Waxes composition of reproduction cork from Quercus suber and its variability throughout the industrial processing

Summary The chemical composition of waxes was studied in cork planks from three different trees of Spanish Quercus suber at four different stages of the in‐dustrial processing of the first transformation: stripping, first rest, boiling fol‐lowed by open air rest and boiling followed by store-room rest. Waxes were extracted with chloroform (CHCl₃) and the extract was submitted to saponification in order to obtain the neutral and acid fractions. The trimethylsilyl derivatives of both fractions were analysed by GC-MS. An important decrease of the total content of waxes -CHCl₃ extract-(average values: 10.13 to 4.91%) and of the acid fraction (average values: 2.51 to 0.96%) was observed throughout the industrial processing. Some differences were also found among trees, two of them being richer in total waxes (average, 8.14 and 7.71%) than the other one (average, 4.67%). Concerning the three groups of identified components (fatty acids, fatty alcohols and triterpenes), the contents of the fatty alcohols and acids suffer an important reduction during the first rest (average values: 30.39 to 17.53% and 58.73 to 31.48%, respectively), while after boiling, there is a decrease of the triterpenes concentrations (79.16 to 58.00% -neutral fraction- and 40.23 to 23.20% -acid fraction-), being greater when the second rest is carried out in a store room. Significant differences among trees were only found in the fatty alcohols contents. The neutral fraction was mainly composed by fatty alcohols (all the even members from C₁₈ to C₂₆, with traces of intermediate odd members and some unsaturated groups, C₂₀ and C₂₁) and triterpenes (14 components, among them friedelin, betulin and cerin were identified), and a very small amount of monocarboxylic fatty acids (C₁₆, C₁₈, C₂₂ and C₂₄ members) was also present. The main group of components of the acid fraction was that of fatty acids (saturated even C₁₂–C₂₄ and odd C₁₅, C₁₇, C₂₁ members, accompanied by a great amount of unsaturated terms and some ω-hydroxyacids, 18-hydroxy-9,12-octadecadienoic and 18-hydroxy-9-octadecenoic acids). Four triterpenes, the C₂₀, C₂₄ and C₂₈ alcohols and ferulic acid were also detected in the acid fraction. Some of these individual components of each fraction were also affected by the industrial processing and presented significant differences among the studied trees. Received 12 December 1997     

Waxes composition of Quercus suber reproduction cork from different Spanish provenances

Summary The chemical composition of waxes was studied in Quercus suber cork planks collected in seven localities of the three main production areas of Spain. Waxes were extracted with chloroform (CHCl₃) and the extract was submitted to saponification in order to obtain the neutral and acid fractions. The trimethylsilyl derivatives of both fractions were analysed by GC-MS. The seven populations cannot be distinguished by their total contents of waxes (Chloroform extract) and of the neutral and acid fraction. This lack of differences could be due to the important variability among samples of each population. The neutral fraction was mainly composed of fatty alcohols (all the even members from C₁₈ to C₂₆, with traces of the intermediate odd members and some unsaturated groups) and triterpenes (11 components, among them friedelin, betulin and cerin were identified) and a very small amount of monocarboxylic fatty acids (C₁₆ and C₂₄ members) was also present. The main group of components of the acid fraction was that of fatty acids (saturated even C₁₄–C₂₄ and odd C₁₅, C₁₇, C₂₁ members, accompanied by a great amount of unsaturated terms and some ω-hydroxyacids, 18-hydroxy-9,12-octadecadienoic and 18-hydroxy-9-octadecenoic acids). Four triterpenes and the C₂₀ and C₂₄ alcohols were also detected in the acid fraction. Significant differences were found among the corks of the various provenances in the global contents of alcohols and triterpenes and in these individual components contents of the neutral fraction. Concerning the differences among the various provenances in the contents of the groups of fatty acids, alcohols, and triterpenes in the acid fraction, a higher number of differences were obtained in the comparisons of the contents of acids and triterpenes, whereas the differences were practically null in the case of the alcohols. When the differences were studied considering the individual compounds, the relation with the behaviour of the contents of the groups of components was not as clear as it happens in the neutral fraction. In this case, the differences were scarcer and they did not show a unique pattern, neither in the components, nor the populations. There is no correspondence in the differences among the provenances regarding the components of the neutral fraction or those of the acid fraction. No clear relationship was found between the geographical proximity of the provenances and chemical similarity of the waxes composition. Received 12 December 1997     

