Vertical Distribution Of Leaf Area Research Articles

Maximizing radiation use efficiency by matching vertical distribution of leaf area and leaf nitrogen in maize canopy: a simulation study

Maximizing radiation use efficiency by matching vertical distribution of leaf area and leaf nitrogen in maize canopy: a simulation study

Evaluation of root lodging resistance during whole growth stage at the plant level in maize

Root lodging due to strong storm wind is a common problem in maize (Zea mays) production, leading to reduced crop yield and quality and harvest efficiency. Little information is available on quantifying effects of vertical leaf area distribution on root lodging in crops such as maize. Anti-lodging index of root was computed by the formula: ALroot = Mroot / Mwind, where AL denotes anti-lodging index, and M moment of force. Mroot, root failure moment of force equals to moment arm times max root side-pulling force measured in situ by means of the digital pole dynamometer, and Mwind, wind resultant moment of force is estimated with vertical leaf area distribution and wind speed. Two maize cultivars were examined at 5 different growth stages from V8 to physiological maturity in 2019 and 2020, in Qingdao, China. Root anti-lodging index in tested cultivars fluctuated to a small extent within any year during whole growth period excluding at V8, while there was an inter-annual shift in index means (1.23 vs 0.84). Both root failure moment and wind resultant moment increased first and then decreased with the growth stage, and their influences on root anti-lodging index varied with the year. At wind grade 6, effect sizes, as contribution to root anti-lodging index, of root moment and wind moment were respectively 0.88 and 0.98. The difference in anti-lodging index between cultivars seemed to be disappearing as wind grade goes up. Root failure moment of force positively related to single root tensile resistance, root-soil ball volume, root number and total root length, whose correlation coefficient was the maximum of 0.94. Root anti-lodging index of maize proved stable from V8 on during whole growth period, and vertical leaf area distribution played a substantial role in maize root lodging in terms of wind resultant moment. Our findings provide the insights into root lodging events in crops such as maize, and would serve an approach to assessing crop root lodging resistance in breeding and cultivation programs.

Nitrogen fertiliser only increases leaf area in the lower crown of mid-rotation Eucalyptus nitens plantations

Nutrient supplementation can stimulate growth of plantation trees through an increase in leaf area, but this link is dependent upon how the leaves are distributed throughout the crown. Due to its effect on the light environment through the crown, vertical leaf area distribution can have important impacts on leaf function with implications for tree growth. We used large-scale fertiliser experiments in three mid-rotation Eucalyptus nitens plantations across Tasmania to investigate the effects of mid-rotation nitrogen and phosphorus fertiliser application on leaf area index (LAI), light extinction and leaf traits. Nitrogen and phosphorus application rates were 0, 300, 450, 600, 1200 and 2200 kg/ha and 0 and 200 kg/ha, respectively. We found a non-linear response of LAI to N application at all sites in the year following fertiliser application, with the greatest increases in leaf area occurring in trees grown under the intermediate N treatments. Using vertical light extinction profiles through the crown to infer leaf area distribution, we found that leaf area increase in the upper crown was relatively consistent across the range of N application levels but that leaf area increase in the lower crown increased linearly with increasing N application up to 600 kg N/ha but did not increase further with higher N application rates. The addition of phosphorus increased LAI production at all sites in the first year but had no effect on light extinction and hence the vertical distribution of foliage through the crown. Leaf traits were largely unaffected by nutrient application. These results demonstrate that nitrogen fertilisers applied mid-rotation can boost LAI in E. nitens plantations, but the extra leaf area is predominantly located lower in the crown and therefore unlikely to contribute substantially to any increase in productivity.

From canopy complementarity to asymmetric competition: The negative relationship between structural diversity and productivity during succession

Abstract Positive relationships between structural diversity and forest productivity have been documented in controlled experiments and early secondary forests, however, negative relationships have also been observed in late successional forests. The mechanisms causing observed relationships between structural diversity and productivity are not well‐established, but complementarity among crowns and asymmetric competition have been suggested. We used LiDAR and repeated census data to examine the relationship between canopy structural diversity and productivity in nine 1‐ha subtropical forest plots along a disturbance gradient in southeastern China. We quantified the relative importance of community composition, species diversity, canopy structural diversity, leaf area index (LAI) and disturbance regime on productivity using piecewise structural equation modelling. We also tested how vertical leaf area distribution affected productivity. Contrary to many prior observations, we found a negative relationship between canopy structural diversity and forest productivity. The negative effect may stem from asymmetric competition between overstorey and understorey leaves, leading to a lower leaf area efficiency (i.e. wood production per leaf area). Asymmetric competition was suggested by a negative relationship between understorey leaf area and total productivity. Changes in community composition over the disturbance gradient, but not species diversity, had a significant effect on productivity. Synthesis. Our study suggests that leaf area and canopy structural diversity have contrasting effects on productivity in this subtropical forest, and this needs to be considered when estimating rates of carbon sequestration in secondary forests. The negative effect of asymmetric competition on productivity is comparable to that of the shift in species composition over succession, highlighting the role of canopy structural diversity in shaping forest productivity.

