Filling the“vertical gap”between canopy tree species and understory shrub species:biomass allometric equations for subcanopy tree species

Subcanopy tree species are an important component of temperate secondary forests.However,their biomass equations are rarely reported,which forms a“vertical gap”between canopy tree species and understory shrub species.In this study,we destructively sampled six common subcanopy species(Syringa reticulate var.amurensis(Rupr.)Pringle,Padus racemosa(Lam.)Gilib.,Acer ginnala Maxim.,Malus baccata(Linn.)Borkh.,Rhamnus davurica Pall.,and Maackia amurensis Rupr.et Maxim.)to establish biomass equations in a temperate forest of Northeast China.The mixed-species and species-specifi c biomass allometric equations were well fi tted against diameter at breast height(DBH).Adding tree height(H)as the second predictor increased the R^(2)of the models compared with the DBH-only models by–1%to+3%.The R^(2)of DBH-only and DBH-H equations for the total biomass of mixed-species were 0.985 and 0.986,respectively.On average,the biomass allocation proportions for the six species were in the order of stem(45.5%)>branch(30.1%)>belowground(19.5%)>foliage(4.9%),with a mean root:shoot ratio of 0.24.Biomass allocation to each specifi c component diff ered among species,which aff ected the performance of the mixed-species model for particular biomass component.When estimating the biomass of subcanopy species using the equations for canopy species(e.g.,Betula platyphylla Suk.,Ulmus davidiana var.japonica(Rehd.)Nakai,and Acer mono Maxim.),the errors in individual biomass estimation increased with tree size(up to 68.8%at 30 cm DBH),and the errors in stand biomass estimation(up to 19.2%)increased with increasing percentage of basal area shared by subcanopy species.The errors caused by selecting such inappropriate models could be removed by multiplying adjustment factors,which were usually power functions of DBH for biomass components.These results provide methodological support for accurate biomass estimation in temperate China and useful guidelines for biomass estimation for subcanopy species in other regions,which can help to improve estimates of forest biomass and carbon stocks.

Developing allometric equations to estimate forest biomass for tree species categories based on phylogenetic relationships

The development of allometric biomass models is important process in biomass estimation because the reliability of forest biomass and carbon estimations largely depends on the accuracy and precision of such models.National Forest Inventories(NFI)are detailed assessments of forest resources at national and regional levels that provide valuable data for forest biomass estimation.However,the lack of biomass allometric equations for each tree species in the NFI currently hampers the estimation of national-scale forest biomass.The main objective of this study was to develop allometric biomass regression equations for each tree species in the NFI of China based on limited biomass observations.These equations optimally grouped NFI and biomass observation species according to their phylogenetic relationships.Significant phylogenetic signals demonstrated phylogenetic conservation of the crown-to-stem biomass ratio.Based on phylogenetic relationships,we grouped and matched NFI and biomass observation species into 22 categories.Allometric biomass regression models were developed for each of these 22 species categories,and the models performed successfully(R^(2)=0.97,root mean square error(RMSE)=12.9​t·ha^(–1),relative RMSE=11.5%).Furthermore,we found that phylogeny-based models performed more effectively than wood density-based models.The results suggest that grouping species based on their phylogenetic relationships is a reliable approach for the development and selection of accurate allometric equations.

Quantification by allometric equations of carbon sequestered by Tectona grandis in different agroforestry systems

Non destructive methods for quantification of carbon seques- tration in tropical trees are inadequately developed. We described a stan- dardized method for estimating carbon stock in teak （Tectona grandis Linn. F.）. We developed linear allometric equations using girth at breast height （GBH）, height and age to quantify above ground biomass （AGB）. We used AGB to estimate carbon stock for teak trees of different age groups （1.5, 3.5, 7.5, 13.5, 18.5 and 23.5 years）. The regression equation with GBH, y = 3.174x - 21.27, r2=0.898 （p 〈0.01）, was found precise and convenient due to the difficulty in determination of height and age in dense natural forests of teak. The equation was evaluated in teak agroforestry systems that included Triticum aestivum （wheat）, Cicer arietinum （gram）, Withania somnifera （ashwagandha）,

Improved allometric equations for tree aboveground biomass estimation in tropical dipterocarp forests of Kalimantan,Indonesia