Fingerprinting and genetic variability in cork oak (Quercus suber L.) elite trees using ISSR and SSR markers

Quercus suber L., is a socially, economically and ecologically important forest species in rural areas of the Mediterranean basin. Fifty three elite-trees from nine stands of four provenance regions in the Community of Extremadura (Spain) were analysed with the aim to establish their DNA-fingerprinting and the genetic relationships among them. Two types of molecular markers, microsatellites and intermicrosatellites, were used for tree genotyping. Microsatellite markers could distinguish 94% of the trees. Observed and expected heterozygosities, and effective number of alleles were correlated with the polymorphism information content (PIC). Intermicrosatellite patterns were used to construct a dendrogram. They showed high levels of genetic diversity in these elite trees, without any clear relationship with provenance regions. Both types of markers detected a high level of intra-stand variability, which is common in Quercus species, probably due to its mating system, the low distance among stands and the small size of stands. Others factors that could affect this result, such as introgression between Q. suber L. and Q. ilex L., are also commented. The high level of genetic variability detected in these elite trees can be useful for improvement programs. Also the usefulness of SSR and ISSR markers to establish the DNA-fingerprinting of each tree could be focused to select clonal lines of commercial interest.     

Morphological evaluation of cork oak (Quercus suber): Mediterranean provenance variability in Tunisia

Twenty-six provenances (2 340 plants) of cork oak (Quercus suber spp.) originating from Portugal, Spain, Italy, Morocco, Algeria, and Tunisia were tested for genetic variation among and within provenances by growth traits. Seven morphometrical characters were measured in 90 plants from each provenance. Analysis of variance showed highly significant differences for all characters. The phenotypic coefficient of differentiation reached 0.24 for the form and 0.22 for height, thus revealing a strong structuring between the provenances. Comparative study of growth among the provenances revealed more vigorous growth and better survival rate for those from Morocco, Spain, and Portugal, which may constitute better materials for afforestation. Furthermore, this variability appeared to be geographically structured and would be mainly genetically controlled, as cork oak provenances were cultivated under the same environmental conditions. Our results should be helpful for guide forest managers in afforestation.     

Factors affecting post-fire crown regeneration in cork oak (Quercus suber L.) trees

Cork oak (Quercus suber) forests are acknowledged for their biodiversity and economic (mainly cork production) values. Wildfires are one of the main threats contributing to cork oak decline in the Mediterranean Basin, and one major question that managers face after fire in cork oak stands is whether the burned trees should be coppiced or not. This decision can be based on the degree of expected crown regeneration assessed immediately after fire. In this study we carried out a post-fire assessment of the degree of crown recovery in 858 trees being exploited for cork production in southern Portugal, 1.5 years after a wildfire. Using logistic regression, we modelled good or poor crown recovery probability as a function of tree and stand variables. The main variables influencing the likelihood of good or poor crown regeneration were bark thickness, charring height, aspect and tree diameter. We also developed management models, including simpler but easier to measure variables, which had a lower predictive power but can be used to help managers to identify, immediately after fire, trees that will likely show good crown regeneration, and trees that will likely die or show poor regeneration (and thus, potential candidates for trunk coppicing).     

