Cork Oak Stands Research Articles

Temporal metabolic profiling of the Quercus suber–Phytophthora cinnamomi system by middle‐infrared spectroscopy

SummaryThe oomycete Phytophthora cinnamomi is an aggressive plant pathogen, detrimental to many ecosystems including cork oak (Quercus suber) stands, and can inflict great losses in one of the greatest ‘hotspots’ for biodiversity in the world. Here, we applied Fourier transform‐infrared (FT‐IR) spectroscopy combined with chemometrics to disclose the metabolic patterns of cork oak roots and P. cinnamomi mycelium during the early hours of the interaction. As early as 2 h post‐inoculation (hpi), cork oak roots showed altered metabolic patterns with significant variations for regions associated with carbohydrate, glycoconjugate and lipid groups when compared to mock‐inoculated plants. These variations were further extended at 8 hpi. Surprisingly, at 16 hpi, the metabolic changes in inoculated and mock‐inoculated plants were similar, and at 24 hpi, the metabolic patterns of the regions mentioned above were inverted when compared to samples collected at 8 hpi. Principal component analysis of the FT‐IR spectra confirmed that the metabolic patterns of inoculated cork oak roots could be readily distinguished from those of mock‐inoculated plants at 2, 8 and 24 hpi, but not at 16 hpi. FT‐IR spectral analysis from mycelium of P. cinnamomi exposed to cork oak root exudates revealed contrasting variations for regions associated with protein groups at 16 and 24 h post‐exposure (hpe), whereas carbohydrate and glycoconjugate groups varied mainly at 24 hpe. Our results revealed early alterations in the metabolic patterns of the host plant when interacting with the biotrophic pathogen. In addition, the FT‐IR technique can be successfully applied to discriminate infected cork oak plants from mock‐inoculated plants, although these differences were dynamic with time. To a lesser extent, the metabolic patterns of P. cinnamomi were also altered when exposed to cork oak root exudates.

Adaptive management and debarking schedule optimization of Quercus suber L. stands under climate change: case study in Chamusca, Portugal

Cork oak stands are one of the major sources of income from Portuguese Mediterranean forests. Future climate is projected to increase temperatures, reduce precipitation and decrease current forests’ productivity and therefore, adapting management, is a key strategy to mitigate impacts of future climate on cork supply. The central objective of this research was to compare conventional and adaptive management regimes under scenarios of climate change. The adaptive management focussed on adopting optimal harvest schedules while considering different management objectives. The study focused on the Chamusca region, one of the most productive areas of cork, and considered four distinct spatial scales for analysis. For each scale, the management objective was defined according to field information and considered the maximization of cork production while targeting different cork harvesting flows: (a) more frequent and regular or (b) less frequent and concentrated. A forest growth model was used to simulate climate change impact on future yield of cork oak stands under different forest management alternatives. A mixed integer programming model was developed to find the most adequate cork debarking cycle calendar for cork oak stands. Our results suggest that (1) business as usual management under climate change scenarios could decrease cork supply and carbon stock in the tree component of the forests by up to 20 and 30 %, respectively, (2) the development of adaptive management strategies, including cork extraction schedule optimization, to address climate change has advantages over traditional practices and (3) may contribute further to increase cork production (up to double productivity in mid-long term) while addressing concerns with the regulation of cork extraction flows through the adaptation of debarking periods. The mixed integer programming allowed the spatial visualization of the debarking cycle delay. Furthermore, results underline the relevance of an approach to develop adaptive management strategies that can consider different management goals, with different constraints to address climate change.

Morphological and genetic diversity of Biscogniauxia mediterranea associated to Quercus suber in the Mediterranean Basin

Biscogniauxia mediterranea is a widespread fungus that causes charcoal disease on cork oak and other hardwood hosts. It had been considered a secondary pathogen causing the disease only in stressed hosts. However, its frequency and severity have been increasing, inclusive in young trees without other decline signs and developing atypical symptoms. The present work aims to assess the fungus' variability in the Mediterranean basin, following cork oak geographical distribution. A collection of 36 isolates originated from cork oak in Portugal and other Mediterranean countries, from other hosts and from trees with different ages and disease expression were analyzed by cultural characteristics, conidial dimensions, and growth rates at different temperatures and by microsatellite-primed PCR profiles. Clustering UPGMA analyses combining different parameters were preformed. All the approaches revealed high level of intraspecific polymorphism among Mediterranean isolates, not allowing relating the disease development with any analyzed features. The results highlighted the variability of this fungus that induces its adaptation ability in the present worrying scenario of climatic change. All the conditions are gathered to favor the aggravation of the disease in cork oak stands.

