36]. The species is susceptible to Bursaphelencus xylophilus, the nematode that causes the pine wilt disease [4, 37], and to root rot pathogen Heterobasidion annosum [38]. Bark beetles (Ips sexdentatus, Orthotomicus erosus, Tomicus piniperda and T. destruens) are also referred to as the main biotic agents causing economic losses to the species [37].

The maritime pine stands sustainability is also linked with climatic change. The increase in temperature and decrease of precipitation may result in a trend to its northwards distribution [28, 29]. Also, it seems that there will be a trend towards a longer dry season in the Mediterranean. One way to mitigate its effects is by reducing density through thinning in maritime pine stands and/or with mixed stands [39, 40] of maritime pine with other conifer or broadleaved species (Section 3.1).

3. Silviculture of maritime pine (P. pinaster)

3.1 Forest systems

Maritime pine is managed in high-forest stands [2] (see Figures 2 and 4). The structure is most frequently even-aged, whether from natural [41, 42, 43, 44] (Figure 3) or artificial [2, 40, 45, 46] regeneration. Traditionally, maritime pine is managed in pure stands. The preference for even-aged stands is related to easier management, promotion of wood quantity and quality [2, 40, 47, 48, 49, 50, 51], and disturbances, mainly fire or harvest events that usually result in one regeneration cohort shortly after disturbance, if seed is available [41, 42, 43, 44].

The uneven-aged structure is less frequent [22, 24, 42] probably due to the specie traits. Uneven-aged stands are more frequently developed with shade-tolerant species. Yet, uneven-aged stands have been successfully developed with shade-intolerant species with few cohorts (1 to 4) [52, 53, 54]. Several studies compare and discuss even and uneven-aged stands of maritime pine [55, 56, 57]. Uneven-aged stands of maritime pine are frequently originated from natural regeneration, whether as pure [22, 42, 47, 58] or mixed stands [23, 24, 59, 60].

The advantages of mixed stands in what concerns the stands’ sustainability while attaining similar or better yields than pure stands [52] enhanced the spread of maritime pine mixed stands. Examples are: P. pinaster and P. sylvestris [46, 50, 60]; P. pinaster and P. pinea, P. sylvestris, P. halepensis or P. nigra [39]; P. pinaster and Quercus pyrenaica [61]; P. pinaster and P. radiata [45]; P. pinaster, Castanea sativa and Quercus robur [23, 24]; and P. pinaster and Eucalyptus spp. [62]. While some mixed stands are originated from plantations [45] others are the result of natural regeneration [24, 42, 62]. Overall, mixed maritime pine stands have higher diversity [24, 50]; soil fertility is enhanced [50]; have a higher water holding capacity [63], and higher yields [60].

The development of maritime pine is determined by four broad factors; water availability, aerial growing space availability, tree “social” status (based on tree’s height relative to surrounding trees), and silviculture practices. Maritime pine stands in the Mediterranean climate are constrained by the available water. Several references [59, 61, 64, 65, 66, 67, 68, 69] indicate that growth occurs mainly in spring and autumn as a result of precipitation [67]. A study on the effect of precipitation on water uptake in maritime pine, stresses the effects of the temporal variability of rainfall and site on the water availability [67]. As the water absorption by maritime pine individuals does not occur immediately after the rainfall but has some delay in time [67, 68], it is better explained by a set of events of rainfall [67]. Also, summer precipitation (from May to September) seems to have low contribution to the absorption of water for two reasons: the precipitation amount is low and it is partially lost through evaporation. In mixed stands of P. pinaster and Quercus pyrenaica, spring growth of maritime pine is promoted in the early spring because leaf area is available prior to the oak’s [59, 61] and due to the maritime pine root system, which is able to develop in depth thus exploring a large volume of soil [70, 71]. Also, when under water deficit, maritime pine ceases growth both in spring and fall [59, 61, 64]. In fall, trees are able to grow if water is larger than what is needed for the rehydration [59, 64]. The geographic origin along with the climate influences the tree growth reaction to drought, with higher growth under Atlantic climates than under the Mediterranean ones, which is related to the xeric climate adaptation of the species [72].

All species have, to a lower or wider extent, plasticity which enables individuals to adapt to the available growing space, by maintaining or increasing light intersection, water and nutrients absorption, and reducing competition. Species plasticity results in the variation of tree allometry, which enables the maintenance of growth. Crown plasticity can be the result of stand structure and/or climatic conditions [52]. For maritime pine individuals the increase in density results in the reduction of crown size due to crowding, when individuals do not have enough aerial growing space, or when branch abrasion occurs. These phenomena constrain the lateral growth of the crown and being maritime pine shade-intolerant, the lower crown under shade dies, resulting in the regression of the crown [39, 73]. Drought also affects crown allometry. In sites prone to drought its crown tends to have a large volume. The larger crown volume can be explained by the stands’ low density, being trees in free growth thus expressing the growing habits characteristic of the specie; and as the main limiting factor is water; it is expected that belowground competition is higher than that above ground. As a consequence, the crown competition and the variability in its allometry are weaker on dry sites and stronger on humid ones [39]. Likewise, the increase of aridity decreases productivity both in pure and mixed stands, whether for volume [73] or for biomass [74].

