Tree Competition Research Articles

Generalized Nonlinear Mixed-Effect Model of Individual TREE Height to Crown Base for Larix Olgensis Henry in Northeast China

ABSTRACT The height to crown base (HCB) of a tree is a necessary variable that includes many growth and yield models as a predictor, and it is important to develop an HCB model due to its contribution to forest management. In this study, we developed an individual tree HCB model for Larix olgensis using a generalized nonlinear mixed-effects model with 2510 Larix olgensis trees on 40 sample plots located on the Dongzhelenghe Forest Farm in northeastern China. According to the evaluated base model, a logistic model that most suited our data was selected as the base model. In addition to height and diameter at breast height, tree and stand level variables that represent tree size, site quality and competition, such as dominant height, basal area of trees larger than subject tree and relative spacing index, were added to the HCB model. The sample plot was set as the random effect, and the parameters describing sample plot-level random effects were included in the HCB model through the mixed-effects model. The variance heteroscedasticity in the residuals was reduced by including a constant plus power variance function in the HCB model. The results showed that the mixed-effects model described a larger part of the HCB variations ( = 0.7642, RMSE = 1.7225) than did the ordinary least square model ( = 0.7063, RMSE = 1.9224).

Doubling of biomass production in European boreal forest trees by a four-year suppression of background insect herbivory

Background insect herbivory, i.e. the minor but chronic plant damage caused by insects, is usually considered ‘negligible’ for plants when compared with the severe defoliation associated with forest pest outbreaks. We experimentally tested the hypothesis that the impacts of background herbivory on tree growth and mortality accumulate over years, resulting in much larger effects than usually assumed. In boreal taiga forests near Arkhangelsk (Northern Russia), application of insecticide at 10-day intervals for four growth seasons (June–September of 2014–2017) decreased foliage losses to insects in our study species (early successional deciduous Betula pubescens and Populus tremula; late successional coniferous Picea abies and Pinus sylvestris) from 2.04–6.35% to 0.72–1.18%. The magnitude of the insecticide treatment effect on plant losses to defoliating insects varied considerably among the study species, with the highest effect observed in white birch and the lowest effects in the two late successional species. Across all tree species, insecticide treatment nearly doubled the increase in tree biomass relative to control plots, demonstrating that background insect herbivory has major negative impacts on tree growth and productivity. The insecticide-treated plots showed the largest increase in biomass in Norway spruce and the smallest increase in European aspen when compared to the control plots. The changes in birch growth following the release from insect herbivory were three times greater than the effects of the same level of simulated herbivory in an earlier experiment, thereby hinting at the importance of herbivore-specific elicitors in the growth suppression of trees damaged by insects. The mortality of late successional species in the treatment plots increased nearly three-fold, whereas the mortality of early successional species did not change relative to controls, suggesting an increase in competitiveness of the early successional trees released from herbivory. Thus, in agreement with an earlier modelling study, we conclude that minor herbivore damage, over the long term, substantially reduces biomass production in North European forest trees. Due to differential effects on coexisting tree species, this damage has a pronounced impact on plant competitiveness and affects both the productivity and the structure of boreal forests.

The effect of prescribed burning on the drought resilience of Pinus nigra ssp. salzmannii Dunal (Franco) and P. sylvestris L.

Prescribed burning applied after a dry year increased (fall burns) or had no effect (spring burns) on pine resilience, measured as the capacity to reach pre-drought growth levels compared with unburned pines. In fall burns, there was a larger increase in resilience when the impact of drought and burning on pines was low and burning caused a significant release from tree competition. Prescribed burning after a dry year can decrease a tree’s resistance (inverse of growth reduction during disturbance); however, burning can increase resource availability and consequently tree resilience (capacity to reach pre-disturbance growth levels). In addition, the burning season can affect the latewood to earlywood ratio (latewood:earlywood) and thus have an impact on tree-ring density. To study the effects of two consecutive disturbances (drought and burning) on Pinus nigra spp. salzmannii and Pinus sylvestris in terms of (i) total tree-ring resistance, (ii) total tree-ring, earlywood and latewood resilience, and (iii) post-stress latewood:earlywood. We selected drought-affected trees (control) and drought-and-burning-affected trees (burned) for tree-ring sampling. For each tree, we measured total tree-ring, earlywood, and latewood widths to determine resilience and resistance indices and calculated pre- and post-stress latewood:earlywood as indicators of tree-ring density. We used linear mixed-effects models to relate these response variables to the pine species, burning characteristics, and competition release. Resistance was higher in control trees than burned trees. P. nigra showed lower resistance than P. sylvestris but higher resilience. Resistance positively influenced resilience. Specifically, as resistance increased, total tree-ring and latewood resilience increased significantly in pines burned during the fall compared with those burned in spring or control pines. In fall burns, the pines’ resilience increased, especially when pines were significantly released from tree competition. In P. nigra, post-stress latewood:earlywood increased after fall burns as pre-stress latewood:earlywood decreased. Prescribed burning can be a valuable management tool for overcoming the effects of a dry year on tree growth by increasing resilience, especially in P. nigra.

