Diversity of tree species and structure have the potential to increase various forest ecosystem services such as stress resistance, biodiversity, and productivity. Recent studies indicate the ...relevance and potential of both diversity of tree species and diversity of stand structure. Here, we analyze the effect of stand structure on the stand-level behavior of even-aged stands of Norway spruce (Picea abies (L.) H. Karst.). Drawing from 11 long-term thinning experiments encompassing 77 plots and 425 surveys, we analyzed the effect of tree size and growth frequency distributions on (i) the self-thinning line, (ii) the Eichhorn rule and yield level, and (iii) the stand density-growth relationship. We revealed an optimum relationship between the inequality of tree size and growth within forest stands and the maximum stand density, standing volume, total yield, and stand density-growth relationship. Highest stand density, standing volume, total volume yield, and stand volume growth occurred at moderate structural diversity, in terms of the inequality of tree size and growth. All else being equal, strongly homogeneous, or very heterogeneous stand structures were found to be suboptimal in terms of growth and yield. Our findings emphasize that incorporating information about tree size frequency distributions can greatly improve upon classical models of stand dynamics, even for even-aged spruce monocultures. The profound influence of stand structure on stand growth and yield, even when the classical stand attributes are analogous, warrants further scientific attention; e.g., neglecting differences in stand structure can lead to skewed interpretations of growth reactions to density regulation or species mixing. From a forest management perspective, our results unveil the untapped potential of structural diversification, even for mono-specific and even-aged stands. We elucidate the growth trade-offs when structural diversity is either neglected or overly emphasized.
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•Tree size variation's effects on growth and yield of even-aged Norway spruce stands.•Focus on max stand density, general and special yield levels, and stand growth.•Unimodal link of stand traits to tree size variation, optimal at mean level.•Spotlights tree size variation's relevance in stand analysis, modeling, management.
Questions: We aim for a better understanding of the different modes of intra- and inter-specific competition in two- and three-species mixed-forests. How can the effect of different modes of ...competitive interactions be detected and integrated into individual tree growth models? Are species interactions in spruce–fir–beech forests more associated with size-symmetric or size-asymmetric competition? Do competitive interactions between two of these species change from two- to three-species mixtures? Location: Temperate mixed-species forests in Central Europe (Switzerland). Methods: We used data from the Swiss National Forest Inventory to fit basal area increment models at the individual tree level, including the effect of ecological site conditions and indices of size-symmetric and size-asymmetric competition. Interaction terms between species-specific competition indices were used to disentangle significant differences in species interactions from two- to three-species mixtures. Results: The growth of spruce and fir was positively affected by increasing proportions of the other species in spruce–fir mixtures, but negative effects were detected with increasing presence of beech. We found that competitive interactions for spruce and fir were more related to size-symmetric competition, indicating that species interactions might be more associated with competition for below-ground resources. Under constant amounts of stand basal area, the growth of beech clearly benefited from the increasing admixture of spruce and fir. For this species, patterns of size-symmetric and size-asymmetric competitive interactions were similar, indicating that beech is a strong self-competitor for both above-ground and below-ground resources. Only for silver fir and beech, we found significant changes in species interactions from two- to three-species mixtures, but these were not as prominent as the effects due to differences between intra- and inter-specific competition. Conclusions: Species interactions in spruce–fir–beech, or other mixed forests, can be characterized depending on the mode of competition, allowing interpretations of whether they occur mainly above or below ground level. Our outcomes illustrate that species-specific competition indices can be integrated in individual tree growth functions to express the different modes of competition between species, and highlight the importance of considering the symmetry of competition alongside competitive interactions in models aimed at depicting growth in mixed-species forests.
Climate change is increasing the severity and frequency of droughts around the globe, leading to tree mortality that reduces production and provision of other ecosystem services. Recent studies show ...that growth of mixed stands may be more resilient to drought than pure stands. The two most economically important and widely distributed tree species in Europe are Norway spruce (Picea abies (L.) Karst) and Scots pine (Pinus sylvestris L.), but little is known about their susceptibility to drought when coexist.
This paper analyses the resilience (resistance, recovery rate and recovery time) at individual‐tree level using a network of tree‐ring collections from 22 sites along a climatic gradient from central Europe to Scandinavia. We aimed to identify differences in growth following drought between the two species and between mixed and pure stands, and how environmental variables (climate, topography and site location) and tree characteristics influence them.