Individual tree social status influences tree allometry and growth. In pure stands, the individuals in the lower social status (dominated) have lower sizes and growth rates, due mainly to the lower availability of growing space, light in particular. In mixed stands of P. pinaster and P. sylvestris, it was found a negative effect on dominated maritime pine individuals, probably due to the shade casted to those individuals. Inversely, in the admixtures of P. pinea and P. pinaster, and P. nigra and P. pinaster, the effects on the dominated trees were positive, which can be attributed to the different crown architecture of the species [39]. In P. pinaster and P. sylvestris, pure and mixed stands [60], maritime pine crowns in mixtures had smaller volumes (related to the specie shade intolerance), than in pure stands, and high competition for light was also found. Inversely, P. sylvestris tends to keep its lower branches (as it is more tolerant to shade). Also, maritime pine tends to increase its height growth to enable the individuals to reach the upper canopy layer, and, thus to reach sunlight. This results in the ascension of its crowns, which is enhanced by the crown regression (i.e., the death of the shaded lower branches) and by the development of branches with steep angles in relation to the stem. The different behavior of the two species might promote the stand vertical stratification and the optimization of the available canopy space [60]. The former and the higher capacity to hold water off the mixed stands [63] may, at least partially, explain the increase in productivity [60], stocking, and total organic carbon [75] found in mixed stands when compared to pure maritime pine stands.

Defoliation in maritime pine individuals results in the reduction of growth, of −0.9% of increment in basal area per 1% reduction of leaf area. For 15–30% of defoliation, the reduction of growth is considerable [49]. In a drought study, Rodriguez-Vallejo [40] observed that leaf area reduction due to drought resulted in the reduction of tree growth and that in natural stands was lower than in plantations. The reduction of growth due to leaf area reduction is related to the decrease of transpiration, hydraulic conductivity, and increase in xylem embolisms as well as competition for water. Thinning reducing competition may mitigate drought impacts on tree vigor and growth in maritime pine plantations [40].

Differences in tree allometry can also be assessed based on the configuration of the tree stem profile and have a direct influence on stem volume. Calçada-Duarte [76] points to a large number of geometric volume shapes for the species, varying from paraboloid to a solid of intermediate features of cone and neiloid (stem form with high tapering), which can result in stem volume differences greater than 25% for trees with equal values of diameter at breast height and total tree height.

3.2 Silvicultural practices

The most frequent silvicultural practices in maritime pine stands are thinning and pruning. Thinning is used to regulate stand density. The goal is to maintain the best trees, that will reach the end of the production cycle and remove those that have lower growth rates (dominated), less desired stem shapes, or are dead or diseased [77] while providing intermediate economic revenues. The most frequent thinning method is from below (e. g., [2, 47]). This method is used because it is suited for shade-intolerant species and for sites with periodical drought season [77], which is the case of the maritime pine stands in the Mediterranean basin with an annual summer drought period. Thinning is of importance in these stands due to its effects on tree and stand growth; wood quality and quantity, especially when associated with pruning; and system sustainability, particularly to disturbances such as fire and drought. Due to its shade intolerance, their release should be done early in stand development [2, 78].

The thinning intensity can be based on empirical rules or defined by objective criteria, being usual to use of Wilson’s spacing factor [79] or Hart-Becking spacing index (H-B), widely used in France for coniferous trees (e.g., [80]), and Stand Density Index [81], the latter based on the self-thinning theory law. Density regulation based on SDI relies on the assumption that in monospecific even-aged populations of trees experiencing complete crown closure, mortality is density-dependent. The natural trajectory of the number of trees per tree size was defined by Luis and Fonseca [82] and revised by Enes et al. [44]. The use of relative values of SDI is suitable for management purposes, as it provides information on the appropriate number of living trees for given tree size, according to the management aims (e.g., optimum growth-density interval, maximization of stand volume, or maximization of mean tree size).

Arellano-Pérez [47] in maritime pine pure even-aged stands, used thinning from below with two intensities, light (removal of 20% of basal area) and heavy (removal of 40% of basal area), and compared them with unthinned plots. The authors observed that growth in diameter was the largest in the heavy thinning plots while total and crown base were similar in all treatments. Six years after thinning basal area was the largest in unthinned plots. The fuel load was lower in thinned plots, but that of the understorey had a slight increase in the thinned plots. Thinning reduces the probability of active crown fire probability but increases passive one. Overall, according to Arellano-Pérez [47] thinning did not affect fire severity and reduced potential fire risk. The effect of density on maritime pine growth is related to competition for growing space. The higher the density attained, the lower the growth, especially in diameter [58, 83]. Stands with high density are exposed to longer periods of hydric stress, especially during the drier months. Inversely, in low density stands, individual trees develop larger (deeper and wider) root systems, thus reaching water stored in the lower soil layers [83].