Influence of slope, aspect and competition index on the height-diameter relationship of Cyclobalanopsis glauca trees for improving prediction of height in mixed forests

Diameter at breast height (DBH) and height (H) of trees are two important variables used in forest management plans. However, collecting the measurements of H is time-consuming and costly. Instead, the H-DBH relationship is modeled and used to estimate H. But, ignoring the effects of slope, aspect and tree competition on the H-DBH relationship often impedes the improvement of H predictions. In this study, to improve predictions of (Thunb.) Oerst. tree H in mixed forests, we compared eleven H-DBH models and examined the influence of slope and aspect on the H-DBH relationship using 426 trees. We then improved Hegyi competition index and explored its effect on the H predictions by including it in the selected models. Results showed 1) There were statistically significant effects of slope and aspect on the H-DBH relationship; 2) The log transformation and exponential model performed best for sunny- and shady-steep, respectively, and the Gompertz’s model was optimal for both sunny- and shady-gentle; 3) Compared with the whole dataset, the division of the data into the slope and aspect sub-datasets significantly reduced the RMSE of H predictions; 4) Compared with the selected models without competition index, adding the original Hegyi and improved Hegyi_I into the models improved the H predictions but only the models containing the improved Hegyi_I significantly increased the prediction accuracy at the significant level of 0.1. This study implied that modeling the H-DBH relationship under different slopes and aspects and including the improved Hegyi_I provided the great potential to improve the H predictions.Cyclobalanopsis glauca

Grazing pressure and tree competition affect cattle performance and native pastures in Eucalypt woodlands of Queensland, north-eastern Australia

Context Well managed grazing pressure will optimise animal and pasture production, and preserve the soil to maintain a viable beef business on native pastures in eucalypt woodlands. Aims A cattle grazing experiment was established to measure animal and pasture performance under management practices used in the Aristida/Bothriochloa native pastures in central Queensland. Methods Performance of Brahman-cross steers and pastures were measured in an experiment with three grazing pressures by two tree densities in a Eucalyptus populnea woodland in north-eastern Australia over 8 years in paddocks of 4–18 ha. Key results At low grazing pressure with trees killed by herbicide (‘cleared’), stocking rate increased 35% as pasture composition and biomass improved over 8 years. At low grazing pressure where treed, stocking rate remained constant, however, at high grazing pressure where treed, it was reduced after 4 years. The annual liveweight gain increased from 0.37 to 0.45 to 0.51 kg/head.day as grazing pressure was reduced from high to medium to low grazing pressure respectively, and across grazing pressures it decreased from 0.49 where cleared to 0.39 kg/head.day where treed. Liveweight gain per hectare increased under low grazing pressure and declined at medium and high pressures. Body condition scores responded positively to lower grazing pressure and a lack of tree competition to pastures. This treatment combination also produced higher animal sale values. Pasture biomass, basal area and ground cover were all affected negatively by increasing grazing pressure. Conclusions Grazing 25% of autumn pasture improved dry matter production, species composition and land condition, and increased steer growth rates, body condition and their market value. This grazing pressure produced an increasing trend in stocking rates relative to the decline at higher grazing pressures. Higher liveweight gain/ha was produced initially at high grazing pressure (75% utilisation), however, after 4 years animal condition and pastures deteriorated, requiring a reduction in stocking rate to maintain the condition of both the remaining animals and the pastures. Managing tree competition to pastures is necessary to maintain the higher animal production potential. Implications This objective information demonstrates the benefits for cattle, pastures and long-term economic outcomes of managing for conservative grazing pressure and controlling tree competition to pasture in this woodland. Applying these findings can improve beef business outcomes and provide management groups with objective educational resources.