Leaf area index estimation using top-of-canopy airborne RGB images

Leaf Area Index (LAI) is one of the most important biophysical properties of a crop, used in detecting long-term water stress, estimating biomass, and identifying crop growth stage. Remote sensing based LAI estimation techniques perform well for early growth stages but tend to produce high error during the crop reproductive stage due to canopy closure. Moreover, estimation of the true LAI from individual leaf measurements remains a challenge. Consequently, two alternate methods have been developed and compared for estimating the LAI of a maize crop using top-of-canopy RGB images collected throughout the growing season using a hexacopter. Both methods used the RGB images to estimate the canopy height and the green-canopy cover together with a ‘vertical leaf area distribution factor’ (VLADF) from allometric relations (using crop height from RBG images and days after sowing). The first method used an empirical approach to estimate the LAI from training a linear function of the above inputs to Licor canopy analyser values of LAI. The method was trialled for a research farm located in a semi-arid area of southern peninsula India and found to have validation results with an R2 of 0.84 and RMSE of 0.36 for the unused portion of the Rabi (post-monsoon) season data of 2018–19, and R2 of 0.77 and RMSE of 0.45 for the Rabi 2019–20 season data when compared with Licor LAI values. While seemingly acceptable, the Licor canopy analyser gives a foliage area index and so the accuracy of this model was very low (R2 of 0.56 and RMSE of 1.34) when evaluated with true LAI from manual measurements of the leaf area. Consequently, a refinement was introduced using only VLADF, green-canopy cover estimates from the RBG images, and a field measured top leaf angle. The output derived from this conceptual model had an R2 of ~0.6 and RMSE of 0.73 when compared with the true LAI values. Importantly, the LAI from this conceptual model was found to be unaffected by canopy closure during the reproductive stage of the crop.

Simulating the effect of flowering time on maize individual leaf area in contrasting environmental scenarios.

The quality of yield prediction is linked to that of leaf area. We first analysed the consequences of flowering time and environmental conditions on the area of individual leaves in 127 genotypes presenting contrasting flowering times in fields of Europe, Mexico, and Kenya. Flowering time was the strongest determinant of leaf area. Combined with a detailed field experiment, this experiment showed a large effect of flowering time on the final leaf number and on the distribution of leaf growth rate and growth duration along leaf ranks, in terms of both length and width. Equations with a limited number of genetic parameters predicted the beginning, end, and maximum growth rate (length and width) for each leaf rank. The genotype-specific environmental effects were analysed with datasets in phenotyping platforms that assessed the effects (i) of the amount of intercepted light on leaf width, and (ii) of temperature, evaporative demand, and soil water potential on leaf elongation rate. The resulting model was successfully tested for 31 hybrids in 15 European and Mexican fields. It potentially allows prediction of the vertical distribution of leaf area of a large number of genotypes in contrasting field conditions, based on phenomics and on sensor networks.

Dynamic Simulation on Canopy Light Distribution and Dry Matter Accumulation of Natural Colored Cotton in Alar Reclamation Area of Xinjiang

Based on Ross’s theory of optical radiation transmission and full consideration of influences of vertical distribution of canopy leaf area and leaf inclination angle distribution of colored cotton on the light distribution, the Gaussian 5-point distance was used to divide the canopy into 5 layers on basis of the leaf area index. The leaf inclination angle on each layer was divided into 6 equal parts by 15°. The types of radiation in canopy, spatial distribution of light radiation, as well as diurnal variation with solar hour angles were quantified in detail. After comprehensively considering influences of temperature, physiological age and other factors on photosynthesis and respiration, the canopy light distribution, photosynthetic production and dry matter accumulation of colored cotton were simulated with strong mechanistic and physiological & ecological significance. The colored cotton samples sown on April 16, 2019 were used to verify the model. The RMSEs of simulated and measured canopy PAR values at Beijing time 10:00, 12:00, 14:00 and 16:00 on July 30 were 58.2, 64.1, 43.4 and 39.7 µmol•m-2•s-1, respectively. The RMSE of simulated and observed values of the dry matter accumulation above ground was 412.6 kgDM•hm-2, reflecting the good predictability of the model.

