Large-scale structural variation and long-term growth dynamics of Juniperus thurifera trees in a managed woodland in Soria, central Spain

? Juniper woodlands have a great conservational and productive importance in Spain. However, basic traits of their structural variation and growth dynamics are unknown.

? To characterize the structural variation and assess both height and radial growth patterns, we sampled and measured stem cross-sections every 0.5 m in height from 107 Spanish juniper (Juniperus thurifera L.) trees, which were selected based on a 350 m regular grid over a whole woodland of 3 300 ha in area. We used dendroecological techniques and spatial analysis to study tree structure and growth.

? Structural traits such as bole diameter and height were mainly determined by tree life-related parameters such as tree age and growth rate variation, while abiotic factors such as altitude played a minor role in determining structural variation. Over the last 300 years, juniper establishment has been continuous in time but discontinuous in space. Large-scale spatial heterogeneity of tree establishment, and the presence of an early growth suppression and abrupt growth changes in junipers > 100 years old are consequences of an intense management that almost ceased in the late 19th century.

? Abrupt growth releases and suppressions were synchronic among different age classes, and coincided with documentary records on livestock decrease and key historical changes in land use. Growth patterns suggest that juniper is a brow sing-tolerant species that is able to survive large periods of intense browsing pressure.

     

Geographically structured and temporally unstable growth responses of Juniperus thurifera to recent climate variability in the Iberian Peninsula

Geographically structured tree-ring networks are needed to fully understand the spatiotemporal variability in climatic sensitiveness of trees and to study their future responses to global warming. We aim to identify the spatially constrained structure of radial-growth patterns of the Spanish juniper (Juniperus thurifera L.) and to assess whether their climate–growth responses were unstable during the late twentieth century. Tree–ring width chronologies were built for 13?J. thurifera stands in Spain using dendrochronological methods and related to monthly climatic data. Sites were grouped according to their growth patterns using hierarchical cluster analysis. The relationships among geographical, climatic and stand features and their influence on radial growth were evaluated using redundancy analysis. The climate–growth relationships and their temporal stability were assessed using Pearson’s and moving bootstrapped correlations, respectively. Stands formed three geographical groups according to their high-frequency growth variation: North West and Centre, North East and South East. We found that J. thurifera radial-growth patterns depended on geographical and climatic factors, but not on the stand structure, and responded to a northwest–southeast gradient of decreasing rainfall and influence of Atlantic Westerlies and Mediterranean cyclonic activity. The positive response to June precipitation was unstable during the late twentieth century and started earlier in populations from western mesic sites than in eastern xeric sites. This pattern may be related to either decreasing water availability in western than in eastern sites or the resilience of J. thurifera growth from xeric sites in response to the increasing summer aridity.     

Gender, neighboring competition and habitat effects on the stem growth in dioecious Fraxinus mandshurica trees in a northern temperate forest

? This study analyses the sex ratios, and the effect of gender, neighboring competition and habitat factors on the stem growth of dioecious F. mandshurica trees in the Changbai Mountains of North-Eastern China.

? The general sex ratio of the flowering trees did not significantly deviate from 1:1. The sex ratio varied among age classes. A female-biased sex ratio was found in the age classes of 65–75 years and 85–95 years, but departure from 1:1 was not significant in other age classes.

? An analysis of variance revealed that the growth rate was extremely sensitive to gender and tree size, as shown by a highly significant gender-related and size-dependent effect. Male trees had a faster stem growth rate than female trees, and the assumption is that trade-offs between reproductive and vegetative processes can induce the difference in stem growth rate between genders.

? Stem growth rates are positively affected by soil moisture and tree size for both genders at all considered neighborhood distances. The growth rates of female trees were negatively affected by neighboring competition from other female trees at all neighborhood distances between 1 and 10 m, but were not significantly affected by male trees. The growth rates of male trees were negatively affected by neighboring competition from male trees for neighborhood distances between 3 and 10 m, but were not significantly affected by female trees.

