François Ningre

Modifying the microclimate around young oaks through vegetation manipulation : Effects on seedling growth and branching

Abstract The objective of this research was to evaluate the influence of vegetation control on the microclimate (light, air temperature, vapor-pressure deficit (VPD)) and on the growth of young planted, sessile oak seedlings. Three types of vegetation control, creating a range of above-ground interaction intensity, were compared. In the open treatment, vegetation was chemically controlled and was maintained at a low height; in the closed treatment, vegetation was manually controlled and a few dominant individuals that overtopped the oak seedlings were cut; and in the sheath treatment, the vegetation in a 0.5-m diameter circle around each seedling was manually cut to a height equal to three quarters of the total seedling height. In all treatments, a strong reduction of incoming radiation was observed, as well as an increase in air temperature and VPD during the day, and a decrease in daily PET, compared to the reference located 4xa0m above the vegetation. However, the neighboring vegetation did not modify the microclimate around the seedling to a level that could have induced a significant reduction in seedling photosynthesis and, thus, in seedling growth. Seven years after planting, seedlings were shortest in the open treatment and tallest in the closed treatment (334, 372, 378xa0cm in the open, sheath and closed treatments, respectively). Seedlings in the open treatment allocated proportionally more biomass to diameter increment than to height growth, compared to seedlings in the sheath and closed treatments. The seedlings averaged 1.77, 1.10 and 1.00 forks in the open, sheath and closed treatments, respectively, and the average height of the lowest fork was 171, 206 and 226xa0cm in the same treatments. The reduction of the number of forks per seedling was not related to a reduction of the appearance of new forks, but rather to a reduction in the persistence of the existing forks. These changes in stem allometry and in fork development seemed to be related to the lateral shade afforded by the vegetation.

Growth and posture control strategies in Fagus sylvatica and Acer pseudoplatanus saplings in response to canopy disturbance

BACKGROUND AND AIMSnForest tree saplings that grow in the understorey undergo frequent changes in their light environment to which they must adapt to ensure their survival and growth. Crown architecture, which plays a critical role in light capture and mechanical stability, is a major component of sapling adaptation to canopy disturbance. Shade-adapted saplings typically have plagiotropic stems and branches. After canopy opening, they need to develop more erect shoots in order to exploit the new light conditions. The objective of this study was to test whether changes in sapling stem inclination occur after canopy opening, and to analyse the morphological changes associated with stem reorientation.nnnMETHODSnA 4-year canopy-opening field experiment with naturally regenerated Fagus sylvatica and Acer pseudoplatanus saplings was conducted. The appearance of new stem axes, stem basal diameter and inclination along the stem were recorded every year after canopy opening.nnnKEY RESULTSnBoth species showed considerable stem reorientation resulting primarily from uprighting (more erect) shoot movements in Fagus, and from uprighting movements, shoot elongation and formation of relay shoots in Acer. In both species, the magnitude of shoot uprighting movements was primarily related to initial stem inclination. Both the basal part and the apical part of the stem contributed to uprighting movements. Stem movements did not appear to be limited by stem size or by stem growth.nnnCONCLUSIONSnStem uprighting movements in shade-adapted Fagus and Acer saplings following canopy disturbance were considerable and rapid, suggesting that stem reorientation processes play a significant role in the growth strategy of the species.

Deer-mediated expansion of a rare plant species

Numerous plant colonizations have been putatively attributed to deer, based on plant species traits, fur brushing or dung analyses. But, in woodlands, direct links between the expansion of zoochorous plant species and ungulate presence have seldom been reported. Based on coupled floristic and browsing surveys, repeated in time, we analysed the causes of the spatio-temporal progression of the epizoochorous species Cynoglossum germanicum over 30xa0years in a network covering an 11000xa0ha forested area in north-eastern France. In this area, deer populations reached a peak in the 1970s, then were reduced in order to meet forest management requirements. Although initially rare and protected locally, C. germanicum has displayed an unexpected fast colonization rate during the last few decades but only in the northern part of the forest, which previously had the highest animal populations. Absent in the initial 1976 survey, C. germanicum occurred in 8% of the plots in 1981, then 46% in 2006. Logistic regression models revealed that the probability of occurrence of C. germanicum in 2006 increased not only with light indicator values, in accordance with its ecological requirements, but also with past deer browsing pressure. This result provides direct evidence of long-lasting impacts of deer populations on plant species distribution. Combining two complementary traits, animal transport and herbivory avoidance, C. germanicum benefited from epizoochorous dispersal and, once established, was protected from deer browsing by the presence of toxic proteins in its tissues. Due to the triggering role of ungulates, this species switched from the status of rare to that of colonizer within only a few decades.

Evaluation and comparison of size–density relationships for pure even-aged stands of ash (Fraxinus excelsior L.), beech (Fagus silvatica L.), oak (Quercus petraea Liebl.), and sycamore maple (Acer pseudoplatanus L.)