Boreal tree-rings are influenced by temperature up to two years prior to their formation: a trade-off between growth and reproduction?

Large spatial and between-tree variability has recently been observed in the response of boreal forests to ongoing climate change, spanning from growth stimulation by increasing temperatures to drought limitation. To predict future responses of boreal forests, it is necessary to disentangle the drivers modulating the temperature-growth interaction. To address this issue, we established two inventory plots (at a treeline and closed-canopy forest) and assembled site chronologies in Picea glauca stands at the transition between boreal forest and tundra in Northern Quebec, Canada. In addition to site chronologies, we established a set of chronologies containing, for each year, exclusive subsets of tree-rings with specific cambial age (young/old), tree dimensions (small/large) and tree social status (dominant/suppressed). All chronologies were correlated with climatic data to identify the course of climatic conditions driving variability in tree-ring widths. Our results show that the growth of P. glauca correlates significantly with summer temperature in tree-ring formation years and during up to two prior summers. Tree-ring width is positively influenced by summer temperatures in tree-ring formation year and two years prior to tree-ring formation. In addition, climate-growth correlations indicate a negative effect of summer temperature one year before tree-ring formation at the closed-canopy forest site. The pattern of climate-growth correlations is tightly synchronized with previously published patterns of climate-reproduction correlations of P. glauca, suggesting a growth-reproduction trade-off as a possible factor modulating the response of boreal forests to summer temperatures. Climatic signal does not differ between pairs of chronologies based on subsets of cambial ages, stem dimensions or tree competition status at the treeline site. However, the response to summer temperatures one year before tree-ring formation is significant only in mature (old, large and dominant) individuals at the closed-canopy site. The inverse pattern of temperature-growth correlations during a sequence of three years challenges predictions of how boreal forests respond to climate change.

Growth and survival dynamics of partially cut northern hardwood stands as affected by precut competition and spatial distribution of residual trees

Abstract Modelling growth and survival dynamics after partial harvesting must take account of the heterogeneous spatial pattern of residual trees that results from the presence of machinery trails. We used data from 23 permanent sample plots in northern hardwood stands to reconstruct the growing environment of individual trees before and after partial harvesting. We modelled harvest probability, growth response and survival probability using a complementary set of explanatory variables that was assembled to reflect the spatial distribution of trees and skid trails prior to and after harvest. Results showed that the distribution of harvested trees was concentrated in skid trails and in their close vicinity. However, this spatial pattern had no significant effect on either the post-cut basal area increment (BAI) or the survival of residual trees. BAI and survival of individual trees were both mostly related to the competitive environment prior to harvest, while post-cut changes in competitive environment had only a marginal effect on growth and survival dynamics. We conclude that selection cuts did not substantially increase the growth and survival of residual trees, likely because tree removal was mostly concentrated near skid trails, where the negative effects of machinery access were highest.

Intraspecific Competition Affects Crown and Stem Characteristics of Non-Native Quercus rubra L. Stands in Germany

Accurate guidelines for silvicultural management of exotic tree species in Germany are sparse. For example, northern red oak (Quercus rubra L.) is the most commonly planted exotic deciduous tree species in Germany, but its response to varying levels of competition intensity has not yet been adequately explored. Here, we used terrestrial laser scanning to non-destructively examine the responses of stem and crown characteristics of Quercus rubra to intraspecific competition. A total of 100 dominant red oak trees were investigated in ten pure red oak stands, located in five federal states of Germany. The external stem quality characteristics namely stem non-circularity and bark anomalies decreased with increasing tree competition. Also, the crown characteristics crown volume, crown surface area, maximum crown area, crown length, and branch length declined by the degree of individual tree competition. We conclude that individual tree properties can be controlled by competition intensity, resulting in improved timber quality as shown for other tree species.