? These results suggest that intrasexual competition (male-male competition and female-female competition) may cause difference in stem growth for both genders. Intersexual differences in sensitivity to neighboring plants are considered to be an important factor driving gender-specific growth patterns.

     

The decreased competition in expanding versus mature juniper woodlands is counteracted by adverse climatic effects on growth

World-wide deforestation is being reversed in Mediterranean continental areas, where abandonment of traditional practises favours the expansion of valuable habitats, like Juniperus thurifera woodlands. We hypothesised that pre-existing trees facilitate establishment in expanding woodlands, whereas in mature woodlands, competition leads to patch disaggregation. We compared the imprint of these processes on growth, demographic and spatial structure of expanding and mature J. thurifera woodlands. We selected plots where we geopositioned, aged and quantified the morphological characteristics of all trees. In the mature woodland, trees arranged in clumps and randomly in the expanding woodland. Competition negatively affected growth, was greater in the mature woodland and led to disaggregation of juvenile clumps. Differences in growth between the mature and the expanding woodland disappeared in climatically unfavourable years, suggesting that adverse climate constrains growth more in expanding than in mature woodlands. We suggest that change in the dispersal agents and a decrease of facilitation underlay differences in spatial patters between the expanding and the mature woodland. Observed effective recruitment in less than 30?years into the expanding woodland evidenced that propagule arrival and sapling survival do not constrain woodland expansion. Furthermore, growth of juveniles established in these new areas is favoured by reduced intra-specific competition. However, we expect growth in expanding woodlands to be negatively impacted by climate change. We conclude that under current global change scenario, conservation of J. thurifera woodlands is favoured by changes in land use, but greater frequency and severity of drier than usual episodes could hamper natural reforestation.     

Disruption of <Emphasis Type="Italic">Juniperus thurifera</Emphasis> woodland structure in its northwestern geographical range: potential drivers and limiting factors

Enhancement of Juniperus thurifera recruitment and colonisation by oak and pine species has been related at the local level to changes in livestock pressure. We used forest inventory data from Castilla y León Autonomous Region (Central Spain), an area comprising 34% of the world range of J. thurifera, to assess whether this process is occurring at a larger scale. We compared tree composition and density in a set of 659 permanent plots over a 10-year period. Logistic models and redundancy analysis were used to assess the effect on this process of parameters such as livestock pressure, propagule availability and climatic conditions. Between 1992 and 2002, juniper woodlands became denser (1.31% juniper stem year⁻¹) and tree diversity increased due to rapid colonisation by oaks and pines (2.21% occupied plots year⁻¹). In addition, the presence of juniper increased in other types of forests at a moderate rate (0.6% y⁻¹). Thus, we observed both a disruption of the borders between current forest types and a generalised increase in α-diversity of tree species. The seed source was the main factor explaining colonisation rate, suggesting that the pace of colonisation is critically constrained by the spatial configuration of the landscape and the local propagule availability of the colonising species. If the current colonisation trends continue, monospecific juniper woodlands will become very scarce by the end of the twenty-first century.     

Stand structure, competition and growth of Scots pine (Pinus sylvestris L.) in a Mediterranean mountainous environment

The relationship between competition and tree growth was studied in four stands of Pinus sylvestris L. occurring in a continental Mediterranean mountain area (in the Guadarrama range, Spain), i.e., an uneven-aged stand, a stand with oak (Quercus pyrenaica Willd.) understorey, a plantation, and a mature even-aged stand. Competition was measured by a simple size-ratio distance-independent index and was negatively associated with tree diameter. This negative association was stronger in the uneven-aged, plantation and mature even-aged stands than in the stand with oak understorey. Competition was also negatively associated with current diameter increment. This relationship was moderately strong in the mature even-aged stand and weak in the uneven-aged stand and the plantation. In the uneven-aged and the mature even-aged stands, a weakly significant relationship was found between diameter growth and tree size, whereas these parameters were not associated in the stand with oak understorey. The competition index provided a better prediction of growth rate than the alternative use of diameter. Both diameter and basal area growth were greater in the uneven-aged than in the even-aged stands.     