Nunes et al. [84] in a thinning from below experiment in maritime pine pure even-aged stands with intensity ranging from light to heavy, highlighted its importance in diameter growth while height growth was not affected. Another study in a mixed stand of P. pinaster and Quercus pyrenaica [59] observed the highest radial increments with heavy thinning intensity. The difference between treatments corresponded to the spring growth (earlywood) and was constrained especially by water availability, i.e., under drought, there was a reduction of radial growth. Inversely, the autumn radial growth (latewood) does not seem to be affected by thinning, probably because it is highly dependent on the precipitation amount [59].

Pruning is a silvicultural practice frequently associated with thinning. Its main goal is to form a knot-free wood stem as high as possible, the reduction of the knotty stem core (both in number and size) and to stop juvenile wood growth [2, 85]. Pruning is recommended for two reasons: to reduce the knots number and size, which is one of the most derogatory wood features when used for nobler applications (e.g., veneer, plywood, structural elements, and furniture), both in the wood appearance characteristics and their mechanical resistance [86, 87]; and the removal of the less photosynthetically efficient branches (frequently the lower), enabling an increase of the carbohydrate availability, thus increasing growth [88, 89]. Yet, pruning removes both dead and live branches, the latter reducing also leaf area, which may also reduce photosynthesis and thus growth [90]. Hevia et al. [45] evaluated the effect of light (12–15% crown removal) and heavy (29–37% crown removal) pruning in young (7–11 years old) pure even-aged stands of maritime pine, and compared the results with unpruned trees. The higher the pruning intensity is, the greater will be the reduction of diameter growth, while lower effects were detected for height growth. Similar results were attained by Courdier et al. [91]. The effects of pruning intensity are related to species traits, namely the architecture of the crown, leaf surface area, photosynthesis, shade tolerance, and growth rates; but also, to edaphic and climatic site characteristics [45]. Hevia et al. [45] observed that the increase of growth post pruning was related to site index, relative spacing index, age, and tree diameter, as well as stand structure prior to pruning. The authors mentioned that the better the site, the older the trees, and the larger the diameter, the higher the growth in diameter and height. The post pruning growth seems to be also linked to the reserves in carbohydrates; the larger the reserves the higher the growth ([45] and references therein).

3.3 Stand regeneration

The regeneration of a stand is linked to its forest system. Clear cutting is associated mainly with artificial regeneration while clear-cutting with standards, clear-cutting by strips and/or patches, and shelterwood systems are frequently linked to natural regeneration [92, 93]. The most frequently used regeneration systems in maritime pine stands are clear-cutting, clear-cutting with standards, and clear-cutting by strips [2, 46].

Natural regeneration encompasses a set of sequential steps, namely seed production, seed dispersal, germination, and seedling establishment. Maritime pine trees are self-fertile. Wind pollination helps to spread their pollen grains from the male sexual organs (cone) to the female ones. Flowering, fruiting, and seed production are dependent on the tree development stage, stand density, and climate. Maritime pine individuals start to fruit at about 10–15 years old, with a periodicity of masting cycles of 3–5 years [2]. Trees with larger dimensions produce higher cone yields. Trees with larger dimensions tend to be in the upper layer of the canopy, are more vigorous and the light crow area is larger, all of which contribute to the increase of cone production [43, 94]. The reduction of density through thinning, reducing competition, and promoting the increase of crown area, especially the outer one where flowering and fruiting occur, increases fruit yield [43, 94].

Cone full development needs 2 years to be achieved [2] and climate, especially precipitation, determines the number of mature cones per year [43, 95]. For maritime pines stands the seed production per year is enough to regenerate the stands, in spite of its interannual variability [94, 96]. Its seeds are mainly wind dispersed; thus, wind direction and intensity are key factors in its dispersal, which occurs in the summer, from June to August [2]. The mean and the maximum dispersal distances of the seed are circa 14–25 m and 54 m, respectively [97].

Germination is related to seed germination rate and predation both before and after dispersal. Maritime pine germination occurs either in spring or autumn [2] and it is dependent on nutrient availability as the seed have few reserves; water, the increase in water stress reduces the germination and survival rates; and light environment, as germination and early development of seedlings is promoted by semi-shade environments that reduce light intensity and soil temperature, and increase soil moisture [78, 94, 98]. Guignabert et al. [94] mentioned that drought in summer was the primary cause of death in seedlings, mainly due to the increase of the deficit in vapor pressure and transpiration of seedlings. Partial cutting reduced water stress, thus promoting seedling survival [94] and a crown cover of about 32% had higher germination and survival rate of seedlings when compared with a crown cover of circa 5% [98].

Guignabert et al. [94] comparing seedlings with partial cutting clearcutting observed that seed production and dispersal were not limiting factors to regeneration. Inversely, the storage and conservation of seed in the seedbank constrained germination because of the high predation after dispersal; harvest residues and litter layer did not allow seeds to reach the soil; the capacity of germination of seeds was lower on clearcutting, and the germination rate was high in the first year after seed rain (previous year to harvest) and drastically reduced in the two following years.

