Key message
This study presents the first attempt to quantify how the thigmomorphogenetic syndrome is involved in
Fagus sylvatica
L. tree growth responses to thinning. An experimental design ...preventing mechanosensing in half of the trees demonstrated that radial growth distribution in roots and along the tree stem is under strong biomechanical control.
Context
Studies on the mechanosensitive control of growth under real forest conditions are rare and those existing to date all deal with conifer species. In the current context of global changes, it is important to disentangle how different biotic and abiotic factors affect tree growth.
Aims
Whereas growth changes after thinning are usually interpreted as responses to decreased competition for resources, this study investigates the importance of how mechanosensing controls growth distribution inside the tree.
Methods
In an even-aged beech stand, 40 pole-sized trees (size class at first thinning) were selected, half of the plot was thinned and, within each sub-plot (thinned and unthinned), half of the tree were guy-wired in order to remove mechanical stimulations to the lower part of the stem. Four years later, all trees were felled and volume increment, ring width distribution along the tree height, and the largest ring width of the structural roots were measured. The effect of mechanical stimulation in the two treatments (thinned and unthinned) was assessed.
Results
Removal of mechanical stimulation decreased the volume increment in the lower part of the stem as well as radial root growth but did not affect axial growth. When mechanical strain was removed, the ring width distribution along the stem height changed drastically to an ice-cream cone-like distribution, indicating a strong mechanosensitive control of tree shape.
Conclusion
In a forest stand, the growth allocation inside the tree is under strong mechanical control. Mechanical stimulations explain more than 50% of the increment stimulated by thinning, whatever the growth indicator. A further challenge is to better understand how cambial cells perceive strains during growth in order to integrate mechanosensing into process-based tree-growth modeling.
Abstract
Forests cover about one-third of Europe’s surface and their growth is essential for climate protection through carbon sequestration and many other economic, environmental, and sociocultural ...ecosystem services. However, reports on how climate change affects forest growth are contradictory, even for same regions. We used 415 unique long-term experiments including 642 plots across Europe covering seven tree species and surveys from 1878 to 2016, and showed that on average forest growth strongly accelerated since the earliest surveys. Based on a subset of 189 plots in Scots pine (the most widespread tree species in Europe) and high-resolution climate data, we identified clear large-regional differences; growth is strongly increasing in Northern Europe and decreasing in the Southwest. A less pronounced increase, which is probably not mainly driven by climate, prevails on large areas of Western, Central and Eastern Europe. The identified regional growth trends suggest adaptive management on regional level for achieving climate-smart forests.
Key message
The Fagacées growth model was originally designed for application in the Northern half of France. It is a robust model with potential applicability to a larger area, though this potential ...has not yet been verified. We added new data to the original parameterization data set and our results show that the Fagacées formalism can be generalized.
Context
The Fagacées growth and yield model was designed for the management of pure even-aged stands of European beech and served as a prototype to build models for other tree species.
Aims
The objective of this study was to improve the growth components of the Fagacées model with additional data from North-Western France to South-Western Germany.
Material and methods
Our model was calibrated on several forest inventory data sets. The first one (F) is the original data set that was used to elaborate the equations in the Fagacées model. The second one (F+) is the original data set extended with additional measurements on the same sites and on new sites in Northern France. The third (G) adds complementary data from a forest network in Southwestern Germany. The last one (A) is the aggregate of all these data sets.
Results
Fitting the original model equations on the extended F+ dataset led us to modify the equation for stand basal area increment. This new equation also fit the German dataset well. The other equations could be applied to all datasets, some with the same parameter values and some after recalibrating according to the dataset.
Conclusion
We conclude that the general form of the model’s equations is appropriate for application to other regions, but that a recalibration of the equations is preferable in order to reflect local conditions. The advantage of our approach is that fewer data are required to recalibrate an existing equation than to establish an entirely new one.
