Cork Quality Research Articles

Mass insulation enhancement of cork stoppers through heat treatment: experimental measurements, modeling, and optimization

Natural cork stoppers, used as bottle sealers for high-end products such as ingredients and flavorings, belong to the highest class in terms of cork quality. Various studies have been carried out to examine the solute diffusion in cork using conventional methods. However, the investigations were limited to natural cork stoppers that had been boiled and cut in a single direction. In this study, the anisotropic cylindrical shape of cork samples was modeled, considering various cutting directions. Additionally, we subjected the stoppers to boiling then to an optimal THT cycle to improve the cork's mechanical qualities. The optimal thermal cycle is determined using TGA analysis. IR and SEM tests are employed to verify the enhancements in the cork. The apparent mass diffusion coefficient (Dapp) is determined by the calibration of the model with the experimental data acquired via the conductometric method. The coefficient values are optimized using the Bat algorithm with a relative error of 10–5. The Dapp was found to be better for the treated cork grown in the Jijel region than for the cork Medea, which is better than of Skikda cork. Regarding the impact of the cutting direction on the Jijel's cork, the results indicate that the mass insulation is higher when the treated stoppers are cut in the tangential-longitudinal direction (DJTLt=4,12.10-13m2s-1) compared to the tangential-radial direction (DJTRt=7,75.10-13m2s-1) and the longitudinal-radial direction (DJLRt=8,53.10-13m2s-1). The treatment, which serves as a solution for the cork industry, showed an improvement in mass insulation 13-fold greater than the natural cork.

Cytisus Legume Association Interests on Oak Growth and Cork Quality

Background: The cork oak is the major forest element of northern Tunisia, especially in Kroumirie. It is a species of greatecological, economic and social value, with a natural distribution limited to the western Mediterranean basin. Following serious biotic and abiotic problems, oak forests have been greatly affected and their natural regeneration has become very difficult, even almost absent these last years marked by last climatic changes. The association of cork oak with autochthonous plant species such as leguminous is an original research to resolve this problem. As part of this reflection, the association of the cork oak with Cytisusvillosus could positively influence its growth and productivity by providing the nutrients necessary for its survival in the face of climatic constraints. Methods: The study was conducted in two sites in the Kroumirie (Ain Draham and El Feija forests). In each site two populations of the same bioclimatic stage are chosen: a population of only cork oak and a cork oak population associated with Cytisusvillosus. Several parameters were measured on trees at each population: morphological (height, circumference, leaf area index LMA) ecophysiological (photosynthesis, stomatal conductance, transpiration) and cork quality (porosity, humidity, quality) parameters. Result: The oaks with legumes in both Ain Drahem and Feija sites showed highly significant differences compared to the other two populations without association; for morphological and ecophysiological parameters. In the same way, statistical analysis has shown that the cork oak trees in Feija population with Cytisus have better cork quality than other populations. We can conclude that the cork oak behaves better and have higher growth and better cork quality in the presence of legumimous species like Cytisusvillosus. Such results could encourage the reintroduction of this legume especially in degraded areas of cork oaks for their maintenance and preservation especially in the current climate conditions.

Quercus suber L. Genetic Resources: Variability and Strategies for Its Conservation

Quercus suber L. is an evergreen cork oak tree that can produce cork, one of the most important valuable natural bioresources in Portugal, with a high impact for the bioeconomy. Given its socio-economic relevance and the upcoming biotic and abiotic threats cork oak faces, it is of extreme importance that genetic conservation of its genetic variability occurs so that cork oaks can adapt to new conditions. This work represents a review of the current knowledge on Quercus suber genetic resources, focusing on the existing genetic variability and the strategies for its conservation. Furthermore, we highlight genetic knowledge on tolerance and response to abiotic and biotic stresses and cork quality, which are useful for further studies on stress response pathways and mechanisms and improvement regarding stress tolerance.