The effect of soil on cork quality.

The present work aimed to contribute for a better knowledge regarding soil features as cork quality indicators for stoppers. Cork sampling was made in eight Cork oak stands (montados de sobreiro) located in the Plio-Plistocene sedimentary formations of Península de Setúbal in southern Tagus River region. The samples used to classify the cork as stopper for wine bottles were obtained in eight cork oak stands, covering soils of different types of sandstones of the Plio-plistocene. In each stand, we randomly chose five circular plots with 30 m radius and five trees per plot with same stripping conditions determined by: dendrometric features (HD- height stipping, PBH- perimeter at breaster height), trees vegetative condition (defoliation degree); stand features (density, percentage canopy cover); site conditions (soil type and orientation). In the center of each plot a pit was open to characterize the soil profile and to classify the soil. Cork quality for stoppers was evaluated according to porosity, pores/per cm2 and cork boards thickness. The soil was characterized according to morphological soil profile features (lithology, soil profound, and soil horizons) and chemical soil surface horizon features (organic matter, pH, macro, and micronutrients availability). Based on the variables studied and using the numerical taxonomy, we settled relationships between the cork quality and some soil features. The results indicate: (1) high correlation between the cork caliber and boron, cation exchange capacity, total nitrogen, exchange acidity, and exchangeable magnesium, potassium, calcium, and sodium in soils of theirs cork oaks; (2) the cork porosity is correlated with the number of pores/cm2 and magnesium soil content; (3) the other soil features have a lower correlation with the caliber, porosity, and the number of pores per cm2.

The effect of soil compaction at different depths on cork oak seedling growth

Soil compaction promoted either by inadequate management (pressure of livestock and machinery) or by soil natural conditions (podzolisation) can influence the growth of cork oak seedlings. We hypothesized that compaction could be related with the lack of natural regeneration and decline on cork oak stands. In this paper, we evaluated the response of cork oak seedlings growth in terms of area and biomass production for above and belowground parts at different compaction depths tested for a sandy-loam soil. This study was done in a greenhouse, with germinated seedlings. Three treatments were applied. One no-compaction treatment (control, C0) and two with a soil compacted layer at 60 cm (C1) and 30 cm depth (C2). The level of compacted layer was 1.37 MPa of mechanical resistance. Results show that tap root length is negatively affected by compaction at 60 and 30 cm depth. Below and aboveground biomass are affected by compaction at 30 cm depth. In addition, the leaf area results demonstrate that compaction is a sensitive factor for this parameter. In this 1-year stage, plants spend more energy in roots production. Due to soil formation and bad management of cork oak stands, soil compaction at depth could be a cause for the observed lack of natural regeneration, affecting the growth at earlier stages and probably for the decline of cork oak populations.

Analysis of raw cork production in Portugal and Catalonia using life cycle assessment

