Radial variation of wood density components and ring width in cork oak trees

The radial variation of ring width and wood density was studied in cork oaks (Quercus suber) using microdensitometry. The observations were made in young never debarked cork oaks (30–40 years of age) and in mature trees under cork production (37–60 years of age). The cork oak wood is very dense (mean ring density 0.86 g.cm^?3, between 0.79 g.cm^?3 and 0.97 g.cm^?3) with a small intra-ring variability (mean earlywood density 0.80 g.cm^?3 and latewood density 0.90 g.cm^?3). The density components decreased from pith to bark more rapidly until the 15th ring, and then only slightly. There were no significant differences in the mean density components between never debarked trees and trees under cork production but their outwards decrease was accentuated in the never debarked trees. The annual growth was high, with a ring width mean of 3.9 mm (4.2 mm in the first 30 years) and the latewood represented 57% of the annual growth.     

Cork oak (<Emphasis Type="Italic">Quercus suber</Emphasis> L.) wood growth and vessel characteristics variations in relation to climate and cork harvesting

Variations in tree ring growth of Quercus suber L. were analysed using dendrochronological techniques on cork oak discs from trees harvested in the cork producing region of Alentejo, Portugal. A tree-ring chronology containing a strong common signal and covering the period from 1970 to 1995 was build for ca. 30-year-old cork oaks never submitted to cork harvesting using 14 trees that crossdated satisfactorily out of 30 sampled trees. The tree ring indices correlated positively with September temperature (r = 0.48, P < 0.05) and very strongly with precipitation totals from previous October until current February (r = 0.82, P < 0.001) showing that the water stored in the soil during the autumn and winter months prior to the growing season has a primordial effect on the growth of the given season. The effects of cork harvesting were analysed by comparing mean ring width, mean annual vessel area, vessel density (n°vessels/mm²), and vessel coverage (percentage of transverse surface occupied by vessels) between three mature cork oak trees and three young trees, for the period from 1987 to 1996, corresponding to the growth between two consecutive cork removals in the case of mature trees. In 1988, 1989 and 1996 (corresponding to the first and second years after cork removal, and 1996 to a year of cork removal), the ratios between ring widths of young versus mature trees was twice that for the rest of the period. However, an effect of cork removal indicated by eventual alterations in vessel size and distribution in the wood rings corresponding to the years 1988, 1989 and 1996 in the mature cork oaks was not observed.     

Chronic cork oak decline and water status: new insights

Chronic decline and Sudden death are two syndromes of cork oak (Quercus suber) dieback. Mortality is associated with water stress, but underlying physiological mechanisms are poorly understood. Here, we investigated the physiological performance of declining trees during the summer drought. Leaf water potential, gas-exchange, fluorescence of photosystem II and leaf and root starch concentration were compared in healthy (asymptomatic) and declining trees. Low annual cork increment in declining trees indicated tree decline for several years. All trees showed similar water status in spring. In summer, declining trees showed lower predawn leaf water potential (?2.0 vs. ?0.8 MPa), but unexpectedly higher midday leaf water potential than healthy trees (?2.8 vs. ?3.3 MPa). The higher midday water potential was linked to by means of strongly reduced stomatal conductance and, consequently, transpiration. This study is pioneer showing that declining trees had high midday water potential. A tendency for lower sap flow driving force (the difference between predawn and midday water potential) in declining trees was also associated with reduced photosynthesis, suggesting that chronic dieback may be associated with low carbon uptake. However, starch in roots and leaves was very low and not correlated to the health status of trees. Declining trees showed lower water-use efficiency and non-photochemical quenching in summer, indicating less resistance to drought. Contrarily to chronic decline, one tree that underwent sudden death presented predawn leaf water potential below the cavitation threshold.     

Variation of endophytic cork oak‐associated fungal communities in relation to plant health and water stress

The main objective of this study was to obtain more comprehensive knowledge about the effect of water stress on endophytic fungal communities in asymptomatic and declining cork oak trees. Six asymptomatic and six declining cork oak trees were randomly selected in a natural cork oak forest located in Sardinia, Italy. In February 2003, the soil around three asymptomatic and three declining trees was covered with a circular plastic film to reduce rain water supply with the intention to induce water stress. The remaining six trees served as controls. Predawn xylematic water potential (PWP) was used as water status indicator and measured seasonally. Between July 2003 and June 2004, fungal endophytes were isolated every 2–4 months from twigs, branches and woody tissues. Significant differences in PWP between covered and control trees were detected mainly in autumn. Gas exchange was greatest in asymptomatic control plants. All tissues were colonized by endophytic fungi. Nineteen fungal species were isolated from 1620 plant fragments. Biscogniauxia mediterranea was the most frequently isolated fungus. Its isolation frequency was significantly higher in declining covered trees than in control trees (p < 0.05). Presence of this fungus in asymptomatic control trees was significantly higher in winter than in summer. Water stress seems to reduce species diversity of the endophytic mycobiota in cork oak and to promote proliferation of some potentially pathogenic endophytes.     

