**2. The sustainable management of cork production**

Cork oak forests are often a part of a multifunctional agroforestry-pastoral system called "montado" that is considered a high nature value farming system, according to the European classification proposed by the European Environmental Agency [5]. These ecosystems are also recognized as habitats of conservation value listed in the Habitats Directive [6].

In this nonwood forest production system, cork harvesting is the major economic activity, and cork is the most valuable product. The entire cork chain from the forest to the consumer relies on the regular and sustainable production of cork. To maintain cork production capacity and provide the mentioned environmental services, it is necessary that cork oak forests are adequately managed, being the sustainability a matter of general concern [3].

The exploitation of the cork oak tree for cork production needs its periodical removal from the stem and branches in a degree that is considered well suited with the maintenance of the tree in good physiological conditions [3]. Cork production yields depend on the tree and cork growth, as well as on management variables such as the intensity of cork extraction and the interval between strippings that are regulated with strict rules by the Portuguese legislation (Decreto-Lei nr. 155/2004).

The production of cork relies on specific forest management and silvicultural model, often called subericulture that is based on the biological characteristics of the cork oak bark development [7]. The extraction of cork, or cork stripping, is done manually by cutting large rectangular planks and pulling them out of the tree when the cork oak is physiologically active in late spring and early summer. By law, young trees can only be stripped when they reach at least 70 cm of the perimeter at 1.3 m of height, corresponding to about 25 years of age. Moreover, cork cannot be stripped above a stem height equal to twice the perimeter of the stem in the first stripping, or not more than three times, for a mature tree in full production, the so-called stripping coefficient. The trees are then debarked every 9 years (the legal minimum allowed in Portugal and Spain) or more, called the production cycle [8]. The decision of longer production cycles is often related to an adjustment to cork growth and productivity in order to achieve the cork plank thickness required by the industry.

This means that cork growth is the main criteria to consider in cork management planning since it determines the thickness of the cork plank that is available for industrial processing, which is primarily oriented toward the production of wine stoppers (requiring a minimum thickness of 27 mm after the cork postharvest boiling operation).

The annual cork growth along the production cycle varies with the number of years in the cork growth sequence (growth is usually higher in the first years of the cycle) and is influenced by environmental and tree conditions [9, 10]. A recent study encompassing a large time span (24 years) and sampling (1584 cork samples), reported an average annual cork ring width ranging from 1.2 to 7.3 mm with an average value of 3.3 mm [11].

**225**

phloem (**Figure 1**).

walled cells at their borders.

*Cork and Cork Stoppers: Quality and Performance DOI: http://dx.doi.org/10.5772/intechopen.92561*

namely, during summer drought [14–16].

**3. The cork formation**

of its activity decreases with age [3].

Cork oaks show high resiliency to interannual precipitation variability, with rapid and complete recovery from extreme dry years or from rainfall exclusion, but have a high sensitivity to the amount and timing of late spring precipitation [12]. In Mediterranean conditions, access to water resources and the relationship to soil-site conditions are key factors for cork oak development [13]. Soils with low depth and high compactness have a negative influence on the development of the cork oak deep root system, thereby diminishing the access to direct groundwater resources,

It is foreseen that in the Iberian Peninsula, spring precipitation will be reduced and more severe droughts will occur as predicted by the Intergovernmental Panel on Climate Changes (IPCC). In such a scenario, cork growth will be slower resulting in narrower annual rings that will decrease the overall thickness of the cork layer at the end of the production cycle. Given the importance of the cork plank thickness for the industrial processing into cork stoppers, it is foreseeable that the silvicultural cork management will require an adjustment to mitigate the effects of drought by postponing the cork striping (i.e., increasing the duration of the cork production cycle) [17].

Cork is a protective tissue located in the outer bark of the cork oaks as part of the periderm. The formation of cork in the periderm results from the activity of a secondary meristem, the phellogen: each phellogen mother cell originates by cellular division cork cells that grow unidirectionally outward in the tree's radial direction and phelloderm cells to the inside [18]. In the cork oak, the first phellogen maintains its activity year after year, producing successive layers of cork. The phellogen may be functional for many years, probably during the tree's life, although the intensity

If the cork layer of the initial periderm (virgin cork) formed in the young cork oaks is removed (by an operation called cork stripping), a new phellogen is formed inside the phloem and rebuilds a traumatic periderm and its subsequent cork layer (second cork). At this time of life in the young cork oaks, the radial growth of the stem is still important, and the second cork external regions are subject to large tangential stress that may result into deep fractures of the cork [19]. If the second cork is removed, the process is repeated with the formation of a new phellogen and the production of a new cork layer (reproduction cork) that will endure few fractures due to the low tangential stress caused by the radial growth of the mature tree. Upon removal of this reproduction cork, the process is repeated, therefore allowing exploitation during the tree's lifetime by successive removals of the

reproduction cork. The second and reproduction corks are covered at the outside by a thin lignocellulosic layer of phloem, corresponding to the part of the phloem that remained to the outside when the traumatic phellogen was regenerated inside the