Seed predation is a primordial factor in maritime pine regeneration. Predation before dispersal occurs when fruits are in the maturation early stages, while predation after dispersal takes place in the ground prior to germination, mainly by birds and insects. Post dispersal seed predation happens mostly in autumn and winter and depends on seed and predators’ number, frequently having a trend towards a high spatial and temporal variability [99]. Ruano et al. [96] observed that predation reduced seed of maritime pine from 400,000–500,000 seeds/ha to 10,000 seeds/ha, and that the seed predation rate increased with the decrease of quantity of seed.

3.4 Stand structure dynamics

Stand structure dynamics is determined by the initial species composition and proportions and structure. The differences in stand structure, even if they are small, may be, and many times are, enlarged in time [25]. These differences are visible both in the estimates of the stand variables and their precision and accuracy, which reinforces the need to develop flexible models that accommodate the variability of growth patterns and interactions between individuals for the variability in stand structure [52]. Alegria [42] and Alegria and Tomé [22] developed growth models for maritime pine uneven-aged stands. In both studies, the authors referred that the existing models (developed for even-aged stands) are not able to accommodate the differences in structure, and the new models outperformed the existing ones. Gómez-García [100] developed height-diameter functions for P. pinaster mentioning that mixed models were able to accommodate the variability in tree allometry as well as the limitations on the available data. Riofrío et al. [46] developed height-diameter functions for P. pinaster and P. sylvestris, pure and mixed even-aged stands. The model was able to accommodate the different patterns between trees and species, and account for the different species traits, allometry, and interactions. Also, the authors reported that these models had better performance than those existing for pure even-aged stands.

In maritime pine even-aged stands, rotation can be defined for a target age or diameter. Rotation age varies between 35 and 45 years [2], though longer rotations have been used, for example in coastal dunes of Mata Nacional de Leiria (see Figure 2), of 70 years for timber and 100–140 years for protection [101]. The target diameter is defined according to the use of wood with 7–14 cm of diameter at breast height for panels and pulp; 14–20 cm for timber and > 35 cm for veneer wood and large dimension timber [2]. Figure 5 presents P. pinaster wood logs, after logging.

Figure 5.

Pinus pinaster wood logs.

Stand structure, tree growth, and silvicultural practices have a key role in wood quantity and quality. High stand density, especially in the early stages of development, promotes height growth in maritime pine stands, which shortens the period of juvenile growth of wood enabling trees to develop mature wood at early stand development stages [87, 102, 103], as well as reducing stem taper and promoting stem straightness that reduces the amount of reaction (compression) wood, thus reducing the undesirable characteristics for most wood uses [104]. However, as it is a fast-growing specie and shade-intolerant, release through non-commercial or commercial thinning should be prescribed [2]. The reasons for the early release of competition are twofold. The release will increase diameter growth and tree mechanical stability. The mechanical tree stability is frequently accessed with the h/d ratio (ratio between total tree height and diameter at breast height, with both variables in the same units). Mechanical stability is attained for h/d lower than 85 ([105] and references therein). As already referred due to its shade intolerance maritime pine individuals, when in dense stands lose their lower branches [2] whether due to shading or branch abrasion, originating the crown regression and reduction of growth [25]. Two structure indices can be used as proxies of potential photosynthetic ability, vigor, and growth: crown ratio (cr: percent of crown length in relation to total height), which is also used for mechanical stability assessment; and linear crown ratio (lcr: percent of the crown in relation to stem diameter). For good vigor and growth cr ≥ 30% and lcr > 50%, while for a good mechanical stability cr ≥ 50% ([105] and references therein).

Spatial tree arrangements have also a determinant role in wood quality. In irregular spacing, especially in dense stands, trees can develop eccentric and tortuous or leaned stems, which reduce mechanical stability, in particular to wind and snow, and depreciate wood quality due to compression wood [85, 104].

Stem taper determines the quantity and quality of wood. Theoretically, trees in free growth tend to have stems more conical while those with narrower spacing tend to be more cylindrical. Also, maximum radial growth is higher near the crown base where carbohydrates are more available due to mechanical stress [85]. Thus, density should be suited to the development of cylindrical stems. Wood quality is also determined by the presence of branches and juvenile wood. Early pruning indicated for maritime pine [2] enables to increase in the length of the cylindrical stem, reduces the knotty stem core, and promotes the formation of mature wood [2, 85, 87, 102, 103]. Pruning in the early stand development stages, with few high-intensity interventions enables an easier and faster recovery of the tree growth. The goal is to attain a knotty stem core of 1/3 or less of the diameter at breast height at the end of the production cycle [2].

Annual radial growth and its variability also determine the quantity and quality of timber. The goals are attaining a radial growth as large and as constant as possible, that maintains good wood technological properties. Thinning, redistributing the growing space by the better-suited trees that are foreseen to reach the end of the production cycle, enables to achieve the two aforementioned goals. Thinning from below and selective (Schädelin) thinning can be used [2]. In the former the trees removed are predominantly the dominated ones, thus maintaining the upper canopy. The latter is characterized by the selection of the future trees which are released from completion in thinning. This results in a trend towards higher growth rates in the latter [77]. Regarding thinning intensity, the higher the larger the radial growth, but also increases annual radial growth variability [77]. Thus, the option is between thinning of lower intensity and higher frequency or of higher intensity and lower frequency.