Virgin cork colour and porosity as predictors for secondary cork industrial quality

Currently, the selection of the trees to be removed in a thinning, carried out in a young cork oak plantation, is determined by a systematic rule to reach a predefined tree density value, in combination with simple criteria regarding individual tree sanitary conditions and stem shape. Since no criteria regarding the cork are available, thinnings can result in the removal of individuals that would later produce high quality secondary cork, leaving tree producers with lower quality cork. This study aims to test the feasibility of using CIELAB colour space coordinates and porosity values, measured in virgin cork samples, as predictors of the industrial cork quality and porosity obtained in secondary cork, thereby providing additional information usable for the classification of trees prior to the first thinning implementation. The L* and b* colour coordinates, measured in the transverse direction, depicted three main cork quality groups. Other L* and b* colour parameters, measured in the radial direction or in the cork back surface, have also shown statistically different average values while measured in different quality cork samples. Cork porosity from virgin and secondary cork showed a positive moderate monotonic relationship. A hyperbolic model for the prediction of secondary cork porosity was developed based on the virgin cork porosity, with one parameter expressed as a function of virgin cork colour coordinates (R2 =0.445). Results were successful in confirming the hypothesis raised. They may be used for the classification of trees prior to thinning.

Analysis of cork quality and cork tree health in stands of western Spain

Good knowledge of the factors that influence cork quality and the health of forests is essential when making decisions regarding the management of productive cork oak forests. The quality of cork planks depends on: plank thickness, plank porosity and the presence of anomalies. So far, no studies have evaluated cork quality based on cork anomalies, partly due to the difficulty of obtaining adequate data. In 1985, it was developed the first Cork Quality Field Assessment Plan (CQFAP) to provide information about the quality of harvested cork by collecting data of very high quality. This work analyzes the anomalies present in 2049 cork sampling units collected from ten stands under the CQFAP for the period 1986–2012 in three consecutive cork stripping cycles. Data on diseases, pests and stripping damages are also analyzed in the 654 trees where cork sampling units were collected in the third stripping cycle. Additionally, mean monthly temperature and precipitation values for the 35 years covering the three stripping periods were calculated. A progressive deterioration of cork quality was observed in the sampled cork oak stands. It was found a positive relationship between cork thickness and temperature indicating that the cork growing period could be extended when temperatures are moderate. Sampled cork oaks were more affected by Cerambycidae attacks and Coraebus undatus indicating their prevalent nature. The presence of Coraebus undatus were higher in stands with good health status. The studied diseases were Biscogniauxia mediterranea, Phytophthora sp. and fungi of the family Botryosphaeriaceae, finding a not previously reported correlation between the presence of Crematogaster scutellaris and the three studied diseases. Stripping damages increase with smaller cork thickness but climate is not a decisive factor for a proper stripping. Although having room for improvement, the CQFAP is proving to be an effective tool for assessing the quality of cork, the health status of cork oak forests and the evolution of these forests over time. The information provided could be also used to identify places worthy of further studying. The main recommendation for improving the CQFAP would be to monitor the same trees in each consecutive sampling for properly monitoring cork thickness and the presence of defects to be used as a proxy for tree health. Regarding forest management, the main recommendations would be to extend the length of stripping cycles to enhance cork production, and to use the new tools and systems for cork stripping to reduce damage.

Phytochemical Composition of Extractives in the Inner Cork Layer of Cork Oaks with Low and Moderate Coraebus undatus Attack

The beetle Coraebus undatus, during its larval stage feeds, and excavates galleries on the cork-generating layer of Quercus suber L. trees, seriously affecting the cork quality with significant economic losses for the cork industry. This work compared the composition of the extracts present in the innermost cork layers (the belly) of cork planks from Q. suber trees with low and moderate C. undatus attack in one stand. The total extractives in the inner cork layer from trees with moderate and low C. undatus attacks were similar (on average 22% of the cork mass) with a high proportion of polar compounds (91% of the total extractives). The chemical composition of the inner cork lipophilic extractives was the same in trees infested and free of larvae, with triterpenes as the most abundant family accounting for 77% of all the compounds, predominantly friedeline. The hydrophilic extractives differed on the levels of phenolic compounds, with higher levels in the inner cork extracts of samples from trees with low attack (90.0 mg GAE g−1 vs. 59.0 mg GAE g−1 of inner cork mass) The potential toxic activity of phenolic compounds may have a role in decreasing the larval feeding.