The cork oak periderm has lenticels that originate from the activity of particular regions of the phellogen, called lenticular phellogen, and differ from the surrounding cork tissue. The activity of lenticular phellogen is maintained year after year, and therefore, the lenticels prolong radially from the phellogen to the external surface of the periderm forming approximate cylinders named lenticular channels [3]. The lenticular channels are loosely filled with a lenticular filling tissue of rigid unsuberified cells with thick walls and show ruptures and intercellular voids to a great extent [3]. The region bordering the lenticular channels has often higher density than the surrounding material due to the presence of lignified and thick*Cork and Cork Stoppers: Quality and Performance DOI: http://dx.doi.org/10.5772/intechopen.92561*

*Chemistry and Biochemistry of Winemaking, Wine Stabilization and Aging*

(Wine Opinions, CTR, Iniciativa Cork, Gfk, Opinion Way).

**2. The sustainable management of cork production**

listed in the Habitats Directive [6].

sustainability a matter of general concern [3].

Cork is a world-renowned material used for sealing wine bottles. Cork is a cellular material with chemical inertia and a set of specific physical and mechanical properties that provide an outstanding performance when in-bottle wine aging is wanted, by combining the required minute oxygen transfer with mechanical sealing of the bottle, durability, and chemical stability [3, 4]. Cork is the closure material preferred by wine consumers, and a bottle corked with a natural cork stopper is indicative of high-quality or very high-quality wine, as shown by recent surveys

Cork oak forests are often a part of a multifunctional agroforestry-pastoral system called "montado" that is considered a high nature value farming system, according to the European classification proposed by the European Environmental Agency [5]. These ecosystems are also recognized as habitats of conservation value

In this nonwood forest production system, cork harvesting is the major economic activity, and cork is the most valuable product. The entire cork chain from the forest to the consumer relies on the regular and sustainable production of cork. To maintain cork production capacity and provide the mentioned environmental services, it is necessary that cork oak forests are adequately managed, being the

The exploitation of the cork oak tree for cork production needs its periodical removal from the stem and branches in a degree that is considered well suited with the maintenance of the tree in good physiological conditions [3]. Cork production yields depend on the tree and cork growth, as well as on management variables such as the intensity of cork extraction and the interval between strippings that are regulated with strict rules by the Portuguese legislation (Decreto-Lei nr. 155/2004). The production of cork relies on specific forest management and silvicultural model, often called subericulture that is based on the biological characteristics of the cork oak bark development [7]. The extraction of cork, or cork stripping, is done manually by cutting large rectangular planks and pulling them out of the tree when the cork oak is physiologically active in late spring and early summer. By law, young trees can only be stripped when they reach at least 70 cm of the perimeter at 1.3 m of height, corresponding to about 25 years of age. Moreover, cork cannot be stripped above a stem height equal to twice the perimeter of the stem in the first stripping, or not more than three times, for a mature tree in full production, the so-called stripping coefficient. The trees are then debarked every 9 years (the legal minimum allowed in Portugal and Spain) or more, called the production cycle [8]. The decision of longer production cycles is often related to an adjustment to cork growth and productivity in order to achieve the cork plank thickness required by the industry. This means that cork growth is the main criteria to consider in cork management planning since it determines the thickness of the cork plank that is available for industrial processing, which is primarily oriented toward the production of wine stoppers (requiring a minimum thickness of 27 mm after the cork postharvest

The annual cork growth along the production cycle varies with the number of years in the cork growth sequence (growth is usually higher in the first years of the cycle) and is influenced by environmental and tree conditions [9, 10]. A recent study encompassing a large time span (24 years) and sampling (1584 cork samples), reported an average annual cork ring width ranging from 1.2 to 7.3 mm with an

**224**

boiling operation).

average value of 3.3 mm [11].

Cork oaks show high resiliency to interannual precipitation variability, with rapid and complete recovery from extreme dry years or from rainfall exclusion, but have a high sensitivity to the amount and timing of late spring precipitation [12]. In Mediterranean conditions, access to water resources and the relationship to soil-site conditions are key factors for cork oak development [13]. Soils with low depth and high compactness have a negative influence on the development of the cork oak deep root system, thereby diminishing the access to direct groundwater resources, namely, during summer drought [14–16].

It is foreseen that in the Iberian Peninsula, spring precipitation will be reduced and more severe droughts will occur as predicted by the Intergovernmental Panel on Climate Changes (IPCC). In such a scenario, cork growth will be slower resulting in narrower annual rings that will decrease the overall thickness of the cork layer at the end of the production cycle. Given the importance of the cork plank thickness for the industrial processing into cork stoppers, it is foreseeable that the silvicultural cork management will require an adjustment to mitigate the effects of drought by postponing the cork striping (i.e., increasing the duration of the cork production cycle) [17].
