Stand Density Index Research Articles

Beyond mean functional traits: Influence of functional trait profiles on forest structure, production, and mortality across the eastern US

Plant functional traits (PFTs) have increased in popularity in recent years to describe various ecosystems and biological phenomena while advancing general ecological principles. To date, few have investigated distributional attributes of individual PFTs and their relationship with key attributes and processes of forest ecosystems. The objective of this study was to quantify the distribution and contribution of various PFTs in determining forest structure, live tree production (volume and biomass), and tree mortality across the eastern US. In total, 16 metrics representing species specific gravity and their shade, flood, and drought tolerance were used to develop a PFT profile for over 23,000 permanent sample plots in the region. Spatial relationships were observed when analyzing not only the mean value of these traits but also measures of PFT complexity: the standard deviation, Shannon’s index (a measure of PFT diversity), and Gini coefficient (a measure of PFT inequality). Results from nonparametric random forests models indicated that variables which formed the PFT profile contributed to explaining broad-scale patterns in the variability in forest structure (volume and biomass of overstory live trees, maximum stand density index, and tree seedling abundance; R2 ranged from 0.09 to 0.78), production (volume [R2=0.16] and biomass accretion [R2=0.11]), and to a lesser degree, tree mortality. Despite the variability in the data employed and the variety of forest management regimes in these stands, this work demonstrates the utility of applying PFT profiles for understanding and predicting patterns of forest structure and production and their role in critical ecosystem processes such as carbon sequestration.

Analyzing size-symmetric vs. size-asymmetric and intra- vs. inter-specific competition in beech (Fagus sylvatica L.) mixed stands

In mixed stands, inter-specific competition can be lower than intra-specific competition when niche complementarity and/or facilitation between species prevail. These positive interactions can take place at belowground and/or aboveground levels. Belowground competition tends to be size symmetric while the aboveground competition is usually for light and almost always size-asymmetric. Interactions between forest tree species can be explored analyzing growth at tree level by comparing intra and inter-specific competition. At the same time, possible causes of niche complementarity can be inferred relating intra and inter-specific competition with the mode of competition, i.e. size-symmetric or size-asymmetric. The aim of this paper is to further our understanding of the interactions between species and to detect possible causes of competition reduction in mixed stands of beech (Fagus sylvatica L.) with other species: pine–beech, oak–beech and fir–beech. To test whether species growth is better explained by size-symmetric and/or size-asymmetric competition, five different competition structures where included in basal area growth models fitted using data from the Spanish National Forest Inventory for the Pyrenees. These models considered either size-symmetry only (Reineke’s stand density index, SDI), size-asymmetry only (SDI of large trees or SDI of small trees), or both combined. In order to assess the influence of the admixture, these indices were introduced in two different ways, one of which was to consider that trees of all species compete in a similar way, and the other was to split the stand density indices into intra- and inter-specific competition components. The results showed that in pine–beech mixtures, there is a slightly negative effect of beech on pine basal area growth while beech benefitted from the admixture of Scots pine; this positive effect being greater as the proportion of pine trees in larger size classes increases. In oak–beech mixtures, beech growth was also positively influenced by the presence of oaks that were larger than the beech trees. The growth of oak, however, decreased when the proportion of beech in SDI increased, although the presence of beech in larger size classes promoted oak growth. Finally, in fir–beech mixtures, neither fir nor beech basal area growth were influenced by the presence of the other species. The results indicate that size-asymmetric is stronger than size-symmetric competition in these mixtures, highlighting the importance of light in competition. Positive species interactions in size-asymmetric competition involved a reduction of asymmetry in tree size-growth relationships.