We found that both the timing and duration of drought drive the different responses between species and compositions. Norway spruce showed higher vulnerability to summer drought, with both lower resistance and a longer recovery time than Scots pine. Mixtures provided higher drought resistance for both species compared to pure stands, but the benefit decreases with the duration of the drought. Especially climate sensitive and old trees in climatically marginal sites were more affected by drought stress.
Synthesis. Promoting Scots pine and mixed forests is a promising strategy for adapting European forests to climate change. However, if future droughts become longer, the advantage of mixed stands could disappear which would be especially negative for Norway spruce.
Mixtures of Scots pine and Norway spruce provided higher drought resistance for both species compared to pure stands, but the benefit decreases with drought duration. Norway spruce showed higher vulnerability to summer drought, with both lower resistance and a longer recovery time. Promoting Scots pine and mixed forests is a promising strategy for adapting European forests to climate change.
Key message
Crown, height and stem allometry vary with stand density and species composition, the plasticity in response to inter- and intra-specific competitions being related to species shade ...tolerances.
Determining the way in which variability in tree allometry is modulated by intra- and inter-specific competitions in different species and stand compositions is of particular interest for forest modelling and practice. In this study, we explore this variability by developing models for tree crown diameter, total height and diameter at a height of 4 m, which include intra- and inter-specific competition terms. More than 19,000 Scots pine, silver fir, sessile oak and European beech trees from 4711 sample plots belonging to the Spanish National Forest Inventory were included in the study, covering both monospecific and two species mixed stands in Northern Spain. Trees growing under conditions of high competition displayed narrower crowns, greater heights and less taper for a given tree diameter, the plasticity in crown and height in response to intra-specific competition being related to species shade tolerance. The inter-specific competition effect on crown diameter and height was related to the difference in shade tolerance between the two species of the mixture, while stem taper did not exhibit this pattern. These results suggest that trees in mixed stands indeed show a modified allometry, which might be related to complementary resource acquisition strategies. The large variability observed in tree allometry indicates the need to consider both intra- and inter-specific competitions in allometric models.
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•Size-asymmetric competition is stronger than size-symmetric one in studied mixtures.•Beech benefits from pine and oak admixtures, more when the latter are larger trees.•Beech has a ...negative effect on pine growth and a variable effect on oak growth.•Fir and beech basal area growth is not modified by species admixture.•Species mixing can reduce competition on smaller trees, improving stand stability.
In mixed stands, inter-specific competition can be lower than intra-specific competition when niche complementarity and/or facilitation between species prevail. These positive interactions can take place at belowground and/or aboveground levels. Belowground competition tends to be size symmetric while the aboveground competition is usually for light and almost always size-asymmetric. Interactions between forest tree species can be explored analyzing growth at tree level by comparing intra and inter-specific competition. At the same time, possible causes of niche complementarity can be inferred relating intra and inter-specific competition with the mode of competition, i.e. size-symmetric or size-asymmetric. The aim of this paper is to further our understanding of the interactions between species and to detect possible causes of competition reduction in mixed stands of beech (Fagus sylvatica L.) with other species: pine–beech, oak–beech and fir–beech. To test whether species growth is better explained by size-symmetric and/or size-asymmetric competition, five different competition structures where included in basal area growth models fitted using data from the Spanish National Forest Inventory for the Pyrenees. These models considered either size-symmetry only (Reineke’s stand density index, SDI), size-asymmetry only (SDI of large trees or SDI of small trees), or both combined. In order to assess the influence of the admixture, these indices were introduced in two different ways, one of which was to consider that trees of all species compete in a similar way, and the other was to split the stand density indices into intra- and inter-specific competition components. The results showed that in pine–beech mixtures, there is a slightly negative effect of beech on pine basal area growth while beech benefitted from the admixture of Scots pine; this positive effect being greater as the proportion of pine trees in larger size classes increases. In oak–beech mixtures, beech growth was also positively influenced by the presence of oaks that were larger than the beech trees. The growth of oak, however, decreased when the proportion of beech in SDI increased, although the presence of beech in larger size classes promoted oak growth. Finally, in fir–beech mixtures, neither fir nor beech basal area growth were influenced by the presence of the other species. The results indicate that size-asymmetric is stronger than size-symmetric competition in these mixtures, highlighting the importance of light in competition. Positive species interactions in size-asymmetric competition involved a reduction of asymmetry in tree size-growth relationships.