基于竞争指数的杉木林分生长可视化模拟研究

以我国南方速生丰产林——杉木纯林为研究对象,采用改进过的Hegyi的简单竞争指数模型对杉木纯林间的竞争强度进行定量分析,并建立单木生长模型,再通过径阶模型将其应用于全林分,将单木生长模型研究扩展到全林分生长模型研究。同时,通过可视化模拟技术,为更加逼真的恢复和重建林分空间结构提供理论依据。     

Modelling growth behaviour in monoculture in subtropical eastern Australia of two eucalypt species that differ in shade tolerance

Growth behaviour of the relatively shade-intolerant species Eucalyptus dunnii and the rather more shade-tolerant Corymbia citriodora was considered during several growth periods over 8–14 years of age in experimental plantation monoculture in subtropical New South Wales. Larger trees dominated growth in any stand; this was consistent with the principle that inter-tree competition in such forests is for light and is asymmetric in that larger trees shade smaller trees but the reverse does not occur. Once competition was established, competition-induced mortality of C. citriodora was less than that of E. dunnii. A model relating tree diameter growth rate to tree diameter, competitive position in the stand and stand basal area was developed. The model showed that smaller, hence partially shaded, trees in stands of C. citriodora were better able to maintain their growth rates than equivalent trees in stands of E. dunnii. It is hypothesised that this reflects the greater shade tolerance of C. citriodora. This pattern of growth led to a more even distribution of tree size classes across the stand in the more shade-tolerant species.     

Growth responses of West-Mediterranean Pinus nigra to climate change are modulated by competition and productivity: Past trends and future perspectives

Positive and negative effects of climate change on forest growth have been observed in different parts of the world. However, much is still unknown about how forest structure and productivity might affect climate-growth relationships in the future. We examined the effects of climate, site quality, and competition on tree basal area growth of black pine (Pinus nigra Arn.) between 1964 and 2005 in 21 sites in the Iberian Peninsula. We used a new approach to simultaneously account for climate-growth relationships, inter-annual growth variability, and stand structural changes, by fitting a linear mixed effects model (LMEM) for basal area increments (BAI) using climate data, tree-ring chronologies, and repeated forest inventory data. This approach showed the potential to improve our understanding of climate effects on tree growth and to include climate in empirical forest growth models. We used the LMEM to make projections of BAI growth under two CO₂ emission scenarios and two global circulation models (GCM). The main climate drivers for growth were precipitation from previous autumn to summer and winter temperature with a positive effect, and temperature in spring-summer which had a negative effect. Tree response to climate was modulated by stand conditions, tree competition, and productivity. The more productive stands showed greater ability to either maintain or increase growth at warmer spring-summer temperatures under different levels of autumn-summer precipitation. Growth projections showed important regional differences. In general, growth under future climate is predicted to decrease although moderate growth increases might be expected in the northern region for highly and moderately productive stands.     

Intensity and timing of warming and drought differentially affect growth patterns of co-occurring Mediterranean tree species

Climate change involves warmer temperatures, altered precipitation patterns, increased climatic variability and, in Mediterranean regions, increased frequency and severity of droughts. Tree species may show different growth responses to these components of climatic change, which may trigger changes in forest composition and dominance. We assessed the influence of recent climatic changes on secondary growth of mature trees from four species co-occurring in a Mediterranean continental forest: Quercus ilex, Quercus faginea, Pinus nigra and Juniperus thurifera. We used dendrochronology to relate radial-growth variables [earlywood and latewood widths, basal area increment (BAI)] to annual and seasonal climatic variables for the period 1977–2007. Our results showed that Q. faginea BAI has declined, whereas J. thurifera BAI has increased over time while Q. ilex and P. nigra have maintained their growth rates. Growth was mainly favored by higher precipitations and tree size for all species. Reduced growth during extremely dry years was observed for all study species, but all of them except Q. faginea recovered their growth levels 2 years after drought. Our findings illustrate how the effects of climatic changes on growth should include analyses of seasonal climatic trends and extreme events such as severe droughts. We conclude that the seasonal timing of warming and precipitation alterations leading to drought events caused contrasting effects on growth of co-occurring Mediterranean tree species, compromising their future coexistence.     