Vertical Distribution of Leaf Area of European Larch (Larix decidua Mill.) and Norway Spruce (Picea abies (L.) Karst.) in Pure and Mixed Stands

The growth effects of mixtures are generally assumed to be a result of canopy structure and crown plasticity. Thus, the distribution of leaf area at tree and stand level helps to explain these mixing effects. Therefore, we investigated the leaf area distribution in 12 stands with a continuum of proportions of European larch (Larix decidua Mill.) and Norway spruce (Picea abies (L.) Karst.). The stands were between 40 and 170 years old and located in the northern part of the Eastern Intermediate Alps in Austria at elevations between 900 and 1300 m asl A total of 200 sample trees were felled and the leaf area distribution within their crowns was evaluated. Fitting beta distributions to the individual empirical leaf area distributions, the parameters of the beta distributions were shown to depend on the leaf area of the individual trees and, for spruce, on the proportion of spruce in the stands. With the equations determined, the leaf area distribution of all trees in the stand, and thus its distribution in the stands, was calculated by species and in 2 m height classes. For the individual trees, we found that the leaf area distribution of larch is more symmetric, and its peak is located higher in the crown than it is the case for spruce. Furthermore, the leaf area distribution of both species becomes more peaked and skewed when the leaf area of the trees increases. The mixture only influences the leaf area distribution of spruce in such a way that the higher the spruce proportion of the stand, the higher the leaf area is located within the crown. At the stand level, a strong relationship was found between the proportion of spruce and the distance between the peaks of the leaf area distributions of larch and spruce.

Changes in the vertical distribution of leaf area enhanced light interception efficiency in maize over generations of selection.

Breeders select for yield, thereby indirectly selecting for traits that contribute to it. We tested if breeding has affected a range of traits involved in plant architecture and light interception, via the analysis of a panel of 60 maize hybrids released from 1950 to 2015. This was based on novel traits calculated from reconstructions derived from a phenotyping platform. The contribution of these traits to light interception was assessed in virtual field canopies composed of 3D plant reconstructions, with a model tested in a real field. Two categories of traits had different contributions to genetic progress. (a) The vertical distribution of leaf area had a high heritability and showed a marked trend over generations of selection. Leaf area tended to be located at lower positions in the canopy, thereby improving light penetration and distribution in the canopy. This potentially increased the carbon availability to ears, via the amount of light absorbed by the intermediate canopy layer. (b) Neither the horizontal distribution of leaves in the relation to plant rows nor the response of light interception to plant density showed appreciable trends with generations. Hence, among many architectural traits, the vertical distribution of leaf area was the main indirect target of selection.

The relationship between leaf and ecosystem CO2 exchanges in a maize field

The relationship between leaf photosynthetic rate (A) in a vegetation canopy and the net ecosystem CO2 exchange (NEE) over an entire ecosystem is not well understood. The aim of the present study is to assess the coordinated changes in NEE derived with eddy covariance, A measured in leaf cuvette, and their associations in a rainfed maize field. The light response-curves were estimated for the carbon assimilation rate at both the leaf and ecosystem scales. NEE and A synchronically changed throughout the day and were greater around noon and persisted longer during rapid growth periods. The leaf A had a similar pattern of daytime changes in the top, middle, and bottom leaves. Only severe leaf ageing led to a significant decline in the maximum efficiency of photosystem II (PSII) photochemistry. The greater maximum NEE was associated with a higher ecosystem quantum yield. NEE was positively and significantly correlated with the leaf A averaged based on the vertical distribution of leaf area. The finding highlights the feasibility of assessing NEE by leaf CO2 exchange because of most of experimental data obtained with leaf cuvette methods; and also implies that simultaneously enhancing leaf photosynthetic rate, electron transport rate, net carbon assimilation at whole ecosystem might play a critical role for the enhancement of crop productivity.