Modeling regional and climatic variation of wood density and ring width in intensively managed Douglas-fir

Characteristics of annual rings are reliable indicators of growth and wood quality in trees. The main objective of our study was to model the variation in annual ring attributes due to intensive silviculture and inherent regional differences in climate and site across a wide geographic range of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco). Ring specific gravity and ring width of Douglas-fir were examined at five long-term Levels-Of-Growing-Stock (LOGS) installations, three in the US and two in Canada, covering a latitudinal gradient between 43°N and 50°N. At each location, increment cores were collected from replicated plots with three levels of stocking: control (unthinned), lightly thinned (70% basal area retention), and heavily thinned (30% basal area retention). X-ray densitometry analysis provided ring specific gravity and width profiles for 5676 rings from 134 trees. The reduction of stand density through repeated entries resulted in decreased ring specific gravity and increased ring width. A four-parameter mixed-effects logistic model was used to predict ring specific gravity using cambial age, stand density (as number of stems per hectare), and two climatic variables: average temperature from March to May and total precipitation from April to August. A three-parameter mixed-effects logistic model was used to predict ring width using cambial age, stand density (as stand density index), and total climatic moisture deficit of June and July. Both models indicated significant site differences that were included in the models through indicator variables. Ring specific gravity increased slightly with increasing average temperature from March to May and decreasing total precipitation from April to August. Predictions of ring specific gravity of Douglas-fir appear to be more sensitive to changes in temperature compared with changes in precipitation.

Effect of species composition on the production rate and efficiency of young Picea glauca–Populus tremuloides forests

Stand stem volume growth, aboveground tree biomass growth, and site occupancy were measured in 20-year-old mixed stands dominated by white spruce (Picea glauca [Moench] Voss) and trembling aspen (Populus tremuloides Michx.) to examine the effects of species composition on the rate and efficiency of forest production. Measures of site occupancy derived from Plant Canopy Analyzer data indicated that sample plots were fully occupied, and that leaf area index and light interception increased with increasing softwood (SW) proportion. Relationships between site occupancy and SW proportion differed among occupancy metrics, suggesting that basal area, stand density index, and crown area index do not adequately represent use of the light resource. No significant effect of species proportion on the rate of volume or biomass production was detected, contradicting the hypothesis that competitive reduction and facilitation might increase the production of mixed stands. Production efficiency based on light interception for both stand components combined varied weakly with SW proportion. Efficiency metrics based on surrogates for light interception showed differing patterns with SW proportion, underscoring the need for caution in interpreting these metrics in relation to resource use.

Regeneration and Recruitment Correlate with Stand Density and Composition in Long-Unburned Aspen Stands Undergoing Succession to Conifer in the Sierra Nevada, USA

Succession of aspen stands to conifer in the Sierra Nevada Mountains of California and Nevada, USA, is being interrupted by forest managers thinning conifers to sustain aspen stands in situ. However, patterns of stand density, species composition, and regeneration prior to management intervention have scarcely been described. We established a grid of sample plots throughout nine aspen stands encircling Lake Tahoe in the central Sierra Nevada. The degree of succession to conifer throughout each aspen stand was spatially heterogeneous. Patches of pure aspen were rare. Stand density index (SDI) reached an upper limit of 1700 in pure aspen. As composition shifted in favor of conifer, SDI approached or attained a maximum of 2500 in some plots. Stand density and species composition data were tested as predictors of conifer and aspen regeneration densities in each plot. Conifer seedlings had an average density of 3261 ha-1; they were most abundant in aspen-dominated areas of any density, and in conifer-dominated areas of higher density. Aspen regeneration had an average density of 3211 ha-1 and was one order of magnitude less frequent in areas of pure conifer versus areas of pure aspen, but remained relatively abundant in most areas. Aspen saplings were rare (average density of 42 ha-1 for saplings 10-15 cm DBH), especially at high stand densities or in areas where conifers dominated. Our findings suggest that forest managers interested in sustaining aspen stands in situ will need to control stand density to promote recruitment of younger aspen to the overstory.

Overstory and Understory Dynamics in a Ponderosa Pine Plantation Vary with Stand Density in the Sierra Nevada: 40-Year Results

