Tree Diameter Increment Research Articles

Radiocarbon ageing of Indigenous culturally‐significant trees for bushfire management on Minjerribah (North Stradbroke Island), southeast Queensland

SummaryPartnerships between Indigenous People and governments for joint management of ancestral lands, designated as protected conservation areas for biodiversity and cultural heritage, provide an opportunity to leverage accumulated traditional land management knowledge with emerging science. In Australia, veteran or large old trees in natural landscapes are of significant ecological importance, and among the Indigenous communities, they are living monuments to historical cultural practices, but their survival, endurance and long lives are rarely acknowledged to afford them protection. In sub‐tropical Australia, the lack of consistent annual growth rings in stem wood makes it difficult to estimate the age of large trees using methods such as the periodic diameter increment. Ngugi et al., 2020 published radiocarbon (14C) dates using wood core (‘pith‐wood’) samples from 12 Indigenous culturally‐significant trees covering five species on Minjerribah (North Stradbroke Island). Due to the imprecise calendar age results of the single samples used in the original study (Ngugi et al., 2020), subsequent radiocarbon dating of an additional three wood core samples from each tree was undertaken to age the trees more precisely. The revised tree ages ranged from 63 to 531 years and suggest an important role of past Indigenous land management practices into protecting Bugari (Cypress Pine, Callitris columellaris F. Muell.) from deadly crown scorching fires. These results underscore the importance of incorporating Indigenous practices in current fire management strategies and plans. Estimating tree ages based on the periodic diameter increment method overestimated the age relative to that derived from radiocarbon dating. New bias correction factors were developed for adjusting the recorded periodic tree diameter increments.

Individual tree diameter growth modelling for natural secondary forest in Changbai Mountains, Northeast China

Individual modelling is a foundational approach to study the natural forest growth and in this paper, we develop a serial distance-depended individual tree model for some species in natural forest which would provide prediction and characteristics for natural species. The data used to develop individual model for natural mixed forests were collected from 712 remeasured 10-year periodic permanent sample plots of in Baihe Forest Bureau of Changbai Mountains, northeast China. Based on analyzing relationship between diameter increment of individual trees with tree size, competitive status, and site condition and finding out the major independent variables, the growth models for individual trees of 15 species in the natural mixed forests, that have good predicting precision, and easily applicable, were developed using stepwise regression method. The individual growth model developed in this study can reflect the tree increment of 15 species and be generally well suited for simulating tree and stand growth for natural mixed forests in Changbai Mountains. The research results for individual trees growth model of each species showed that main variable to influence on diameter increment of individual trees for natural mixed forests were tree size (D) and then competition index. The site condition was not related with diameter increment. The natural logarithm of DBH (lnD) and square diameter (D2) were included in the predicting models of diameter increment for all 15 species. The diameter increment was directly proportional to tree diameter for each species. For the competitive indexes in growth model, the relative diameter (RD), canopy closure (P), and the ratio of diameter of subject tree with maximum diameter (DDM) were related to diameter increment and the stand density measures were not significantly influenced on diameter increment. As canopy closure increase, tree increment decreases. The site conditions were performance less of factors in increment predictions in the model.

The impact of natural constraints in linear regression of log transformed response variables

Abstract In linear regression, log transforming the response variable is the usual workaround regarding departures from the assumption of normality. However, the response variable is often subject to natural constraints, which can result in a truncated distribution of the residual errors on the log scale. In forestry, allometric relationships and tree growth are two typical examples a natural constraint; the response variable cannot be negative. Traditional least squares estimators do not account for constrained response variables. For this study, a modified maximum likelihood (MML) estimator that takes natural constraints into account was developed. This estimator was tested through a simulation study and showcased with black spruce tree diameter increment data. Results show that the ordinary least squares estimator underestimated large conditional expectations of the response variable on the original scale. In contrast, the MML estimator showed no evidence of bias for large sample sizes. Departures from distributional assumptions cannot be overlooked when the model is used for predictive purposes. Both Monte Carlo error propagation and prediction intervals rely on these assumptions. In this context, the MML estimator developed for this study can be used to properly propagate the errors and produce reliable prediction intervals.