Background： Currently, the common and feasible way to estimate the most accurate forest biomass requires ground measurements and allometric models.Previous studies have been conducted on allometric equations development for estimating tree aboveground biomass（AGB） of tropical dipterocarp forests（TDFs） in Kalimantan（Indonesian Borneo）.However, before the use of existing equations, a validation for the selection of the best allometric equation is required to assess the model bias and precision.This study aims at evaluating the validity of local and pantropical equations; developing new allometric equations for estimating tree AGB in TDFs of Kalimantan; and validating the new equations using independent datasets.Methods： We used 108 tree samples from destructive sampling to develop the allometric equations, with maximum tree diameter of 175 cm and another 109 samples from previous studies for validating our equations.We performed ordinary least squares linear regression to explore the relationship between the AGB and the predictor variables in the natural logarithmic form.Results： This study found that most of the existing local equations tended to be biased and imprecise, with mean relative error and mean absolute relative error more than 0.1 and 0.3, respectively.We developed new allometric equations for tree AGB estimation in the TDFs of Kalimantan.Through a validation using an independent dataset,we found that our equations were reliable in estimating tree AGB in TDF.The pantropical equation, which includes tree diameter, wood density and total height as predictor variables performed only slightly worse than our new models.Conclusions： Our equations improve the precision and reduce the bias of AGB estimates of TDFs.Local models developed from small samples tend to systematically bias.A validation of existing AGB models is essential before the use of the models.

Stand Structure,Allometric Equations,Biomass and Carbon Sequestration Capacity of Acacia mangium Wild.(Mimosaceae)in Cote d’Ivoire

In addition to bioenergy production, Acacia magium, a fast growing species, plays a major role in climate change mitigation through carbon sequestration from the atmosphere. The objective of this study was to improve estimates of aboveground biomass of 3, 7 and 11 years old stands of Acacia mangium set up through natural regeneration at Anguédédou in C?te d’Ivoire. Tree measurements were done in circular plots of 615 m2 located at the center of each stand. 24 trees of circumference at breast height (cbh) between 31 and 116 cm were felled, weighed and measured. Multiple linear regressions were used to develop allometric equations linking aboveground biomass of trees to cbh and/or height. The carbon stock and sequestration capacity of each stand was assessed using these predictive models. The average cbh was 39.4 cm, 73.5 cm and 91.4 cm respectively for 3, 7 and 11 years old stands with a density ranging between 845 trees·ha-1 and 553 trees·ha-1. The allometric equations for biomass estimation were Btotal aboveground = exp(-3.455 + 2.081 × ln(C)), Btrunk = exp (-5.153 + 1.681 × ln(C) + 1.056 × ln(H)), Bbranches = exp(-2.005 + 0.498 × ln(C2 × H)), Bleaves = exp(-2.415 + 1.339 × ln(C)). Total height had no influence on total and leaf biomass but increased precision of trunk and branch biomass. The carbon sequestration capacity of aboveground biomass was highest in Acacia mangium stand of 7 years old with 45.14 teqCO2·ha-1·year-1 and lowest in the 3-year stand with 33.90 teqCO2·ha-1·year-1.

Allometric equations for aboveground biomass estimation of Diospyros abyssinica (Hiern) F. White tree species

Introduction:Quantifying forest biomass requires the application of allometric equations which is a fundamental step.Generalized allometric equations have been applied to quantify aboveground biomass(AGB)of forests.But,adopting generalized allometric equations to quantify AGB of different forests creates uncertainty.Therefore,developing species-and sitespecific allometric equations is essential to accurately quantify the biomass.The study was aimed to develop species-specific allometric equations for Diospyros abyssinica(Hiern)F.White in Yayu Coffee Forest Biosphere Reserve using the Semi-destructive method.The vegetation types of Yayu Coffee Forest Biosphere Reserve is categorized to Moist Evergreen Montane Rainforest of Ethiopia.Results and discussion:Evaluating statistical relationships of AGB against predictor variables,eight allometric equations were formulated.AGB was regressed against trunk diameter(D),total height(H),and wood density(ρ)individually and in combination.Selection of allometric equations was employed using model performance statistics.Equations with a higher coefficient of determination(adjusted R^(2)),lower residual standard error,and Akaike information criterion(AIC)values were found best-fitted.Relationships of AGB and independent variables were found statistically significant(p<0.000).Overall,formulating species-and site-specific allometric equations is significant for accurate estimation of forest biomass and carbon stock budget.