Plasticity and climatic sensitivity of wood anatomy contribute to performance of eastern Baltic provenances of Scots pine

The efficiency of water use and transport are among the main factors affecting competitiveness, growth, and distribution of trees under warming climate. The phenotypical and genetic plasticity of tree populations is considered as an indicator of their adaptive capacity under changing environment. Climatic changes are expected to affect growth of Scots pine (Pinus sylvestris L.), and selection of reproductive material among the populations suitable for future climates has been advised for sustaining productivity of stands. In this regard, provenance trials can serve as source of comprehensive information about growth plasticity and climate-growth interactions of diverse populations. Quantitative wood anatomy can provide detailed information about xylogenesis and factors affecting it, which are crucial for long-term predictions. Wood anatomy of two top- (Gustrow and Rytel), two low-performing (Eibenstock and Dippoldiswalde), and one local (Kalsnava) provenances of Scots pine from the eastern Baltic region growing in three provenance trials in Latvia was studied using mixed models, accounting for the experimental design, as well as using the time-series approach. Provenance had a significant effect on the studied wood anatomical proxies, indicating genetic adaptation of xylogenesis. The top-performing provenances, which originated from warmer and drier conditions, had tracheids with larger lumens and thinner walls, thus indicating adaptation to water deficit. The top-performing Rytel provenance showed the highest phenotypical plasticity of lumen cross-section area and cell wall thickness of stemwood tracheids. The studied low-performing provenances, which originated from the Orr Mountains, had tracheids with thicker wall and smaller lumens, likely to ensure mechanical durability. The local provenance showed intermediate values of the studied wood anatomical proxies. The effect of provenance on wood anatomical proxies showed some variations among the trials, which differed by continentality, likely due to ecological transfer distance. The studied anatomical proxies were affected by weather conditions prior and during formation of a tree ring, yet these relationships differed by trial and provenance. In general, wood anatomy of earlywood was affected by temperature in the dormant period and beginning of summer, as well as precipitation in the end of the previous vegetation period. Proxies of latewood showed correlation with temperature (negative) and precipitation (positive) in summer, suggesting response to the availability of water. Considering the observed relationships, the top-performing provenances, particularly Rytel, have a high potential to sustain productivity of stands within the region in the future.

Analyzing Spatial Distribution Patterns of European Beech (Fagus sylvatica L.) Regeneration in Dependence of Canopy Openings

The use of natural regeneration techniques is one of the key elements of modern (close-to-nature) forestry. In natural forests, changes in canopy cover, such as the emergence and successive re-closure of canopy gaps are particularly important, as they influence the light availability on the forest floor. Creating canopy gaps of different size is a promising silvicultural tool allowing the regulation of the light availability in managed forests in order to control regeneration composition and development. In this study, we used terrestrial laser scanning data to investigate the relationship between canopy-gap dimensions and emerging natural regeneration along a gradient of management in forests dominated by European beech (Fagus sylvatica, L.). We analyzed the spatial distribution and height of regeneration patches in dependence of gap characteristics. Mean regeneration height decreases progressively from the gap polygon over a transition zone towards the area under the canopy, while the tallest regeneration plants were placed in positions midway between center and gap edge, and not directly in the gap center as we initially assumed. The centers of regeneration patches were not displaced when compared to the associated canopy gap centers, as has been reported in other studies conducted on the northern hemisphere for various tree species. The observed patterns did not depend on management strategies, indicating that regeneration responded equally to naturally created gaps and gaps that were caused by logging. We conclude that establishment and development of shade-tolerant European beech regeneration in forest stands is driven by gap openings, but not necessarily direct radiation. If at all, pronounced direct radiation mainly occurs at the northern edge of large gaps. Neither regeneration patch center, nor regeneration tree height pointed in that direction. Our study suggests that in the investigated beech-dominated forests the effect of increased light availability at the northern edge of a gap is overruled by other factors increasing towards the gap edge, such as increased belowground competition of the overstory trees.

Multifactor relationships between stand structure and soil and water conservation functions of Robinia pseudoacacia L. in the Loess Region.

Ninety-six sample plots were established for a tree census to explore the multifactor relationships between the soil and water conservation functions and the stand structure in a typical black locust (Robinia pseudoacacia L.) plantation in the Caijiachuan watershed of the Loess Plateau, Western Shanxi Province, China. Based on the observational and experimental data, a topography-structure-function model was built using a structural equation modeling (SEM) approach. The latent variables were the topographical factors, horizontal structure, vertical structure, soil and water conservation, and sediment reduction. The results indicated that the horizontal structure of the Robinia pseudoacacia L. forest was the most obvious latent variable, which was expressed in the path coefficient (pc = 0.85) corresponding to the sediment reduction; the stand density and tree competition index were the major drivers of the structure, with path coefficients of −0.96 and −0.92 and influence coefficients of −0.997 and −0.998. These factors are easily regulated. Among these factors the stand density of the arbor layer is recommended to be kept stable within the range from 1600 to 1700 trees/hm2. These relationships showed that reducing the tree competition index and changing the microtopography could effectively enhance the soil and water conservation functions in this ecologically significant loess area.