Neighbor and Height Effects on Crown Properties Associated with the Uniform-Stress Principle of Stem Formation

According to the uniform-stress principle of stem formation, the amount of leaf area a tree carries and the leverage it exerts on the stem determine the stem dimensions. Within an even-aged monoculture, the leaf area per tree and the leverage placed on the stem are functions of tree density and tree height. The uniform-stress principle presents the means to translate density effects on crown characteristics into stem dimensions and total standing volume. This approach is truly a top-down method of simulating growth tree and stand growth because leaf area and other crown properties must be determined before stem size and taper can be calculated. Each crown property influences either the sail area or the leverage placed on the stem, but the degree to which a specific crown property affects these parameters changes with stand density and height. Leverage is the more complicated of the two variables, being a function of the height to the base of the live crown and the vertical distribution of leaf area. The purpose of this brief review is to summarize the effects of stand density on the height to the base of the live tree and the vertical distribution of leaf area and the various ways these variables have been quantified.

Phenotypic plasticity to altered apical bud temperature in Cucumis sativus: more leaves‐smaller leaves and vice versa

Many studies investigated temperature effects on leaf initiation and expansion by relating these processes to air temperature or the temperature of a specific organ (e.g. leaf temperature). In reality plant temperature is hardly ever equal to air temperature or spatially uniform. Apical bud temperature (Tbud ), for example, may greatly differ from the temperature of the rest of the plant (Tplant ) dependent on the environment. Recent research in Cucumis sativus showed that Tbud influences leaf initiation independent of Tplant . These findings trigger the question if such spatial temperature differences also influence leaf expansion and plant phenotype. In a 28 day study, we maintained temperature differences between Tbud and Tplant ranging from -7 to +8 °C using a custom-made bud temperature control system. Leaf expansion did not only depend on leaf temperature but also on the difference between bud and leaf temperature. Differences between Tbud and Tplant considerably influenced vertical leaf area distribution over the shoot: increasing Tbud beyond Tplant resulted in more and smaller leaves, while decreasing Tbud below Tplant resulted in less and larger leaves. The trade-off between leaf number and leaf area resulted in phenotypic alterations that cannot be predicted, for example, by crop models, when assuming plant temperature uniformity.

Tree level hydrodynamic approach for resolving aboveground water storage and stomatal conductance and modeling the effects of tree hydraulic strategy

AbstractThe finite difference ecosystem‐scale tree crown hydrodynamics model version 2 (FETCH2) is a tree‐scale hydrodynamic model of transpiration. The FETCH2 model employs a finite difference numerical methodology and a simplified single‐beam conduit system to explicitly resolve xylem water potentials throughout the vertical extent of a tree. Empirical equations relate water potential within the stem to stomatal conductance of the leaves at each height throughout the crown. While highly simplified, this approach brings additional realism to the simulation of transpiration by linking stomatal responses to stem water potential rather than directly to soil moisture, as is currently the case in the majority of land surface models. FETCH2 accounts for plant hydraulic traits, such as the degree of anisohydric/isohydric response of stomata, maximal xylem conductivity, vertical distribution of leaf area, and maximal and minimal xylem water content. We used FETCH2 along with sap flow and eddy covariance data sets collected from a mixed plot of two genera (oak/pine) in Silas Little Experimental Forest, NJ, USA, to conduct an analysis of the intergeneric variation of hydraulic strategies and their effects on diurnal and seasonal transpiration dynamics. We define these strategies through the parameters that describe the genus level transpiration and xylem conductivity responses to changes in stem water potential. Our evaluation revealed that FETCH2 considerably improved the simulation of ecosystem transpiration and latent heat flux in comparison to more conventional models. A virtual experiment showed that the model was able to capture the effect of hydraulic strategies such as isohydric/anisohydric behavior on stomatal conductance under different soil‐water availability conditions.

Rainfall interception modelling: Is the wet bulb approach adequate to estimate mean evaporation rate from wet/saturated canopies in all forest types?

The Penman–Monteith equation has been widely used to estimate the maximum evaporation rate (E) from wet/saturated forest canopies, regardless of canopy cover fraction. Forests are then represented as a big leaf and interception loss considered essentially as a one-dimensional process. With increasing forest sparseness the assumptions behind this big leaf approach become questionable. In sparse forests it might be better to model E and interception loss at the tree level assuming that the individual tree crowns behave as wet bulbs (‘‘wet bulb approach”). In this study, and for five different forest types and climate conditions, interception loss measurements were compared to modelled values (Gash’s interception model) based on estimates of E by the Penman–Monteith and the wet bulb approaches. Results show that the wet bulb approach is a good, and less data demanding, alternative to estimate E when the forest canopy is fully ventilated (very sparse forests with a narrow canopy depth). When the canopy is not fully ventilated, the wet bulb approach requires a reduction of leaf area index to the upper, more ventilated parts of the canopy, needing data on the vertical leaf area distribution, which is seldom-available. In such cases, the Penman–Monteith approach seems preferable. Our data also show that canopy cover does not per se allow us to identify if a forest canopy is fully ventilated or not. New methodologies of sensitivity analyses applied to Gash’s model showed that a correct estimate of E is critical for the proper modelling of interception loss.