Dry-season leaf flushing of Enterolobium cyclocarpum (ear-pod tree): above- and below-ground phenology and water relations

Above- and belowground phenology and water relations of Enterolobium cyclocarpum Jacq. trees in the dry forest of Santa Rosa National Park, Costa Rica were studied during two consecutive phenological cycles, from November 1998 to June 2000. Aboveground phenological activity, including leaf shedding, growth and maturation of dormant fruits, new leaf flushing and flowering, occurred during the dry season. Measurements of leaf water potential, stomatal conductance and sap flow indicated that stomata of newly flushed leaves remained essentially closed until the onset of the first rains, suggesting that the main factor accounting for the favorable water balance of dry-season flushed leaves was their capacity to restrict water loss. Evidence of a contribution from stem and root water stores to shoot expansion was mixed because only the first dry-season flushing episode monitored was accompanied by a marked decrease in stem and root water potentials. Fine root production did not precede leaf flushing, occurred only after the onset of the rainy season and stopped under drought conditions, suggesting that soil water content was the most important variable controlling fine root dynamics in this species.     

Nitrogen sources for current-year shoot growth in 50-year-old sessile oak trees: an in situ (15)N labeling approach

We used long-term in situ (15)N labeling of the soil to investigate the contribution of the two main nitrogen (N) sources (N uptake versus N reserves) to sun shoot growth from bud burst to full leaf expansion in 50-year-old sessile oaks. Recovery of (15)N by growing compartments (leaves, twigs and buds) and presence of (15)N in phloem sap were checked weekly. During the first 2 weeks following bud burst, remobilized N contributed ~90% of total N in growing leaves and twigs. Nitrogen uptake from the soil started concomitantly with N remobilization but contributed only slightly to bud burst. However, the fraction of total N due to N uptake increased markedly once bud burst had occurred, reaching 27% in fully expanded leaves and 18% in developed twigs. In phloem sap, the (15)N label appeared a few days after the beginning of labeling and increased until the end of bud burst, and then decreased at full leaf expansion in June. Of all the shoot compartments, leaves attracted most of the absorbed N, which accounted for 68% of new N in shoots, whereas twigs and new buds accounted for only 28 and 3%, respectively. New N allocated to leaves increased from unfolding to full expansion as total N concentration in the leaves decreased. Our results underline the crucial role played by stored N in rapid leaf growth and in the sustained growth of oak trees. Any factors that reduce N storage in autumn may therefore impair spring shoot growth.     

Variability of the compression properties of cork

The variability of the compression properties of cork was determined after field sampling covering the main production area in Portugal (10 sites) with samples taken from 20 trees per site at the time of cork stripping. In all cases, cork showed compressive stress–strain curves typical for cellular materials, which are characterized by an approximately “elastic” region up to a 5 % strain, followed by a large plateau up to 70 % strain caused by progressive buckling of cell walls, and a steep stress increase for higher strains corresponding to cell collapse. The radial direction of compression offered higher strength. The Young’s moduli averaged 10.4 and 9.2 MPa for radial and non-radial directions, respectively, spreading from 3.5 to 22.5 MPa for the non-radial and 4.2 to 21.5 MPa for the radial directions. The geographical location of cork production was the major factor of variability. Density, annual growth ring width and chemical composition influenced compression. Cork samples with higher density showed overall larger resistance to compression. The energy absorbed per unit volume to achieve the maximal deformation with full densification of cork is higher when the average annual ring width is smaller. Cork samples with relatively higher suberin content required less stress for deformation. The results encompass the natural variability of cork and are the most extensive to characterize cork. They allow a better insight into the differences that may explain the variation in cork properties and strengthen its use either in the known applications, i.e. as a sealant, or in novel applications.     

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.     

