► We evaluated the carbon balance of three forest management scenarios. ► We used a system of models to represent the whole forest-wood product chain. ► The harvested wood product carbon pools were ...10times smaller than the forest carbon pools. ► Regardless of the scenario, most harvested wood products were short lived. ► Scenarios with higher stem densities over longer rotations improved the carbon balance.
In this study, we evaluated three different management scenarios in relation to their ability to maximize the carbon balance. The study was carried out within a context of managed even-aged sessile oak stands, and the management scenarios were contrasted in terms of stem density and rotation length. A system of models designed to represent the whole forest-wood product chain was used to assess the carbon balance of each scenario. The boundaries of this system were extended as far as possible to prevent any carbon leaks. Dead organic matter and disposed harvested wood products (HWPs) were taken into account in the assessment. The scenarios were compared in terms of their average stock in the forest and the HWP carbon pools, as well as some additional fluxes. These included: (i) substitution for fossil fuel; (ii) carbon emissions due to HWP processing; (iii) the accumulation of non-degradable carbon in the landfill; and (iv) methane emissions resulting from anaerobic decomposition of disposed HWPs.
Results showed that the HWP carbon pools were about 10times smaller than the forest carbon pools. The substitution for fossil fuel constituted the largest positive flux, whereas methane emissions in absence of methane recovery facilities accounted for 40% of the negative fluxes. The burning of wood products for energy was preferable to HWP disposal at the landfill site. As long as the risk of extreme climate events remains marginal, management scenarios that maintain higher stem densities over longer rotation lengths appeared to be favorable to carbon sequestration.
Key message
We studied the size-density trajectories of pure even-aged unthinned beech stands in the ranges of 625–40,000 trees per hectare initial densities and of 12–33 years of age. A new ...piecewise polynomial function family was fitted to the trajectories, giving way to various applications. Initial number of stems per hectare (
N
0
) and 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 line. The two former parameters were tied by a linear relationship, which allowed the prediction of trajectories not considered in this study. Furthermore, the generic trajectory equation fitted the trajectories of thinned stands not used in the estimation of the parameters.
Context
This paper models the size-density trajectories of pure even-aged beech stands, including the early development stage, which is not as well documented as are the later stages.
Aims
The work reported in this paper concerns the development of a novel approach to size-density trajectories, considered as a mortality model to provide references to managers of beech forests.
Material and methods
A 33-year-old beech spacing trial beginning at 12 years of age provided the opportunity to study the size-density trajectories of unthinned stands of this species. The beech data helped us to develop a new piecewise function to model these trajectories. The model we chose was a polynomial segment smoothly joining two linear functions.
Results
The fits of this model allowed us to estimate the parameters of the size-density trajectories of all stands, which were the quadratic mean girths at mortality onset and at maximum density. A linear relationship between these characteristics allowed us to reduce the number of parameters needed to fit the trajectories and made it possible to predict a stand trajectory from any initial density not observed in the experimental stands.
Conclusion
A single-parameter function family could be used to fit the size-density trajectories of beech stands. The predicted trajectories have various applications in beech silviculture and growth simulators.
Due to the fact that forest ecosystems can potentially mitigate the impact of climate change, the carbon balance of managed forests has caught the attention of a large scientific community. Some ...authors conclude that extending rotation lengths would actually favour the climate change mitigation effect since more carbon would be stored in the biomass on the average. However, when the occurrence of catastrophic disturbances such as windstorms is not considered, the advantage of extending the rotation length might be overestimated for some species. In this study, we addressed this issue by coupling a growth model, a windstorm damage model and a carbon assessment tool. The evolution of an even-aged European beech (Fagus sylvatica L.) stand was simulated under three different rotation lengths. Simulations including stochastic windstorm events were run and compared with deterministic simulations with no catastrophic disturbance. Our results indicate that when disturbances caused by storms were not taken into account, the carbon balance was actually overestimated in some cases and that this overestimation increased with the rotation length. In our case study, omitting windstorm damage resulted in an overestimation as large as 8% for the longer rotation length. Nevertheless, when windstorm damage was taken into account in the simulation, the longer rotation length still stored more carbon on the average than shorter rotation lengths. However, the marginal gain in carbon storage induced by the increase of the rotation length was reduced.
•
Introduction
Size-density relationships define the maximum number of stems that even-aged stands of a given species can hold in relation to the mean size of trees. They are used to derive stand ...density measures and are useful tools used to control tree mortality.
•
Objectives
Size-density relationships were already available in France for beech and oak. The objective of this study was to extend these relations to younger development stages and test if specific relations are needed to be established for a set of species of different shade tolerance, including beech, ash, sycamore maple, and oak.