Response of leaf area and dry matter of crop, weeds and cover crops to competition and fertilizer resources

Plasticity of plants to allocate leaf area and dry matter to upper layer of canopy play important role in canopy architecture and competition. In order to study the vertical distribution of leaf area and dry matter of corn (Zea mays L.), cover crops and weeds canopy in different fertilizer condition and competition, a randomized complete block design experiment with 8 treatments and 3 replicates was conducted at Sari Agricultural Sciences and Natural Resources University in 2012. Treatments were included corn with soybean (Glycine max (L.) Merr.) as cover crop without fertilizer application, corn with soybean as cover crop with chemical fertilizer application, corn with soybean as cover crop with compost fertilizer application, corn with wheat (Triticum aesitivum L.) as cover crop without fertilizer application, corn with wheat as cover crop with chemical fertilizer application, corn with wheat as cover crop with compost fertilizer application and corn monoculture both in weedy and weed free conditions. The results showed that weed infestation reduced total leaf area and dry matter of corn. Corn distributed more leaf area and dry mater of canopy to the upper layer in weedy conditions. Between cover crops, soybeans allocated corn leaf area and dry mater to the higher layers of canopy than wheat. Also, soybean reduced leaf area and dry mater production of weeds more than wheat. Soybean as cover crop with the use of compost treatment was more efficient in reducing of weed biomass and corn yield production.

Intra- and inter-specific differences in crown architecture in Chinese subtropical mixed-species forests

Absorption of photosynthetically active radiation (APAR) is fundamental for tree growth and is strongly influenced by crown architecture. The aim of this study was to quantify the intra- and inter-specific variability in crown architecture in monospecific and mixed-species subtropical Chinese forests. A total of 68 trees, including Castanopsis eyrei, Castanopsis sclerophylla, Cunninghamia lanceolata, Cyclobalanopsis glauca and Liquidambar formosana were destructively sampled and their crown architectures were quantified in terms of the vertical distribution of live branch diameter, individual branch leaf area, leaf area and leaf-area density. The vertical distributions were fitted by a two-parameter right truncated Weibull distribution. Inter-specific variability was assessed using ANCOVA and post hoc Tukey tests and intra-specific variability was assessed by fitting linear and linear mixed effect models. The peak in the vertical distribution of leaf area was highest for the least shade tolerant species, L. formosana (relative depth into the crown of 0.5), intermediate for C. sclerophylla (0.55), C. glauca (0.55) and C. lanceolata (0.6) and lowest for C. eyrei (0.75). For all species, the vertical distribution of leaf area was influenced by tree size except for C. glauca. For L. formosana and C. lanceolata, the distribution of leaf area or branch sizes shifted upwards as tree diameter increased, possibly to overtop neighbouring trees. In contrast, as C. eyrei stem diameter increased, the vertical distribution of mean branch diameter shifted downwards, indicating that larger C. eyrei trees invested a higher proportion of their crown growth into their lower crown when compared to smaller trees. The vertical distribution of leaf-area density varied between species but not within a given species. Crown architectures were not influenced by stand density (basal area) or the species composition of the plot. This intra-specific consistency is useful for modelling light in forests. This study shows that there is a significant inter-specific variability in the crown architectures of the co-occurring species in these subtropical forests. There is also significant intra-specific variability related to tree size and this relationship varies between species. This crown architectural variability and its effect on stand structure are likely to influence the light absorption of these stands.