Grey alder at the regeneration stage: Long-term effects on soil nitrogen and carbon pools and Norway spruce growth

Increasing soil N availability is an effective way to enhance soil organic carbon (SOC) sequestration and biomass production in N-limited ecosystems such as boreal Norway spruce forests. Alder has a root nodule symbiosis with Frankia, an N2-fixing bacterium, and can thus add considerable amounts of N to soil mainly through its N-rich leaf litter. The objective of this study was to determine whether the presence of grey alder at the regeneration stage had any long-term effects on soil C and N cycling and stocks and spruce growth. The study sites were two relatively fertile 40-year-old Norway spruce stands (Luhdansuo and Porkkola) in Southern Finland. At both study sites, half the spruce stand had an admixture of grey alders for approximately 10–15 years after regeneration (alder treatment), but no alder had been present in the other half (control treatment). Alders were cut from the alder treatment 25 (Luhdansuo) or 30 (Porkkola) years before the measurements. At Porkkola, alder treatment received ash fertilisation 13 years ago. We observed no significant differences in the plant-available N fluxes, measured with a microdialysis technique, or SOC stocks between treatments. The total N stock (organic horizon + 0–30 cm mineral soil) and organic horizon thickness were larger, while the microbial biomass C:N and C mineralisation rate were lower in the alder versus the control treatment at Luhdansuo. The annual ring widths of spruces were larger in the alder versus the control treatment for up to six years after alder removal at both sites, but no differences were observed in the current tree diameters or diameter increments. In conclusion, most of the current soil or tree properties did not differ significantly between treatments, possibly because the effect of alder had faded out over the 25–30 years since its removal, and because the sites were initially relatively fertile.

The Growing Dynamic of Pure Scots Pine Stands Using Different Thinning Regimes in Lithuania

The aim of this study is to relate initial stand density and thinning intensity to tree diameter, height, and volume increment based on continuous 30-year observations in thinning experimental stands of Scots pine (Pinus sylvestris L.). The thinning experiments on Scots pine were established in 1990 and 1992, and the experimental model in each trial comprised five density variants: control and four plots with tree densities up to 3.0–4.4, 2.0–2.4, 1.0–1.2, and 0.5–0.6 thousand trees per hectare after the initial thinning. Since the establishment, only the dead trees were removed in control plots, and other initial density plots were thinned according to predefined thinning treatment plans. Our study shows that the lowest increment in diameter was found in control plots and the highest in the most intensive thinning plots, in which the diameter increment was 1.5 times higher. However, the intensification of thinning and the signs of growth stabilization in the control stands cannot guarantee them the position of highest productivity and cumulative volume in the future.

Functional traits related to competition for light influence tree diameter increments in a biodiversity manipulation experiment

Functional traits related to competition for light influence tree diameter increments in a biodiversity manipulation experiment

Effects of two different thinning methods on the diameter and basal area increments of silver lime (Tilia tomentosa Moench) target trees in Fruška Gora (Serbia)

This paper analyses the cumulative effects of selective thinning and thinning from below on diameter and basal area increments of target trees after 25/26 years in 52, 69- and 86-year-old silver lime (Tilia tomentosa Moench) stands. Two target tree collectives were analysed: (1) elite trees selected between 1993–1994 from permanent sampling plots (selective thinning method), and (2) a ‘comparable collective’ of target trees selected in 2019 (25/26 years later) according to the same criteria as the elite trees, in the same stands thinned from below. Elite trees that were selectively thinned had: higher diameter, basal area and volume per tree, higher diameter and basal area increment for a given time period, and lower slenderness coefficients compared to the target trees that were thinned from below at 52 and 69 yr. While diameter increment decreases with age, and differences between elite trees of different ages are clearly delineated, diameter increments of trees thinned from below are not significantly different at 52 and 69 yr. In addition, basal area increment of trees is highest at 69 yr with selective thinning. When thinning from below, there were no significant differences in basal area increments between trees at 69 and 86 yr. Silver lime shows a strong growth response after selective thinning at ages 25/26 and 44 yr. However, our results show that this response is less pronounced when selective thinning begins at 61 yr.

