Felipe Crecente-Campo

Aboveground stand-level biomass estimation: a comparison of two methods for major forest species in northwest Spain

ContextThe scaling-up approach (which requires the use of individual tree biomass equations and data) is one of the most commonly used methods for estimating stand biomass at a local scale. However, biomass prediction over large management areas requires more efficient methods.AimsTwo methods of estimating aboveground stand biomass were developed and compared: stand biomass equations (SBE) including observed stand variables, and SBE including biomass expansion factors (BEF) and stand volume.MethodsTwo types of systems of additive equations were fitted simultaneously for components and total aboveground stand biomass, to ensure additivity. Inherent correlations among biomass components were also taken into account in the fitting process.ResultsThe systems explained a high percentage of the observed variability. The SBE systems that included observed stand variables provided more accurate estimates than those that included BEF and stand volume. However, the latter were found to be more precise for stem wood and total aboveground biomass prediction.ConclusionsBoth approaches provide a direct link between forest inventory data, outputs from whole-stand growth models, and biomass estimates at stand level. Taking into account that the inventory effort is similar for both alternatives, the choice of which to use will depend on the data available and on the relative importance of the biomass components for the end-users.

Modelling canopy fuel variables for Pinus radiata D. Don in NW Spain with low-density LiDAR data

Crown fire initiation and spread are key elements in gauging fire behaviour potential in conifer forests. Crown fire initiation and spread models implemented in widely used fire behaviour simulation systems such as FARSITE and FlamMap require accurate spatially explicit estimation of canopy fuel complex characteristics. In the present study, we evaluated the potential use of very low-density airborne LiDAR (light detection and ranging) data (0.5 first returns m-2) - which is freely available for most of the Spanish territory - to estimate canopy fuel characteristics in Pinus radiata D. Don stands in north-western Spain. Regression analysis indicated strong relationships (R2 = 0.82-0.98) between LiDAR-derived metrics and field-based fuel estimates for stand height, canopy fuel load, and average and effective canopy base height Average and effective canopy bulk density (R2 = 0.59-0.70) were estimated indirectly from a set of previously modelled forest variables. The LiDAR-based models developed can be used to elaborate geo-referenced raster files to describe fuel characteristics. These files can be generated periodically, whenever new freely available airborne LiDAR data are released by the Spanish National Plan of Aerial Orthophotography, and can be used as inputs in fire behaviour simulation systems.

A merchantable volume system for Pinus sylvestris L. in the major mountain ranges of Spain

Abstract• Accurate estimation of stem volume makes it possible to estimate the monetary value of one of the many commodities and services that forests provide to society, i.e. timber.• In the present study a compatible volume system for Scots pine in the major mountain ranges of Spain (the Pyrenees, Southern Iberian Range, Northern Iberian Range, Soria and Burgos Mountains, Central Range and Galician Mountains) was developed with data from 2 682 destructively sampled trees. Several well-known taper functions were evaluated. A second-order continuous-time autoregressive error structure was used to correct the inherent autocorrelation in the hierarchical data, allowing the model to be applied to irregularly spaced and unbalanced data.• The compatible segmented model of Fang et al. (2000) best described the experimental data. It is therefore recommended for estimating diameter at a specific height, height to a specific diameter, merchantable volume, and total volume for the six mountain ranges analyzed.• The non-linear extra sum of squares method indicated differences in mountain range-specific taper functions. A different taper function should therefore be used for each mountain range in Spain.Résumé• L’estimation précise du volume des tiges permet d’estimer la valeur du bois qui est un des nombreux produits et services que la forêt apporte à la société.• Dans cette étude, nous avons développé un modèle compatible de volume du pin sylvestre avec les mesures obtenues pour 2 682 arbres abattus dans les principales zones de montagne en Espagne, les Pyrénées, le sud et le nord de l’Ibérie, les montagnes de Soria et Burgos, le centre et les montagnes de Galice. Nous avons évalué plusieurs équations de profil de tiges bien connues. Pour corriger les autocorrélations inhérentes aux données hiérarchiques, nous avons utilisé un modèle d’erreur autoregressif à l’ordre 2 en temps continu, ce qui permet au modèle d’être appliqué à des données irrégulièrement espacées dans le temps et non équilibrées.• Le modèle segmenté compatible de Fang et al. (2000) décrit le mieux les données expérimentales. Il est donc recommandé pour estimer le diamètre à une hauteur donnée, la hauteur à un diamètre donné, le volume commercial et le volume total pour les six zones montagneuses étudiées.• La méthode des moindres carrés généralisés non linéaires indique des différences entre équations de profils de tiges des différentes zones montagneuses. Il faut donc utiliser une équation de profil de tige par zone de montagne en Espagne.