Development of branch, crown, and vertical distribution leaf area models for contrasting hardwood species in Maine, USA

Branch, crown vertical leaf area distribution models were developed for naturally regenerated hardwood species and planted hybrid poplar clones. Species-specific differences were found at all levels of investigation. Coexistence in mixed-species stands is strongly influenced by species differences in leaf area production and distribution. The majority of leaf area models in the literature are focused on conifer species, which have substantially different crown forms than hardwood species. Therefore, the goal of this investigation was to develop branch, crown, and vertical leaf area distribution models for various hardwood species that accounted for their greater crown complexity. A nonlinear model including branch diameter, branch tip height, and height to the start of the foliage was the best fit for branch leaf area. Branch leaf area ranged from 0.05 to 0.37 m2 for Populus grandidentata and Betula populifolia for an averaged sized branch, respectively. The best fit model for crown leaf area was a nonlinear form accounting for stem diameter and crown length. Crown leaf area ranged from 3.26 to 9.85 m2 for Populus tremuloides and Betula populifolia for an averaged sized tree, respectively. Vertical leaf area distribution was best fit by a right-truncated Weibull distribution and showed a peak in the middle third of the crown for most of species. In addition, leaf area production varied among four hybrid poplar clones in plantations, suggesting a strong genetic control over crown form. Overall, leaf area varied among species at all levels of investigation, suggesting that coexistence of hardwood saplings in this investigation was strongly influenced both by inherent species-specific leaf area production and vertical distribution.

A pseudo-3D model to optimise the target traits of light interception in short-rotation coppice willow

Fast growing short rotation coppice willows are being developed as biomass crops for bioenergy and biofuels. The UK Perennial Bioenergy Crops programme (BSBEC-BioMASS) aims to optimise biomass yield and wood composition of willow. Towards this goal, new trials were established in 2009 of contrasting genotypes (Endurance, Resolution, Terra Nova and Tora) in two different environments (wet and dry) for intensive studies and to increase our process understanding. From data of the first growth cycle we developed a model for light interception, representing the canopy structure, and accounting for both the vertical and horizontal heterogeneity of willow canopies. The key components of light interception were considered as: leaf inclination distribution and its effects on the extinction coefficient; clumping index; and leaf area index as a function of canopy height. The model realistically represents the varietal differences observed in the field and predicts well light interception for willow (r2 ranges from 0.91 to 0.99) when compared with measured data. The extinction coefficient proved to be well represented using a standard ellipsoidal distribution function (r2 ranges from 0.81 to 0.97). Vertical leaf area distribution and clumping index were found to be the most important traits for light interception and, unlike the extinction coefficient, the clumping index can be expressed as a function of the leaf area. We conclude that the new model provides a useful and simple approach to characterise the efficiency of light interception in willow genotypes and can help to identify target traits for the selection of varieties with an improved architecture for light interception.

Leaf Area Index as Vegetation Density Measure for Hydraulic Analyses

The correct parameterization of vegetation using measurable plant characteristics is essential for reliable flow resistance estimates. This paper focuses on the impact of density and distribution of the leaf mass on the drag force. Direct drag force measurements on flexible artificial plants were conducted in a stand. The experiments were novel in that the leaf area index (LAI) was varied in two ways: (1) the distances between the plants were altered, and (2) the leaf area of the plants was doubled and tripled. In addition, the vertical leaf area distribution was examined. The drag forces depended on the leaf mass in a similar fashion regardless of the horizontal distribution of the leaf mass, but the vertical distribution had a more significant impact. Shading of the leaves slightly reduced resistance for bushy plants because the leaf mass hidden behind the frontal projected area exerted drag. The experiments confirmed LAI to be a useful measure in estimating vegetative resistance.

The effects of spacing on growth, morphology and biomass production and allocation in two hybrid poplar clones growing in the boreal region of Canada

Intra-clonal competition was studied in young hybrid poplar plantations to assess the effects of spacing on growth, biomass production and allocation, and morphological characteristics of above- and below-ground tree parts. Three spacings were used as whole-plots (1 × 1 m, 3 × 3 m and 5 × 5 m), with two hybrid poplar clones as subplots (BT747, Populus balsamifera L. × P. trichocarpa Torr. & Gray; MB915, P. maximowiczii A. Henry × P. balsamifera L.) in a split-plot design. After six growing seasons, diameter at breast height (dbh) increased by about 120% from the 1 × 1 m to the 5 × 5 m spacing for clone MB915, while there was no significant change in dbh for the other clone. The effect of spacing on height growth was opposite for the clones; it increased by about 175% from the narrowest to the widest spacing for clone MB915, while it decreased by about 27% for clone BT747. Estimates of above-ground biomass production after six growing seasons were significantly reduced with increasing spacing, with 29.6, 4.9 and 3.2 MgDM ha−1 on average from the narrowest to the widest spacing. Branch traits and the vertical distribution of leaf area were the most affected by spacing for both clones, while live crown ratio and percentage of syllepsis did not change. Spacing also affected proportions of biomass allocated to stem, leaves, and branches, but allocation to roots did not change.