Quality control of natural cork stoppers by image analysis and oxygen transmission rate

Abstract This work presents the evaluation of the porosity by image analysis, the quantitative analysis of the cell morphology from images obtained by scanning electron microscopy (SEM) and the oxygen transmission rate (OTR) of natural corks of different visual quality grades. Due to the natural variability of cork stoppers, statistically significant differences could not be established in the porosity of the corks according to their commercial quality. However, the determination of the surface porosity coefficient by image analysis in the tangential and axial sections of the corks allowed us to distinguish between high, medium and low quality classes. The cells in the tangential section were shaped between circular and hexagonal, with very regular perimeters regardless of the cork quality. While the cells of the radial and axial sections showed a square and rectangular shape, with more irregular perimeters, mainly in the lowest quality corks and in the axial section. Corks commercially classified as “flower”, “second” and “third” had the lowest OTR values and presented a similar statistical distribution in their cell perimeters in the axial section. While the corks with higher OTR values (superior and fourth qualities) corresponded with those with greater cell perimeters and greater dispersion in their distribution.

Influence of water supply on cork increment and quality in Quercus suber L.

Abstract Cork oak (Quercus suber L.) grows in the Mediterranean basis including Portugal and is the main species producing cork which is used prevailingly in stopper industry. In our paper, cork from Quercus suber L. over three consecutive harvests from a traditional rainfed plot, between 1999 and 2017, and cork from an irrigated plot, harvested in 2017, were studied. We applied two X–ray image analysis technologies – X–ray micro-computed tomography and X–ray microdensitometry. Cork development, related with intern porosity, growth and density was studied with the objective of understanding the cork characteristics evolution over the years and with a different water regime. The outcomes of this study suggested an increase in density and porosity over harvests and a slight decrease of the cork growth. Cork samples from the irrigated plot, compared with cork from the same year of extraction in the rainfed plot, showed higher growth rate and higher porosity. The results demonstrated the contribution of climatic factor of precipitation as well as the silviculture model in cork characteristics, showing the relevance of the present work for the definition of the management practices. These may be determinant for enhancing cork quality and quantity production through silviculture measures. Our findings can be particularly useful for stakeholders especially under the conditions of Portugal in terms of increasing the value of the industrial chain of cork.

Kinship Analysis and Pedigree Reconstruction of a Natural Regenerated Cork Oak (Quercus suber) Population

Cork oak (Quercus suber L.) is a valuable forest species in the western Mediterranean Basin due to its ecological value and the production of cork (a renewable natural material). Cork quality depends on the genetic background and cork oak environment, which has long been recognized. As no cork oak genetic trials with pedigree information were available, the inference of the genetic relatedness between individuals from molecular markers can potentially be applied to natural populations. This work aimed to investigate the potential of performing kinship prediction and pedigree reconstruction by SNP genotyping a natural cork oak population. A total of 494 trees located in Portugal were genotyped with 8K SNPs. The raw SNP set was filtered differently, producing four SNP sets that were further filtered by missing data, genotype frequency, and minor allele frequency. For each set, an identity by descent (IBD) matrix was generated to perform the relationship prediction, revealing from 22,114 to 23,859 relationships. Familial categories from the first to the third degree were able to be assigned. The feasibility of SNP genotyping for future studies on the kinship analysis and pedigree reconstruction of cork oak populations was demonstrated. The information produced may be used in further breeding and conservation programs for cork oak.

Study of the cork quality indices in the north-west region of Algeria

ABSTRACT Cork quality indices were characterised in eight cork oak stands in the production region of Oranie (North-West, Algeria), based on field sampling of 75 trees/stand. The superposition of the quality indices provided by the Coveless CQ05™ onto the measurements of the technological cork features carried out in the laboratory showed a perfect homogenisation between the two processes. The best classes of quality indices (QI-A (19.5, 19, 12.75, 12)) include thickness (26.90 mm), annual growth (2.11 mm.year−1), the volumetric density (198.94 kg.m−3) and the porosity coefficient (2.65%). Regarding the poor quality indices classes (IQ-C (1.5), these averages tend to increase so as to reduce the market value of cork, in particular the density (355.11 kg.m−3), porosity (7.50%) and defects (70% for earthy cork and 30% for inclusions) The results showed the preponderant role of quality indices in the good marketing of cork and the classification of production areas.

