Mariola Sánchez-González

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.RésuméDans cette étude, nous avons mesuré l’influence de diverses variables sur l’épaisseur du liège des forêts de chênes-lièges. Dans ce but nous avons d’abord appliqué un modèle linéaire mixte pour prédire l’épaisseur moyenne du liège, et on a alors identifié les co-variables explicatives pour expliquer leur possible corrélation avec des effets aléatoire. Le modèle prédisant l’épaisseur moyenne du liège peut être décrit comme un processus stochastique où un modèle fixe et déterministe explique la valeur moyenne, tandis qu’une variabilité résiduelle inexpliquée est décrite et modélisée par l’inclusion de paramètres aléatoires relevant de la parcelle, de l’arbre, de la récolte de liège par parcelle et aux niveaux résiduels des arbres prenant en compte la covariance de la structure spatiale entre les arbres d’une même parcelle. Le calibrage a été réalisé en employant la théorie BLUP (Best linear unbiased predictor ou Meilleur prédicteur linéaire non biaisé) On a essayé différentes options pour trouver la dimension optimale de l’échantillon et on a trouvé qu’il était opportun d’utiliser quatre arbres par parcelles. Finalement le modèle a été appliqué pour calculer la production de liège et a été comparé avec le poids de liège obtenu avec le modèle employé d’habitude en Espagne.

Base-age invariant cork growth model for Spanish cork oak (Quercus suber L.) forests

A cork growth model for Spanish cork oak forests was developed using data from 432 cork samples. Ten dynamic equations derived from the generalised algebraic difference approach (GADA) were considered for analysis, and both numerical and graphical methods were used to compare alternative models. All of the equations are base-age invariant and directly estimate accumulated cork thickness in complete years at any moment of the cork rotation. The fittings were done using the nested iterative method of the stochastic regression approach. The GADA formulation derived by Krumland and Eng (2005) from the Richards model by considering a1 and a3 as related to site productivity was finally selected. The cork growth model developed in this study allows the total accumulated cork thickness in complete years to be estimated in the fourth year of cork rotation with close to 80% reliability for any debarking period, increasing to nearly 90% from the eighth year onwards.

Response to the interaction of thinning and pruning of pine species in Mediterranean mountains

Pruning allows knot-free timber to be obtained, thereby increasing the value of the highest-value wood products. However, the effect of pruning on growth is under discussion, and knowledge about the tree response to the simultaneous development of thinning and pruning is scarce. The objective of this study was to analyze the effect of the interaction of thinning and pruning on tree and stand level and the annual radial growth of two pine species native to Mediterranean mountains. We used long-term data of three trials installed in pine stands where several combinations of pruning and thinning were developed. Five inventories were carried out for each trial, and the mean dasometric features of the different treatments were compared using linear mixed models including a competition index. In addition, we collected cores from ten trees per plot in order to evaluate the annual response of trees to the thinning and pruning. We analyzed the annual radial growth using a semiparametric approach through a smooth penalized spline including rainfall and temperature covariates. Pruning did not show any effect on growth. However, larger diameter and increased annual radial growth were found in thinned plots, both with and without pruning, as compared to unthinned plots. Also, we found significant effects of climate on annual radial growth. We recommend the application of thinning and pruning in stands of Mediterranean mountains in order to get knot-free timber since growth reduction was not found in thinned stands.

Basal Area and Diameter Distribution Estimation Using Stereoscopic Hemispherical Images

Abstract In recent years, proximal sensing data has increasingly been used to optimize forest inventories. In this paper we present a forest inventory methodology based on stereoscopic hemispherical images. An automated pixel-based approach and a user-guided “region growing” approach have been developed for image matching. To estimate the basal area, number of trees and mean diameter, the sampling probability is determined for each tree. The accuracy and precision of the estimates derived from stereoscopic hemispherical images was analyzed for a set of National Forest Inventory plots. The results revealed that tree matching depends on the species, the distance to the target tree and the diameter. The Pearson correlation coefficient was 0.86 for the mean diameter and 0.89 for the basal area, whereas for the number of trees per hectare it was 0.59. The proposed methods may be used in large scale forest inventories as a cost-efficient way of obtaining data on diameter distribution and basal area from field surveys following a two-stage scheme combined with remote sensing techniques.

Alternative approaches to assessing the natural regeneration of Scots pine in a Mediterranean forest

Key messageIn modelling regeneration patterns, parametric regression is recommended because it can account for the spatial and temporal correlation present in the data, whereas decision trees allow more complex interactions and can be used to reduce the number of variables.ContextThe establishment of seedlings after regeneration fellings is key to guaranteeing the development and persistence of the forest. Depending on the objective pursued, data available or type of forest, a number of different methods have been employed to assess the relationship between seedling establishment and both environmental and stand factors. Most authors have conducted their analyses using parametric regression or point pattern analysis.AimWe analysed the way in which light, stand conditions, edaphic and topographic variables affect the regeneration of Pinus sylvestris L. in Central Spain. We used different methods to analyse the same data set. The strengths and weaknesses of each method were discussed.MethodsWe used two parametric approaches: generalized linear mixed model regression using a negative binomial followed by the variant explanatory variables reduction prior to regression as well as three nonparametric approaches not commonly employed in forest regeneration: nonmetric multidimensional scaling, regression trees and random forests algorithm.ResultsThe parametric regression identified a larger number of variables associated with the regeneration process and the inclusion of a random effect in the model allowing the consideration of the spatial variability among plots. However, decision trees captured the complex interaction among variables, which typical parametric methods were unable to detect.ConclusionDifferent statistical methods gave similar insights into the underlying ecological process. However, different statistical premises with inference implications can be noticed. This may give misinterpretation of the model depending on the nature of the data. The choice of a given method should be made according to the nature of the data and the achievement of desirable results.