Modeling of the height–diameter relationship using an allometric equation model:a case study of stands of Phyllostachys edulis

Understanding the relationship between tree height （H） and diameter at breast height （D） is vital to forest design, monitoring and biomass estimation. We developed an allometric equation model and tested its applicability for unevenly aged stands of moso bamboo forest at a regional scale. Field data were collected for 21 plots. Based on these data, we identified two strong power relationships： a corre- lation between the mean bamboo height （Hm） and the upper mean H （Hu）, and a correlation between the mean D （Din） and the upper mean D （Du）. Simulation results derived from the aUometric equation model were in good agreement with observed culms derived from the field data for the 21 stands, with a root-mean-square error and relative root-mean-square error of 1.40 m and 13.41%, respectively. These results demonstrate that the allometric equation model had a strong predictive power in the unevenly aged stands at a regional scale. In addition, the estimated average height-diameter （H-D） model for South Anhui Province was used to predict H for the same type of bamboo in Hunan Province based on the measured D, and the results were highly similar. The allometric equation model has multiple uses at the regional scale, including the evaluation of the variation in the H- D relationship among regions. The model describes the average H-D relationship without considering the effects caused by variation in site conditions, tree density and other factors.

Development of Allometric Models for Estimating the Biomass of <i>Sclerocarya birrea</i>(A.Rich) Hoscht and <i>Boscia senegalensis</i>(Pers.) Lam. ex Poir.

The objective of this study was to develop allometric models for estimating the biomass of Sclerocarya birrea (A.Rich) Hoscht and Boscia senegalensis (Pers.) Lam. ex Poir. The inventory of these ligneous was carried out at Widou Thiengoly (North of Senegal). The sample consists of 43 individuals of Boscia senegalensis and 15 individuals of Sclerocarya birrea. The selected individuals were dendrometrically characterized before being cut, compartmentalized (trunk, branches, and twigs) and weighed entirely. Simple regression tests were performed to examine the most explanatory dendrometric parameter (x) for biomass (y) according to two types of models: the linear model (y = ax + b) and the polynomial model of degrees 2 (y = ax2 + bx + c). The criteria for selection and validity of the models are based firstly on the tests of normality, nullity, heterogeneity and autocorrelation of the residues. The results showed that the most explanatory dendrometric parameter of the biomass was the crown surface for Boscia senegalensis and the 1.30 m diameter for Sclerocarya birrea of all the tests performed, the second-order Polynomial model is the best predictor of above ground biomass for these two species. Thus, the allometric models established to predict the biomass of these two species are: y = 0.0023x2 + 0.4851x - 0.0519 for Boscia senegalensis and y = 0.35x2 + 10.35x - 12.90 for S. birrea;with very significant correlation coefficients (R2) of 0.85 and 0.94 respectively. These results can be used for a sequestered carbon assessment study and will play a role in monitoring the carbon market in Africa.

Developing individual tree-based models for estimating aboveground biomass of five key coniferous species in China

Estimating individual tree biomass is critical to forest carbon accounting and ecosystem service modeling.In this study,we developed one-（tree diameter only） and two-variable（tree diameter and height） biomass equations,biomass conversion factor（BCF） models,and an integrated simultaneous equation system（ISES） to estimate the aboveground biomass for five conifer species in China,i.e.,Cunninghamia lanceolata（Lamb.） Hook.,Pinus massoniana Lamb.,P.yunnanensis Faranch,P.tabulaeformis Carr.and P.elliottii Engelm.,based on the field measurement data of aboveground biomass and stem volumes from 1055 destructive sample trees across the country.We found that all three methods,including the one-and two-variable equations,could adequately estimate aboveground biomass with a mean prediction error less than 5%,except for Pinus yunnanensis which yielded an error of about 6%.The BCF method was slightly poorer than the biomass equation and the ISES methods.The average coefficients of determination（R^2） were 0.944,0.938 and 0.943 and the mean prediction errors were 4.26,4.49 and 4.29% for the biomass equation method,the BCF method and the ISES method,respectively.The ISES method was the best approach for estimating aboveground biomass,which not only had high accuracy but also could estimate stocking volumes simultaneously that was compatible with aboveground biomass.In addition,we found that it is possible to develop a species-invariant one-variable allometric model for estimating aboveground biomass of all the five coniferous species.The model had an exponent parameter of 7/3 and the intercept parameter a_0 could be estimated indirectly from stem basic density（a_0= 0.294 q）.