We periodically measured overstory ponderosa pine (Pinus ponderosa) growth and understory cover and abundance in a long-term study on the west slope of the Sierra Nevada, California, USA. The study was established in 1969 in a 20-year-old plantation, thinned to basal areas of 9, 16, 23, 30, and 37 m2 ha−1 and rethinned three times. The objective was to determine the effect of stand density regimes on productivity, understory vegetation, aboveground carbon storage, and mortality caused mainly by Dendroctonus infestations. Results showed that without mortality, basal area and volume increments were not affected by thinning the plantation to 16 m2 ha−1 from 37 m2 ha−1. With Dendroctonus actively attacking trees, the plantation could be thinned to 9 m2 ha−1 without sacrificing volume increment, because the level of mortality increased with stand density index. The thinning intensity did not affect the abundance of understory vegetation but did affect the cover of understory trees and graminoids. Intensively managed plantations could sequester and store the same amount of carbon as less intensively managed plantations. However, because fire is a major component in a pine-dominant ecosystem, carbon in larger diameter trees would be more resilient to wildfire than carbon in small diameter trees or understory vegetation.

Estimation of forest structure and canopy fuel parameters from small-footprint full-waveform LiDAR data

Precise knowledge of fuel conditions is important for predicting fire hazards and simulating fire growth and intensity across the landscape. We present a methodology to retrieve and map forest canopy fuel and other forest structural parameters using small-footprint full-waveform airborne light detection and ranging (LiDAR) data. Full-waveform LiDAR sensors register the complete returned backscattered signal through time and can describe physical properties of the intercepted objects. This study was undertaken in a mixed forest dominated by Douglas-fir, occasionally mixed with other conifers, in north-west Oregon (United States). We extracted two sets of LiDAR metrics using pulse detection and waveform modelling and then constructed several predictive models using forward stepwise multiple linear regression. The resulting models explained ~80% of the variability for many of the canopy fuel and forest structure parameters: aboveground biomass (R2=0.84), quadratic mean diameter (R2=0.82), canopy height (R2=0.79), canopy base height (R2=0.78) and canopy fuel load (R2=0.79). The lowest performing models included basal area (R2=0.76), stand volume (R2=0.73), canopy bulk density (R2=0.67) and stand density index (R2=0.66). Our results indicate that full-waveform LiDAR systems show promise in systematically characterising the structure and canopy fuel loads of forests, which may enable accurate fire behaviour forecasting that in turn supports the development of prevention and planning policies.

Reevaluating the self-thinning boundary line for ponderosa pine (Pinus ponderosa) forests

The self-thinning rule has been used extensively to predict population dynamics under intraspecific and interspecific competition. In forestry, it is an important silvicultural concept for maintaining stand health in the face of climate change and biotic stress, but uncertainty exists because traditional self-thinning limits were set subjectively without regard to site quality. We addressed this by analyzing ponderosa pine (Pinus ponderosa Lawson & C. Lawson) data from 109 research plots measured repeatedly and 59 inventory plots measured once across California. Self-thinning boundaries were fitted to the data with quantile regression and stochastic frontier function (SFF) techniques with and without site index (SI) as a covariate. The models from both methods fitted the data well with either research plots or all plots. Slopes for size-density trajectories were –0.45 with the 0.99 quantile and –0.47 for SFF. Maximum stand density indices (SDI) were 1250 trees per hectare (TPH) with the 0.99 quantile and 1050–1060 TPH with SFF. Mortality occurred when site occupancy from SFF reached 0.75, suggesting a zone of imminent mortality. Curvilinear trends in maximum SDI across SI for both methods indicate that self-thinning varies with site quality. Any management regimes that increase site quality and productivity will increase the self-thinning boundary.