When the objective of forest stands is the production of quality wood, it is advisable that they be installed with reduced spacing. With this practice, the height growth is promoted (in detriment to diameter growth), in order to release the influence of the crown at the lower levels of the stem as soon as possible, reducing the amount of juvenile wood in the stem and promoting the early development of the mature wood (of better quality) in the lower levels of the stem [87, 102, 103], which are the most valuable due to their larger dimension in diameter. At the same time, the stem taper is reduced and its straightness is increased, thus also reducing the amount of compression wood, which presents undesirable characteristics for most wood uses [104].

A profile of a radial section of maritime pine wood is shown in Figure 6.

Figure 6.

Radial section of Pinus pinaster wood.

3.5 Growth rate vs. wood quality

Given the great importance of the effect of the growth rate on wood quality, this topic has been studied for a long time, without, however, maintaining a great controversy, even allowing any bibliographic review to be forwarded to support any of the preconceived views. Initially, it was generally accepted that, in softwoods, rapid growth was associated with low densities, but this idea was based on a simple analysis of the cross-section of the stem by comparing the wide rings with low density, located in the center of the tree (juvenile wood), and the narrow rings with high density, located close to the bark (mature wood). However, the effects of ring width and age were confounded, so that most of the problems thought to be related to wide rings were, after all, due to the age of wood formation, that is, due to juvenile wood versus mature wood [106]. Regardless of the ring width, the juvenile wood is characterized by presenting a low density, which contrasts with the high density of the mature wood. Although the juvenile wood of softwood normally presents wide rings, the narrow rings of the juvenile wood also have low densities, as well as the wide rings of the mature wood show high densities [102]. Thus, the true effect of growth rate on density (as well as on other properties) can only be well evaluated in rings of the same age [106]. Currently, it is consensual that it is the occurrence of juvenile wood (age of the growth rings) and not the growth rate in diameter (ring width) that produces the worst quality wood.

Numerous studies carried out with resinous species in Portugal and Spain have repeatedly demonstrated the absence of correlation between ring width and wood quality characteristics [107, 108, 109, 110, 111, 112, 113, 114, 115, 116, 117] which are sufficiently clear to stop fearing, for this species growing in these regions, any hypothetical antagonism between the vigor and the wood quality. In this regard, also worth mentioning the work carried out by Fernandez-Golfin and Diez [107] on the influence of the ring width on the wood density and other physical-mechanical properties of wood in different species (among which P. pinaster). In addition to corroborating the reduced predictive capacity of ring width for wood density, the authors draw attention to the fact that the first research teams on wood technology were North European, so the most widespread wood quality standards came from studies carried out in these latitudes with slow and homogeneous species, as a result of reduced interannual variability. However, according to these authors, the woody material produced in Southern Europe is characterized by an enormous variability in the ring width, essentially induced by the great variability of precipitation, which, in this region, is the main limiting factor for growth. Thus, “The wood of these species and origins must be classified according to standards that take into account their growth characteristics and not using standards made to classify other species and/or provenances. In this sense, the use of the ring width as a limiting factor of wood quality (imposed by many European classification standards) only results in an unfounded technical barrier to wood from fast-growing species and/or from European southern climates, and open the doors to slow-growing species from more northern regions” [107].

4. Growth models, simulators, and decision support systems (DSS) for maritime pine (P. pinaster)

The importance of the maritime pine, both in area and yield, has led to the development of a large number of growth and production models to support the management of this forest resource. The first growth models for maritime pine - in the form of Yield Tables - were developed in Portugal, for the Leiria National Forest, by Santos Hall [118], and in Spain, in the 1940s, by Echeverría and De Pedro [119] in the Atlantic area. In France, the production tables developed by Décourt and Lemoine [120] for the pinewoods of the SW region (Landes) were the first models published for the specie. Significant development of models followed, attesting to the interest shown in this field of modeling applied to the species by researchers and technical experts. The evolution of the models since the production tables reflected the state of the art in the respective research area at the time, and documents the contemporary approach to forest growth prediction. In general, the models that have been proposed are empirical, at the stand or tree level, aiming at the application to pure and regular P. pinaster stands. The Dryads model [121], for uneven-aged, pure or mixed stands of P. pinaster and hardwoods (Castanea spp. and Quercus spp.), the PBIRROL model [122], for uneven-aged stands, should be highlighted here, due to their distinctive application, as well as the tests performed with the hybrid models, physiologically based of FOREST-BGC [123] and 3-PG [124], calibrated for the species by Lopes [125] and Alexandre [126]. Additional information about growth models can be found in Fonseca [127] and Bravo et al. [128]. Fonseca [127] presents a list of 30 models developed for the species in Portugal, and Bravo et al. [128] summarize the main models developed for the Atlantic and the Mediterranean maritime pine forests in Spain. The FORMODELS database (available at http://www.iefc.net/formodels_database_forest_modeles_liste/) contains a comprehensive list of 20 models developed for the species for different ranges of applicability in Portugal, Spain, and France, most of them referring to growth models and a few of other categories (biomass, mushrooms and fire behavior).