This study aims to (1) evaluate the environmental impacts associated with the three types of raw cork produced in Portuguese cork oak woodlands (in Alentejo region) considering two alternative practices for stand establishment (plantation and natural regeneration), (2) compare the environmental impacts of raw cork production in Portuguese cork oak woodlands and in Catalonian cork oak forests, and (3) assess the influence of different allocation criteria for partitioning the environmental impacts between the different types of raw cork produced. A cradle-to-gate approach was adopted starting with stand establishment up to cork storage in a field yard. The system boundaries include all management operations undertaken during the following stages: stand establishment, stand tending, cork stripping, and field recovery. The allocation of the environmental impacts to reproduction, second, and virgin cork was based on mass and market price criteria. An alternative allocation approach was simulated by allocating environmental impacts also to the wood produced in the cork oak stands. The impact assessment was performed using the characterization factors recommended by the International Reference Life Cycle Data System (ILCD). In Portugal, cork produced from naturally regenerated stands has a better environmental performance than cork produced from planted stands, but the differences are smaller than 10 %. Different management models of cork oak stands in Portugal and Catalonia (agro-silvopastoral system and forest system, respectively) originate different impact levels, which tend to be significantly lower in Catalonia. The environmental hot spots in the two regions are also distinct. In Catalonia, they are associated with cleaning, road maintenance, and worker and cork transport. In Portugal, they are fertilization, pruning, and cleaning. The two allocation criteria affect significantly the results obtained for virgin cork in Portugal and for virgin and second cork in Catalonia. Besides, when impacts are also allocated to wood, mass allocation should be avoided as it would not create incentives for a sustainable management of cork oak stands. The environmental impact from Catalonian cork may be reduced by decreasing mechanized shrub cleaning and road maintenance operations through the introduction of livestock in cork oak forests, and also by a better planning of management operations. For the Portuguese cork, improvements may be achieved by optimizing fertilizer dosage, planting nitrogen-fixing crops and pastures that improve soil quality, avoiding unnecessary operations, improving the efficiency of management operations, and increasing tree density.

Identifying signatures of natural selection in cork oak (Quercus suber L.) genes through SNP analysis

Cork oak (Quercus suber L.) is an evergreen tree species endemic to the western Mediterranean Basin with a major economical, social and ecological relevance, associated with cork extraction and exploitation. In the last years, cork oak stands have been facing a significant decline, which may be aggravated by the climate changes that are predicted to occur within cork oak distribution range during this century. Under this scenario, the assessment of adaptive genetic variation is essential to understand how cork oak may cope with these threats and to delineate strategies for the management of its genetic resources. In this study, six candidate genes possibly significant for environmental adaptation were analysed in cork oak populations from its entire distribution range. Signatures of natural selection were investigated using population genetic statistics and environmental association tests under alternative scenarios of population genetic structure. Signals of balancing selection were detected in the putative non-expressor of pathogenesis-related gene 1 (NPR1), involved in plant defence response against pathogens, in auxin response factor 16 (ARF16), a gene implicated in root development, in RAN3, also involved in developmental processes, and in glutamine synthetase nodule isozyme (GS), involved in nitrogen fixation. Furthermore, for ARF16, a class I heat shock protein (sHSP) and GS, associations were found between SNP allele and haplotype frequencies and several spatial and climatic variables, suggesting that these genes may have a role on cork oak local adaptation. In this study, the first steps were taken into gathering information on cork oak adaptation to environmental conditions.

Land cover change and afforestation of marginal and abandoned agricultural land: A 10 year analysis in a Mediterranean region

In Mediterranean countries land abandonment, the loss of traditional land-use systems and the action of fire are causing forests degradation, increasing erosion risk and desertification. The Rural Development Programme–Afforestation of Agricultural Land (RURIS–AAL) is an afforestation programme for marginal and abandoned agricultural land which intends to contribute to the rehabilitation of degraded lands and to mitigate the effects of desertification.The study was conducted in a Mediterranean region under desertification risk. The goal of the study was twofold: (i) to explore the main drivers of land cover change and the impact of RURIS–AAL on both agriculture area loss and on native oaks area recovery and (ii) to assess the appropriateness of the species used and the levels of afforestation success of RURIS–AAL from 2002 until 2011. The analysis was conducted through spatial analysis in a Geographical Information System and by statistical analysis using both nonparametric correlations and Categorical Principal Components Analysis (CATPCA).The results proved that the study area’s land cover change (2000–2006) was mainly due to forest fires and as result, a decrease in forest areas (ground cover higher than 30%) as opposed to an increase of open forests (ground cover between 10% and 30%), cuts and new plantations areas was observed. The impact of RURIS–AAL on agricultural area loss was found to be very weak (−1009ha; −0.6%). The species used in RURIS–AAL were well selected in relation to theirs ecological zoning and the levels of afforestation success were high (76%). A total of 3363ha were forested mainly with pure cork oak and mixtures of cork oak and holm oak (86%) which was very positive to recover oak forest area. Moreover, the CATPCA proved that the combination of the necessary biophysical conditions for each species together with the most appropriated procedures for stand establishment was verified in RURIS–AAL. High afforestation success rates were obtained for both pure cork oak stands (79%) and mixed stands of cork oak (89%), namely mixed umbrella pine and cork oak stands (95%).Both species, cork oak and umbrella pine, are very well adapted to Mediterranean environments and offer non-wood products (cork and cone) of high market value which will both improve farm’s multi-functionality and provide extra incomes in the long-term. These findings are important guidelines to be kept in consideration for successful afforestation in future programmes in Mediterranean regions.