Carbon sequestration in Chir-Pine(Pinus roxburghii Sarg.) forests under various disturbance levels in Kumaun Central Himalaya

We studied variations in tree biomass and carbon sequestration rates of Chir Pine （Pinus roxburghii. Sarg.） forest in three categories of forest disturbance, protected, moderately disturbed, and highly disturbed. In the first year, total biomass was 14.7.t.ha-1 in highly disturbed site, 94.46 t.ha-1 in moderately disturbed forest, and 112.0 t.ha-1 in protected forest. The soil organic carbon in the top 20 cm of soil ranged from 0.63 to 1.2%. The total rate of carbon sequestration was 0.60 （t/ha）.a-lon the highly disturbed site, 1.03 （t/ha）a-1 on the moderately disturbed site, and 4.3 （t/ha）.a-1 on the protectedsite. Keywords： carbon sequestration, soil organic carbon （SOC）, disturbed forest, vegetation analysis, allometric equations

Composition and partition of biomass in a smith fir (Abies georgei var.smithii) timberline forest on the Sergyemla Mountain,southeastern Tibetan Plateau

The southeastern Tibetan Plateau has the highest timberlines in the world and climate change affects this area to a considerable extent. Quantification of vegetation biomass in the forests at the timberline is essential for understanding carbon balance, forest structure and functional changes in the forests at timberlines. We describe biomass and its constituent components of a smith fir tim- berline forest occurring at an elevation of 4320 m on the Sergyemla Mountain in the southeastern Tibetan Plateau. The results show that, in the tree layer, the average diameter at breast height （DBH） was 35.6 cm, the density 251 trees per hectare with a total biomass of 309.9 t.ha 1. Of the total biomass, the tnmks accounted for 70.49%, branches for 9.09%, leaves for 5.57% and roots for 14.85%. The total biomass in the shrub layer was 77.9 t＇ha-~ of which the biomass of trunks, branches, leaves and roots accounted for 42.8, 14.9, 4.6 and 15.6 t.ha-1, respectively. The total biomass in the herb layer was 3.0 t.ha-1. The total vegetation biomass in the smith fir timberline forest was 390.9 t-ha-l, to which the tree layer contributed the most （79.3%）, followed by shrubs （19.9%） and herbs （0.8%）. We estimate that the smith fir forest stand at the timberline had a higher total biomass than other fir or spruce forest stands in the surrounding area.

Fractal analysis of canopy architectures of Acacia angustissima, Gliricidia sepium, and Leucaena collinsii for estimation of aboveground biomass in a short rotation forest in eastern Zambia

A study was conducted at Msekera Regional Agricultural Research Station in eastern Zambia to （1） describe canopy branching properties of Acacia angustissima, Gliricidia sepium and Leucaena collinsii in short rotation forests, （2） test the existence of self similarity from repeated iteration of a structural unit in tree canopies, （3） examined intra-specifie relationships between functional branching characteristics, and （4） determine whether allometric equations for relating aboveground tree biomass to fractal properties could accurately predict aboveground biomass. Measurements of basal diameter （Din0） at 10em aboveground and total height （H）, and aboveground biomass of 27 trees were taken, but only nine trees representative of variability of the stand and the three species were processed for functional branching analyses （FBA） of the shoot systems. For each species, fractal properties of three trees, includ- ing fractal dimension （Dfract）, bifurcation ratios （p） and proportionality ratios （q） of branching points were assessed. The slope of the linear re- gression ofp on proximal diameter was not significantly different （P 〈 0.01） from zero and hence the assumption that p is independent of scale, a pre-requisite for use of fraetal branching rules to describe a fraetal tree canopy, was fulfilled at branching orders with link diameters 〉1.5 cm. The proportionality ration q for branching patterns of all tree species was constant at all scales. The proportion of q values 〉0.9 （fq） was 0.8 for all species. Mean fraetal dimension （Df^ct） values （1.5-1.7） for all species showed that branching patterns had an increasing magnitude of intricacy. Since Dfraet values were 〉1.5, branching patterns within species were self similar. Basal diameter （D10）, proximal diameter and Dfraet described most of variations in aboveground biomass, suggesting that allometric equa- tions for relating aboveground tree biomass to fractal properties could accurately predict aboveground biomass. Thus, assessed Acacia, Gliri- cidia and Leucaena trees were fractals and their branching propertiescould be used to describe variability in size and aboveground biomass.