Factors driving growth responses to drought in Mediterranean forests

We lack information regarding the main factors driving growth responses to drought in tree species with different vulnerability against this stressor and considering sites with contrasting climatic conditions. In this paper, we identify the main drivers controlling growth response to a multi-scalar drought index (Standardized Precipitation Index, SPI) in eight tree species (Abies alba, Pinus halepensis, Quercus faginea, Pinus sylvestris, Quercus ilex, Pinus pinea, Pinus nigra, Juniperus thurifera). We sampled forests growing across a pronounced climatic gradient under Mediterranean conditions in north-eastern Spain. To summarize the patterns of growth responses to drought, we used principal component analysis (PCA). To determine the main factors affecting growth responses to drought, correlation and regression analyses were carried out using a set of abiotic (climate, topography, soil type) and biotic (Normalized Difference Vegetation Index, Enhanced Vegetation Index, tree-ring width, diameter at breast height) predictors and the PCs loadings as response variables. The PCA analysis detected two patterns of growth responses to drought corresponding to xeric and mesic sites, respectively. The regression analyses indicated that growth responses to drought in xeric forests were mainly driven by the annual precipitation, while in mesic sites the annual water balance was the most important driver. The management of Mediterranean forests under the forecasted warmer and drier conditions should focus on the main local factors modulating the negative impacts of drought on tree growth in xeric and mesic sites.     

杉木人工林单木断面积生长动态模拟

应用面板数据固定效应模型对杉木人工林单木断面积生长规律进行模拟,在模拟过程中,将胸径(DBH)和活冠比例(LCR)作为自变量,又分别加入密度因子和不同类型的竞争指数,同时引入立地条件和林龄效应来解释单木断面积生长过程中的异质性。结果表明:虽然密度因子与竞争指数有较强的相关性,但是在单木断面积生长中都具有不可忽略的重要影响。立地条件与林龄对单木断面积的拟合偏差在不同的林分密度下略有不同,随立地指数或者林龄增加,其对平均单木断面积拟合偏差的影响也增大。     

Spatial point-pattern analysis for detecting density-dependent competition in a boreal chronosequence of Alberta

In the boreal forest of Alberta, fire and wind often open gaps in the canopy where late-successional species can establish and over time cause a shift in the species distribution from deciduous (e.g., trembling aspen) dominated to mixedwood, to shade-tolerant conifer (e.g., white spruce) dominated stands. This study attempted to understand the change of density-dependent competition in a boreal chronosequence and the role of tree competition in affecting stand structure and mortality. Four 1-ha stem-mapped plots were established to represent a chronosequence comprised of aspen dominated, mixedwood, and spruce dominated stands in Alberta. Second order spatial point-pattern analysis using Ripley's K(t) function showed that intraspecific competition is a prevailing force causing conspecific tree mortality and thus shaping the stand structure. The results of bivariate K(t) function analysis did not reveal sufficient evidence of interspecific competition. This suggested that competitive interaction among heterospecific trees was not strong enough to cause significant tree mortality, but the analysis of marked correlation function revealed that interspecific competition could have a negative impact on tree growth. This study highlights the importance of density-dependent competition in understanding stand dynamics of boreal forests over succession.     