Methods of Forest Structure Research: a Review

The forest structure generally refers to the configuration and distribution of different plant species and sizes. Investigation and analysis of forest structures help us to understand the history, current status, and future development of forest ecological systems. This paper aims at a systematic summary of the quantitative analysis methods of forest structure. The marked second-order characteristic method has obvious advantages in explaining the relationships among tree species and the dynamic relationship between tree size differentiation and scale. The quantitative analysis method of spatial structure based on the relationships of nearest neighbor trees, compared with traditional non-spatial indices or functions, does not only analyze four important aspects of the forest structure (spatial distribution pattern diversity, species diversity, size diversity, crowding degree diversity), but also demonstrates its strength in elaborating fine-scale spatial stand structure. This nearest-neighbor analytical method also bridges the gap between stand structure parameters and tree competition indices, especially through the multivariate distribution of structural parameters. This nearest-neighbor analytical method provides an in-depth, multi-faceted interpretation of forest structure at different levels. Structure-based forest management has been proposed based on this analytical method of spatial structure, and is a proven way to effectively improve forest quality.

Quantification of Individual Tree Competition Index Taking Chinese-Fir Plantations in Subtropical Low Hilly Area as an Example

Competition among trees is a fundamental interaction process within plant community, which is the theoretical basis of thinning. Plant competitive intensity is generally measured using a competition index (CI) that can be classified into two major categories: distance-independent and distance-dependent. The current study used Cunninghamia lanceolata (Lamb.) Hook as the test subject and used Hegyi's CI (distance-dependent), to quantify individual CI and their relationship with tree diameter at breast height (DBH). Five different criteria were used to select potential competitors for the calculation of CI. Seven basic linear and nonlinear mathematical functions were used to test and quantify the relationships between DBH of the target tree and the individual CI. Results showed that individual CI was negatively correlated with target tree DBH: as DBH increased, competition intensity weakened. The adjusted R2 with five different criteria of selection competitors simulated by seven functions ranged from 0.30 to 0.82. Considering the root mean square error (RMSE), P-value, and adjusted-R2, our results suggested that the best model to simulate the relationship between individual CI and focal tree DBH was power function (CI = 43.98 × DBH-1.08, adjusted R2 = 0.81) and with the Voronoi diagram method as the criteria for selecting competitors. These results can demonstrate a clearer understanding of the spatial structure of forests, and can be used to guide the selection of thinning trees in the process of thinning practice.

Hydrologic Modifications Challenge Bottomland Hardwood Forest Management

AbstractBottomland hardwoods are floodplain forests along rivers and streams throughout the southeastern United States. The interrelations among hydrology, soils, geomorphic landforms, and tree species composition are the foundation of forest management in bottomland hardwoods, and historically their correspondence has allowed for somewhat predictable forest responses based upon the hydrogeomorphic setting. However, extensive hydrologic and geomorphic modifications in floodplains have disrupted these interrelations and, on many sites, have created novel disturbance regimes resulting in unpredictable forest responses. Reduced or altered timing of surface flooding and groundwater declines are common in the region and have favored increases in stem densities, particularly of species less tolerant of flooding and more tolerant of shade. In these highly modified systems, more process-level understanding of floodplain hydrology, soil moisture dynamics, interspecific tree competition, and regeneration is needed to develop more effective management prescriptions and for forestry to be represented in integrated water-resource management decisions.

Competition of key tree species with selective cutting at different intensities in broadleaved-korean pine mixed forest in the Changbai Mountain, China.