Analysis of variables influencing tree cork caliper in two consecutive cork extractions using cork growth index modelling

The influence of tree and stand variables, debarking intensity, and precipitation on the caliper of cork produced by a tree and on the evolution of cork caliper between consecutive cork extractions was researched. A total of 370 cork samples were collected in 23 permanent plots distributed across the cork production area in Portugal, covering a period from 1984 to 2010. Cork growth was evaluated using the cork growth index (cgi), defined as the radial width of the first eight complete years of cork growth after stripping. The differences in mean cgi at plot level between two consecutive cork growth periods (cgp) were assessed using nonparametric tests. A mixed model approach was used due to the nested structure of the data for modelling cgi value at tree level. The cgi of two consecutive cork extractions is linearly related at tree level. At plot level, the mean value of cgi decreased in 5 out of the 23 plots and increased in 2 plots for α = 0.05. The number of debarked branches and the variation of precipitation between two cgp were the variables that explained the largest part of the cgi evolution. However, significance of plot random parameters indicates that other variables are involved in the tree cgi evolution, pointing out to the need of further research. Tree size and precipitation during the cgp are related to the individual tree cgi. The effect of increasing stand density and debarking intensity on cork growth was not clear. Long term research based on permanent plot measurements and research trials is needed to clarify the impact of tree competition and debarking intensity.     

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.     

Quantile regression for modelling the impact of climate in cork growth quantiles in Portugal

The annual growth and the thickness of cork are known to be highly variable between trees located in the same geographical location. Researching how climate variables affect different trees within the same site is a step forward for the management of cork production since current knowledge focusses only on the average tree response. Quantile regression methodology was applied for the first time to a large data set containing measurements of cork growth, sampled in 35 stands across the cork oak distribution area in Portugal. This methodology proved to be useful for testing the hypothesis raised: does climate affect differently the annual cork growth, and ultimately cork thickness of individual trees located in the same stand? Estimating the amount of cork produced by one stand that has the required thickness for the production of natural cork stoppers is essential to support cork oak management. However, no model, before this work, had been developed to provide managers with this information. A downward parabolic relationship between annual cork growth and annual precipitation was determined for all quantiles, with optimum annual average precipitation value ranging from 1103 to 1007 mm. April to August monthly temperatures, spring average temperature or summer average temperature, showed a negative relationship with annual cork growth, in particular for lower quantiles. Maximum annual temperature was shown to negatively affect annual cork thickness, in particular for the trees under the 6th quantile. The ratio between annual precipitation and average temperature, that define the Lang index (LI), showed a downward parabolic relationship with annual cork growth. Best cork growth conditions are found for Lang index values around 60, corresponding for the transition between semi-arid climate and humid climate. The application of the final model developed for estimating cork thickness of an eight years’ cork growth period allowed the prediction and mapping of the percentage of cork suitable for natural cork stopper production. It showed that higher values are expected in the Southern and Central coastal regions and along the Tagus River basin. The Northern coastal and mountain regions, characterised by Lang index values higher to 60 (humid climates), present lower estimated values for the percentage of cork suitable for natural cork stopper production. The estimated values are expected to be reduced under climate change scenarios in the Southern and Central coastal regions.

     

Nutrients return from leaves and litterfall in a mediterranean cork oak (<Emphasis Type="Italic">Quercus suber</Emphasis> L.) forest in southwestern Spain

The knowledge of the cycle of nutrients is fundamental for the correct comprehension of the tree–soil relationship and for an adequate forest management. In order to analyse the nutrients return from leaves and litterfall in a Mediterranean cork oak forest in southwestern Spain, 12 trees were randomly selected and litterfall collected for 2 years. Samples were taken monthly and separated in different fractions (leaves, twigs, catkins, acorns and miscellaneous), then leaves nutrients were analyzed. Simultaneously, we analyzed the nutrient content of living leaves from the same trees in each season during 1 year. The analyzed nutrients were C, N, P, K, Ca, Mg, Fe, Mn, S, Cu, Zn and Mo. Annual patterns of each nutrient in fallen leaves were characterized and compared with seasonal values of these nutrients in living leaves. Leaves fall has two annual maximum, first and most important in spring around April coinciding with renewal of foliar cover and second around October. Main concentration patterns of N, P and K are related with phenological patterns, in consequence minimum concentration in leaves fall were obtained in periods of growing and maximum litterfall. Concentrations of Ca, Fe and Mn increase with the age of the leaves and maximum concentrations were obtained before periods of maximum litterfall while concentrations of Cu, Mo and Mg stay stable. Seasonal analysis of nutrients in living leaves collected from the same trees in four different periods of the year allowed to corroborate the patterns of leaves fall and the probable osmotic function of K.     

Occurrence of Phytophthora cinnamomi in cork oak forests in Italy

An increasing decline and mortality of cork oak trees have been recently observed in central Italy and Sardinia Island. Following surveys conducted in three declining cork oak forests, a Phytophthora species was consistently isolated from soil samples collected from trees displaying different level of decline. Based on morphological features, growth rates at different temperatures and analysis of DNA sequences of the ITS region, all isolates were identified as Phytophthora cinnamomi Rands. This pathogen caused large brownish lesions on inoculated freshly cut branches of cork oak. It was re‐isolated from all infected tissues. These findings represent the first report of P. cinnamomi on cork oak trees in Italy.     