Abstract• IntroductionSize-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.• ObjectivesSize-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.• ResultsWe 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.

Xylem traits in European beech (Fagus sylvatica L.) display a large plasticity in response to canopy release

Key messageThe position of trees in the canopy impacts xylem structure and its inter-annual variation. After canopy release, the increase in the hydraulic conductivity of growth rings was driven by an increase in radial growth in large trees, and by both an increase in radial growth and changes in xylem structure in saplings.ContextForest canopies are frequently subjected to disturbances that allow understory trees to access the upper canopy. The effect of canopy release on xylem anatomy has been assessed in juvenile trees and saplings, while the potential acclimation of larger trees remains poorly documented.AimsWe estimated the potential hydraulic conductivity of growth rings in large understory trees compared to overstory trees, and evaluated the responses to canopy release in large trees and in saplings.MethodsWe recorded radial growth, wood density, and vessel structure in beech trees according to their position within the canopy and their size. Xylem traits were followed during 6xa0years after canopy release for large trees, and during 2xa0years for saplings. Vessel diameter and frequency as well as ring area were used to compute the potential annual ring hydraulic conductivity.ResultsLarge understory trees displayed lower radial growth increments and lower potential annual ring hydraulic conductivity than overstory trees. After canopy release, potential annual ring hydraulic conductivity increased in large trees, due exclusively to increased radial growth without any change in specific hydraulic conductivity. It increased in saplings due to both increased radial growth and increased specific conductivity.ConclusionTree size impacted xylem structure and resulted in plasticity of the potential hydraulic conductivity of the annual tree ring following canopy release.

La fourchaison du Frêne en plantation : causes, conséquences et contrôle

En préambule, il faut rappeler la définition que nous avons retenue de la fourche . Une fourche se caractérise par un développement réduit du bourgeon terminal relativement à celui des bourgeons latéraux inférieurs ou par labsence de pousse terminale ; la prise de « relais » est dès lors assurée par un ou plusieurs bourgeons latéraux inférieurs de croissance initiale voisine mais à potentiel de développement variable.

Modeling size-density trajectories for even-aged beech (Fagus silvatica L.) stands in France

Key messageWe 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 12–33xa0years of age. A new piecewise polynomial function family was fitted to the trajectories, giving way to various applications. Initial number of stems per hectare (N0) and mean girth at breast height at the onset of mortality (Cg0) 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.ContextThis 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.AimsThe 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 methodsA 33-year-old beech spacing trial beginning at 12xa0years 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.ResultsThe 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.ConclusionA 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.

Time shifts in height and diameter growth allocation in understory European beech (Fagus sylvatica L.) following canopy release

Key messageIn 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.AbstractUnderstory 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.

A carbon accounting tool for complex and uncertain greenhouse gas emission life cycles

Abstract Software applications for life cycle assessment of greenhouse gas (GHG) emissions have become popular over the last decade. Their objective is to provide insight into how GHG emissions could be reduced in the sectors defined by the UNFCCC. However, boundaries between these sectors are not closed and current tools are not designed to represent this complexity or to assess the numerous sources of uncertainties. In this paper, we present CAT v1.0, software developed for managed forests in the LULUCF sector, but whose emission life cycle is linked to that of other sectors. While the structure of the software follows IPCC Guidelines, it also contains additional features such as an embedded Monte-Carlo error propagation technique and a user-friendly flux manager that allows for complex cradle-to-grave representations of the wood transformation industry. The flexibility of the software is illustrated through two case studies in northeastern France.

Trajectoires d’autoéclaircie du Douglas (Pseudotsuga menziesii ) en France

Les relations taille-densité sont des équations de prédiction du nombre d’arbres vivants par unité de surface en fonction d’une caractéristique moyenne de peuplement. En notant Ln le logarithme népérien, Cg la circonférence quadratique moyenne à 1,30 m, N le nombre de tiges à l’hectare, la trajectoire suivie dans l’espace (Ln(Cg), Ln(N)) par les peuplements non éclaircis depuis les premiers stades de développement a pour propriété bien connue d’être linéaire lorsque la densité maximum a été atteinte, et, dans une certaine mesure, dépend peu de la productivité de la station (Reineke, 1933). C’est sous cette forme que les données de cette trajectoire ont été considérées pour les travaux présentés ici, avec un échantillon de peuplements de Douglas équiennes purs provenant de plusieurs régions de France. L’occasion d’établir ces trajectoires est rarement rencontrée, peu d’espèces bénéficiant du support expérimental nécessaire et en particulier des mesures régulières de la croissance dès l’installation des peuplements. En France, ces trajectoires n’ont été élaborées que pour une seule autre espèce : le Hêtre (Ningre et al., 2016).