Effects of cadmium on cork oak (Quercus suber L.) plants grown in hydroponics

Cork oak (Quercus suber L.) is an autochthonous tree species that is being used for reforestation in heavy-metal-contaminated areas in Spain. A hydroponics experiment was carried out to characterize the effects of Cd on several morphological and physiological parameters in this species, including shoot length, nutrient concentrations and allocation in different organs, leaf pigment concentrations, photosynthetic efficiency, root ferric chelate reductase (FCR) activity and organic acid concentrations in xylem sap. Four different Cd treatments were applied, adding Cd chelated with EDTA or as chloride salt at two different concentrations (10 and 50 μM Cd). After 1 month of Cd treatment, plant growth was significantly inhibited in all treatments. Results indicate that Cd accumulates in all organs 7- to 500-fold when compared with control plants. The highest Cd concentration was found in the 50 μM CdCl(2) treatment, which led to concentrations of ~30, 123 and 1153 μg Cd g(-1) dry weight in leaves, stems and roots, respectively. In the strongest Cd treatments the concentrations of P and Ca decreased in some plant parts, whereas the Mn leaf concentrations decreased with three of the four Cd treatments applied. The concentrations of chlorophyll and carotenoids on an area basis decreased, whereas the (zeaxanthin plus antheraxanthin)/(total violaxanthin cycle carotenoids) ratio and the non-photochemical quenching increased significantly in all Cd treatments. Cadmium treatments caused significant increases in the activity of the enzyme FCR in roots and in the concentrations of organic acids in xylem sap. Some of the physiological changes found support the fact that Cd induces a deficiency of Fe in cork oak, although the plant Fe concentrations were not reduced significantly. At higher concentrations the effects of Cd were more pronounced, and were more marked when Cd was in the free ion form than when present in the form of Cd-EDTA.     

Ectomycorrhizal inoculation with Pisolithus tinctorius increases the performance of Quercus suber L. (cork oak) nursery and field seedlings

Mediterranean ecosystems are characterized by large arid areas where the patchy distribution of trees offers little protection against harsh climate conditions for seedling establishment. Climate change is predicted to result in an increase in these arid regions, with pronounced effects on vegetation. Production of seedlings with developed ectomycorrhizas is a promising strategy for minimizing the initial transplant shock, thereby increasing plant survival and growth during the first, most critical years of a plantation. One important species in the Mediterranean basin is Quercus suber (cork oak), which occurs, together with other evergreen oak species, in an agro-silvo-pastoral system that represents an example of sustainable land use in Europe. In this study, a Pisolithus tinctorius isolate was used for ectomycorrhizal colonization of cork oak nursery seedlings, and the effects on aboveground plant growth and leaf structural and physiological parameters were investigated. Ectomycorrhizal development resulted in a significant increase in leaf area, dry weight, nitrogen content, and photosynthetic pigments, and mycorrhizal plants showed a higher photosynthetic capacity and water use efficiency. Nursery-inoculated plants established in the field showed increased survival and growth during the first year after transplant. These results indicate a potential for further enhancing the use of mycorrhizal inoculation as a cultivation practice in forest nurseries. Considering the difficulty of soil restoration under limiting environmental conditions, nursery inoculation with ectomycorrhizal fungi can be an important advantage for improving the quality of seedling stock and its performance after out-planting in the field, benefiting the regeneration of arid regions and the reintroduction of inocula of ectomycorrhizal fungi into these areas.     

Cellular structure and chemical composition of cork from the Chinese cork oak (Quercus variabilis)

Quercus variabilis Blume, the Chinese cork oak, is an oak species with a thick cork outer bark. The cork is exploited at a limited scale in China and considered of lower quality than the commercial cork from Quercus suber. We studied an industrial cork granulate feedstock of Q. variabilis in relation to cellular structure and chemical composition and compared it to Q. suber cork under a material’s perspective. The cork of Q.variabilis has 1.1 % ash, 9.6 % extractives, 34.8 % suberin and 19.1 % lignin. The monosaccharide composition with shows a predominance of hemicelluloses: glucose 42.8 % of total neutral sugars, xylose 27.5 %, arabinose 15.4 %, galactose 9.0 %, mannose 4.0 %, rhamnose 1.2 %. The FT-IR spectrum shows the indicative peaks of suberin. The composition is overall similar to that of Q. suber cork. Q. variabilis cork has the typical cellular characteristics of bark cork tissues with a regular and radially aligned structure of cells without intercellular voids. Solid volume fraction was estimated at approximately 16 %. Compared with Q. suber, the Q. variabilis cork cells are smaller, the cell wall undulation and the overall row alignment less homogeneous, the cell surface is irregular and the solid volume proportion higher. The characteristics of Q. variabilis cork support its use as a cellular material for sealing, insulation and energy absorption, but the overall quality is lower than that of Q. suber cork. The negative impact of the higher density and structural lower uniformity at tissue and cell level should be evaluated for processing and product performance.     