Modelling canopy fuel variables in Pinus pinaster Ait. and Pinus radiata D. Don stands in northwestern Spain

ContextThe fuel complex variables canopy bulk density and canopy base height are often used to determine crown fire initiation and spread. Direct measurement of these variables is impractical, and they must be estimated indirectly.AimsThe objectives of the present study were to model the vertical profile of available crown fuel in maritime and radiata pine stands in NW Spain, using data from destructively sampled trees; to compare the values of the canopy fuel variables estimated by two different methods and to estimate these variables from common stand descriptors.MethodsSystems of equations were fitted simultaneously to address the inherent correlations between available crown fuel components and between the canopy fuel variables. Heteroscedasticity and autocorrelation were also taken into account in the fitting process, where necessary.ResultsThe values of the canopy fuel variables varied greatly depending on the estimation method used. Models for predicting the variables at stand level explained a high percentage of the observed variability. Crowning is more likely in maritime pine than in radiata pine stands.ConclusionsAlthough the approach used in this study provides a realistic depiction of the vertical distribution of crown fuel, it departs from the requirements of Van Wagner’s model. The method used to estimate canopy fuel variables affects fire modelling results and thus the design of fuel treatment projects.

Selection of mixed-effects parameters in a variable–exponent taper equation for birch trees in northwestern Spain

ContextTaper equations predict the variation in diameter along the stem, therefore characterizing stem form. Several recent studies have tested mixed models for developing taper equations. Mixed-effects modeling allow the interindividual variation to be explained by considering both fixed-effects parameters (common to the population) and random-effects parameters (specific to each individual).AimsThe objective of this study is to develop a mixed-effect variable exponent taper equation for birch trees in northwestern Spain by determining which fixed-effects parameters should be expanded with random-effects parameters.MethodsAll possible combinations of linear expansions with random effects in one and in two of the fixed-effects model parameters were tested. Upper stem diameter measurements were used to estimate random-effects parameters by the use of an approximate Bayesian estimator, which calibrated stem profile curves for individual trees.ResultsParameter estimates for more than half of the mixed models investigated were nonsignificant. A first order autoregressive error structure was used to completely remove the autocorrelation between residuals, as mixed-effects modeling were not sufficient for this purpose.ConclusionThe mixed model with the best fitting statistics did not provide the best calibration statistics for all upper stem diameter measurements. From a practical point of view, model calibration should be considered an essential criterion in mixed model selection.

Spatial Distribution Patterns in the Very Rare and Species-Rich Picea chihuahuana Tree Community (Mexico)

The very rare Mexican Picea chihuahuana tree community covers an area of no more than 300 ha in the Sierra Madre Occidental. This special tree community has been the subject of several studies aimed at learning more about the genetic structure and ecology of the species and the potential effects of climate change. The spatial distribution of trees is a result of many ecological processes and can affect the degree of competition between neighbouring trees, tree density, variability in size and distribution, regeneration, survival, growth, mortality, crown formation and the biological diversity within forest communities. Numerous scale-dependent measures have been established in order to describe spatial forest structure. The overall aim of most of these studies has been to obtain data to help design preservation and conservation strategies. In this study, we examined the spatial distribution pattern of trees in the P. chihuahuana tree community in 12 localities, in relation to i) tree stand density, ii) diameter distribution (vertical structure), iii) tree species diversity, iv) geographical latitude and v) tree dominance at a fine scale (in 0.25 ha plots), with the aim of obtaining a better understanding of the complex ecosystem processes and biological diversity. Because of the strongly mixed nature of this tree community, which often produces low population densities of each tree species and random tree fall gaps caused by tree death, we expect aggregated patterns in individual Picea chihuahuana trees and in the P. chihuahuana tree community, repulsive Picea patterns to other tree species and repulsive patterns of young to adult trees. Each location was represented by one plot of 50 x 50 m (0.25 ha) established in the centre of the tree community. The findings demonstrate that the hypothesis of aggregated tree pattern is not applicable to the mean pattern measured by Clark-Evans index, Uniform Angle index and Mean Directional index of the uneven-aged P. chihuahuana trees and P. chihuahuana tree community and but to specific spatial scales measured by the univariate L-function. The spatial distribution pattern of P. chihuahuana trees was found to be independent of patches of other tree species measured by the bivariate L-function. The spatial distribution was not significantly related to tree density, diameter distribution or tree species diversity. The index of Clark and Evans decreased significantly from the southern to northern plots containing all tree species. Self-thinning due to intra and inter-specific competition-induced mortality is probably the main cause of the decrease in aggregation intensity during the course of population development in this tree community. We recommend the use of larger sampling plots (> 0.25 ha) in uneven-aged and species-rich forest ecosystems to detect less obvious, but important, relationships between spatial tree pattern and functioning and diversity in these forests.