Growth stresses and strains in cork

Summary A detailed study of the growth stresses and strains in the cork shell of the cork-oak was undertaken based on experimentally determined constitutive relations for cork in tension and compression. The stresses depend on the thicknesses of the cork shell and of the back layer around the cork shell, on the radius of the trunk and on its increase due to growth. The circumferential stresses in the cork shell and back layer are tensile and increase with increasing distance to the tree axis. The radial stresses are compressive and decrease with increasing distance to the tree axis. The strains due to growth are not recovered when the cork boards are removed, unless the boards are heated, for example, by immersion in boiling water. Other consequences of the growth stresses are analysed, such as the occurrence of corrugations in the lateral cell walls of cork, the variation of width of the successive growth rings and the occurrence of cracks in the back layer and outer cork layers.     

Differences of raw cork quality in productive cork oak woods in Sicily in relation to stand density

Differences of raw cork quality, in terms of thickness and porosity, were observed in productive cork oak (Quercus suber L.) stands in the Nebrodi and Iblei mountains of Sicily in relation to stand density. Five study areas were chosen across a range of different stand density satisfying specific sampling requirements among two managed cork oak woods.Analysis of variance using Fisher–Snedecor‘s F test (p < 0.05) were used to identify statistically significant differences of cork quality and sylvicultural, dendrometric and cork stripping parameters between study areas within each stand. Relationships between cork quality and sylvicultural and dendrometric parameters were also tested using Pearson's correlation coefficient (r). Results shown that sylvicultural practices are a key factor influencing cork quality: statistically significant differences in cork thickness and crown area were observed between areas at different stand density; positive correlations were found between cork thickness and crown area in lower density study areas favouring a significant vegetative activity; negative correlations were found between parameters of cork stripping (cork stripping coefficient, cork stripping surface, maximum height of cork stripping) and cork thickness. No correlations were found between cork porosity and dendrometric or sylvicultural parameters, demonstrating the probable genetic origin of cork porosity. Results shown that cork quality parameters and stand attributes may be used in sustainable management models of cork oak forests.     

Predicting mature cork biomass with t years of growth from one measurement taken at any other age

Cork is a natural product that is extracted from the outer bark of the cork oak tree. According to Portuguese legislation, the interval between two consecutive cork extractions on the same tree must be equal or greater than 9 years. Although the majority of the cork oak stands are debarked at the end of this period, this rotation may not be the optimum in many cases. The existing models for cork weight prediction can only be used for trees debarked at a 9 years or, in one model, at a 10-years rotation period, since the data used for its development was limited to these growth periods. The development of a method that allows for the prediction of the mature cork biomass with t years of growth, based in one measurement taken at any other age, was the main objective of this work. The method is based on the knowledge that the density of the cork tissue is constant between the inner and outer cork rings, being significantly different from density of the cork back. It can be implemented using two different equations that were developed during this work: a model to estimate cork biomass with 9 years of age and a model to estimate the cork back weight proportion at 9 years of age. For the first model, four different alternative models were developed, considering different levels of information collected during forest inventory. The model to estimate the cork back weight proportion leads to the biomass of cork tissue. Cork biomass at t years is obtained by decreasing or increasing the biomass of cork tissue according to the difference in cork thickness between t and 9 years of growth. The proposed method was evaluated by comparing the observed and estimated values of cork biomass from an independent data set with corks with 9, 10 and 11 years of age. The results showed similar precision for corks with 9, 10 or 11 years of age. As expected the precision of the predictions increases when the model to estimate cork biomass with 9 years of age uses more information. The presented method should be an important tool for cork oak stand management, for the prediction of the evolution of carbon stocks in cork oak stands, and will allow analyzing the impact in cork biomass production of decreasing or extending the interval between two consecutive cork extractions.     

Variables influencing cork thickness in Spanish cork oak forests: A modelling approach

In this study, we evaluate the influence of different variables on cork thickness in cork oak forests. For this purpose, first we fitted a multilevel linear mixed model for predicting average cork thickness, and then identified the explanatory covariates by studying their possible correlation with random effects. The model for predicting average cork thickness is described as a stochastic process, where a fixed, deterministic model, explains the mean value, while unexplained residual variability is described and modelled by including random parameters acting at plot, tree, plot × cork harvest and residual within-tree levels, considering the spatial covariance structure between trees within the same plot. Calibration is carried out by using the best linear unbiased predictor (BLUP) theory. Different alternatives were tested to determine the optimum subsample size which was found to be appropriate at four trees. Finally, the model was applied and its performance in the estimation of cork production was tested and compared with the cork weight model traditionally used in Spain.     