In this section we identify the simulators available for the species, presenting the references as to authorship or their reference documents and availability to users. Some of the models are hosted on platforms, namely, the CAPSIS (Computer-Aided Projection of Strategies in Silviculture) platform, see [129], the platform “Qforestry” (Quantitative forestry), the web-based application to simulate alternatives for sustainable forest management SIMANFOR [130], and the “sIMfLOR” platform, where the StandSim.dd simulator is located (Table 1) [141].

Simulator	Reference	Main charac-teristics and access (code)	Platform
PBRAVO	[131, 132]	Stand level with disaggregation by diameter classes (Weibull function)	CD Rom (Pbravo vs. 2.0).
ModisPinaster	[32, 133, 134]	Stand level with disaggregation by diameter classes (Johnson SB)	CAPSIS (http://www.inra.fr/capsis)
PBIRROL	[22, 122]	Tree level, distance-dependent	StandSim.dd simulator (http://www.isa.utl.pt/cef/forchange/fctools)
PINASTER	[135]; Additional references in sIMfLOR	Tree level, distance-dependent.	StandSim.dd simulator (http://www.isa.utl.pt/cef/forchange/fctools)
FlorNExT	[136]	Online application developed for the simulation of the growth and production at stand level. Combines several models developed for the species. Additional references in FlorNExT. Application of ForesMTIS. Web product	ForesMTIS. (http://flornext.esa.ipb.pt/)
GesMO	GesMo 2005 1.0 na§; GesMO 2.0; [137]	Growth simulator and product classification for several species, including maritime pine	CD-Rom.
SIMFOR	na§	Simulator for maritime pine located on Qforestry Platform for results transfer related to quantitative methods for forest management	Qforestry (https://www.qforestry.com/)
SIMANFOR	[130, 138]	Support system for the simulation of sustainable forest management alternatives which includes modules for maritime pine	SIMANFOR (www.palencia.uva.es/SIMANFOR)
PP3	[139]	Tree level, distance-independent	CAPSIS (http://www.inra.fr/capsis)
Lemoine	[140]	A stand growth model	CAPSIS (http://www.inra.fr/capsis)
Afocelpp	na§	Tree level, distance-independent	CAPSIS (http://www.inra.fr/capsis)
Pinus pinaster	na§	Tree level, distance-independent. Adaptation of PP3, for the integration of spatialized processes	CAPSIS (http://www.inra.fr/capsis)
SilmarS	na§	Growth model	CAPSIS (http://www.inra.fr/capsis)

Table 1.

Simulators and web products for Pinus pinaster.

^§

Specific reference not available; see the Web reference for details.

Although each model has its own specificities, the models produced to describe the dynamics of growth and several of them make it possible to anticipate the results of silvicultural options or management scenarios, according to predefined objectives or those to be achieved.

To support forest management, optimization models are used, usually anchored in Decision Support Systems (DSS), with the objective of obtaining optimal solutions for a given objective - usually wood production - subject to a set of constraints. Examples of optimization models for P. pinaster are found in Pasolodos-Tato [142], Fonseca [143], Rodil [144], and Petucco et al. [145]. In terms of supporting decision, Costa et al. [146] and Garcia-Gonzalo et al. [147] present case studies of DSS to generate management plans aimed at the production of wood for common lands and national forests, respectively, in Portugal. Other references are Falcão and Borges [148] and Garcia-Gonzalo et al. [148, 149].

5. Wood traits

5.1 Anatomy

Concerning the anatomical characterization, P. pinaster wood shows particularly longer tracheids than most resinous woods, which gives it great axial cohesion during its mechanical performance in use. For example, while P. pinaster wood presents an average tracheids length of 4.35 ± 0.50 mm [150], P. nigra and Cupressus lusitanica, also growing in Portugal, present average values of 3.74 ± 0.45 mm and 1.60 ± 0.16 mm, respectively [151, 152], P. sylvestris 1.73 ± 0.12 mm in Finland [153] and Picea abies with average values of ~2.75 mm [154], much lower than P. pinaster. Another important anatomical wood feature is the dimension of the lumen diameter of the earlywood tracheids, which in maritime pine is approximately 33 μ, a significantly higher value than that of Picea abies (27μ) and P. sylvestris wood (29 μ) [155]. This characteristic is reflected in the good performance of P. Pinaster wood in its drying behavior and preservation treatments.

5.2 Physical properties

The usual air-dry wood density values of approximately 0.566 g/cm³ in 30-year-old trees are worth mentioning [156], but which can reach average values of 0.657 g/cm³ at 70 years old [150]. These values are identical to those of P. nigra (0.588 ± 0.096 g/cm³) [116, 117] and P. sylvestris (0.588 ± 0.101 g/cm³) [114, 115] with identical ages and growing in Portugal, but higher than P. sylvestris wood from Sweden, France and the Czech Republic (0.391–552 g/cm³) [157, 158, 159, 160], Picea abies (0.410–516 g/cm³) [157], and Abies balsamea (0.351 g/cm³) [161].