Fitting and calibrating a three-level mixed effects cork growth model

Cork value is determined mainly by its thickness at the moment of debarking but is also affected by the presence of any discontinuities related to porosity characteristics and/or the presence of anomalies in the cork plank. Cork quality is closely related to the final price of the raw material, so inventories conducted prior to the cork debarking are aimed at identifying the characteristics of the cork and therefore its potential price. Besides information from these cork inventories, forest managers also need a reliable prediction of the final cork thickness to help them determine the most profitable moment to harvest. In this work, a three-level nonlinear mixed effects cork growth model was developed for Spanish cork oak forests using 1027 cork samples taken in nine regions of provenance into which Spain is divided for identification of forest reproductive material. The fitting step revealed that the region and the stand level explained much less variability than the tree level in the three-level mixed-effects models. Therefore, subject–specific predictions were obtained with the joint use of region-, stand-and tree-level random effects. The subject–specific predictions were compared with three alternative prediction options when no additional measurements are available. The precision of predictions with calibrated mixed-effects model using region-, stand-, and tree-level random effects was 0.1 mm when taking one additional measurement at any time during the rotation period. The developed model is more precise and has a wider range of application than the previous model, being suitable for predicting accumulated cork thickness in more than 90% of the cork oak distribution area in Spain. However, it should only be applied to make predictions within a given cork rotation period and not to simulate cork growth over potential successive cork rotations. In the future, it would be necessary to develop an annual cork growth model which includes site, stand and tree variables and which is capable of providing long-term predictions under different climatic and management scenarios. The model developed in this study could provide the basis for such a model because its random effects could be used to identify the main variables related to cork growth through correlation analysis.

Irrigation of Young Cork Oaks under Field Conditions—Testing the Best Water Volume

This study is the beginning of the first long-term study on cork oak irrigation under field conditions, with a structural-functional approach. Cork oaks are currently facing disturbances affecting cork quality and quantity, jeopardizing the future of the economic sector. There is a need for new production techniques that maximize cork oak growth and vitality. In this study, irrigation was implemented in a new intensive cork oak plantations to test the best irrigation volume. The long-term goal is to improve tree growth with minimum water requirements. A 6 ha intensive plantation was installed in Coruche, Portugal. The experimental plot consisted of a subsurface drip fertigation system, buried 40 cm deep; with five independent irrigation treatments. It was tested four irrigation volumes during the dry period—21 weeks in the summer of 2016—ranging from 1.88 mm to 5.62 mm a week. Information on meteorological conditions, soil moisture profile and leaf stomatal conductance were gathered periodically and dendrometric measurements were performed before and after the treatments. Cork oaks’ structural and functional parameters were associated with irrigation volume Response to irrigation showed an inflection point in treatment 2, corresponding to a water supply of 3.12 mm per week: below the inflection point, stomatal conductance was reduced by 15% and relative diameter growth at the base was reduced by 10%. Stomatal conductance also showed a positive relationship with soil moisture below the irrigation tubes and with plants’ stem diameter. In conclusion, irrigation supply during the period of water stress improved function and structure of cork oaks seedlings under field conditions. These results suggest that irrigation can be a viable alternative to improve cork oak growth in afforestation and reforestation.

Étude de l’influence des facteurs du milieu sur la croissance et la production du liège de la suberaie du Parc national de Theniet El Had (Tissemsilt, Algérie)

La variation des capacités de production de liège en relation avec les facteurs du milieu a été évaluée à partir de 53 placettes temporaires installées (763 arbres) dans la forêt montagneuse de chêne-liège du Parc national de Teniet El Had (Algérie). Trois paramètres de production sont retenus : l’épaisseur (mm/an), le rendement d’un arbre moyen (kg) et la productivité d’un peuplement (kg/ha). Le présent travail montre l’effet des principaux facteurs du milieu sur l’épaisseur et la production de liège, identifié à partir de modèles linéaires généralisés. Les résultats ont montré que la croissance du liège est lente (2,8 mm/an) et qu’elle est davantage affectée par des variables propres à l’arbre (état de santé, diamètre et hauteur d’écorçage) que par les sites (exposition). Cependant, le rendement est influencé par les conditions de croissance des arbres, exprimées par leur hauteur totale mais aussi par leur état de santé. À l’échelle des peuplements, deux qualités de sites sont proposées en fonction de la valeur de la hauteur dominante exprimée par le modèle : d’une part, les sites de bonne qualité (Hd > 10 m ; 2 900 kg/ha) caractérisés par des arbres de bonne croissance en largeur et, d’autre part, les sites de mauvaise qualité avec une productivité réduite de plus de moitié (Hd < 10 m ; 1 700 kg/ha). Ces résultats fournissent une orientation de gestion permettant au forestier de planifier la récolte de liège de sa forêt selon la rotation pratiquée et la qualité de la planche recherchée.