叶林模式杜仲生物量的动态研究

通过对叶林模式杜仲6—10月的生物量动态变化的调查分析,结果表明:杜仲枝条长度和粗度前期增长较快,而到8、9月以后,增长缓慢,在10月份达到最大值,分别为115.57 cm和1.971 cm。6—8月为杜仲枝条皮干重的缓慢增长期,8—10月为杜仲枝条皮干重的快速增长期,在10月份最高,达到15.99 g;不同月份杜仲枝条皮含水率从高到低顺序为:6月>8月>9月>7月>10月,6月份最高可达74.7%,而10月份最低为68.88%。6—7月杜仲剥皮枝条干重增长不明显,7—10月为杜仲剥皮枝条干重的快速增长期,在10月份最高,达到111.63 g。不同月份杜仲剥皮枝条含水率从高到低顺序为:6月>7月>8月>9月>10月,6月份最高可达66.95%,而10月份最低为50.16%。6—9月为杜仲叶(单枝)总干重的增长期,9—10月为稳定期,9月份最高,达到48.96 g;不同月份杜仲叶(单枝)含水率从高到低顺序为:9月>8月>6月>10月>7月,9月份最高可达72.44%,而7月份最低为69.91%。不同月份杜仲叶片数量(单枝)从多到少顺序为:8月>9月>10月>7月>6月,8月份最多可达59片,而6月份最少为39.7片。建议以杜仲叶子为原料的加工产业,应在9月采摘叶子。     

Cambial Anatomy and Annual Rhythm of Secondary Xylem Development in the Twigs of Azadirachta indica A. Juss. (Meliaceae) Growing in Different Forests of Gujarat State

Abstract

Seasonal behavior of vascular cambium and development of secondary xylem were studied in the 2-3-years-old branches of Azadirachta indica A. Juss. (Meliaceae) growing in moist deciduous (MDF), dry deciduous (DDF) and scrub land (SF) forests of Gujarat State. Development of xylem began with the sprouting of new leaves in January in both MDF and DDF. Xylem production culminated in July in MDF whereas it reached peak in April and July in DDF. Cambial cells ceased to divide with the maturation of leaves in November and October in MDF and DDF, respectively. Mature xylem and phloem derivatives surrounded the cambium in December in MDF and November in DDF. Radial growth in the branches of trees growing in SF was found continuous throughout the year with peak cambial activity in April, July and November. In all the three forests, maximal radial growth was encountered in July when the rains were heavy. Cambial activity and xylem development showed significant correlation with the phenology of the trees in MDF and DDF whereas cambium was found active throughout the year and no correlation was found between the cambial activity and phenology in SF.     

Observations on bud burst phenology in a field trial established with Poplar (Populus spp.)

Bud flushing is very important for the survival and growth of trees, a phenomenon matched each year with the annual course of temperature and the timing of bud flushing in the spring. Essentially it represents a serious ecological and evolutionary tradeoff between survival and growth. The most suitable timing of bud burst permits trees to begin growth sufficiently early to take advantage of favorable spring conditions, but late enough to decrease the risks of tissue damage from late frost. In the present study bud burst spring phenology of poplar (Populus tremula and P. tremuloides) from eight different provenances, originating from Europe and the USA, was observed during March and April, 2009. The experimental plot was located at Solling, Germany (51°44′0″ N, 9°36′0″ E). A six stage subjective scoring system of bud burst phenology was used to identify the phenological stages of the seedlings, where each plant was observed twice a week. The aim of the study was to predict phenotypic variation in poplar, originating from regions between 42° and 60° N latitude, growing in similar environments. Timing of bud flushing of poplar was recorded. It was found that seedlings of provenance 3, which originated from 42.35° N latitude, started and completed flushing significantly earlier than those of other provenances, while seedlings of provenance 5, originating from 54.29° N latitude, started flushing very late and only a few plants reached top scoring at the end of the experimental period. Analysis of variance showed statistically highly significant differences (p < 0.05) in bud flushing among the provenances. The correlation between scoring and flushing periods was very strong within provenances although the flushing pattern differed among provenances (origin of the planted seedlings). Bud flushing showed a negative correlation with the origin of the planted seedlings. Given the field experience gained with this experiment, it is recommended that seedlings from provenances 5 and 8 could be used for future plantations where late frost may be a problem for the young shoots of seedlings.