STAND DENSITY INDICES FOR FOREST PLANTATIONS

Stand density can be defined as the quantitative measure of stocking expressed either absolutelyin tenus of number of trees, basal area, or volume per unit area or relative to some standcondition. It is an important measure to schedule thinning regimes especially in the even-agedstands. There are a number of indices available toquantify the stand density. However, matchingdensity indices are mostly depends on species and its growth behaviour. In this study fourdifferent stand density indices were compared on Eucalyptus grandis stands grown in theNuwara-Eliya divisionThe first index is the basal area, which is a conventional method to measure stand density. Inmanaged plantations due to the lack of continuity, it is not so widely used among the even agedstands, though it is the major tool to measure the density in natural forests. The maximumobserved basal area was 74.6m2/ha and the minimum was 3.6m2/ha. The second index is therelative spacing (RS). Plantations are more suitable to indicate their stand density by measuringrelative spacing .The minimum observed relative spacing was 0.088 and the maximum was0.458.The third index Crown Competition Factor (CCF), describes the relationship betweencrown surface area and the stand area. This can be interpreted; as when the crown surface areafor a unit land area is high then the stand density is high .It is a good measure to interpret thestand density situation for a dense crown species. The fourth popular stand density measure isStand Density Index (SOl). Under the proper management system, it produces a basal area of75m2/ha with 195 stems/ha and mean diameter of 70 emThere is no unique density index available for a particular tree species. The suitability of densityindices depends not only on species but also on their growth behaviour. In this study, crowncompetition factor showed very less accessibility especially for E. grandis. Stand basal area ismore descriptive and regardless of age, for the comparison of stands, it is inferior. Relativespacing is more appropriate for the stands where no thinning takes place. This explains how thenumber of stems/ha affects the RS with the age. This method is good to use in the spacing trialrather than to quantify the thinning. Stand density index (SOl) appeared to be the most prolificdensity index for this species.

Thinning ponderosa pine (Pinus ponderosa) stands reduces mortality while maintaining stand productivity

We analyzed 45 years of data collected from three ponderosa pine (Pinus ponderosa Douglas ex P. Lawson & C. Lawson) levels-of-growing-stock installations in Oregon (OR) and northern California (CA), USA, to determine the effect of stand density regimes on stand productivity and mortality. We found that periodic annual increment (PAI) of diameter, basal area (BA), volume, and aboveground dry mass were significantly related to stand density index (SDI) and stand age at start of the period; the quadratic trends varied among sites. Precipitation departure from the normal for each period explained a significant amount of residual variation in all PAI variables except diameter. BA production did not change significantly as SDI exceeded 270 trees·ha−1 at the OR sites and 320 trees·ha−1 at the CA site. Stand productivity was the highest at Elliot Ranch (CA) and the least at Blue Mountains (OR). A similar trend held in growth efficiency under lower stand densities (SDI < 600). Most of the mortality was caused by Dendroctonus bark beetles in stands that exceeded SDI of 500 trees·ha−1. Limiting SDI was about 900 trees·ha−1, although plots at Elliot Ranch reached much higher than that. The results demonstrate that silvicultural control of stand density can be a powerful tool for reducing bark beetle caused mortality without sacrificing stand productivity.

Fire-caused tree mortality in thinned Douglas-fir stands in Patagonia, Argentina

In 2003 in a municipal park near Esquel, Patagonia, Argentina, plots within a 21-year-old Douglas-fir (Pseudotsuga menziesii) afforested area were subjected to three silvicultural treatments (thinning to Reineke’s Stand Density Index (SDI) of 900, 700, 500). In March 2007 all plots were burned by a wildfire that presented extreme fire behaviour. Three weeks after the wildfire we assessed mortality, height of scorch and percentage of crown scorch, and during three subsequent growing seasons we measured mortality and growth parameters. At the end of the study, mortality differed significantly among treatments and an untreated control, and ranged from 100% in the untreated control to 25, 10 and 5% in the SDI 900, 700 and 500 treatments. The highest growth parameters and lower mortality rates were achieved at SDI indices of 700 or 500 (i.e. in the least dense plots). Trees thinned to these densities not only appear to withstand extreme fires, at least under the conditions presented, but also to achieve the highest growth rates.

Electrophysiological characteristics of undergrowth shrubs in the forest-steppe zone of western Ukraine

The article reviews the electrophysiological characteristics of pre-cambial phloem material in undergrowth shrubs. The electrophysiological properties of hazel (Corylus avellana L.), elder (Sambucus nigra L.) and other un- dergrowth shrubs are compared, as well as how these characteristics change depending on stand density indices. Differences in electrophysiological characteristics reveal the crucial role of light for the growth, development, and persistence of undergrowth shrubs. I suggest that a quantitative assessment of electrophysiological characteristics should be undertaken when managing forest stands for the creation of desirable undergrowth.