Local and generalized height-diameter models with random parameters for mixed, uneven-aged forests in Northwestern Durango, Mexico

BackgroundWe used mixed models with random components to develop height-diameter (h-d) functions for mixed, uneven-aged stands in northwestern Durango (Mexico), considering the breast height diameter (d) and stand variables as predictors.MethodsThe data were obtained from 44 permanent plots used to monitor stand growth under forest management in the study area.ResultsThe generalized Bertalanffy-Richards model performed better than the other generalized models in predicting the total height of the species under study. For the genera Pinus and Quercus, the models were successfully calibrated by measuring the height of a subsample of three randomly selected trees close to the mean d, whereas for species of the genera Cupressus, Arbutus and Alnus, three trees were also selected, but they are specifically the maximum, minimum and mean d trees.ConclusionsThe presented equations represent a new tool for the evaluation and management of natural forest in the region.

Evaluation of age-independent methods of estimating site index and predicting height growth: a case study for maritime pine in Asturias (NW Spain)

Site index is the most common measure of forest site productivity used worldwide, serving as an important baseline for forest-level planning and helping to formulate silviculture strategies. Site index is usually obtained through measurement of dominant height and stand age. However, age is not available in many cases, such as in most national forest inventories or when it is not a main determinant of growth. Age-independent methods have been developed to overcome this problem, allowing modelling of the dominant height growth and estimation of site index in the absence of age data. The purpose of this study was to compare the most widely used age-dependent method and several age-independent alternatives, namely the growth intercept method, the age-independent difference equation method and an iterative method proposed in this study. We used stem analysis and permanent sample plot data from Pinus pinaster Ait. stands in Asturias, Spain, to test the alternative methods. It was found that for site index estimation, a 7-year growth intercept was the most accurate method. However, for prediction of dominant height growth, an age-independent equation that depends on climatic factors performed best, even better than the traditional age-dependent site quality curves. The iterative method proposed in this study proved to be the least accurate alternative, although it performed well in predicting height growth for long intervals starting from low heights. Finally, we provide recommendations regarding when the different methods should be used.

Can random components explain differences in the height–diameter relationship in mixed uneven-aged stands?

Abstract• ContextTree height prediction is an important issue in forest management since tree heights are usually measured only in a sample of trees. Although numerous model approaches have been used for this purpose, no agreement on which one is more appropriate has been achieved.• AimsTo analyse the random effects of basic and generalised height–diameter (h–d) models fitted to multi-species uneven-aged forest stands, and to establish their ability to explain differences between ecoregions, plots and species.• MethodsHeight and diameter measurements for 29,084 trees from 187 sample plots located in the state of Durango (Mexico) were used. Basic and generalised h–d models were fitted in a mixed-models framework. The variability between ecoregions, plots and species was considered in the random effects definition. Model calibration for different height sampling designs and sampling sizes was also analysed.• ResultsRandom components performed well in explaining the differences in the h–d relationship between the different plots and species; however, no significant variance for the random effects was found for the different ecoregions. A calibrated basic h–d model produced similar results to a fixed-effects generalised h–d model when a sufficiently large number of trees was used in the calibration process.• ConclusionFrom a practical point of view, if no calibration is carried out, different models should be used for the different species, so that at least the variation among species is captured.

Tarifas de biomasa aérea para abedul (Betula pubescens Ehrh.) y roble (Quercus robur L.) en el noroeste de España

El objetivo de este trabajo fue el desarrollo de modelos de estimacion de biomasa aerea para abedul (Betula pubescens Ehrh.) y roble (Quercus robur L.) en Galicia (noroeste de Espana). Para ello se emplearon datos obtenidos mediante el muestreo destructivo de 50 abedules y 50 robles, en los que se obtuvo el peso seco de biomasa total y por fracciones: madera del fuste, corteza del fuste, ramas mayores de 2 cm, ramas de 0,5 cm a 2 cm, ramas menores de 0,5 cm, y hojas. En un primer paso se seleccionaron los modelos que presentaban el mejor ajuste para cada fraccion de biomasa arborea considerada. Posteriormente, y para asegurar la aditividad, se realizo un ajuste simultaneo de las ecuaciones de estimacion de biomasa por fracciones junto con la de biomasa total, empleando el procedimiento estadistico denominado NSUR (Nonlinear Seemingly Unrelated Regressions). Tambien fue necesario un ajuste ponderado para corregir la existencia de heterocedasticidad. El numero de condicion verifico que no existian problemas graves de multicolinealidad. Al final se obtuvo, para cada especie, un sistema de siete ecuaciones de estimacion de biomasa aerea para las distintas fracciones y para la biomasa total. Estas ecuaciones explicaron como minimo 79% de la variabilidad observada, y en el caso de las ecuaciones de biomasa total 98% para abedul y 97% para roble. Se recomienda la utilizacion de las ecuaciones desarrolladas en este estudio en sustitucion de las ecuaciones de biomasa existentes para la region.