Abstract
A wealth of recent research has improved our understanding of the structure, growth and yield of mixed-species stands. However, appropriate quantitative concepts for their silvicultural ...regulation remain scarce. Due to the species-specific stand densities, growing area requirements and potential over-density, the density and mixing regulation in mixed stands is much more intricate than in monospecific stands.
Here, we introduce the species-specific coefficients: density equivalence coefficients (DEC), for density equivalence; and density modification coefficient (DMC), for density modification in mixed species stands. DEC is suitable for the conversion of the stand density and growing area requirement of one species into that of another species. DMC estimates the modification of maximum stand density by tree species mixing using as reference the maximum stand density of one of the species.
First, we introduce the theoretical concept of these coefficients. Second, we derive the mean values of these coefficients based on long-term experiments using different mixtures of European beech. Third, we apply DEC and DMC for flexible regulation of the stand density and mixing proportion. Thus, silvicultural regulation of monospecific stands and mixed-species stands forms a continuum, where monospecific stands represent an extreme case of mixed-species stands.
Lastly, we discuss the advantages and limitations of these concepts. Future directions comprise the inclusion of additional species, their integration in guidelines and simulation models, and their establishment for the quantitative regulation of experimental plots and the practical implementation in forest stands.
•Foliar decay rates were species-specific in unmanaged plots of Mediterranean mixed-oak pine.•Thinning reduces the decay rate of oak leaves.•C concentration was not affected by thinning, whereas ...heavy thinning reduced N and P concentration.•Heavy thinning of Mediterranean sites might have an aridification effect negatively affecting the decomposition process.
Leaf litter decomposition is a major pathway for nutrient recycling and a chief factor controlling ecosystem primary productivity. The aim of this study was to analyze the effect of forest thinning on the early-stage foliar decomposition rate and nutrient immobilization-release pattern in a mixed Quercus pyrenaica-Pinus pinaster forest growing in the western Mediterranean basin. Two thinning treatments, differing in the intensity of canopy reduction, were compared with a control (unthinned) situation. Oak leaves showed faster decomposition rates than pine needles in unthinned plots. Intense canopy reduction (40% of basal area removed) reduced decomposition rates in both species whereas intermediate reduction (25% of basal area removed) increased decomposition of needles to a rate similar to that of oak leaves. C and N transfer from N-rich to N-poor foliar litter was not detected, indicating a marginal role of canopy reduction in this process. A decoupling of the decomposition rate from immobilization-release pattern was found for mobile elements (K and Mg) at intermediate levels of canopy cover whereas Ca dynamics indicated that pine needles had not started the lignin-mediated degradation phase two years after incubation. We finally hypothesized that strong reductions of canopy cover in dry Mediterranean sites might have an aridification effect on litter decomposition.
•Both size-symmetric and size-asymmetric competition influence crown allometry.•Crown allometry in pines is modified by aridity conditions.•Crowns react more strongly to changes in competition in ...humid sites.•Species interactions have a significant effect on crown allometry.
Crown dimensions such as diameter or crown projection area are important indicators for tree vigour, growing stage or production efficiency and can be used as proxies for the assessment of wood quality, leaf area and leaf biomass, biodiversity, wildlife habitat and disturbance risk to forest stability. Crown allometry can vary with site and competition conditions, but variation patterns are not still well understood.
The main aim of this study was to analyze this allometry variation in the most common pine species along the aridity gradient of the Iberian Peninsula, studying the crown response to aridity and to competition, and determining whether the presence of inter-specific competition modifies crown allometry.
A set of 40,402 sample trees from 12,283 sample plots, along with the Martonne aridity index as an indicator of climatic conditions, were used to develop allometric models, the parameters of which were generalized by including aridity and competition indicators. The plots were located in monospecific or mixed stands of Pinus sylvestris, Pinus nigra, Pinus pinaster, Pinus pinea and Pinus halepensis.
According to our results, both aridity conditions and competition status are important factors modifying the crown allometry of the studied species. Moreover, we found that both size-symmetric competition and size-asymmetric competition should be taken into account when modelling the crown size, both types leading to smaller crown diameter. The crown plasticity of the studied pine species in response to competition presented a low range of variation, the greatest being that of maritime pine, while Scots pine and black pine showed the smallest. The way in which aridity and competition influence crown size differs among the studied species, indicating differences in crown plasticity patterns. Moreover, in humid sites, where competition for light is more important, the crown allometry response to changes in competition is greater, being most evident in P. pinea and P. halepensis.
