Philippe Balandier

Influence of several tree traits on rainfall partitioning in temperate and boreal forests: a review

Abstract• In forests, rainfall is partitioned into intercepted water (IW), throughfall (TF) and stemflow (SF). We reviewed the majority of published works comparing water flows among tree species in temperate and boreal forests to test the effect of several tree traits on water flows.• We hypothesized that water flows differed between evergreen and deciduous species, and according to successional status and bark roughness. We also investigated that water flows would be explained by stand variables such as basal area.• Linear mixed models fitted on reviewed data showed that evergreens had a lower TF than deciduous trees (decrease of 13.9% of total precipitation year-round and 8.4% over the growing period). Similar results were found for conifers compared to broadleaves. TF also declined with the successional status from pioneer to late-successional tree species. SF decreased with bark roughness from smoother to rougher bark. Evergreens had water flows that were dependent on age of the stand, especially for TF which increased by 15.6% of total precipitation from young to adult forests.• The large scale of TF differences between tree genera together with specific transpiration amounts and rooting features highlighted in other studies should result in significant differences in soil water content among tree species. This may have consequences on stand fitness and growth, and understory vegetation.Résumé• En forêt, l’eau de pluie se fractionne en eau interceptée (IW), égouttements (TF) et écoulements le long des troncs (SF). Nous analysons les publications comparant les flux d’eau selon les essences d’arbres, pour les forêts tempérées et boréales, et nous testons l’effet de quelques traits des arbres sur ces flux d’eau.• Nous faisons l’hypothèse que les flux d’eau diffèrent entre les essences à feuilles persistantes et les essences à feuilles caduques, et diffèrent selon le statut successionnel de l’essence et la rugosité de son écorce. Nous testons aussi l’effet de variables du peuplement telles que sa surface terrière.• L’analyse des données (par modèle linéaire à effets mixtes) montre que les essences à feuilles persistantes ont des égouttements (TF) plus faibles que les essences à feuilles caduques (diminution de 13,9 % des précipitations totales en moyenne sur toute l’année et de 8,4 % pour la période estivale). Les résultats sont similaires en comparant les conifères au feuillus. Les égouttements diminuent aussi avec le stade successionnel de l’essence, des pionnières vers les dryades. Les écoulements (SF) diminuent nettement avec la rugosité de l’écorce, des plus lisses vers les plus rugueuses. Les essences à feuilles persistantes ont des flux d’eau qui dépendent de l’âge du peuplement : les égouttements augmentent de 15,6 % des précipitations totales en passant des jeunes forêts aux forêts adultes.• L’amplitude des différences d’égouttements entre essences que nous avons observée, conjointement avec les différences spécifiques de transpiration et de caractéristiques du système racinaire mentionnées dans la littérature, induisent probablement des différences de contenu en eau du sol entre les essences forestières. Cela peut avoir des conséquences sur la santé et la croissance des peuplements, et sur la végétation des strates inférieures.

The state of forest vegetation management in Europe in the 21st century

COST (COST is an intergovernmental framework for European cooperation in science and technology. COST funds network activities, workshops and conferences with the aim to reducing the fragmentation in European research) Action E47, European Network for Forest Vegetation Management—Towards Environmental Sustainability was formed in 2005 and gathered scientists and practitioners from eighteen European countries with the objective of sharing current scientific advances and best practice in the field of forest vegetation management to identify common knowledge gaps and European research potentials. This paper summarizes the work of the COST action and concludes that although diverse countries have by necessity adopted different means of addressing the challenges of forest vegetation management in Europe in the 21st century, some common themes are still evident. In all countries, there is a consensus that vegetation management is a critical silvicultural operation to achieve forest establishment, regeneration, growth and production. It appears that herbicides are still in use to some degree in all the countries reviewed, although at a lower intensity in many of the northern countries compared to other regions. The most common alternatives to herbicides adopted are the use of mechanical methods to cut vegetation and achieve soil cultivation; overstorey canopy manipulation to control vegetation by light availability; and in some instances the use of mulches or biological control. Any reductions in herbicide use achieved do not seem to have been driven solely by participation in forest certification schemes. Other factors, such as national initiatives or the availability of additional resources to implement more expensive non-chemical approaches, may be equally important. The development of more cost-effective and practical guidance for managers across Europe on non-chemical control methods can best be brought about by future collaborative research into more sustainable and holistic methods of managing forest vegetation, through the identification of silvicultural approaches to reduce or eliminate pesticide use and through gaining a better understanding of the fundamental mechanisms and impacts of competition.