Short Communication. Effect of the health status and geographical origin on the cork production characteristics of Western Algeria cork oak stands

Aim of study: To analyze the effect of health status on cork production, analyzing if this influence is uniform or is affected by site conditions.Area of study: Two Western Algerian cork tree forests have been studied: M’Sila located in the coastal plains under semiarid climate, and Zarieffet, located in the mountainous interior under sub-humid climate.Material and Methods: 40 trees were selected in each forest and classified according to their health status as healthy, weakened, or decaying. A sample of cork from each tree has been obtained to measure the key variables related to cork production. A two-way ANOVA was performed considering two factors: site and health status.Main results: Quercus suber L. productivity is affected by the vitality of trees in the same way in both sources, showing values between 5.96 ± 7.1 kg • m-2 (coast, weakened trees) and 8.13 ± 0.45 kg • m-2 (mountain, healthy trees). The health status also affects the number and area of pores, especially in the cork oak groves of the coast, where the coefficient of porosity ranges from 3.79 ± 0.84% (healthy trees) to 8.11 ± 1.91% (decaying trees). The variables where the site has presented a stronger effect are those related to the amount of cork produced by the phellogen (density -kg·l-1, p<0.000- and productivity -kg·m-2, p=0.001-), and pore density (1·cm-2, p=0.001). Scrap thickness (mm) and porosity (%) show a smaller effect although still representative (p=0.041 and 0.038 respectively). Porosity and pore density show interaction site*health status. They all have higher values in the mountain (Zarieffet) than in the coast (M’Sila). The effect of tree vitality on the formation of pores in the cork oak phellogen is lower in the mountain than in the coast. No significant effects were found for any of the two factors neither on the annual growth rate nor on the thickness of the cork.Research highlights: Results lead to the conclusion that the effect of health status on traumatic phellogen formation and activity is clear but not uniform. Further studies are necessary for a deeper understanding of the effect of stress situations on pore formation and characteristics.Keywords: mean annual growth; density; productivity; porosity; decay

The application of the Liocourt model to uneven-aged cork oak stands

The purpose of this study is to determine the applicability of the Liocourt model to cork oak (Quercus suber L.) stands in the south-western Iberian Peninsula, and to review the general applicability of the Liocourt method. Seven uneven-aged stands in a densely structured cork oak forest (basal area ≥ 10 m2/ ha) were selected and analysed. The results indicate that the Liocourt expression suffers from theoretical limitations which make it difficult to obtain through regression analysis. Additionally, the paper concludes that the Liocourt model is inapplicable to cork oak stands in conditions similar to those of the study.

Contribution of cork oak plantations installed after 1990 in Portugal to the Kyoto commitments and to the landowners economy

Cork oak stands are a part of the agroforestry ecosystem in Portugal, characterized by a low crown cover from cork oak trees, managed towards cork production, and sometimes in combination with grazing. In recent years, European Union policies gave impetus to a large area of new cork oak plantations, which have been established mainly for cork production purposes, and consequently with higher stand density than traditional agroforestry systems. These plantations are important not only for cork production but also for the carbon sequestered by these slow growing forests that won't be harvested for wood production. Thinning operations will be needed to avoid excessive inter-tree competition and wood extracted from these thinning may also provide income for the owners. In the present study, carbon sequestered and wood volumes resulting from thinning were estimated for the next 70years. Three scenarios of different annual afforestation rates and different site indexes were tested. The resulting values for the considered scenarios show that, if the plantation rates are maintained, new cork oak plantations will have an important contribution to the Portuguese commitments to providing CO2 offsets under the Kyoto protocol. Additionally, due to the increasing values of initial density in new cork plantations, cork oak forests will produce a significant volume of wood that may become an important contribution to the landowners' income.

