Even-aged Monospecific Stands Research Articles

Development of Spatiotemporal Whole-Stem Models for Estimating End-Product-Based Fibre Attribute Determinates for Jack Pine and Red Pine

The objective of this study was to develop spatiotemporal whole-stem wood quality prediction models for a suite of end-product-based fibre attribute determinates for jack pine (Pinus banksiana Lamb.) and red pine (Pinus resinosa Aiton): specifically, for wood density (Wd), microfibril angle (Ma), modulus of elasticity (Me), fibre coarseness (Co), tracheid wall thickness (Wt), tracheid radial diameter (Dr), tracheid tangential diameter (Dt), and specific surface area (Sa). Procedurally, these attributes were determined for each annual ring within pith-to-bark xylem sequences extracted from 610 jack pine and 223 red pine cross-sectional disks positioned throughout the main stem of 61 jack pine and 54 red pine sample trees growing within even-aged monospecific stands in central Canada. Deploying a block cross-validation-like approach in order to reduce serial data dependency and enable predictive performance assessments, species-specific calibration and validation data subsets consisting of cumulative moving average values were systematically generated from the 27,820 jack pine and 11,291 red pine attribute-specific annual ring values. Graphical, correlation, regression and validation analyses were used to specify, parameterize and assess the predictive performance of tertiary-level (ring-disk-tree) hierarchical mixed-effects whole-stem equations for each attribute by species. As a result, the jack pine equations explained 46, 66, 74, 63, 59, 72, 42 and 48% of the variation in Wd, Ma, Me, Co, Wt, Dr, Dt and Sa, respectively. The red pine equations explained slightly higher levels of variation except for Me: 50, 71, 31, 83, 72, 78, 56 and 71% of the variation in Wd, Ma, Me, Co, Wt, Dr, Dt and Sa, respectively. Graphical assessments and statistical metrics related to attribute and species-specific residual error patterns and goodness-of-fit, lack-of-fit and predictive error metrics, revealed an absence of systematic bias, misspecification or aberrant predictive performance. Consequently, the resultant parameterized models were acknowledged as acceptable functional descriptors of the intrinsic spatiotemporal cumulative developmental patterns of the studied end-product fibre attribute determinates, for these two pine species. Although predicted development patterns were similar between the species with the greatest degree of nonlinearity occurring before a cambial age of approximately 30 years, irrespective of attribute, jack pine exhibited a greater degree of nonlinearity in the Wd and Dt developmental trajectories, whereas red pine exhibited a greater degree of nonlinearity in the Ma, Me, Co, Wt, Dr and Sa developmental trajectories. Potential biomechanical linkages underlying the observed attribute distribution patterns, as well as the potential utility of the models in forest management, are also discussed.

Dividing the pie: A quantitative review on plant density responses.

Plant population density is an important variable in agronomy and forestry and offers an experimental way to better understand plant-plant competition. We made a meta-analysis of responses of even-aged mono-specific stands to population density by quantifying for 3 stand and 33 individual plant variables in 334 experiments how much both plant biomass and phenotypic traits change with a doubling in density. Increasing density increases standing crop per area, but decreases the mean size of its individuals, mostly through reduced tillering and branching. Among the phenotypic traits, stem diameter is negatively affected, but plant height remains remarkably similar, partly due to an increased stem length-to-mass ratio and partly by increased allocation to stems. The reduction in biomass is caused by a lower photosynthetic rate, mainly due to shading of part of the foliage. Total seed mass per plant is also strongly reduced, marginally by lower mass per seed, but mainly because of lower seed numbers. Plants generally have fewer shoot-born roots, but their overall rooting depth seems hardly affected. The phenotypic plasticity responses to high densities correlate strongly with those to low light, and less with those to low nutrients, suggesting that at high density, shading affects plants more than nutrient depletion.