Selective cutting changes tree species composition and stand structure, modifies tree competition intensity in the stand, with consequences on tree growth and population dynamics. Key tree species play a crucial role in maintaining community structure and ecosystem function. To clarify the competitive characteristics of three key species (Pinus koraiensis, Tilia amurensis, and Fraxinus mandshurica, which accounted for about 70% of growing stock) of typical forest types in broadleaved-Korean pine mixed forest, Hegyi competition index was used in four permanent 1-hm2 plots, i.e. virgin forest (VF), low-intensity cutting forest (LCF), moderate-intensity cutting forest (MCF) and high-intensity cutting forest (HCF). Compared with VF, the stand density and mean DBH of big trees (DBH≥20 cm) significantly decreased in MCF and HCF, but the stand density of young trees (2 cm≤DBH<10 cm) increased, but no significant changes of stand or DBH in LCF. In all the four forest types, individual competition index (CI) of the three species decreased with increasing DBH. There was a power function relation between CI and DBH. CI curve turned to be gentle at DBH reaching 20 cm, and this rule was not affected by selective cutting. In VF, LCF and HCF, most of competition pressure of young trees of all key species derived from other tree species in secondary canopy or understory, but the competition pressure of small trees (10 cm≤DBH<20 cm) and big trees of P. koraiensis originated mainly from intra-species and other species in secondary canopy or understory. The competition stress of F. mandshurica was mainly affected by P. koraiensis and other species in secondary canopy or understory. T. amurensis was mainly affected by intra-species and P. koraiensis. Betula platyphylla contributed most of the competition stress (over 50%) to all three key species in HCF. Our results suggested that removing the trees which are in secondary canopy or understory but suppress the three key trees species intensively would be beneficial to the regeneration and growth of key species before tree DBH exceeds 20 cm. We proposed to regulate the density of key tree species in overstory to cultivate large diameter timber according to the competition among the key trees species. Once tree DBH exceeds 20 cm, tree growth would not be affected by competition. This study had guiding significance for the cultivation of key tree species and rapid recovery of natural forests after selective cutting.

Competition and climate influence growth of black spruce in western boreal forests

The effects of competition and climate on the growth of black spruce (Picea mariana (Mill.) BSP.) were examined using long term measurement data collected across the western Canadian boreal forest. Non-linear mixed effect models were developed and used to examine tree growth responses to competition and climate across the range of upland forest types where black spruce is found in this region including: spruce/fir, deciduous and pine forest types. Basal area of competitor trees larger than the subject tree, and at least one of the following climate variables: frost free period, mean annual precipitation and mean annual temperature, were selected as predictors of diameter and height growth. Spruce/fir competition had a stronger negative effect on the growth of black spruce than pine competition. However, deciduous competition had a positive effect on diameter and height growth of black spruce. Growth was generally sensitive to changes in climate across the three forest types. Increases in temperature (except in pine forest stand) and reductions in precipitation (except in deciduous forest stand) lead to increases in diameter and height growth; however, our model also predicted that increasing temperature when pine stand densities are high will lead to a reduction in growth. These results indicated that forest types and competition will determine the impact of climate on the growth of black spruce trees. Climate sensitive growth functions developed for black spruce in this study will be useful in modeling and managing pure and mixed stands of black spruce across the western boreal forest.

Analysing tree–neighbourhood interactions in ecotones of montane evergreen and deciduous forests in China

AbstractQuestionsDo neighbourhood interactions in trees result in discernible spatial and phylogenetic signals among their nearest neighbours in a montane evergreen and deciduous forest? Are neighbourhood patterns of different species associated with their abundances and traits?LocationBadagongshan National Nature Reserve, Hunan, China.MethodsWe used four nearest‐neighbour indices: average spatial distance to the nearest conspecific cohort‐mate (SNCC), average spatial distance to the nearest conspecific adult (SNCA), average phylogenetic distance to the nearest heterospecific cohort‐mate (PNHC), and average phylogenetic distance to the nearest heterospecific adult (PNHA). We focused on 77 abundant species in a 25‐ha plot. We evaluated whether the changes in SNCC, PNHC, SNCA and PNHA deviated significantly from expected values of spatially random mortality during two life stage transitions.ResultsFor more than one‐fourth of the tested species, the changes in SNCC and PNHC deviated significantly from the expectations of random mortality. Nearly one‐third of the tested species showed greater SNCC at the juvenile stage than expected of random mortality, which may be mainly explained by conspecific tree competition for space and resources. Approximately 60% and 40% of the tested species showed significantly smaller SNCA and PNHA than expected by random mortality, respectively, which suggests the effects of facilitation and habitat filtering. In addition, abundant and taller species were suppressed by conspecific cohorts to a greater degree than locally rarer and shorter species. Deciduous species were more negatively influenced by conspecifics and closely related heterospecifics than evergreen species.ConclusionsOur study focused on long‐term responses of individual species to their conspecific and phylogenetic neighbourhoods and revealed the associations between species traits and neighbourhood patterns. Such trait–neighbourhood interactions provide support for stabilizing mechanisms driving species coexistence in this species‐rich subtropical forest.