Tree Allometry in Tropical Forest of Congo for Carbon Stocks Estimation in Above-Ground Biomass

The research was aimed to estimate the carbon stocks of above-ground biomass (AGB) in Lesiolouna forest in Republic of Congo. The methodology of Allometric equations was used to measure the carbon stock of Lesio-louna tropical rainforest. The research was done with six circular plots each 40 m of diameter, with a distance of 100 m between each plot, depending on the topography of the site of the installation of these plots. The six studied plots are divided in two sites, which are: Iboubikro and Ngambali. Thus, in the six plots, there are three plots in Iboubikro site and three plots in Ngambali site. The results of this study show that the average carbon stock of aboveground biomass (AGB) in six plots was 170.673 t C ha-1. So, the average of carbon stock of aboveground biomass (ABG) in Iboubikro site was 204.693 t C ha-1 and in the Ngambali site was 136.652 t C ha-1. In this forest ecosystem, the high stock of carbon was obtained in Plot 3, which was in Iboubikro site. Plot 3 contains 20 trees and an average DBH of 24.56 cm. However, the lowest carbon stock was obtained in Plot 4, which was in Ngambali site. Also, Plot 4 contains 11 trees and an average DBH of 31.86 cm. The results of this research indicate that, the forests in the study area are an important carbon reservoir, and they can also play a key role in mitigation of climate change.

Estimation of above-ground biomass and carbon stock of an invasive woody shrub in the subtropical deciduous forests of Doon Valley,western Himalaya,India

This study describes the different parameters used to derive the allometric equation for calculating the biomass of an invasive woody shrub Lantana camara L.from the subtropical conditions of western Himalaya.It identifies the most accurate and convenient method for biomass calculation by comparing destructive with nondestructive methodology.Different parameters were measured on a wide range of Lantana from different community levels for the non-destructive calculation of total aboveground biomass.Different explanatory variables were identified and measured such as basal diameter either as a single independent variable or in combination with plant height.The other suitable combinations of available independent variables include crown length,crown width,crown area,crown volume and coverage of the plant.Amongst the wide range of allometric equations used with different variables,the equation with D2 H as a variable was found to be the most suitable estimator of biomass calculation for Lantana.Sahastradhara,being the most disturbed area due to its high tourist activity round the year,showed maximum coverage（58.57 % ha-1）,highest biomass（13,559.60 kg ha-1） and carbon density（6,373.01 kg ha-1）of Lantana.The degree of Lantana’s invasiveness in subtropical conditions was also calculated on the basis of importance value index（IVI）.The maximum IVI（22.77）and mean coverage（26.8 % ha-1） was obtained from the areas near Jolly Grant airport,indicating that physically disturbed areas are more suitable for the growth of Lantana,which may significantly increase shrub biomass.The importance of incorporating allometric equations in calculation of shrub biomass,and its role in atmospheric carbon assimilation has thus been highlighted through the findings of this study.

Re-estimating the changes and ranges of forest biomass carbon in China during the past 40 years

Background: In recent decades the future of global forests has been a matter of increasing concern, particularly in relation to the threat of forest ecosystem responses under potential climate change. To the future predictions of these responses, the current forest biomass carbon storage(FCS) should first be clarified as much as possible,especially at national scales. However, few studies have introduced how to verify an FCS estimate by delimiting the reasonable ranges. This paper addresses an estimation of national FCS and its verification using two-step process to narrow the uncertainty. Our study focuses on a methodology for reducing the uncertainty resulted by converting from growing stock volume to above-and below-ground biomass(AB biomass), so as to eliminate the significant bias in national scale estimations.Methods: We recommend splitting the estimation into two parts, one part for stem and the other part for AB biomass to preclude possible significant bias. Our method estimates the stem biomass from volume and wood density(WD), and converts the AB biomass from stem biomass by using allometric relationships.Results: Based on the presented two-step process, the estimation of China’s FCS is performed as an example to explicate how to infer the ranges of national FCS. The experimental results demonstrate a national FCS estimation within the reasonable ranges(relative errors: + 4.46% and-4.44%), e.g., 5.6–6.1 PgC for China’s forest ecosystem at the beginning of the 2010 s. These ranges are less than 0.52 PgC for confirming each FCS estimate of different periods during the last 40 years. In addition, our results suggest the upper-limits by specifying a highly impractical value of WD(0.7 t·m-3) on the national scale. As a control reference, this value decides what estimate is impossible to achieve for the FCS estimates.Conclusions: Presented methodological analysis highlights the possibility to determine a range that the true value could be located in. The two-step process will help to verify national FCS and also to reduce uncertainty in related studies. While the true value of national FCS is immeasurable, our work should motivate future studies that explore new estimations to approach the true value by narrowing the uncertainty in FCS estimations on national and global scales.