Chloroanisoles and Other Chlorinated Compounds in Cork from Different Geographical Areas.

Cork quality is crucial for the fabrication of corks intended to be used to seal wine bottles. This work has focused on the determination of chloroanisoles (CAs)—exogenous compounds with a low perception threshold—in cork. The identification and quantification of these compounds was carried out with Bond Elut-ENV solid phase extraction and gas chromatography with mass spectrometry detection. Cork samples were obtained from oaks from Catalonia, Extremadura and Italy, and the presence of CAs was evaluated. Moreover, cork affected by the presence of yellow stains (a defect present in cork, mainly originated from the growth of the fungus Armillaria mellea) was analysed separately. The results obtained from cork macerates revealed the presence of trichloroanisole (TCA) in Catalan and Italian cork. Furthermore, TCA concentration was not statistically different when comparing cork affected and non-affected by the growth of A. mellea. Other chlorinated compounds were identified by comparison of their mass spectra with the data from the NIST library.

Inter-tree competition analysis in undebarked cork oak plantations as a support tool for management in Portugal

Scheduling the first thinning in young cork oak stands is an important decision in management that is usually subjectively defined by each manager. An analysis of competition in cork oak plantations is extremely relevant as a support for this important decision. This study aimed to understand the relationship between stand characteristics and tree growth efficiency, and subsequently help identify the presence or absence of an inter-tree competition pattern in cork oak stands that were never debarked. The inter-tree competition was assessed by looking to several stand characteristics: (1) comparison with the self-thinning line; (2) stand density measures, such as stand spacing coefficient and relative spacing; (3) stand structure measures evaluated through the skewness and variability; and (4) tree relative growth rate pattern over tree size. Data used for this analysis were gathered from permanent plots that were established in even-aged stands located across the cork oak distribution area in Portugal. The study shows that the most common stand densities on young cork oak plantations might not lead to serious inter-tree competition before the first cork extraction. This information is relevant to support the potential timing and severity of the first thinning in order to retain more trees until the second cork extraction when cork quality can be taken into account in the selection of the trees to be removed.

Phellem Cell-Wall Components Are Discriminants of Cork Quality in Quercus suber.

Cork is a renewable, non-wood high valued forest product, with relevant ecological and economic impact in the Mediterranean-type ecosystems. Currently, cork is ranked according to its commercial quality. The most valuable planks are chosen for cork stoppers production. Cork planks with adequate thickness and porosity are classified as stoppable quality cork (SQC). The chemical composition of cork is known, but the regulation of metabolic pathways responsible of cork production and composition, hence of cork quality, is largely unknown. Here, we tested the hypothesis that post-genomic events may be responsible for the development of SQC and N-SQC (non-stoppable quality cork). Here, we show that combined proteomics and targeted metabolomics (namely soluble and cell wall bound phenolics) analyzed on recently formed phellem allows discriminate cork planks of different quality. Phellem cells of SQC and N-SQC displayed different reducing capacity, with consequential impact on both enzymatic pathways (e.g., glycolysis) and other cellular functions, including cell wall assembly and suberization. Glycolysis and respiration related proteins were abundant in both cork quality groups, whereas the level of several proteins associated to mitochondrial metabolism was higher in N-SQC. The soluble and cell wall-bound phenolics in recently formed phellem clearly discriminated SQC from N-SCQ. In our study, SQC was characterized by a high incorporation of aromatic components of the phenylpropanoid pathway in the cell wall, together with a lower content of hydrolysable tannins. Here, we propose that the level of hydrolysable tannins may represent a valuable diagnostic tool for screening recently formed phellem, and used as a proxy for the quality grade of cork plank produced by each tree.

Characterisation of the cork growth and quality of cork oak woodlands, North-West Algeria

ABSTRACT Cork growth and quality were characterised in eight cork oak stands within the production area of Oran (Oranie, Fr) (North-West, Algeria), based on a field sampling on 75 trees/stand. Cork thickness was on average 26.50 mm, with 44% of the samples in the commercial classes unsuitable for production of stoppers. The annual average cork growth for a production cycle with 10 complete growth years was 2.43 mm. For this region, a production cycle of 12 years is not adequate for raw-material industrial requirements. The aspect of cork is characterised by 11 intrinsic and extrinsic defects. The average density of cork is 256.77 kg/m3 and the porosity, measured by image analysis, 4.6%. The quality of cork according to the Instituto de Promocion de Corcho (IPROCOR) standards categorises the thin cork as the most-valued product, intended for the manufacture of the discs. This is in conformity with the slow growth of cork in this region.