•
Results
We relied on stands growing at maximum density and used selected data coming from the inventories of permanent control plots and specifically established temporary plots. A multiple comparison procedure was used to differentiate between the parameters of the relations. Two size–density relations were retained with a common slope and different intercepts for ash and beech on one hand, and for oak and sycamore maple on the other hand. Stands of shade-intolerant species like oak appeared able to hold less trees of a given mean size, but shade tolerance did not seem to influence the mortality rate which appeared to be the same.
Key message
A piecewise polynomial function already used to represent the size-density trajectories of pure even-aged stands of beech, oak, and Douglas-fir proved its ability to represent the ...size-density trajectories of a new species, ash. The widespread ash dieback caused departures from the expected size-density trajectories. These abnormalities can be used to detect an extra level of mortality due to infection by
Hymenoscyphus fraxineus
in pure even-aged ash stands
.
Context
The size-density trajectories allow quantifying more precisely the density of stands and can help the forest manager to decide of the opportunity of thinnings. This study helped to quantify extra mortality in pure even-aged stands by using the size-density trajectories established for stands evolving at maximum density.
Aims
This study was conducted to establish size-density trajectories of pure even-aged ash stands and compare them with those recently established for beech and oak in France, in particular concerning the onset of density-dependent (regular) mortality. The additional effect of ash dieback on mortality was also an issue.
Material and methods
We used permanent and semi-permanent unthinned ash plots installed in the north of France and where inventories of trees were performed at more or less regular intervals: measurements included tree status (dead or alive) and diameter or girth at breast height for all trees and total height for a sample of living trees. The size-density trajectories of plots describing the course of the number of living trees in relation with the mean stand girth, in logarithmic scales, were modeled with a piecewise polynomial function fitted with a mixed-effects model. A permanent sample of trees was also selected for ash dieback and extra mortality monitoring.
Results
The piecewise polynomial function already used proved its ability to represent the size-density trajectories of even-aged ash stands of various initial densities and fertility levels. As for beech and oak, the trajectories were modeled so that mortality onset occurred at a constant relative density. This level appeared to be much higher for ash (RDI = 0.58), revealing that ash survived with less growing space than beech and oak and appeared to be more efficient. Ash dieback caused an additional mortality in the experimental ash stands studied, and this excess of mortality appeared predictable on the basis of observed departures from the expected size-density trajectories.
Conclusion
A single parameter function family could be used to predict the size-density trajectories of even-aged ash stands, on the basis of the results obtained previously on oak and beech. Mortality onset and space requirements of ash could be compared with those of beech and oak and show that ash can survive at higher densities and is a more efficient species. Predicted size-density trajectories proved also useful to detect and quantify the excess of mortality due to
H. fraxineus
on ash. This approach could be extended to other diseases and species with predictable size-density trajectories.
Reducing stand density by thinning intensification has been emphasized as an efficient strategy of forest adaptation to climate change as it improves stand resistance to drought. Yet, it is still ...unclear how it could affect litter carbon (C) cycling processes. Recent evidence indicates that the plasticity of an oak tree species can lead to a decline in its leaf litter quality and decomposability following thinning. The consequences for litter decomposition and forest floor C storage at the ecosystem scale remain largely unexplored.
In this study, we took advantage of a regional‐scale, multi‐site network of long‐term thinning experiments in temperate oak (Quercus petraea) forests to address this issue. We measured ecosystem properties related to forest floor C cycling in 19 plots across eight experimental sites covering a large gradient of stand density and age. Though we expected thinning to affect in situ litter decomposition by altering oak leaf litter quality, we conducted complementary experiments exploring additional mechanisms, that is alterations of microenvironmental conditions and soil faunal activity.
Thinning intensification induced a strong decline in tree canopy leaf area index, above‐ground tree litter production and forest floor decomposition rate in early “aggradation” stage of forest development. This slower litter decomposition was mainly driven by plasticity of oak trees that produced leaf litter of poorer quality and decomposability following thinning, for example, litter richer in secondary metabolites such as condensed tannins. Change in microenvironmental conditions also contributed to the slowdown of litter decomposition, likely as a result of the less buffered microclimate associated with larger tree canopy opening. No change in soil faunal effect induced by thinning was observed.
Thinning intensification resulted in a limited decrease in forest floor C stock. Indeed, the slower litter decomposition offset nearly half of the forest floor C loss associated to the reduced litterfall in “aggradation” stage.
Our study demonstrated that phenotypic plasticity in leaf litter traits of a dominant tree species can strongly affect ecosystem functioning by slowing forest floor decomposition following thinning intensification, in turn partly mitigating the negative effect of thinning on forest floor C storage.