Modeling Tree Growth Responses to Climate Change: A Case Study in Natural Deciduous Mountain Forests

Climate change has significant effects on forest ecosystems around the world. Since tree diameter increment determines forest volume increment and ultimately forest production, an accurate estimate of this variable under future climate change is of great importance for sustainable forest management. In this study, we modeled tree diameter increment under the effects of current and expected future climate change, using multilayer perceptron (MLP) artificial neural networks and linear mixed-effect model in two sites of the Hyrcanian Forest, northern Iran. Using 573 monitoring fixed-area (0.1 ha) plots, we measured and calculated biotic and abiotic factors (i.e., diameter at breast height (DBH), basal area in the largest trees (BAL), basal area (BA), elevation, aspect, slope, precipitation, and temperature). We investigated the effect of climate change in the year 2070 under two reference scenarios; RCP 4.5 (an intermediate scenario) and RCP 8.5 (an extreme scenario) due to the uncertainty caused by the general circulation models. According to the scenarios of climate change, the amount of annual precipitation and temperature during the study period will increase by 12.18 mm and 1.77 °C, respectively. Further, the results showed that the impact of predicted climate change was not very noticeable and the growth at the end of the period decreased by only about 7% annually. The effect of precipitation and temperature on the growth rate, in fact, neutralize each other, and therefore, the growth rate does not change significantly at the end of the period compared to the beginning. Based on the models’ predictions, the MLP model performed better compared to the linear mixed-effect model in predicting tree diameter increment.

Litter removal impacts on soil biodiversity and eucalypt plantation development in the seasonal tropics

The little layer of tree plantations provides primary nutrients for uptake, buffers changes in soil moisture, and provides habitat and substrate to soil epigeic fauna. However, this layer in eucalypt plantations is often removed to reduce fuel load during the fire season in the Brazilian savanna (Cerrado). Therefore, it is necessary to quantify the effects of changes in litter dynamics on the function of these plantations, on key nutrient cycling processes and on epigeic fauna diversity and abundance. In two adjacent stands (one juvenile and one mature), the consequences of two years of litter removal were quantified as monthly litterfall, leaf and fine wood litter decomposition, epigeic fauna abundance and diversity, soil biogeochemical variables, and tree diameter and basal area increments. Monthly litterfall rates in juvenile and mature stands did not change with litter removal over the study period. Annual litterfall ranged from 4.1 to 4.9 Mg ha−1a−1 in litter removal plots and from 3.9 to 4.8 Mg ha−1a−1 in control plots. Fine wood litter decomposition was slower in litter removal plots compared to controls, while leaf decomposition rates were similar in both. Two years of litter removal in the juvenile stand did not affect topsoil biogeochemical parameters but decreased available phosphorus at 20–40 cm depth relative to controls. In the mature stand, total cation exchange capacity (0–20 cm) was higher in controls (6.4 cmolc dm−3) relative to litter removal plots (6.3 cmolc dm−3), while soil moisture (0–40 cm depth) was lower in litter removal (25.45 m3 m−3) compared to control plots (26 m3 m−3) in the dry season. A non-metric multidimensional scaling ordination revealed an increased homogeneity in epigeic fauna where litter was removed. Litterfall, decomposition, diameter increment, four soil physical parameters and fourteen chemical parameters at 0–20 and 20–40 cm depth explained the differences in soil epigeic fauna composition between litter removal and control plots. Diameter increment decreased with litter removal only in the juvenile stand, which had reached its growth peak. The results indicate that removing excess litter to decrease fuel volume can alter soil biodiversity and edaphic conditions that negatively affect nutrient cycling and tree growth.