Differences between intra- and inter-specific competition strategies in mixed stands account for higher productivity. The mechanisms of competition as they relate to stand structure remain poorly ...explored. Climatic effects have also been overlooked in most approaches aimed at studying tree growth. These effects were investigated through the case study of beech-oak mixtures in France. A single-tree diameter-increment mixed model was fitted to data gathered from the Laboratoire d'Etude des Ressources Foret-Bois (LERFoB) network of permanent plots. In order to take stand structure into account, the model was able to distinguish between inter-and intra-specific competition at tree level. Differences in intra-and inter-specific competition are mainly detected in the understorey of oak-dominated stands, where suppressed beech trees performed better than suppressed oak trees. From a climatic point of view, diameter increment was sensitive to the mean temperature of the vegetative period. Concerning competition, our results demonstrate that optimal growth induced by mixtures is not only a matter of species proportion but also depends on stand structure. Regarding climatic effects, they need to be considered in growth predictions, especially given the future climatic conditions predicted for continental Europe. These findings could serve as guidelines for designing strategies to alleviate stress in oak-beech stands.
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•Stem volume loss by natural mortality in unmanaged forest stands was analyzed.•Volume losses of 30–40% of total volume production were observed up to an age of 100–150 years.•Average ...annual biomass loss was 0.8–2.1 t ha−1 (0.4–1.1 tC ha−1).•Loss fraction of gross stand growth increased continuously with stand age.•Trade-off identified between wood utilization and deadwood accumulation for biodiversity.
Even-aged stands can regenerate with many thousand seedlings per hectare before the density declines to just a few hundred trees per hectare 100 years later; management practices can lead to even lower tree numbers due to quality selection and thinning. In other words, during the development of unmanaged stands, the majority of individuals die naturally due to competition. Despite the far-reaching consequences for structural and genetic diversity, dead wood and fuel wood accumulation, we have only limited quantitative knowledge about the continuous mortality of trees and the wood volume loss over longer timespans.
For this study, we used a unique set of 476 unmanaged, monospecific experimental plots of Norway spruce (Picea abies (L.) H. Karst.), silver fir (Abies alba Mill.), Scots pine (Pinus sylvestris L.), European larch (Larix decidua Mill.), Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco)), European beech (Fagus sylvatica L.), and oak (Quercus robur L. and Quercus petraea (Matt.) Liebl.) throughout Europe to analyze the competition-based mortality of trees and its dependency on age and site conditions.
First, we show that the total stem volume production, standing stock, and mortality were continuously increasing until an age of 100–150 years. The accumulated competition-caused stem volume loss at that age amounted to 500–1000 m3 ha−1.
Second, the net growth of the stands (share of the growth that is accumulated in the standing stock) strongly decreased with increasing age even when the gross growth was still high. The proportion of the net growth versus gross growth continuously decreased with increasing age regardless of site quality.
Third, we show a degressive decrease of the annual relative tree number mortality rates from 0.05 to 0.20 in young down to 0.01–0.02 in mature stands. For some species, we found these rates to be site dependent with different directions of the site effect. The interplay of decreasing mortality rates and increasing average volume of the dead trees resulted in unimodal mortality curves over time of the annual mortality, peaking at 3–12 m3 ha−1 yr−1 at ages of about 75–150 years.
Over the whole rotation, the average annual biomass loss from mortality ranged between 0.8 and 2.1 t ha−1 yr−1 with a carbon content of 0.4–1.1 t C ha−1 yr−1. We discuss the relevance of the results for measuring, understanding, modelling, and managing forest stands. Our results reveal that the withdrawal of forest management and setting aside (previously managed) forests over a rotation time of 100–150 years means that about one third of the total production in monospecific stands would flow to the debris pool rather than being exploited for carbon sequestration and related emission savings in harvested wood products. The mortality related loss fractions of above ground biomass we quantified in this study indicate the trade-off between wood production and setting aside forest to allow deadwood accumulation and associate changes in biodiversity.
•The estimated stochastic distribution of windstorm severity makes part of the model.•Interspecific competition and interspecific competition effects on mortality differ.•Windstorms are the main ...cause of catastrophic tree mortality in French oak–beech stands.•Drought significantly increases tree mortality in French oak–beech stands.•Mortality in beech understories decreases in the presence of oak overstories.