Thinning increases cone production of stone pine (Pinus pinea L.) stands in the Northern Plateau (Spain)

ContextEdible stone pine (Pinus pinea L.) nut is a forest product which provides the highest incomes to the owners of stone pine forests.AimThe objective of this work is to evaluate the effect of first thinning on growth and cone production in an artificially regenerated stand in order to determine optimum intensity.MethodsA thinning trial was installed in 2004 to compare two thinning regimes (heavy and moderate) and a control treatment. From 2004 to 2012, six inventories of forest attributes were carried out, and the cone crop was harvested annually. We evaluated the effect of thinnings on growth using repeated measures analysis of variance with a mixed model approach. With regards to cone production, we first estimated the probability of finding cones in a tree by applying a generalized mixed model and then estimated cone production by using a mixed model, including climatic variables.ResultsWe found that thinning had a positive influence on tree diameter increment. Thinning increased the probability of finding cones and cone production. However, significant differences between heavy and moderate thinnings were not found.ConclusionWe recommend early silvicultural treatments in stone pine stands to favor the development of trees and larger edible pine nut production.

Quality grading of cork stoppers based on porosity, density and elasticity

An objective control method for grading cork stoppers is presented using a cork stopper quality index based on porosity, density and elasticity, these being the properties which have the greatest influence on the closure capacity of the stopper. The elasticity of the cork stopper is measured through the relaxation ratio, which is defined by the relationship between the relaxation force exerted by the cork in the bottleneck and the compressive force exerted by a caliper to fit the stopper in the bottle. The relaxation ratio, defined in this way, represents the part of the compression force which is applied to the stopper on insertion and which is recovered in the form of the relaxation force to achieve closure. The calculation of the relaxation ratio involves the measurement of the relaxation force of the fitted stopper. This force has been measured rigorously and precisely using a device developed in the Cork Laboratory at the INIA-CIFOR and which is presented for the first time in this paper.ZusammenfassungVorgestellt wird ein objektives Kontrollverfahren zur Klassifizierung von Korken anhand eines Qualitätsindex für Korken basierend auf den Eigenschaften, die den größten Einfluss auf die Verschlussleistung des Korkens haben, namentlich Porosität, Dichte und Elastizität. Die Elastizität des Korkens wird anhand des Relaxationsverhältnisses gemessen. Dies ist das Verhältnis zwischen der vom Korken im Flaschenhals ausgeübten Relaxationskraft und der Druckkraft, die erforderlich ist, um den Korken in die Flasche einzuführen. Das so definierte Relaxationsverhältnis stellt den Teil der beim Einpressen aufgewandten Druckkraft dar, der als Relaxationskraft, mit der die Flasche verschlossen wird, wiedergewonnen wird. Zur Berechnung des Relaxationsverhältnisses muss die Relaxationskraft des eingesetzten Korkens gemessen werden. Diese Kraft wurde mittels eines im Cork Laboratory bei INIA-CIFOR entwickelten Gerätes, das in dieser Arbeit zum ersten Mal vorgestellt wird, präzise gemessen.

Correlation between porosity of cork planks before and after boiling using near infrared spectroscopy

The aim of this study was to evaluate the potential of near infrared spectroscopy (NIRS) in predicting the coefficient of porosity of raw and boiled cork samples used mainly for producing cork stoppers for wine bottling. The classification of cork planks is currently performed visually in the last stage of preprocessing after boiling in water. In this study, calibration models have been developed on 279 cork samples before and after boiling. Three models were obtained: one which predicts the coefficient of porosity of unprocessed samples, a second one that predicts porosity in boiled samples and finally a model that predicts the coefficient of porosity of boiled samples from spectra of raw samples. The models obtained showed a coefficient of determination of 0.58, 0.59 and 0.52, respectively. These results show that the modified partial least squares models based on NIRS technology can be used for screening cork porosity, not only at the two stages of processing, but also to predict boiled porosity. This suggests that NIRS technology could be used for determining the coefficient of porosity in boiled cork planks, and could also provide a useful tree selection tool.

Regeneration dynamics of mixed stands of Pinus pinaster Ait. and Pinus pinea L. in Central Spain

The dynamics of mixed stands are more complex and less studied than those of monospecific stands. The objective of this work was to analyze the variables involved in seedling occurrence and seedling survival in mixed stands of Pinus pinaster and P. pinea in Mediterranean areas. From 2011 to 2016, regeneration of both species was monitored at two sites located in Central Spain. We installed 72 regeneration plots where seedling dynamics were monitored. All the trees in the study areas were measured and mapped. Additionally, we took hemispherical photographs in each regeneration plot. The average density of P. pinea seedlings over the study period was almost 20 times larger than that of P. pinaster. Our results indicate that the seedlings of both species grow under moderate light conditions. In addition, we found that the occurrence of seedlings of both species was related to the structure of the stand. P. pinea seedlings grew where the density and size of P. pinaster trees were low and where P. pinea trees provided moderately sheltered conditions, whereas the number of P. pinaster seedlings was related to under intermediate densities of P. pinaster trees. Furthermore, seedling survival was positively associated with age of the seedlings and negatively with the August average maximum temperature. The temporal continuity of mixed stands of P. pinea and P. pinaster in the study area is compromised by the observed lack of regeneration of P. pinaster.