Abstrait
La production et la décomposition des litières sont des processus clés du cycle du carbone dans les écosystemes terrestres. Comprendre comment ceux‐ci sont affectés par les facteurs du changement global est essentiel pour évaluer les conséquences potentielles sur la dynamique du carbone et les boucles de rétroactions avec le climat. Les forêts tempérées sont fortement menacées par le phénomène croissant des vagues de chaleur et de sécheresses estivales, qui peuvent entraîner une mortalité massive des arbres. La gestion forestière pourrait être utilisée pour rendre les peuplements forestiers mieux adaptés au changement climatique. La réduction de la densité des peuplements en intensifiant les éclaircies est par exemple mise en avant comme une stratégie d’adaptation efficace pour renforcer la résistance des peuplements aux sécheresses. Dans cette étude portant sur des forêts de chênes sessiles, nous avons examiner comment cette adaptation de la gestion forestière pourrait affecter les processus écosystémiques liés au recyclage du carbone dans les couches organiques du sol forestier, à savoir le tapis forestier.
Nous avons sans surprise observé qu'une coupe partielle des peuplements par les opérations d'éclaircie entrainait une ouverture de la canopée arborée et une réduction de la production de litière aérienne. Toutefois, l'ampleur du déclin ultérieur de la masse du tapis forestier et de son stock de carbone était étonnamment faible. En effet, nous avons également observé que l’intensification des éclaircies entraînait un ralentissement de la décomposition des litières. Des investigations complémentaires ont révélé que ce ralentissement s’expliquait principalement par la production de feuilles de chêne de moindre qualité biochimique après éclaircies. Cette litière était plus riche en métabolites secondaires récalcitrants tels que les tannins, et était donc moins décomposable. La plus grande instabilité du microclimat lié à l'ouverture de la canopée arborée semble également contribuer à cette décomposition plus lente de la litière.
Dans l'ensemble, notre étude suggère que plusieurs mécanismes contribuent à atténuer en partie les conséquences de l'intensification des éclaircies pour le stockage du carbone dans les tapis forestiers. Parmi eux, la plasticité phénotypique des arbres apparaît comme un facteur sous‐estimé mais essentiel, qui devrait donc être mieux pris en compte pour comprendre la réponse du fonctionnement des écosystèmes forestiers au changement global.
A plain language summary is available for this article.
Plain Language Summary
In forestry, harvest models have become popular for forecasting thinning under business-as-usual scenarios. There are two binary processes involved in thinning operations: (i) whether a plot is to be ...thinned and (ii) whether a particular tree within that plot is to be harvested. These processes can be modeled using logistic regressions. The data used to fit such models come from forest inventories, where the observations are not usually independent. Random effects can be used to deal with these correlations. However, fitting the plot-level and tree-level models independently hinders the estimation of the covariance between the random effects of both models. The objective of this paper was to develop a statistical method for the simultaneous prediction of harvest probabilities at the plot and tree levels in a single mixed-effects model. We developed a maximum likelihood estimator based on the joint distribution of the probability that a given plot is thinned and the probability that a given tree within that plot is harvested. The estimator was derived from a zero-altered binomial form, but it assumed distinct harvest probabilities for each single tree. The estimator was tested in the case study of mixed stands of oak (Quercusspp.) and beech (Fagus sylvatica L.) in Northern France.
Size-density relations are equations for predicting the number of live trees as a function of an average stand characteristic. They are usually only known for the maximum density (RDI equal to 1). ...Based on the size-density trajectory followed by the values (Cg, N) for mean girth and number of live stems per hectare, we were able to establish a new size-density relation model for a pure even-aged Douglas-fir stand —in the form of a piecewise polynomial function— representative of the development of an unthinned stand, from initial to maximum density. It appears that the flex points in the various trajectories that are characteristic of the onset of mortality have a linear relation with each trajectory being a function of initial stand density alone. The maximum size-density line and the line for onset of mortality can be used to build density diagrams that could be helpful in establishing rules for stand density management.
Les relations taille-densité sont des équations de prédiction du nombre d’arbres vivants en fonction d’une caractéristique moyenne de peuplement. Elles ne sont généralement connues que pour la densité maximum (densité relative RDI égale à 1). Sur la base de la trajectoire taille-densité suivie par les valeurs (Cg, N) de la circonférence moyenne et du nombre de tiges vivantes à l’hectare nous avons pu établir pour le Douglas en peuplement pur et équienne un nouveau modèle de relation taille-densité — sous la forme d’une fonction polynomiale par morceaux — pour représenter le développement d’un peuplement non éclairci, depuis l’installation jusqu’à la densité maximum. Il apparaît que les points d’inflexion des différentes trajectoires, caractérisant le début de la mortalité, sont liés par une relation linéaire, chaque trajectoire étant seulement fonction de la densité initiale du peuplement. La droite taille-densité maximum et celle d’apparition de la mortalité permettent d’envisager la construction de diagrammes de densité pour aider à établir des règles de décision sylvicole (stand density management).
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