Extreme climatic events, such as windstorms and drought, and competition are some of the main factors underlying tree mortality in Atlantic and Central European forest stands. However, current empirical tree mortality models are not adapted to separately consider the different causes of mortality. In addition, these approaches do not distinguish between intra- and inter-specific competition. In this study, we present a comprehensive empirical single-tree mortality model that incorporates all of the aforementioned features. On the one hand, extreme events can enter the model via fixed effects and random effects. The latter allows taking the stochasticity of the process into consideration. The distribution of these random effects can be seen as an extreme event severity distribution, which is of great interest for carrying out stochastic simulations on tree mortality. On the other hand, intra- and inter-specific competition is taken into account through linear interactions of species-specific competition indexes and species factors. In order to test this approach, we selected the beech–oak mixture in France as a case study. Beech–oak mixed forests are a common type in France which provides owners with significant economic benefits. Our findings confirm that drought and, especially, windstorm occurrence are major causes of tree mortality in these forests. The model was able to capture the stochasticity of windstorm events by means of random effects. In terms of competition, the probability of mortality in beech understories was expected to decrease in the presence of oak overstories with respect to pure beech stands. This result reveals the importance of complementarity processes in tree mortality.
Key message
We studied the size-density trajectories of pure even-aged unthinned experimental sessile oak (
Quercus petraea
(Matt.) Liebl.) stands in the ranges of 994–135,555 trees per hectare ...initial densities, observed from the ages of 5 to 38. We compared them to unthinned beech (
Fagus sylvatica
L.) stands from the same experimental area. An original piecewise polynomial function was fitted to the trajectories, giving way to various applications. For each species, the initial number of trees per hectare (
N
0
) and the mean girth at breast height at the onset of mortality (Cg
0
) were parameters of the trajectory model, in addition to the parameters of the maximum size-density lines. The two former parameters (Cg
0,
N
0
) were tied by a linear relationship, which allowed the prediction of trajectories for initial densities not included in the study data. For oak and beech, mortality onset occurred at a constant relative density (RDI), for all initial stand densities, respectively, 0.35 and 0.29. The comparison of the size-density trajectories of oak and beech allowed to establish that oak needs more space than beech for comparable mean girth, and then is less efficient than beech in its space requirements.
Context
This paper models the size-density trajectories of pure even-aged sessile oak stands, including the early development stage. It compares the oak results with those on common beech on the same site from a previous study.
Aims
A novel approach to size-density trajectories, with an original polynomial piecewise function previously used for beech stands on the same site, was satisfactorily used again as a mortality model to provide references to managers of oak forests.
Material and methods
A 38-year-old oak spacing trial, re-measured from year 5 to year 38, provided the opportunity to study the size-density trajectories of unthinned stands of this species.
Results
The fit of the piecewise polynomial function allowed us to estimate the parameters of the size-density trajectories of all stands, which were the initial number of trees per hectare (
N
0
) and the mean girth at breast height at the onset of mortality (Cg
0
), in addition to the intercept (
a
) and slope (
b
) of the maximum size-density line. A linear relationship between Ln(
N
0
) and Ln(Cg
0
) (where Ln is the Neperian logarithm) allowed us to reduce the number of parameters needed to fit the trajectories and made it possible to predict a size-density trajectory from any initial density not observed in the experimental stands. Moreover, this later line appeared to be parallel to the maximum size-density line, and new data allowed to establish that this was also the case for the beech stands on the same site. This parallelism feature translates to the onset of mortality occurring at the same relative density for stands of every initial density that is 0.35 for oak and 0.29 for beech.
Conclusion
Given the parameters of the maximum size-density line, a single-parameter function family could be used to predict the size-density trajectories of oak stands. The predicted trajectories have various applications in oak silviculture and growth simulators. The oak data and new data for beech stands on the same site allowed to compare the two species and draw conclusions on similitudes and differences concerning mortality and space requirements of both species.