Comparison of maximum size–density relationships based on alternate stand attributes for predicting tree numbers and stand growth

Quantification of site occupancy or stand density is essential for modeling forest stand mortality, growth and yield. A variety of measures of stand density have been proposed. Diverse tree and stand attributes have been employed, but direct comparisons of the effectiveness of various measures for estimating number of trees for fully stocked stands and for predicting growth for stands at different stages of development are not possible due to the varying forms of the measures.Maximum size–density relationships are a widely-used type of stand density measure. Reineke’s stand density index (SDI), which is based on number of stems per unit area and quadratic mean stem diameter, is the most commonly-applied stand density measure of this type. In the study reported here, stand density indices were developed by employing the structure of Reineke’s index but using the stand attributes of mean stem volume and mean stand height, as well as mean stem diameter. When estimating number of trees and periodic volume growth per unit area using data from sample plots in planted stands of loblolly pine, the SDI based on mean diameter performed best with mean stem volume being second best and mean stand height third.

Modeling the Effects of Stand Development, Site Quality, and Silviculture on Leaf Area Index, Litterfall, and Forest Floor Accumulations in Loblolly and Slash Pine Plantations

Leaf area index (LAI), needlefall (NF), and forest floor (FF) dynamics are tightly linked with stand productivity, nutrient cycling, and carbon, water, and energy exchange. We analyzed a long-term data set to quantify the impacts of stand development, site quality, and silviculture on LAI and litterfall (LF) in loblolly and slash pine plantations. LAI was significantly correlated with stand density index (SDI) for each stand studied (P 0.001), and the parameters of the fitted sigmoidal function were correlated with site index independently for each species. The maximum LAI that a loblolly or slash pine stand attained was linearly correlated with site index (P 0.001), and the slope of that relationship was different for each species (P 0.003). Soil resource availability affected the relationship between SDI and LAI. When weed control or fertilizer treatments were applied, the maximum attainable mean yearly LAI and the value of SDI that corresponded with the attainment of 50% of the maximum LAI (inflection point) were increased (P 0.05). NF production was linearly related to the previous year's LAI (P 0.001), and this relationship was independent of resources availability (P 0.086); however, the relationship was different for both species (P 0.001). Comparison of simulations of NF, LF, and FF with diverse data sets from the literature, encompassing the natural ranges of both species, indicated that these relationships captured the primary drivers of variation, and therefore the models provide a robust synthesis and prediction system for these important variables. FOR .S CI. 58(5):457-471.

Relationship between above-ground biomass allocation and stand density index in Populus x euramericana stands

The relationships between above-ground stem, branch, leaf masses and stand density index (SDI) were analysed in Populus×euramericana (Dode) Guinier stands in Jiangsu Province, China. The allometric equations developed in this study accurately estimated masses of above-ground and specific above-ground tree components in stands with different SDIs. The relationship between the mean above-ground mass w and the ratio of stem to the above-ground (wS/w) was examined. With increasing w, the stem mass ratio wS/w increased. Mean masses of above-ground and tree components decreased significantly with increasing SDI at age 9 years, indicating the presence of competition-induced limitations to growth. The mean above-ground mass‐SDI and mean component mass‐SDI were strongly correlated with above-ground mass-SDI and tree component mass‐SDI, respectively. An equation describing the relationship between the mean leaf area u and SDI was developed, which fitted well for the u-SDI data. Stem mass increment exponentially increased with increasing leaf area index.

A Density Management Diagram for Even-Aged Sierra Nevada Mixed-Conifer Stands

We have developed a density management diagram (DMD) for even-aged mixed-conifer stands in the Sierra Nevada Mountains using forest inventory and analysis (FIA) data. Analysis plots were drawn from FIA plots in California, southern Oregon, and western Nevada which included those conifer species associated with the mixed-conifer forest type. A total of 204 plots met the selection criteria for analysis, which were for even-agedness and species composition. Even-agedness was characterized by a ratio between two calculations of stand density index. Species composition included admixtures of the species characterizing the Sierra Nevada mixed-conifer type with up to 80% of stand basal area contributed by ponderosa and Jeffrey pines. The DMD is unbiased with respect to species composition and therefore should be broadly applicable to the mixed-conifer type. The DMD is intended for use in even-aged stands, but may be used for uneven-aged management where a large-group selection system is used. Examples of density management regimes are illustrated, and guidelines for use are provided.