The effect of white mistletoe (Viscum album subsp. abietis (Wiesb.) Abromerit) on diameter increment in Kazdağı fir stands

In this study, the effects of white mistletoe (Viscum album subsp. abietis (Wiesb.) Abromerit) on diameter increment in Kazdağı fir (Abies nordmanniana subsp. equi-trojani (Asc. & Sint. ex Boiss.) Coode & Cullen) stands distributed in Kastamonu region of Turkey were investigated. For this purpose, a total of 183 sample trees (77 healthy (uninfected) and 106 infected) were selected from mistletoe-infected stands. Mistletoe infected sample trees were classified according to the mistletoe intensity groups as lightly-, moderately- and heavily-infection levels. Increment cores were taken from the sampled trees, and annual ring widths were measured for evaluation period which is the last 30 years of sample trees. Underbark diameter increments were determined for the last 10, 20 and 30 years and for also last three 10-year periods. Mistletoe infected trees were compared to healthy trees according to the mean diameter increments. As a result, it was determined that there were statistically significant differences between underbark diameter increments of infected and uninfected trees. Increment losses of infected trees compared to uninfected trees were calculated as 21%, 28% and 33% for the last 30, 20 and 10 years, respectively. Similarly, significant differences were also observed for the last three 10-year periods and it was revealed that the underbark diameter increment losses were 11%, 24% and 33%, respectively. The results show that increment losses increase gradually with infection and the presence of white mistletoe negatively affects diameter increments with an increasing acceleration from the past to the present.

Tree growth and mortality of 42 timber species in central Africa

Tree growth and mortality are two central processes in mixed and structurally complex moist tropical forests, yet accurate estimates of the variables needed to model them remain sparse and scattered. It is thus still difficult to predict forest evolution at a local scale and build reliable management plans. To help fill this gap, for 1–7 years we annually monitored 21,180 trees belonging to 42 species exploited for timber production in Central Africa. We made new species-specific estimates of diameter increments and mortality rates, and investigated how tree growth varied with tree size and logging history. We compared our results with the legal values of diameter increments, mortality rates, and minimum cutting diameters used to build forest management plans in Cameroon. Diameter increment was found to vary with tree size for most of the species studied. The relationships between diameter increment and tree size were mostly humpback-shaped. The trees with diameters close to or lower than the reference minimum cutting diameter generally grew faster than the average. We also found that tree growth could slow for 1–2 years after timber exploitation and was then spurred for at least 5 years. The tree growth response to logging was nevertheless species-specific. This study provides new estimates of tree diameter increments and mortality rates that could help make more accurate forest projections and draw up sustainable management plans in Africa.

Primerjava različnih regresijskih modelov za napovedovanje debelinskega priraščanja jelke

We present seven alternative statistical models for modelling tree diameter increment with data from permanent sampling plots. In addition to the polynomial regression model, we present a regression model with added random noise, a mixed linear model, regression with natural splines, and three models with limited dependent variables: truncated regression, tobit regression and grouped data regression. The models may be used when dealing with truncated or censored variables, biased estimation of the increment due to censoring and rounding down, or when having multilevel data. The parametrization of the models was done using 21,013 fir trees on 4,405 plots in the period 1990–2014 in uneven-aged Dinaric fir-beech forests. All models showed a similar effect of tree diameter, stand basal area, basal area of larger trees, diameter structure diversity, altitude and slope. There were only minor differences in the regression coefficients and fit measures. The highest increment predictions were given by the tobit model. The mixed model fit the data best and, compared to the other models, predicted a slower decrease in the growth of large-diameter trees after growth culmination.