KEY MESSAGE : Density was more important in shaping crown structure than neighbor species identity. Both species showed high crown plasticity at alternative levels, which may explain species ...coexistence in mixed broadleaved forests with functionally similar species. Understanding crown response to local competition is essential to predicting stand development in mixed stands. We analyzed data from an 8-year-old field experimental plantation mixing two species according to a crossed gradient of density and species proportion to quantify the effect of a broad range of local neighborhood conditions on the development of young trees at multiple crown levels. We used Fagus sylvatica and Acer pseudoplatanus, as two model deciduous species. They are considered functionally equivalent at the young stages, but with contrasting architectural patterns. For both species: (1) changes in density explained more of the variation on crown development than species proportion (2) much of the effect of competition was accounted for by variables at the stem level, while branch and leaf development within crowns were not directly altered by competition. Both species were able to modify their crowns at the stem level to compete with intra- and inter-specific neighbors: Acer and Fagus were taller with a highest proportion of Fagus as neighbors; Fagus displayed a lower crown base when the proportion of Fagus decreased, while Acer had a lower crown base when the proportion of Fagus around it increased. Both species showed common shapes in allometric relationships but contrasting responses at alternative crown levels. Acer exhibited broader intra-specific variation in its height–diameter relationship and in its crown length, while Fagus displayed higher individual variation of branch development and leaf area than Acer. This study demonstrates that differences in crown development strategy of each species in response to changes in local neighborhood conditions are an important factor in maintaining species coexistence in broadleaved forests and designing mixtures that persist over time.
Key message
In understory trees, height–diameter allocation is subject several constraints: maximal height growth is limited, height growth is never null and, in terms of time dependency, diameter ...growth always precedes height growth.
Understory trees experience a succession of canopy opening and closure events during their lifetime. Preferential allocation of their biomass to height or diameter growth is part of the acclimation process to their immediate environment. In this study, we investigated annual height and diameter increments in
Fagus sylvatica
understory trees submitted to canopy release. Annual height and diameter increments were obtained from retrospective stem analysis from the top of the tree to the stump on 39 understory trees. The relationship between height and diameter increments was investigated and temporal correlations among successive annual height and diameter increments were analyzed. An asymptotic relationship between annual height and diameter increment indicates that stem height growth was limited in understory trees. The intercept significantly differed from zero suggesting that height growth never stopped even when diameter growth was null. Following canopy release events, immediate diameter growth increase was observed while the height growth response was delayed, resulting in a time shift in the growth allocation strategy. Strong and asymmetric temporal correlations between annual height and diameter increments were observed: past annual diameter growth was positively correlated to present height growth. In understory trees, tree height is the main factor determining their potential growth since it determines their access to above-canopy light. However, the precedence of diameter growth over height growth suggests that tree growth is driven by diameter instead of height. This apparent discrepancy may be explained by the fact that, under closed canopy conditions, stem allometry reaches a functional threshold that forces tree to grow in diameter before growing in height.
Plant–soil interactions are increasingly recognized to play a major role in terrestrial ecosystems functioning. However, few studies to date have focused on slow dynamic ecosystems such as forests. ...As they are vertically stratified by multiple vegetation strata, canopy tree removal by thinning operations could alter forest plant community through tree canopy opening. Very little is known about cascading effects on soil biodiversity.
We conducted a large‐scale, multi‐site assessment of collembolan assemblage response to long‐term canopy tree removal in sessile oak Quercus petraea temperate forests. A total of 33 experimental plots were studied covering a large gradient of canopy tree basal area, stand age and local abiotic contexts.
Collembolan abundance strongly declined with canopy tree removal in early forest successional stage and this was mediated by negative effect of understory plant community composition changes, i.e. shift from moss and forb to tree seedling, fern, shrub and grass species. Negative effect of this composition shift on collembolan species richness was largely offset by positive effect of the increase in understory plant species richness. This gives support to both the plant mass‐ratio and functional diversity hypotheses. Collembolan functional groups had contrasting response patterns, which were mediated by different ecological factors. Epedaphic (r‐strategist) abundance and species richness increased with canopy tree removal in relation with the increase in understory plant species richness. In contrast, euedaphic (K‐strategist) abundance and species richness declined with canopy tree removal in early forest successional stage in relation with changes in understory plant community composition and species richness, as well as microclimatic conditions.
Overall, our study provides experimental evidence that forest plant community can be a strong driver of collembolan assemblages. It also emphasizes the role of trees as foundation species of forest ecosystems that can shape soil biodiversity through their regulation of understory plant community and ecosystem abiotic conditions.
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