Effect of thinning on above-ground biomass accumulation in a Douglas-fir plantation in southern Italy

We investigated the effects of a long-term thinning experiment on the distribution of above-ground biomass of Douglas-fir （Pseudotsuga menziesii [Mirb.] Franco var. menziesii） in a plantation in southern Italy. Allometric equations were used to estimate biomass and partitioning to stem and crown compartments. Variation in biomass stock estimated with allometric equations were evaluated according to seven thinning treatments： geo- metric-systematic （1 row every 3）, selective （light-moderate-heavy）, mixed systematic-selective （1 row every 4, 1 row every 5）, unthinned （control）. Over the experimental period of 13 years, current annual increments of carbon were lower （3.4 Mg ha^-1 year^-1） in control plots than in treated plots. At age 30, plots subjected to light selective thinning showed higher values of above-ground biomass （249.7 Mg ha^-1）. The biomass harvested with this treatment was 29.3 Mg ha^-1, and the mean annual increment of carbon over 13 years was 4.8 Mg ha^-1. Our results showed that light thinning stimulated increase in carbon stock, with a minimal loss of carbon during the treatment and a current annual increment of carbon higher than in control sub-plots and sub-plots thinned using systematic methods. This treatment yielded least carbon emissions and we affirm it has discrete global warming mitigation potential.

Growth and nutrient accumulation of Brazil nut trees(Bertholletia excelsa) in agroforestry at different fertilizer levels

The Brazil nut tree（Bertholletia excelsa） is a frequent component of agroforestry systems in the Amazon because of its adaptation to nutrient-poor upland soils and multiple uses.We investigated the aboveground biomass production（kg dry weight）,nutrient uptake and requirements（N,P,Ca,Mg,K） of Brazil nut trees of different sizes grown under agroforestry conditions and fertilized at different levels.Eight of 70 experimental trees with different size were harvested and stem,branches and leaves were separated.Nutrient contents were determined for three trees of varying size.Average tree growth was fast,but variability was high,suggesting considerable potential for the improvement of this economically important species.The trees responded to increased levels of fertilizer and lime with significantly increased foliar nutrient contents and growth,probably because of the improved availability of Mg and Ca for which the species seems to have a relatively high demand.In contrast to Brazil nut trees grown in forest or dense plantations,the agroforestry trees invested a substantial part of their biomass and nutrients in large branches and developed spreading crowns.To improve stem form,reduce competition with associated crops for light and recycle nutrients,regular pruning of lower branches or planting arrangements that favor self-pruning are recommended.These measures would also increase the recycling of Ca and Mg,large quantities of which are contained in the branches.

Analyzing Accuracy of the Power Functions for Modeling Aboveground Biomass Prediction in Congo Basin Tropical Forests

Allometric equation is the common tools for quantifying and monitoring the amount of carbon stored in forest ecosystems. The model used can be one of the major sources of errors that need to be considered for wood biomass estimations. The power function of plants has been questioned by comparing sixteen models. Some adjustment and model selection criteria and prediction of uncertainties have been computed. Published data on biomass studies and plot inventory were used for this analysis. The results highlight that power function is the best model for modeling aboveground biomass and additional effect on logarithm scales of the predictor variables must be prioritized. The power of the logarithm of diameter as predictor variable must be avoided because this leads to worst adjustment and higher prediction uncertainty. Tree height as a third predictor variable gives the best adjustment and reduces the uncertainty on the biomass prediction around 8 t/ha less than model with the two other predictor variables, the diameter and the wood specific density. The adjustment criteria are sufficient for the appreciation of the prediction quality of the models. The exponent of wood density as predictor variable needs better understanding.