A dynamic whole-stand growth model, derived from allometric relationships

Growth and yield tables are constrained by a standard production regime and the stand-level dynamic models are an attractive alternative for the even-aged monospecific stands. The objective of this study is to derive a parsimonious and biologically sound whole-stand dynamic growth model and to validate its consistency and relevance for prediction of stand growth and yield. The state-space modelling approach was employed, introducing several novelties in comparison with its current usage. The model consists of a three-dimensional state vector, defined by dominant stand height, number of trees per hectare and mean stem volume, and three transition functions. A site index model was incorporated for height growth projection and transition functions for stand density and mean stem volume with respect to dominant height increase were derived from simple allometries and biological rationale. The model was examined with data from 79 permanent sample plots in Scots pine plantations. Nonlinear Seemingly Unrelated Regression was used to account for cross-equation correlations, and the Base-Height-Invariant dummy variable method was employed to estimate dynamic-form equations. Model consistency was validated in terms of accuracy of predictions and applicability to both thinned and unthinned stands. The new dynamic growth model is a parsimonious biometric whole-stand model that simulates the expected stand development for a broad spectrum of potential management alternatives and can be incorporated in a computer program for further use. It may be especially useful for application when a scarcity of longitudinal data prevents the use of more complicated modelling approaches.

Effects of experimental warming on growth, biomass allocation, and needle chemistry of Abies faxoniana in even-aged monospecific stands

The response and adaption mechanisms of seedlings under long-term warming have remained largely unknown. In this study, we investigated the effects of warming for 6 years on growth, and needle carbon, nitrogen, chlorophyll, and carbohydrate levels in a coniferous tree species, Abies faxoniana. Seedlings were grown in even-aged monospecific stands under ambient and warming (ambient +2.2°C) temperature in climate control chambers. Warming caused statistically significant increases in the specific leaf area, leaf area ratio, root biomass, leaf biomass, branch biomass, stem biomass, and total mass of the seedlings, and reduced the root/shoot ratio. Warming also increased total chlorophyll concentrations, specific chlorophyll pigments, and Chlorophyll a/b ratios in both studied needle age classes. In addition, C/N ratios of current-year and 1-year-old needles increased by warming. In contrast, warming decreased the levels of N, sugar, cellulose, and starch in needles, while warming had no effect on the height, stem diameter, needle mass ratio, root mass ratio, and root/needle ratio. We conclude that warming increases branch growth and changes needle chemistry, which enhances the light capture potential of seedlings.

Density Effects on Plant Height Growth and Inequality in Sunflower Populations

Comparisons between competing and non-competing sunflower (Helianthus annuus L.) populations demonstrate pronounced effects of density on plant height growth, height-to-crown width ratio, and a population's height inequality. In the present study, non-destructive measurements of height and the projected crown area of sunflower plants were taken at seven times from emergence to fruit maturation in even-aged monospecific stands with initial densities of 1, 4, 16, and 64 plants/m2. The mean height of populations increased and then decreased with increasing population density; the height inequalities of uncrowded populations decreased during stand growth, whereas the height inequalities of crowded populations decreased first and then increased during stand development. The interindividual relationships between the relative height growth rate and height within uncrowded populations became significantly negative during population growth, whereas these relationships were negative first and then became positive during the development of crowded populations. In the uncrowded populations, the static interindividual relationship between height-to-crown width ratio and volume was positive, whereas for the crowded population these relationships became negative with increasing competition for light. The data suggest that the plastic responses of plant height and height-to-crown width ratio to light competition will become more intense with increasing competition intensity. The results of the present study argue strongly for the importance of size-dependent individual-level plastic responses due to size-asymmetric light competition in generating the variations in population height inequality. (Managing editor: Ya-Qin Han)

Stand dynamics modelling approaches for multicohort management of eastern Canadian boreal forests

The objective of this paper is to discuss approaches and issues related to modelling stand dynamics for multi-cohort forest management in eastern Canadian boreal forests. In these forests, the interval between wildfires can be rather long, and the development of natural forest stands may include the establishment, growth and mortality of several cohorts of trees. Later cohorts are characterised by increasing structural complexity, including spatial heterogeneity and irregular tree size distribution. A multi-cohort forest management framework has been proposed to maintain this complexity, and associated biodiversity, on the landscape. Multi-cohort forest management planning requires forecasts of the development of stands with complex structure in response to silvicultural treatment and to natural disturbance, but current stand dynamics models in the region are applicable mainly to even-aged mono-specific stands. Possible modelling approaches for complex stands include i) the adaptation of current whole-stand growth and yield models, ii) distance-independent, empirically-derived individual-tree models, such as the USDA Forest Service Forest Vegetation Simulator, and iii) distance-dependent, empirically-derived or process-oriented individual-tree models. We conclude that individual-tree models are needed because observational data for fitting whole-stand models are not available for the full array of silvicultural treatments and natural disturbances encompassed by multi-cohort forest management. Predictive accuracy is a concern with individual-tree models, and the incorporation of coarse-scale constraints into these models is a promising means to control error.