Development of cork oak (Quercus suber L.) seedlings in response to tree shelters and mulching in northwestern Tunisia

The need for reforestation in cork oak (Quercus suber L.) areas is challenged by difficulties. Principal among these is herbivory of young plants, vegetative competition, and slow growth rates of cork oak seedlings. We evaluated the early development of cork oak seedlings treated using tree shelters and mulching in northwestern Tunisia. We tested three tree shelter treatments (non-vented, vented, and control) to shield seedlings from animal damage and five mulch types to control competing vegetation (Italian Stone Pine, Lentisk, combination of Italian Stone Pine and Lentisk (organic mulches), gravel (inorganic mulch) and no mulch). At the end of the two-year experiment, sheltered seedlings were 89-99% taller than unshelteredseedlings and had higher numbers and lengths of shoot growth flushes. In contrast, both stem diameter growth and dry weight biomass (from samples extracted after two years) were significantly reduced inside tree shelters. Root-to-shoot ratio was not significantly different in sheltered vs. unsheltered seedlings, suggesting that tree shelters do not adversely affect this parameter. Mulching alone did not favour growth, but could be beneficial when combined with tree shelters. The combination of vented tree shelters and gravel mulch was the most effective treatment for promoting diameter, height and stem volume growth.     

Decline and dieback of cork oak(Quercus suber L.) forests in the Mediterranean basin: a case study of Kroumirie,Northwest Tunisia

Assessing the vulnerability of forest ecosystems in the climate change context is a challenging task as the mechanisms that determine this vulnerability cannot be directly observed. Based on the ecological interrelationships between forests and climate, the present review focused on providing current information about vulnerability assessments of cork oak(Quercus suber L.) forests in the Mediterranean basin, especially, in the Kroumirie region(northwest Tunisia), currently under historic extreme drought conditions. From comparing recent findings in this region, we synthesized data on cork oak decline and mortality collected during the historic drought years 1988–1995 period. Climate change impacts cork forest decline, with special interest shown in elevated temperatures and drought; cork oak forest regeneration, and the adaptation of the Kroumirie forest to climate change, are reviewed herein. The studied region has been influenced largely by frequent prolonged drought periods, especially from 1988 to 1995. Droughts were found to consistently have a more detrimental impact on the growth and mortality rates of cork oak populations. Cork oak mortality was recorded for up to 63,622 trees. In the future, more research studies and observational data will be needed, which could represent an important key to understand ecosystem processes, and to facilitate the development of better models that project climate change impacts and vulnerability. The study is useful for researchers and forestry decision makers to develop the appropriate strategies to restore and protect ecosystems, and to help anticipate potential future droughts and climate change.     