Diameter, height and volume increment single tree models for improved Sitka spruce in Great Britain

Abstract The British forestry sector lacks reliable dynamic growth models for stands of improved Sitka spruce, the most important commercial forest type in Great Britain. The aim of this study is to fill this gap by trialling a new modelling framework and to lay the foundations of a future dynamic growth simulator for that forest type. First, we present single tree diameter and height increment models that are climate sensitive and include explicit competition effects. The predictions from the increment models are pooled to project diameter and height at a given age. These projections are then used as inputs to an integrated taper model from which stochastic tree volume predictions are obtained. Retrospective data from over 1400 trees collected in two extensive genetic trials in Scotland and Wales were used for the purposes of this study. Diameter increment and height increment predictions were highly accurate and diameter and height projections proved consistent. The predicted volume at the time of harvesting also exhibited a high degree of accuracy, which shows the robustness of our approach. Further data will be needed in the future to recalibrate the present models and extend their range of validity to the whole of Great Britain.

The effect of slope aspect on growth attributes of Shorea leprosula Miq. in a rehabilitated hilly landscape

Shorea leprosula Miq. is one of Indonesia’s native red meranti species with a higher growth increment than other meranti species. Gunung Dahu Research Forest is a rehabilitated hilly landscape planted with various species of Dipterocarpaceae. This study aims to analyze the growth performance of 24 years old of S. leprosula stands in various slope classes of line planting technique at Gunung Dahu Research Forest. Growth analysis of S. leprosula was carried out by measuring the stem diameter, total tree height, basal area, diameter increment, height increment, and Leaf Area Index (LAI). Slopes were divided into three classes: mild/gentle (15-25%), steep (>25-45%) and very steep (>45%). The results showed that slope class significantly affected height growth and the canopy cover of S. leprosula, but it did not affect the diameter growth. A very steep slope provided the best growth to diameter (average diameter 30.07 cm; MAI 1.25 cm/year) and height (average height 23.7 m; MAI 0.99 m year−1). Furthermore, a linear relationship was formed between the crown cover and slope class in which denser crowns were established in response to a steeper slope (LAI 3.4). Thus, planting S. leprosula as a rehabilitation effort in steep slope landscape delivers beneficial as they showed better growth performance.

Silvicultural intensification has a limited impact on tree growth in forest restoration plantations in croplands

Investigating the impact of silvicultural practices on tropical forest restoration plantations is highly relevant, as there is a growing global demand for large-scale ecological restoration. The objective of this study was to test the hypothesis that native trees respond positively to the intensification of fertilization and weed control in a forest restoration plantation on a site previously used as cropland in the Atlantic Forest region of Brazil. We carried out a study with 22 native tree species, 11 of which were classified as shade species and the other 11 classified as diversity species. We implemented a randomized experimental design in a split-plot design, with six replicates of four treatments: intensive weed control (glyphosate spraying every 3 months) with and without fertilization, and non-intensive control (mechanical control every 6 months), with and without fertilization. We performed fertilization by supplying the site with 53 kg ha−1 of nitrogen, 35 kg ha−1 of phosphorus, and 51 kg ha−1 of potassium. We evaluated individual tree crown areas and increments in basal diameter and height 16 months after planting. In general, growth varied more among species than in weed control and fertilization treatments. Overall, the method of controlling weeds did not influence tree growth, but interacted with species for all growth variables. Fertilization increased tree diameter increment, and interacted with species, leading to diameter and height increments; however, most of the species did not respond to fertilization. Contrary to our expectations and previous findings, silvicultural intensification did not promote higher overall tree growth, probably because the restoration planting was conducted on a cropland with decades of intensive management, which increased soil fertility and reduced weed abundance through periodical maintenance prior to restoration planting. Our results highlight the importance of considering local and historical conditions to decide whether and when management intensification is required to accelerate tree growth in tropical forest restoration plantations.