Another important aspect is that the difference between the wood density of the earlywood and the latewood is not very high, which results in a considerable homogeneity of density within rings [108, 151], with very advantageous repercussions in terms of its workability, namely in its transformation into sheet to plywood and veneer and in the easiness of receiving connection elements (e.g., nails, screws).

The fact that P. pinaster wood has a relatively high density, has consequently a great dimensional instability caused by the gain or lose water during the wood drying (sorption/desorption processes), which results in tangential shrinkage values (T) between 9.1% at 10.1%; Radial (R) between 4.7% and 6.0%; Axial (L) between 0.0% and 1.0% and volumetric (V) between 14.5% and 16.7% [156, 162]. This aspect may be particularly critical in situations where wood is used outdoors, heavily exposed to adverse weather conditions. Comparatively, in softwoods it is common to find lower shrinkages, whose mean T values are usually between 5.6% and 8.3%; R between 3.1% and 5.3%, and V between 9.4% and 13.4% [163, 164]. In this way, it is imperative not only special care during the drying process but also that it only be applied after its moisture content is stabilized in the air. Additionally, it is also recommended to periodically apply insulating products (e.g., paints, varnishes) to reduce these shrinkages [87, 103].

5.3 Chemical properties

In relation to chemical properties, the wide range of studies carried out on this theme has been unanimous in demonstrating a reduced variability, not only between different conifers species but even between trees of the same species. This lack of variability is notable not only in terms of variations in the macromolecule contents (cellulose, hemicelluloses, and lignin), but also in terms of the elemental chemistry. The only difference that is sometimes identified is related to the extractive content of some species, whose range values are usually from 1.5 to 5% [165, 166, 167]. In the case of maritime pine in Portugal, it usually presents relatively higher contents, between 4.2% and 9.6% [113, 168, 169, 170].

Even so, these values for P. pinaster are lower than those reported for the P. sylvestris (10.7–15.4%) and P. nigra wood (6.6–12.9%) growing in Portugal [115, 117]. In terms of the use of P. pinaster wood, these high extractive values give it some natural resistance to biodegradation (but do not prevent the need to apply preservative products in situations of outdoor use) but may cause some problems in surface finish operations.

Regarding the elementary chemistry contents, several studies have shown that the woody biomass of the P. pinaster, not only contain high heating value (HHV), between 20.15 and 21.60 Mj/kg, but also low undesirable elements contents, such as N, S, K, Na, Ca, Mn, Ni, Cr, Cu, F, Cl, and ashes [171, 172, 173, 174, 175, 176, 177]. Thus, the P. pinaster wood is one of the most suitable types of biomass for energy purposes, namely through combustion processes, given the high HHV and the low risk of sintering and corrosive effect of chloride salts and HCl on metal parts in furnace and boiler, that occurs when the halogen elements (F and Cl) are high [178, 179, 180, 181, 182, 183, 184, 185, 186]. Likewise, the low values of N and S also indicate a reduced risk of formation and release to the atmosphere of NOx and SOx [180, 187, 188, 189, 190].

5.4 Genetics and breeding

Although the studies on genetic improvement of P. pinaster in Portugal had started in the 60s of the last century, they were focused on the characteristics of growth, form, and resistance to pests and diseases, and only in the last 25 years did the first study on the genetic control and improvement of the wood qualitative characteristics. At the moment, there is enough knowledge to recognize the existence of high genetic variability (essential to ensure good genetic gains through an improvement program) for some wood characteristics. For example, there was a high genetic control of the characteristics associated with wood density (heritability between 0.60 and 0.98), much higher than that verified for the growth characteristics in diameter (between 0.15 and 0.17), height (0.34), as well as for other wood features, such as lignin content (0.34), Radial Modulus of Rupture (0.34) and Radial Modulus of Elasticity (0.30) [108, 110, 111, 113]. Furthermore, when analyzed separately, the earlywood (formed in spring) exhibits much greater genetic dependence and is controlled over several years by the same set of genes, being the one that better results will provide in the future selection and improvement programs. In the opposite situation, the latewood, showing the lowest and most unstable heritability values, reveals that this type of wood is more strongly affected by environmental conditions than the earlywood [108, 110, 111, 113].

With regard to ring width, no adverse genetic correlations were detected between this and the wood density components. The fact that ring width is genetically and consistently positively correlated with the ring density, earlywood density, latewood percentage, and negatively with the heterogeneity index, allows us to contest, once again, the erroneous idea, but unfortunately still deeply rooted in the thinking of many researchers and wood users, that trees with higher radial growth (higher ring width) produce lower wood quality, namely lower density and latewood percentage in xylem [109, 110, 113].