Water-use efficiency in cork oak (Quercus suber) is modified by the interaction of water and light availabilities

We studied the interaction of light and water on water-use efficiency in cork oak (Quercus suber L.) seedlings. One-year-old cork oak seedlings were grown in pots in a factorial experiment with four light treatments (68, 50, 15 and 5% of full sunlight) and two irrigation regimes: well watered (WW) and moderate drought stress (WS). Leaf predawn water potential, which was measured at the end of each of two cycles, did not differ among the light treatments. Water-use efficiency, assessed by carbon isotope composition (delta(13)C), tended to increase with increasing irradiance. The trend was similar in the WW and WS treatments, though with lower delta(13)C in all light treatments in the WW irrigation regime. Specific leaf area increased with decreasing irradiance, and was inversely correlated with delta(13)C. Thus, changes in delta(13)C could be explained in part by light-induced modifications in leaf morphology. The relationship between stomatal conductance to water vapor and net photosynthesis on a leaf area basis confirmed that seedlings in higher irradiances maintained a higher rate of carbon uptake at a particular stomatal conductance, implying that shaded seedlings have a lower water-use efficiency that is unrelated to water availability.     

Allocation of 14C assimilated in late spring to tissue and biochemical stem components of cork oak (Quercus suber L.) over the seasons

Carbon distribution in the stem of 2-year-old cork oak plants was studied by (14)CO(2) pulse labeling in late spring in order to trace the allocation of photoassimilates to tissue and biochemical stem components of cork oak. The fate of (14)C photoassimilated carbon was followed during two periods: the first 72 h (short-term study) and the first 52 weeks (long-term study) after the (14)CO(2) photosynthetic assimilation. The results showed that (14)C allocation to stem tissues was dependent on the time passed since photoassimilation and on the season of the year. In the first 3 h all (14)C was found in the polar extractives. After 3 h, it started to be allocated to other stem fractions. In 1 day, (14)C was allocated mostly to vascular cambium and, to a lesser extent, to primary phloem; no presence of (14)C was recorded for the periderm. However, translocation of (14)C to phellem was observed from 1 week after (14)CO(2) pulse labeling. The phellogen was not completely active in its entire circumference at labeling, unlike the vascular cambium; this was the tissue that accumulated most photoassimilated (14)C at the earliest sampling. The fraction of leaf-assimilated (14)C that was used by the stem peaked at 57% 1 week after (14)CO(2) plant exposure. The time lag between C photoassimilation and suberin accumulation was ～8 h, but the most active period for suberin accumulation was between 3 and 7 days. Suberin, which represented only 1.77% of the stem weight, acted as a highly effective sink for the carbon photoassimilated in late spring since suberin specific radioactivity was much higher than for any other stem component as early as only 1 week after (14)C plant labeling. This trend was maintained throughout the whole experiment. The examination of microautoradiographs taken over 1 year provided a new method for quantifying xylem growth. Using this approach it was found that there was more secondary xylem growth in late spring than in other times of the year, because the calculated average cell division time was much shorter.     

Dieback of cork oak (Quercus suber) in Catalonia (NE Spain) caused by Botryosphaeria stevensii

Botryosphaeria stevensii Shoemaker (anamorph: Diplodia mutila Fr. apud Mont.) is reported as the cause of canker and dieback of cork oak (Quercus suber L.) in Catalonia (NE Spain). It also causes wilting of trees after cork is removed for industrial purposes. Symptomatology and details of morphology for both anamorph and teleomorph are given.     

Low temperature during winter elicits differential responses among populations of the Mediterranean evergreen cork oak (Quercus suber)

Populations of cork oak (Quercus suber L.) were assessed for seasonal and inter-population variability in, and temperature responses of, the ratio between light-induced variable and maximum fluorescence of chlorophyll, Fv/Fm, considered a surrogate for the maximum photochemical efficiency of photosystem II (PSII). Seedlings from 10 populations throughout the distribution range of Q. suber in the Mediterranean basin were grown in a common garden in central Spain. The Fv/Fm ratio of dark-adapted leaves was measured at dawn every month for 2 years. Air temperature was recorded at a nearby climatic station. During the summer, when maximum air temperatures reached 40 degrees C, there were no significant differences in Fv/Fm among populations, but significant differences were seen during the winter. In colder months, Fv/Fm ranged in all populations between 0.5-0.6 and 0.2-0.3 in 2001 and 2002, respectively. The variance explained by the population effect was greatest during winter months, especially in 2002, reaching a peak value of 10% when minimum air temperature was below -10 degrees C. Populations originating from warmer sites showed the largest decline in Fv/Fm between the end of 2001 and the beginning of 2002. Thus, a negative linear relationship was established between mean annual temperature at the population source and population mean Fv/Fm recorded in the coldest month in 2002 and normalized by the Fv/Fm spring measurement.     