Using the International Tree-Ring Data Bank (ITRDB) records as century-long benchmarks for global land-surface models

Abstract. The search for a long-term benchmark for land-surface models (LSMs) has brought tree-ring data to the attention of the land-surface modelling community, as tree-ring data have recorded growth well before human-induced environmental changes became important. We propose and evaluate an improved conceptual framework of when and how tree-ring data may, despite their sampling biases, be used as century-long hindcasting targets for evaluating LSMs. Four complementary benchmarks – size-related diameter growth, diameter increment of mature trees, diameter increment of young trees, and the response of tree growth to extreme events – were simulated using the ORCHIDEE version r5698 LSM and were verified against observations from 11 sites in the independent, unbiased European biomass network datasets. The potential for big-tree selection bias in the International Tree-Ring Data Bank (ITRDB) was investigated by subsampling the 11 sites from European biomass network. We find that in about 95 % of the test cases, using ITRDB data would result in the same conclusions as using the European biomass network when the LSM is benchmarked against the annual radial growth during extreme climate years. The ITRDB data can be used with 70 % confidence when benchmarked against the annual radial growth of mature trees or the size-related trend in annual radial growth. Care should be taken when using the ITRDB data to benchmark the annual radial growth of young trees, as only 50 % of the test cases were consistent with the results from the European biomass network. The proposed maximum tree diameter and annual growth increment benchmarks may enable the use of ITRDB data for large-scale validation of the LSM-simulated response of forest ecosystems to the transition from pre-industrial to present-day environmental conditions over the past century. The results also suggest ways in which tree-ring width observations may be collected and/or reprocessed to provide long-term validation tests for land-surface models.

Modeliranje višinske in debelinske rasti dominantnih dreves ter ocenjevanje indeksov produkcijske sposobnosti gozdnih rastišč

Modeling the height and diameter growth of trees is an important part of forest management. Growth models provide the basis for determining the thinning regime, target tree dimensions and optimal proportions of developmental phases of forest stands. We developed individual height growth models for dominant Norway spruce (Picea abies (L.) Karst) and European beech (Fagus sylvatica L.) in two forest types (sessile oak-European beech forests and pre-Alpine silver fir-European beech forests). Based on the models, the site productivity index (SPI), defined as the dominant tree height at a diameter of 45 cm, was determined for spruce and beech in both forest types. Based on the diameter increment of the dominant trees, the age of trees in regard to their diameter was calculated, which was the basis for Height-Age modeling. The site productivity index (SPI) of spruce in sessile oak-beech forests and pre-Alpine silver fir-European beech forests is higher than that of beech: 31.3 and 29.7 vs 28.7 and 27.9, respectively. Estimated site indices (SI; dominant tree height at the age of 100 years) in sessile oak- European beech forests and pre-Alpine silver fir-European beech forests were 33.4 and 32.0 for spruce, and 29.0 and 27.0 for beech, respectively. Using the described procedure, it is possible to determine indices of site productivity of spruce and beech (SI and SPI) in the selected forest habitat types. Testing the procedure in other forest types and for other tree species is suggested.

Can understory functional traits predict post-harvest forest productivity in boreal ecosystems?