Tree morphology responds to neighbourhood competition and slope in species-rich forests of subtropical China

Trees are able to respond to their local biotic and abiotic environment with morphological adjustments which improve resource acquisition and, thus, growth. In forests, light is broadly recognised as one of the major factors determining growth, and morphological responses comprise changes in crown architecture and stem stature. On sloping terrain, the interplay of phototropism and gravitropism may further affect morphological growth characteristics. However, different tree species are expected to show species-specific responses. In this study, we analysed three growth characteristics of tree individuals belonging to four species of two functional groups (evergreen: Schima superba, Castanopsis eyrei, deciduous: Quercus serrata var. brevipetiolata, Castanea henryi) in a species-rich Chinese subtropical forest. Crown projection area, relative crown displacement and stem inclination were related to biotic (local species richness, functional richness, competition, stand age) and abiotic (slope aspect and inclination, soil depth) variables in the local neighbourhood of the tree individuals. We hypothesised that (i) there are species-specific differences in the morphological response of crown architecture and stem stature and (ii) that crown size and asymmetry as well as stem inclination are influenced by both, biotic and abiotic factors. In contrast to our expectations we were unable to reveal any species-specific differences in any of the three growth characteristics. The results of mixed effects models showed that crown area was mainly affected by the target tree's dbh and biotic variables related to neighbours (competition, functional diversity), whereas stem inclination was mainly influenced by slope. Relative crown displacement was influenced by both, biotic and abiotic variables. We conclude that growth responses resulting in crown displacement and stem inclination seem to be an important mechanism to ameliorate foraging for light in our study area, but that these responses appear to be species-independent. The interplay of stem inclination and crown displacement allows for a plastic response of tree individuals in biotically and abiotically heterogeneous environments. Our results indicate that forest management in this region should focus on functionally diverse stands which are promoting crown area positively resulting in increased growth rates of individual trees.     

Tests of age-independent competition indices for individual trees in natural hardwood stands

Recent studies have demonstrated that simple indices of competition that incorporate competitor size and inter-tree distances generally perform as well in predicting individual tree growth as more complex approaches of assessing spatial pattern. A major limitation of diameter-distance indices, however, is that their numerical values decrease in a given stand over time even when the stocking level remains constant. In this paper two modifications are proposed which make the index essentially independent of age, thus necessitating only one competition-growth regression for each species on a given site and allowing comparisons between different stands. Tests of several index designs in three evenaged temperate hardwood stands indicated that the correlation between competition and growth is optimal over a wide range of competition radii and that the inclusion of inter-tree distances is of little value despite considerable small-scale variability in the stocking level around individual trees. Highest correlations were obtained when competitors were defined to be only those trees of equal or higher crown class than the subject tree. In these hardwood stands a comparison of the size of a subject tree to that of the competitors was necessary for reasonable correlations with growth. These correlations varied greatly among species even within a single stand and appear to be related to the shade tolerance of the species. For general use the index (ΣD_j)/D_i is recommended, where D_j is the diameter of competitor j and D_i is the diameter of subject tree i. This index can be computed rapidly in the field and does not require mapping of stem positions.     

Explaining growth of individual trees: Light interception and efficiency of light use by Eucalyptus at four sites in Brazil