Effects of partial defoliation on carbon and nitrogen partitioning and photosynthetic carbon uptake by two-year-old cork oak (Quercus suber) saplings

At the end of the growing season in late July, 20-month-old cork oak (Quercus suber L.) saplings were partially defoliated (63% of leaf area) to evaluate their ability to recover leaf area after defoliation. At 18 and 127 days after defoliation, changes in starch and nitrogen pools were determined in leaves and perennial organs, and variations in photosynthetic carbon uptake were investigated. To determine the role of stored nitrogen in regrowth after defoliation, plant nitrogen was labeled in the previous winter by enriching the nutrient solution with 15N. Plants recovered the lost leaf area in 127 days. Although there was remobilization of starch and nitrogen from leaves and perennial organs, the availability of resources for growth in the following spring was not decreased by defoliation. On the contrary, starch concentration in coarse roots was higher in defoliated saplings than in control saplings, presumably as a result of the higher net CO2 exchange rate in newly developed leaves compared with pre-existing leaves.     

Root system of Quercus suber L. seedlings in response to herbaceous competition and different watering and fertilisation regimes

Different management practices are commonly applied to increase pasture yield of Mediterranean open woodlands, but the consequences of increasing competition for soil resources with these practices on tree recruitment are still unknown. In a greenhouse study, belowground competition of Quercus suber L. seedlings growing together with natural (Oak_NP) or improved pasture (Oak_IP) was evaluated, and their root systems compared with those of seedlings growing in bare soil (Oak_BS). Two watering levels and two regimes of P₂O₅ fertilisation were also tested. Because of competition, the Oak_IP seedlings had their fine root mass density, coarse root length, and shoot mass reduced by up to 40, 36, and 39%, respectively, when compared to Oak_NP seedlings. Oak_NP and Oak_BS seedlings showed similar average root density parameters and shoot mass values, indicating that Q. suber seedlings growing with natural pasture is a low competitive system. High availability of water and phosphorus did not mitigate the strength of competition between herbaceous plants and oak seedlings, and favoured the pasture to the detriment of the trees. Our findings suggest that P₂O₅ fertilisation and irrigation practices performed to improve herbaceous productivity will negatively influence recruitment of Q. suber seedlings.     

Carbon sequestration of modern Quercus suber L. silvoarable agroforestry systems in Portugal: a YieldSAFE-based estimation

Modern alley cropping designs, with trees aligned in rows and adapted to operating farming machinery, have been suggested for Europe. This paper explores the potential for adoption of cork oak (Quercus suber L.) agroforestry in Portugal and estimates the potential carbon sequestration. Spatial modeling and Portuguese datasets were used to estimate target areas where cork oak could grow on farmland. Different implementation scenarios were then modeled for this area assuming a modern silvoarable agroforestry system (113 trees ha^?1 thinned at year 20 for establishing 50 trees ha^?1). The YieldSAFE process-based model was used to predict the biomass and carbon yield of cork oak under low and high soil water holding capacity levels. Approximately 353,000 ha are available in Portugal for new cork oak alley cropping. Assuming implementation rates between 10 % of the area with low soil water capacity (60 mm: 15 cm depth, coarse texture) and 70 % of the area with high soil water holding capacity (1,228 mm: 200 cm depth, very fine texture), then carbon sequestration could be 5 × 10⁶ and 123 × 10⁶ Mg CO₂ respectively. Due to higher yields on more productive land, scenarios of limited implementation in high productivity locations can sequester similar amounts of carbon as wide implementation on low productivity land, suggesting that a priori land classification assessments can improve the targeting of land and financial incentives for carbon sequestration.