Understory vegetation plays an important role in maintaining biodiversity, soil nutrient cycling and carbon stocks, yet the ability of understory functional traits to predict forest productivity after harvesting disturbance is unknown. Our objective was to investigate the utility of individual traits (the community-weighted trait mean) and combined information from multiple traits (functional diversity indices) of understory plants in predicting tree diameter growth of black spruce (Picea mariana (Mill.) B.S.P.) after two harvesting methods (clearcutting and careful logging). We used vegetation and tree growth data from a network of field plots (171 plots on 43 sites) established in black spruce dominated forests across Ontario and Quebec, in northeastern Canada. Functional traits (11 traits) of dominant understory species that reflect plant morphology, regeneration strategy, and resource utilization were collected. We then used generalized linear mixed models (GLMMs) to model the response of tree diameter increment to functional diversity indices, community-weighted trait means and their interactions with harvesting method. Our results showed that: 1) at the functional diversity level, functional richness had a positive relationship with tree growth in clear-cut forests, while functional evenness showed a negative and functional divergence a positive relationship with tree growth in careful logging forests; 2) at the individual trait level, understory species with relatively large morphological trait values often promoted tree growth in both forest types, with stronger relationships in careful logging than in clear-cut forests. For regeneration traits, heavier seed weight or permanent seed bank persistence of understory plants had negative relationships with tree growth, and those relationships were either only found in careful logging forests or did not depend on harvesting type. A positive relationship was found between vegetative propagation species and tree growth, which was stronger in careful logging than in clear-cut forests. Species preferring either humid or xeric habitat had positive relationships with tree growth; both shade-tolerant and shade-intolerant species also positively correlated to tree growth. In conclusion, our study shows the important roles of understory traits in predicting tree growth based on both niche complementarity and mass-ratio hypotheses. Moreover, disturbance severity influenced the balance or degree of the positive and negative interactions that occurred between understory plants and black spruce tree growth. We suggest that identifying the threshold of harvesting disturbance severity that can activate facilitation interactions between understory plants and crop trees would be an important topic for future study.

Developing an individual tree diameter increment model of oaks using indicator variables and mixed effects in central China

ABSTRACT Developing an individual tree diameter increment (ΔDBH) model is the basis of near-natural management of mixed, uneven-aged oak forests. This analysis used remeasurement data (2009–2014) comprising 6154 observations from 112 permanent plots in central China to develop and compare an indicator variable model (IVM) and a mixed-effect model (MEM) to estimate ΔDBH. First, a basic model was estimated using 12 potential explanatory variables. Geographical regions (GR), competition intensities (CI) and species compositions (SC) were introduced into the basal model as indicator variables or mixed effects, step by step, and then the prediction accuracy of IVM and MEM was compared. The results showed that (1) the independent variables significantly affecting ΔDBH included the reciprocal of DBH, basal area, altitude, and mean annual rainfall; (2) the introducing GR could not improve the accuracy of estimating ΔDBH, but the CI and SC could. (3) Compared with the basic model and IVM, the percentage mean absolute deviation of MEM decreased by 2.07% and 1.11%, while the root mean square error decreased by 0.06 and 0.04, respectively. The MEM including CI and SC as a random effect showed the best predictive performance and can be applied to improve the prediction of individual oak trees ΔDBH.

Growth Relationships in Silver Fir Stands at Their Lower-Altitude Limit in Romania

This study presents the biometric relationships among various increments that is useful in both scientific and practical terms for the silvicultural of silver fir. The increments recorded in the biometric characteristics of trees are a faithful indicator of the effect of silvicultural work measures and of environmental conditions. Knowing these increments, and the relationships among them, can contribute to adaptations in silvicultural work on these stands with the purpose of reducing risks generated by environmental factors. We carried an inventory based on tree increment cores. The sample size was determined based on both radial increment and height increment variability of the trees. The sample trees were selected in proportion to their basal area on diameter categories. Current annual height increment (CAIh) was measured on felled trees from mean tree category. For CAIh we generated models based on the mean tree height. Percentages of the basal area increment and of form-height increment were used to compute the current annual volume increment percentage (PCAIv). For the mean tree, the CAIh estimated through the used models had a root-mean-square error (RMSE) of 0.8749 and for the current annual volume increment (CAIv) the RMSE value was 0.1295. In even-aged stands, the mean current volume increment tree is a hypothetical tree that may have the mean basal area of all the trees and the form-height of the stand. Conclusions: The diameter, height, and volume increments of trees are influenced by structural conditions and natural factors. The structures comprising several generations of fir mixed with beech and other deciduous trees, which have been obtained by the natural regeneration of local provenances, are stable and must become management targets. Stable structures are a condition for the sustainable management of stands.