Tree type and forest management effects on the structure of stream wood following wildfires

Wildfires are an increasingly common disturbance influencing wood recruitment to streams, and thereby affecting their physical and biological condition. Mediterranean countries such as Portugal, where more than 25% of the land area has burned since 1990, are ideal areas to study impacts of wildfire effects on streams. We evaluated the physical structure of 2206 downed wood pieces (DWP) across 27 first- to third-order streams in central Portugal, all of which had experienced recent wildfires. The streams flowed through monospecific upland forests of Eucalyptus, Maritime pines, or Cork oaks and were fringed by a mixture of riparian tree species. DWP structure differed between tree types and between burned and unburned pieces. Post-fire timber-production forests (Maritime pines and Eucalyptus) contributed a higher quantity of thinner, longer and straighter DWP to streams than Cork oak stands. Pieces from Maritime pines had more rootwads and branches than DWP from the other tree types. Pieces from Cork oak and riparian species generally had a bent form, were shorter and had no rootwads. Burned DWP in streams were often from riparian trees. Relative to unburned DWP, the burned DWP occurred more frequently, were larger and straighter, had branches less often, and were more decayed. With more complex branches, rootwads, and a larger diameter, inputs from burned Maritime pine forests are more likely to change stream hydraulics and habitat complexity, relative to inputs from Eucalyptus forests with their simpler structure. This study shows that, less than a decade after wildfires, structure of downed wood in and near streams is strongly influenced by wildfire, but also still reflects intrinsic species characteristics and respective silviculture practices, even after the effects of fire have been accounted for. Under an anticipated shift in landscape cover with higher shrubland proportions and more mixing of Maritime pine and Eucalyptus forests, our results suggest that instream large wood will become scarcer and more structurally homogeneous.

Nonlinear fixed and random generalized height–diameter models for Portuguese cork oak stands

The objective of the research was to develop a generalized height–diameter model for Quercus suber L. in Portugal, which can be applied both to undebarked and debarked trees, with diameter at breast height over cork larger than 2.5 cm. A nonlinear fixed effects model (NLFEM) and a nonlinear mixed effects model (NLMEM) approaches were used. Parameters estimates were obtained using the SAS macro NLINMIX, which uses a linear approximation to the marginal likelihood function by expanding it with a first-order Taylor series on the random effects. The option of expanding on the random effects at their current empirical best linear unbiased predictors (EBLUP) was used. The fitted models were evaluated using an independent data set, together with an existing model specific for undebarked trees. To obtain subject specific predictions with the NLMEM, a conventional and an improved calibration procedures were applied, considering four different tree sub-sampling designs. Both proposed models included dominant height and stand density as covariates to explain plot variability. Validation indicated that, even in the situations where the NLMEM calibration is not possible, this model should be preferred. The differences between the validated models, which were more evident for young stands, were considered. No large differences in predictive accuracy were found between the calibrated NLMEM using the conventional or the improved calibration procedures, for all the considered sub-sampling designs.

Di Bérenger's method application in southwest iberian cork oak stands

L’articolo analizza l’applicazione del metodo di Di Berenger ai popolamenti di sughera della Spagna sud-occidentale (AB > 10 m2/ha). i risultati indicano la difficolta a ottenere la curva di equilibrio dalle specie di Quercus suber L. poiche la relazione tra area di crescita e diametro a petto d’uomo ha un’alta variabilita. Comunque, in generale, e possibile applicare il metodo ad altri popolamenti con area di copertura delle chiome inferiore al 100%. La curva di Di Berenger e stata spiegata come una funzione potenza.

Characterization of Sardinian cork oak (Quercus suber L.) genetic resources for economically important traits

Considering the very high socio economic value of cork oak (Quercus suber L.) in Mediterranean areas, a better knowledge of local genetic resources is necessary in order to reach a good balance between the conservation issues and the need of an efficient cork production network. In such frame, local germplasm of Q. suber from the main cork production areas of Sardinia was analysed for DNA and isozyme polymorphisms and for cork quality in relation to some environmental parameters. A total number of 24 sample stands of cork oak were selected, representing typical vegetation, sylvicultural and cork quality features within the eight identified cork growing areas in Sardinia. Considerable variation was found between cork oak stands of the different areas. Results of the principal component analysis performed on the cork quality data and environmental characters of the eight areas, showed that the first three components explain 72.2% of the variation. The major characters involved in this differentiation were cork quality characters such as dimensional recover, moisture and Mg content of the cork and also elevation of stands m a.s.l. for the first component. The second component appears to be determined by some climatic parameters (average annual temperature and average of the minimum temperatures of coldest month) and by the Fe and Zn in the cork. For the third component, major characters involved in differentiation are average annual rainfall, bark thickness and Ca content in the cork.