The species mixing effects had less influence on pine crown allometry than the total competition effect, although the effect was always significant. The effect of size-symmetric competition from admixed species was positive or neutral, except for the effect of P. sylvestris on P. nigra crowns. Size-asymmetric competition also plays an important role in the species mixing effects, these effects on intermediate or suppressed trees varying depending on the species mixture. The models developed reveal significant species-specific effects on crown allometry, pointing to the ability of pine species to adapt their crowns to different growing conditions.
•Tree crown morphology was significantly influenced by interspecific competition.•Species mixing induced changes in stand structure compared to monocultures.•Structural heterogeneity in mixed stands ...results from emerging mixing effects.•Relative productivity increased as the difference in expected heights increased.•Crown plasticity and size-distribution are possible mechanism enhancing productivity.
Mixed-species stands have been studied extensively due to their potentially superior productivity, multi-functionality benefits and high ecological value compared to pure stands. The higher structural heterogeneity in mixed stands that can emerge from species interactions could be linked to the relationship between species diversity and ecosystem functions. We tested whether changes in stand structure also occur in mixtures of species with similar traits and whether they explain over-yielding patterns. Based on research with 12 triplets of Scots pine (Pinus sylvestris L.) and Maritime pine (Pinus pinaster Ait.) in the northern Iberian Peninsula (Spain), we provide evidence that species mixing increased structural heterogeneity and may induce over-yielding in mixed-species stands compared to monospecific stands. In this mixture of two light-demanding species, we observed that (i) stand composition influenced the inter-specific crown allometric variation, (ii) structural heterogeneity in mixed stands was caused by both specie-specific traits and species interactions, and (iii) intraspecific and interspecific differences in both crown size plasticity and size-distribution differentiation were associated with the increased relative productivity of mixed stands. We detected that crown complementarity and vertical stratification in the canopy space is a crucial mechanism for enhancing ecosystem productivity in light-demanding species and could be related to light interception and light-use. This work improves our understanding of emerging properties in mixed stands and introduces considerations for properly scaling and tracing mixing effects at individual tree, size distribution and stand levels.
3D crown shape and tree ring development are autobiographies of the growth conditions. With advancements in terrestrial laser scanning, modeling growth from 3D crown structure offers insights into ...trees’ structural and functional responses in a non-destructive and repetitive way. However, how the trees in different growing conditions respond in 3D structure and manifest their responses into tree rings remains unexplored, which would help to better explain tree-level growth dynamics and forest management. To enhance this understanding, we tested a set of hypotheses: (HI) that 3D crown shape (CS) and tree ring (width) patterns (TRP) are correlated across different growing conditions or forest stands like monospecific, provenances trials, and mixed forest stands; (HII) that stand types influence the CS-TRP link; and (HⅢ) local neighborhood competition (LNC) modulates the CS-TRP link. (HⅣ) 3D crowns manifest local growth conditions; therefore, 3D crown structures can be used to predict tree ring growth. We assessed these hypotheses by employing terrestrial laser scanning-based 3D crown shape and dendrochronology-based tree ring width patterns from Norway spruce (Picea abies L. Karst.) trees growing in pure spruce (unthinned and thinned), provenances trial, and mixed-species trial (with European Beech, Fagus sylvatica L.) stands covering a large-scale competition gradient. We first show that 3D crown shape and TRP metrics differed significantly across forests (p<0.05) but were correlated (p<0.05). Neighborhood competition among the forest types influences the link between 3D crowns and tree rings. Pathway-based analyses revealed that neighborhood competition indirectly influences ring variability by modifying crown structure (p<0.05), suggesting local growth conditions are mostly manifested into crown shapes, leading to 3D crown shape-based low-error growth predictions (0.44 mm) across forest types. However, incorporating competition legacy information (competition over the last 30 years) in the model slightly improved the prediction performance (error reduced to 0.41 mm), further explaining that the crown likely loses growth information due to growing conditions (competition it faces and species with it is growing). This study reveals how trees in different growing conditions differed structurally and mechanized their responses in tree rings, providing crucial insights into tree-level growth dynamics and management.
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•3D crown structure (CS) and tree ring patterns (TRP) are significantly correlated across forest stands.•The CS-TRP link is influenced by neighborhood competition and stand types.•CS mostly manifested and indicated the local growing conditions (competition).•3D crown shape is a good and novel predictor of tree growth.
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