The ratio of NPP to GPP: evidence of change over the course of stand development.

Using Scots pine (Pinus sylvestris L.) in Fenno-Scandia as a case study, we investigate whether net primary production (NPP) and maintenance respiration are constant fractions of gross primary production (GPP) as even-aged mono-specific stands progress from initiation to old age. A model of the ratio of NPP to GPP is developed based on (1) the classical model of respiration, which divides total respiration into construction and maintenance components, and (2) a process-based model, which derives respiration from processes including construction, nitrate uptake and reduction, ion uptake, phloem loading and maintenance. Published estimates of specific respiration and production rates, and some recent measurements of components of dry matter in stands of different ages, are used to quantify the two approaches over the course of stand development in an average environment. Both approaches give similar results, showing a decrease in the NPP/GPP ratio with increasing tree height. In addition, we show that stand-growth models fitted under three different sets of assumptions-(i) annual specific rates of maintenance respiration of sapwood (mW) and photosynthesis (sC) are constant; (ii) m(W) is constant, but sC decreases with increasing tree height; and (iii) total maintenance respiration is a constant fraction of GPP and s(C) decreases with increasing tree height-can lead to nearly identical model projections that agree with empirical observations of NPP and stand-growth variables. Remeasurements of GPP and respiration over time in chronosequences of stands may be needed to discern which set of assumptions is correct. Total (construction + maintenance) sapwood respiration per unit mass of sapwood (kg C (kg C year)-1) decreased with increasing stand age, sapwood stock, and average tree height under all three assumptions. However, total sapwood respiration (kg C (ha year)-1) increased over the course of stand development under (i) and (ii), contributing to a downward trend in the time course of the NPP/GPP ratio after closure. A moderate decrease in mW with increasing tree height or sapwood cross-sectional area had little effect on the downward trend. On the basis of this evidence, we argue that a significant decline in the NPP/GPP ratio with tree size or age seems highly probable, although the decline may appear insignificant over some segments of stand development. We also argue that, because stand-growth models can give correct answers for the wrong reasons, statistical calibration of such models should be avoided whenever possible; instead, values of physiological parameters should come from measurements of the physiological processes themselves.

Effects of Plant Density on Frequency Distributions of Plant Height in Chenopodium album Stands: Analysis Based on Continuous Monitoring of Height-growth of Individual Plants

Height-growth of individual plants of Chenopodium album in even-aged monospecific stands with initial densities of 400, 800, and 3600 plants m-2 was followed from emergence to fruit maturation to elucidate the factors responsible for size hierarchy formation. Many individuals stopped growing rather abruptly at relatively early stages in the vegetative phase and some of them eventually died. The other plants grew until fruit maturation and attained large final heights. Depending on whether they terminated or continued height-growth during the vegetative phase, the plants were called 'lower' and 'upper' plants.The numbers of the 'upper' plants per unit area were about 100 plants m-2 irrespective of the initial plant densities, and thus the number of 'lower' plants increased with the increase in the plant density. However, there was an upper limit to the number of the surviving 'lower' plants (400-700 plants m-2). The shapes of the final frequency distributions of plant height were largely determined by the ratio of the number of 'upper' plants to that of surviving 'lower' plants, and were bimodal for the 400 plants m-2 plot and L-shaped for the 800 and 3600 plants m-2 plots. From these, it is concluded that the segregation of the plants into 'upper' and 'lower' plants is an essential factor responsible for the size hierarchy formation. The present results also indicate that both bimodal and L-shaped distributions can result from one-sided competition for light, depending on the initial plant densities.

Stand development in aspen clones

The growth of trembling aspen (Populustremuloides Michx.) sprouts in two separate clones was measured annually for 6 years and at year 12 (site 1) and for 5 years and at year 11 (site 2) following sprout initiation. Over 98% of the stems were established within the first growing season after cutting. By the end of the first growing season, height-frequency histograms were positively skewed and coefficients of variation of height were 56 and 60% for sites 1 and 2, respectively, reflecting the presence of many small and few large stems. By the 3rd year after establishment, over 50% mortality of stems occurred and the coefficients of variation of height had declined to approximately 47% for both sites. Although the results of this study may not have widespread implications because of the limited number of clones, the pattern of stand development observed in aspen was similar to that documented for even-aged monospecific stands of nonclonal woody species, with the possible exception that self-thinning occurred at a more rapid rate in aspen.