Evidence of a seasonal trade-off between growth and starch storage in declining beeches: assessment through stem radial increment, non-structural carbohydrates and intra-ring δ13C.

Forest decline is reported in recent decades all over the world. However, developing a clear vision of the associated tree dysfunctioning is still a challenge for plant physiologists. In this study, our aim was to examine the seasonal carbon adjustments of beech trees in the case of a long-term drought-induced decline. We compared healthy and declining trees in terms of stem radial growth, phloem sugar content and δ13C, together with xylem carbohydrates and intra-ring δ13C patterns. The radial growth of declining trees was clearly reduced by lower growth rates and shorter growing season length (44 days compared with healthy trees). The soluble sugar content was higher in the xylem of declining trees compared with the healthy ones, but similar in the phloem except at the end of their growth. Declining trees increased their levels of xylem starch content from budburst until the date of maximal growth rate. These reserve dynamics revealed an early trade-off between radial growth and starch storage that might be the result of an active or passive process. For declining trees, the slight decrease of intra-ring cellulose δ13C pattern during the early growing season was attributed to the synthesis of 13C enriched starch. For healthy trees, δ13C patterns were characterized by a progressive 13C increase along the ring, attributed to increased water-use efficiency (WUE) in response to decreased water availability. Individual variations of the crown area were negatively correlated to the intra-ring δ13C amplitude, which was ascribed to variations in canopy WUE and resource competition for healthy trees and partly to variations in the amount of reserves accumulated during spring for declining ones. Our study highlights the carbon physiological adjustment of declining trees towards reducing spring growth while storing starch, which can be reflected in the individual intra-ring cellulose δ13C patterns.

Influence of contrasting stocking densities on the dynamics of above-ground biomass and wood density of Eucalyptus benthamii, Eucalyptus dunnii, and Eucalyptus grandis for bioenergy in Uruguay

Short-rotation plantations have been suggested to develop renewable energies in South America. Our study evaluated the biomass production of Eucalyptus benthamii, E. dunnii and E. grandis plantations at stocking densities of 2220, 3330, 4440 and 6660 trees ha−1 in the North (Tacuarembó) and West (Paysandú) of Uruguay, over a 76-month period. The species survival was not related to planting density, and the highest mortality rates occurred at Tacuarembó. The effects of tree competition were more evident for E. grandis. At Tacuarembó site, the average survival of the species were: 57, 57 and 46% at age 76 months for E. benthamii, E. dunnii and E. grandis, respectively. Survival rates were similar for the different planting densities, with ranges between 50 and 57%. At Paysandú site, the average survival of the species were: 83, 86 and 75% for E. benthamii, E. dunnii and E. grandis, respectively, with a range from 78 to 84% for different planting densities. Weather conditions (temperature and rainfall) were similar at both sites throughout the evaluation period. Wood density and individual and total biomass weight (kg tree−1 and Mg ha−1, respectively) were evaluated sampling destructively 2592 trees and logarithmic biomass equations were developed using the diameter at breast height (cm) and total height (m) of the trees. Wood density increased with the age of the crop only at Paysandú. On average, wood density increased from 0.405 g cm−3 at age 18 months to 0.497 g cm−3 at age 76 months. The highest wood density was observed at 76 months on E. benthamii (0.413 and 0.431 g cm−3) and E. dunnii (0.496 and 0.539 g cm−3) at Tacuarembó and Paysandú, respectively. Age had no effect on the wood density at Tacuarembó. The stem biomass showed an inversely-proportional relationship with the planting density. Individual stem biomass was higher for E. grandis (81 and 74 kg tree−1 on average at Tacuarembó and Paysandú, respectively) than for the other species. The highest biomass per hectare was achieved for the highest planting density, specifically for E. dunnii (193 Mg ha−1) and E. grandis (203 Mg ha−1) at Paysandú and for E. dunnii (157 Mg ha−1) at Tacuarembó. At both locations, the effects of contrasting planting densities on biomass production increased throughout the rotation.

Andropogon virginicus L.

<i>Andropogon virginicus</i> L.