The growth of wood in trees and forests depends on the acquisition of resources (light, water, and nutrients), the efficiency of using resources for photosynthesis, and subsequent partitioning to woody tissues. Patterns of efficiency over time for individual trees, or between trees at one time, result from changes in rates photosynthesis and shifts in the relative partitioning to wood. We measured the production ecology (stem growth, light interception, and light use efficiency) to explain patterns of growth among trees within plots through stand development, and tested three hypotheses: (1) dominant trees have higher light use efficiency than subordinate trees; (2) lower variation in the size distribution of trees within plots allows higher light use efficiency; and (3) uniform stand structure and high light use efficiency reduce the age-related decline in tree growth. The experiment used clonal plantations of Eucalyptus at four locations in eastern Brazil. Irrigation and fertilization treatments ensured the major resource limitation for tree growth would be light supply. The influence of variation in the sizes of trees within plots was tested by comparing plots with all trees planted in a single day (uniform treatment) with plots where planting was spread over 80 days (heterogeneous treatment). Light interception per tree was simulated with the MAESTRA model. Across sites, treatments and whole-rotation stand development, dominant trees showed higher rates of stem growth, light interception, and light use efficiency than subordinate trees (supporting the first hypothesis). For example, dominant trees (80th percentile rank) at the end of the rotation grew four-times faster than suppressed trees (20th percentile rank), as a result of 2.1-fold greater light interception, and 1.8-fold greater stem growth per unit of light interception. In some cases, greater variation among tree sizes within plots led to lower efficiency of light use by average-size trees, providing mixed evidence for the second hypothesis. Greater uniformity of sizes of trees within plots did not substantially mitigate the decline in stem growth from mid-rotation to the end of the rotation, refuting the third hypothesis. The high efficiency of dominant trees underscores the marginal contribution of subordinate trees to total stand growth, and should spur further work on thinning to increase growth and lengthen rotations for dominant trees.     

Evaluating tree competition indices as predictors of basal area increment in western Montana forests

Fire hazard reduction treatments are commonly applied to mixed-species coniferous forests in western Montana, USA, to modify fuels structures and alter the competitive environments of individual trees. An improved understanding of how competition can be measured and how it conditions individual tree growth is needed for projecting the development of these forests, with and without treatment. Numerous studies have evaluated how competition affects tree growth and many indices have been developed to quantify the competition an individual tree experiences. These studies suggest that no single competition index or a single class of indices is universally superior; indices perform differently according to forest type and forest conditions. We chose several widely used distance-independent and distance-dependent competition indices, and also derived anisotropic distance-dependent indices from estimates of light interception by tree crowns. We evaluated the effectiveness of these competition measures for predicting basal area increment (BAI) of Pinus ponderosa, Pseudotsuga menziesii, and Larix occidentalis in western Montana. The best distance-dependent competition indices explained a larger proportion of growth variation than the best distance-independent indices (64% vs. 56%). This result indicates that competition is an important growth determinant in these forests and that competition varies locally, with variable tree densities and relatively complex stand structures creating heterogeneous neighborhood conditions. Competition indices derived from light interception models were only weakly correlated with other indices and performed poorly in terms of predicting tree growth. This result accords with previous observations that competition for light is not the primarily growth limitation for trees in the semi-arid conditions of western Montana. More sophisticated light availability models could be used to better assess variability in light interception and its marginal contribution to predictive accuracy of radial tree growth. Diameter and distance-dependent BAI models were developed for growth prediction at the species level and for all species combined.     

Development of GIS-based models to predict plant community structure in relation to western juniper establishment

During the past 130 years, western juniper (Juniperus occidentalis Hook.) has rapidly expanded into sagebrush communities impacting plant structure and function. Predictions of intercanopy shrub density and cover with woodland development can provide insight into site condition and ecological resiliency in western juniper encroached areas. The purpose of this study is to model shrub density and cover in relation to juniper competition across a heterogeneous landscape in a southeast Oregon watershed. Independent variables included in model development were derived from high-resolution color imagery, a 10 m digital elevation model, and field-based vegetation and soil moisture measurements. Juniper cover and three intercanopy classes, representing different levels of juniper competition, were delineated from 1:5000 scale color (RGB) imagery. A competition index was produced from the classified image representing a gradient of potential competition with juniper. An integrated moisture index was generated representing variability in soil moisture due to hillslope solar radiation, curvature of the landscape and flow path direction and flow accumulation (flow path density). Plant density, plant cover, and percent soil moisture (gravimetric) were measured from 10 m × 10 m plots located throughout the watershed study area. Multiple stepwise regression produced the best-fit model for predicting plant density or cover for mountain big sagebrush (Artemesia tridentata ssp. vaseyana Beetle), bluebunch wheatgrass (Pseudoroegneria spicata (Pursh)), and all shrub species combined. The prediction of total shrub density was significantly greater than expected by chance (p < 0.001), with a 0.865 coefficient of determination. Total shrub cover also had a high correlation between observed values and environmental gradients (R² = 0.773). These data can be used by land managers to assess the change in plant structure with increasing juniper canopy cover, a measure important for monitoring impacts from juniper woodland establishment.     