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.

Performance di recenti imboschimenti con quercia da sughero in ex-coltivi

Recently, significant forestry activities have taken place in Sardinia thanks to EU Regulation 2080/92 funds. Some 80% of the afforestated surface has been planted with holm oak and cork oak. The latter also characterizes 89% of the reforestation area. Given the funding source, plantations have been established on farmlands. Growth performances of these recent cork oak stands have been quantitatively evaluated and compared with the performances of two experimental plots. In Gallura (north-east of Sardinia), that is the traditional cork production area and still is economically the most important cork district of the island, these new cork oak plantations have an average size of 28 ha. They have been established on lands that, before plantation, were either pastures (30%) or arable lands (70%). Plantation failures are limited to 8.8% of the total (in term of mass) and seem independent of environmental factors or plantations species composition (conifers have been frequently used as secondary species). Average growth of the stem, measured above cork at collar height, is in the range 4 to 8mm/year with a mean value of 5.5mm/year. No correlation appears with either environmental conditions or species composition of the plantations. In the first experimental plot, soil management practices (natural vegetation removal vs. its cutting and mulching) does not differentiate young plants growth trends. In the control subplots (no removal) stem collar diameter is 20% smaller. Localized manual hoeing around trunk base increased the diameters by 13% but reduced cork thickness by 21%. The second experimental plot allows comparisons among 27 Mediterranean proveniences of cork oak. The trial exhibits reduced genetic influence: diameters and heights growth are significantly different only among extreme groups. In conclusion, reduced growth performances of the plantations established in farmlands is due, to some extent, to the limitations inherent with private land management (constrained to costs minimization) and, on the other hand, to the generally very limited thickness of Gallura soils. Demand for good quality commercial cork is steadily raising. To sustain the request, in the short term, it would be necessary to extend financial support for plantations care, from 5 to 10 years, or even up to the first stripping (virgin cork).

Age-independent difference equations for modelling tree and stand growth

Modelling growth of trees or stands when age is not available is often necessary. This is the case in national forest inventories or when age is not a main determinant of growth (e.g., trees growing in uneven-aged stands). Even if age is not known, functions used to model growth should follow the required pattern, with a maximum and a slow decline after the maximum is attained. There are some empirical functions with these properties; however, direct modelling with difference equations derived from the so-called theoretical growth functions has not been used for this purpose, as they are age dependent. This paper presents a methodology to formulate growth functions as age-independent difference equations. These can be used when age is not available or is not relevant. The proposed equations have the advantage of allowing the direct modelling of yield instead of growth. If the parameters are expressed as a function of site variables, the equations are invariant for projection length and therefore can be used when data is not equally spaced in time, as is the case of most data sets. The methodology is applied to the Lundqvist and Richards growth functions, the most commonly used in growth and yield modelling. The use of the age-independent growth functions is illustrated by using two case studies in Portugal: dominant height growth of eucalyptus (Eucalyptus globulus Labill.) plantations and individual tree growth in diameter at breast height in sparse cork oak (Quercus suber L.) stands.

Forest fires in cork oak (Quercus suber L.) stands in Portugal

Fire and cork can be considered both the most serious problem and the most valuable resource of Portuguese forests. The existence of important areas of burned cork oak stands resulting from severe fire seasons in recent years is an environmental problem which deserves special attention. This paper makes a multidisciplinary survey of the problem of burned cork oak stands focusing on: the dimensions of the problem in Portugal, the remarkable resistance of cork oak to fire, the post‐fire management of cork oak stands, and the economic and ecological effects of fire in cork oak stands. Particular emphasis is given to knowledge gaps in this context.