These results should be sufficiently enlightening for us not to fear, for this species, any possible antagonism between the vigor and wood quality. On the contrary, it is expected that selection by the ring width will have a correlated effect in a slight increase in ring density, earlywood density, and latewood percentage (which should make it possible to reconcile good radial growths with high density), but not being accompanied by any significant changes in the latewood density, which will indirectly allow to increase the homogeneity of the growth rings. This fact is one of the most valued attributes by some of the wood processing industries. For example, the greater the homogeneity within the rings, the easier and more profitable will be the production of veneers, the greater its mechanical strength, the easier it receives the connecting elements (nails and screws), and the lower the risk of wood cracking [109, 113].

One of the places where the genetic improvement of P. pinaster is most advanced is in Australia, which began in 1957 and is currently in its fourth phase. The first phase was the establishment of a preliminary test of provenances that took place between 1964 and 1984 which revealed that in the growing conditions of West Australia, the provenances from Leiria (Portugal) were the most vigorous, confirming, once again, the superiority of the Atlantic provenances for growth [191]. The development in height and diameter at 10 and 20 years old was much higher in the 2 origins from Leiria, compared to those from Corsica, Landes, and Italy and, in terms of volume, the origins from Leiria presented a value greater than twice that of any of the other provenances. Furthermore, the provenances from Leiria were also the most resistant to drought (0.8% mortality, compared to 9.7% for the Landes and 10.1% for Corsica), but little to frost and with frequent stem bifurcations. The provenances from Corsica were superior in the stem straightness, while those from Leiria did not differ significantly from those from Spain and did not show a good performance in this parameter.

In the second phase of the improvement program, an attempt was made to combine, in the same individuals, the vigor characteristic of the provenances from Leiria with the stem straightness of those from Corsica, having crossed these two provenances. However, the hybrids obtained by this cross kept these 2 characteristics apart in the same individuals: either a high vigor, or a good stem configuration, but never both, simultaneously [191].

Faced with this setback, the next phase aimed to improve the stem shape while maintaining its high vigor, using material from 86 selected trees in the Leiria pine forest, which provided considerable genetic gains. According to Butcher and Hopkins [192] and Hopkins and Butcher [193] at this stage of the program, an increase in total volume production of +36% was obtained, which represents, by itself, an average increase of about 3.5 m³ ha⁻¹ year⁻¹ and which, complemented by a significant improvement in the stem quality by increasing their straightness by around 40% and by reducing the size of the branches by 25%, allows for an even greater increase in the total volume of usable wood.

For the fourth phase of the program, which is still in progress, the main objectives were to improve the characteristics of the branches (reduction of the insertion angle and size) and to increase the wood density, having been selected the best individuals from the best families obtained in the previous phase of the program that showed good configuration of the stem and crown, and whose average density of juvenile wood was equal to or greater than 0.430 g/cm³ [193, 194, 195].

Thus, the current knowledge about the properties and characteristics of P. pinaster wood allowed to identify it as a type of wood with potential for a wide range of uses, which go beyond those with less added value (packaging, pallets, and briquettes). In fact, this wood has suitable characteristics for more noble applications, such as structural applications, floors, carpentry and furniture, veneer, particleboard and plywood, poles, and sleepers.

6. Conclusions

Maritime pine is a plastic species widely distributed. Its traits and stand structures as well as the quantity and quality of its wood allow a wide range of uses. The stands are managed for wood, non-woody products, and services, thus recognizing its importance both economical and as a provider service demanded by society, thus contributing to its well-being.

The large representation of the species, particularly in southern Europe, has allowed advanced research on silvicultural systems and cultural practices, and their effects on wood properties, providing clarification on less well-perceived aspects of wood quality, particularly when considering the development of the species in the Mediterranean region. In parallel with silvicultural studies, several growth models and simulators have been developed and proposed to support management.

The challenges facing the species in the future are known, including severe weather conditions, especially drought, rural fires, storms, pests, and diseases. In addition, the systems are under pressure due to the high demand for woody material. From the extensive review carried out on maritime pine, it is noticed these challenges are part of research conducted or underway and of joint initiatives through international research projects (e.g., ForManRisk, https://formanrisk.eu/) to ensure the definition and update of management guidelines for the sustainability of maritime pine systems in the long term.

Acknowledgments

Thanks are due to the Pinus Competence Center (CCPB), Pinus Center (Centro Pinus), and International Union of Forest Research Organizations (IUFRO), namely Division 1 (Silviculture), unit 1.01.10 Ecology and Silviculture of Pine, for promoting fruitful discussions on the silviculture and management of pine forests that have contributed to the organization of this chapter.

Conflict of interest

The authors declare no conflict of interest.

Funding

For the author integrated with the research center Forest Research Centre (CEF), the research was financed by National Funds through the Portuguese funding agency, FCT (the Portuguese Foundation for Science and Technology), within project UIDB/00239/2020. For the author integrated with the MED research center, this work is funded by National Funds through FCT—Foundation for Science and Technology under the Project UIDB/05183/2020. For the author integrated with the CITAB research center, it was supported by National Funds by FCT—Portuguese Foundation for Science and Technology, under the project UIDB/04033/2020.