Silviculture guidelines for developing mixed forests in old coniferous plantations under long-rotation stand density control

This study presents guidelines for developing mixed broadleaf–conifer forests in old coniferous plantations under a long-rotation silviculture system. We established nine square sample plots in Chamaecyparis obtusa (hinoki) plantation forests of various stand ages (22–116 years) and densities (250–1650 trees ha−1). Tree height and diameter at breast height were recorded for all standing trees taller than 1.3 m. This step revealed the varying diameters of coniferous plantation trees and natural broadleaf trees and showed that the density of large trees increased with stand age. The older the plantation forest, the smaller its canopy openness. The standard deviation of canopy openness ranged from 20–30%, with the largest values observed in the middle-aged forests. We estimated the stand density index under which understory vegetation may be established; the maximum stand density index (1028) in the forest studied was considered to be the critical upper limit of stand density control for introducing understory vegetation. Finally, we reviewed previous studies to confirm the validity of our recommendations of long rotation and low stand density control for the development of mixed old plantation forests.

Relationships between forest structure, understorey light and regeneration in complex Douglas-fir dominated stands in south-eastern British Columbia

A quantitative understanding of the effects complex residual forest structure has on the understorey light environment is useful in guiding strategies that facilitate regeneration in irregular stands. In this study we evaluate several methods for estimating the understorey light environment in an uneven-aged mixed conifer forest in the Interior Douglas-fir zone of south-eastern British Columbia, Canada. Measurements were collected 15years after selection harvesting, which was designed to create four different levels of residual basal area (8m2/ha, 16m2/ha, 24m2/ha and uncut). Understorey light availability was highly variable across all levels of residual basal area with estimates of stand structure, including density (N), basal area (G) and stand density index (SDI) explaining 37.2–52.1% of population level variation. While relatively large trees effectively reduced light levels below 40% open sky at a stand level; intermediate and smaller sized trees were found to have the greatest influence on light attenuation at the micro-site scale. Stand variables that heavily weighted the influence of larger trees on light attenuation were weak predictors of understorey light at the micro-site scale. Incorporating variables describing composition and inequality of tree size increased the predictive power of the models resulting in the ability to explain 55% of the variation in light levels. Small plot sizes used in this study are likely limiting the ability to describe variation in light levels and we recommend the use of larger plots (with a radius between 1.0 and 1.7times tree height) for future studies of this nature. The abundance and growth of natural regeneration was strongly related to both light (as influenced by overstorey) and understorey vegetation in these stands.

Development of Planting Density‐Specific Density Management Diagrams for Loblolly Pine

For density management diagrams (DMDs), it is usually assumed the relative value between Reineke’s stand density index (SDI) where self-thinning is expected to begin and a maximum size-density relationship species boundary line is constant regardless of planting density. Using estimated size-density trajectories of loblolly pine plantations in the southeastern United States, the quadratic mean diameter at which the onset of self-thinning occurs was determined. An equation was then fitted to estimate the quadratic mean diameter at which self-thinning is expected to begin relative to planting density. DMDs that accounted for planting density showed that self-thinning began at 40 –72% of the species maximum SDI for planting densities ranging from 605 to 2,722 seedlings per acre.

Differences in growth dynamics of loblolly and slash pine plantations in the southeastern United States

A species comparison study was established in 1995 in the Lower Coastal Plain of southern Georgia and northern Florida, in which loblolly and slash pine plots were paired for combinations of planting density (741–3706 trees/ha) and management intensity (operational and intensive) at nine locations. With the age 2–15year data, species comparisons were conducted using a mixed-effect model for split-plot experiment design with repeated measurements, for average DBH, average height, average live crown length and crown ratio, cronartium infection, survival, stand density index, stand basal area and total volume. Loblolly pine consistently had greater average height and live crown length than slash pine. Average DBH did not differ between species. Loblolly pine outperformed slash pine in terms of stand basal area and total volume, partially due to the higher cronartium infection and mortality of slash pine. More intensive management significantly increased species differences in average height, stand basal area and total volume. Planting density significantly affected species difference only in live crown length.