Evaluation of 10-year temporal and spatial variability in structure and growth across contrasting commercial thinning treatments in spruce-fir forests of northern Maine,USA

Key message

Detailed measures of growth pattern and structural heterogeneity applied in this study helped to quantify the immediate effects of various thinning regimes on forest structure and the resulting alterations in tree size as well as observed longer term stand dynamics.

Context

Forest management, stand structure, and tree growth are highly inter-correlated. Prior analyses, however, have resulted in mixed outcomes with limited success in revealing ecological mechanisms.

Aims

The study aimed at evaluating the relationship between forest structure and stand dynamics by applying several sophisticated measures of growth pattern and structural heterogeneity.

Methods

Data from a controlled and fully stem-mapped commercial thinning experiment with seven contrasting treatments including a non-thinned control at six locations across the Acadian Forest of Maine, USA, was used. Stand-level attributes examined included tree size and growth heterogeneity, spatial tree distribution, and growth dominance.

Results

Thinning generally reduced stand structural heterogeneity compared to the non-thinned control. In addition, the spatial arrangement of trees changed from fully random (non-thinned control) to a more clustered (removal of dominant and co-dominant individuals) or regular distribution (removal of intermediate and suppressed individuals). Overall, stand growth exhibited increasing (non-thinned control, removal of intermediate and suppressed individuals) or decreasing growth dominance of large trees (removal of co-dominant competitors). Forwarder trails increased basal area growth of individual trees up to a distance from the trail of approximately 5 m.

Conclusion

Findings of this study validate an earlier insight according to which interactions between management practices, forest structure, and tree growth form a permanent feedback loop.

     

Interannual variation in competitive interactions from natural and anthropogenic disturbances in a temperate forest tree species: Implications for ecological interpretation

Competition is a major determinant of plant growth and is often used in studies of tree growth and species coexistence. However, these approaches are usually temporally static, i.e., assessed at a single point or period in time. While constantly changing forest conditions due to natural and human-induced disturbances potentially alter competition among individuals, static approaches cannot qualify the temporal variability of competitive interactions. Here we present a longitudinal analysis of competitive interactions among trees and discuss the implication of our results for ecological interpretation.Spatially-explicit tree growth data were obtained from 18 study plots (0.4 ha each) in sugar maple (Acer saccharum Marsh.) stands in Quebec, Canada. During the studied period (1980-2003), these stands had been disturbed by insect outbreaks (forest tent caterpillar, Malacosoma disstria Hubner) and by commercial partial harvest. We analyzed radial growth rates (outcome of competition) on an annual basis and as a function of tree biology (bole diameter, crown position), competition (above- and belowground competition from neighbours) and environmental conditions (light availability, harvest disturbance).Competitive interactions changed throughout the studied period. Canopy disturbance from partial harvest interacted with defoliators and influenced competition symmetry by favoring smaller trees.Competitive interactions seemed to have switched from below- to above-ground following canopy recovery after harvest. Release from competition due to partial harvest increase neighbourhood size (radius of effective competition) and enhanced the competitive pressure from larger individuals.The temporal variability in parameter estimates may be used for setting confidence intervals on competitive success (growth rates), thereby yielding a more robust basis for ecological interpretation. Our results also show that temporal variability in competitive interactions could contribute to the maintenance of high tree species diversity and structural complexity in some ecosystems by temporally altering species-specific responses to environmental change and disturbance.