Quercus suber cork as a keystone trait for fire response: A flammability analysis using bench and field scales

Quercus suber (cork oak tree) is an excellent epicormic resprouter that can keep cambium alive after high-intensity fires because of its plan traits, mainly its thick cork. Although tree bark harvesting is an important source of income, it may reduce protection against wildfires. This study aims to identify stem and cork characteristics that affect the probability of reaching lethal cambial temperature and hence the probability of survival of living tissues. Cork flammability was analyzed using a sample set comprised of 120 cork specimens and 120 cork pieces. Five experimental fires were used to test the results obtained at bench scale.Bench-scale flammability experiments could not simulate the real heat spread and presence of highly volatile isoprenoids. The results revealed differences in cambial temperature and lethal temperature rate (LTR) at both scales according to corkback roughness and cork quality. Peak cambial temperature and LTR show an increase in stems characterized by maximum roughness and worse cork quality. The LTR measurements showed a 30% difference between bench and field scales (0.35 mm s−1 and 0.5 mm s−1, respectively). The findings indicate that corkback roughness, cork thickness, cork porosity and cork quality could influence the vulnerability of cork oak forests to wildfires. A negative correlation was found between total thickness and cambial temperature. A total thickness of over 3.7 cm prevented reaching lethal cambial temperature. Forest managers can use this information to define silviculture and design forest treatments with a view to reducing fire severity and mitigating the ecological and socioeconomic impacts of fire in cork oak forests.

Transcriptional profiling of cork oak phellogenic cells isolated by laser microdissection.

The phenylpropanoid pathway impacts the cork quality development. In cork of bad quality, the flavonoid route is favored, whereas in good quality, cork lignin and suberin production prevails. Cork oaks develop a thick cork tissue as a protective shield that results of the continuous activity of a secondary meristem, the cork cambium, or phellogen. Most studies applied to developmental processes do not consider the cell types from which the samples were extracted. Here, laser microdissection (LM) coupled with transcript profiling using RNA sequencing (454 pyrosequencing) was applied to phellogen cells of trees producing low- and good quality cork. Functional annotation and functional enrichment analyses showed that stress-related genes are enriched in samples extracted from trees producing good quality cork (GQC). This process is under tight transcriptional (transcription factors, kinases) regulation and also hormonal control involving ABA, ethylene, and auxins. The phellogen cells collected from trees producing bad quality cork (BQC) show a consistent up-regulation of genes belonging to the flavonoid pathway as a response to stress. They also display a different modulation of cell wall genes resulting into a thinner cork layer, i.e., less meristematic activity. Based on the analysis of the phenylpropanoid pathway regulating genes, in GQC, the synthesis of lignin and suberin is promoted, whereas in BQC, the same pathway favors the biosynthesis of free phenolic compounds. This study provided new insights of how cell-specific gene expression can determine tissue and organ morphology and physiology and identified robust candidate genes that can be used in breeding programs aiming at improving cork quality.

Towards assessment of cork production through National Forest Inventories

The economic importance of cork in the Mediterranean region demands an accurate assessment of its production. Cork production is currently estimated by aggregating information provided by Regional Forest Services, but this approach results in a lack of robustness at the national level. The objective of the present study is to analyse the role of the Spanish National Forest Inventory (SNFI) as a potential source of data for quantifying cork weight at national level and the scope of National Forest Inventory data to define national trends in cork yield as well as to characterize the main types of cork producing forest stands in Spain. Data from the Second and Third SNFI are used together with the Spanish Forest Map. The results point to the suitability of SNFI to quantify cork production as long as the two main variables defining cork weight, namely cork thickness and debarking height, were correctly recorded in inventories. Although the Second SNFI complied with these requirements, from the Third SNFI the methodology changed, preventing the accurate estimation of cork weight. Based on this study recommendations are made to improve the estimation of cork weight at national level, e.g. to measure cork thickness in all the cork oaks in the plot and to take a sample of cork from the inventoried trees. This information would also make it possible to assess the cork quality in terms of cork thickness growth and to classify cork production in terms of suitability for producing high quality cork products.