Forest vegetation management under debate: an introduction

is anintegral part of silvicultural practices in many parts of theworld (Wagner et al. 2006; Richardson et al. 2006; Newton2006). However, there are substantial differences betweenthe continents with regard to the preferred methods. Theimplementation of tending measures to control woodycompetitors is common in European even-aged stands as inother parts of the world. In contrast, the use of herbicidesfor weed control is much less common in Europe than forexample in North America, South Africa, Australia andNew Zealand where chemical vegetation control, inparticular, is used and promoted strongly in plantationforestry (Newton 2006). If at all, in Europe, herbaceousvegetation is controlled predominantly by mechanical sitepreparation, mulching or other techniques (McCarthy et al.2010, this issue). An overwhelming amount of literaturehas shown that chemical vegetation control can result inhuge gains in wood volume (Wagner et al. 2006). This hadlet to favourable reports of chemical vegetation control asthe following statements may indicate: ‘in most instances,forests cannot be managed economically without herbi-cides if the goal is to grow seedlings at the potential of thesite and the plant community includes sprouting hardwoodsand shrubs of rhizomatous forbs and ferns’ (McDonald andFiddler 1993); ‘most regeneration efforts around the worldwould fail or be severely delayed without effective forestvegetation management … primarily using herbicides’(Wagner et al. 2006); ‘reducing competition for desirabletrees with modern chemicals has less impact on soil andwildlife habitat and lowers human health risk per unitof effectiveness than mechanical or manual methods’(Newton 2006). In contrast, in Europe at present there ispolitical consensus for a reduction in the use of herbicidesas much as possible (i.e. The EU Thematic strategy on theSustainable Use of Pesticides). Society perceives Europeanforests as the last quasi-natural compartments of a land-scape which has been entirely manipulated for more than2,000 years. In this context, herbicides are view by thepublic as a serious threat for the maintenance of the set ofmultiple functions that forests provide (Merlo and Croitoru2005; Schmithu¨sen 2007; Ammer and Puettmann 2009).

Light-dependent development of two competitive species (Rubus idaeus, Cytisus scoparius) colonizing gaps in temperate forest

Forest regeneration can be inhibited by competition for environmental resources (water, nutrients, light) between tree seedlings and some competitive species that are generally light-demanding species developing in gaps. The study’s aim was to quantify the development of two competitive species (Rubus idaeus and Cytisus scoparius) present in the chaîne des Puys, France, relatively to light in gaps inside Picea abies stands. On 29 transects linking the stand to the gap centre (223 points), light intensity was measured (0–80% of relative light) and floristic measurements (cover and height of the different species) were done. Development of both competitive species is positively connected to light, with a bell-shaped curve with a maximum of 40–50% for R. idaeus and a quite constant increase to 80% (maximum of light recorded in the experimentation) for C. scoparius. These results are discussed relatively to understorey vegetation management in order to favour forest regeneration.RésuméEn forêt, la compétition pour la captation des ressources (eau, nutriments, lumière) entre certaines plantes très colonisatrices et les jeunes arbres peut mettre en péril la régénération forestière. Ces espèces sont généralement héliophiles et se développent donc dans les trouées, environnements également favorables aux semis. Le but de l’étude était de quantifier le développement de deux espèces colonisatrices (Rubus idaeus et Cytisus scoparius) de la chaîne des Puys (France) en fonction de l’intensité lumineuse dans des trouées situées en forêt d’épicéas. Des mesures de lumière ont été réalisées sur 29 transects reliant l’intérieur du peuplement au centre d’une trouée (223 points au total, 0–80 % d’éclairement relatif) en parallèle à des mesures floristiques (taux de recouvrement et hauteur des différentes espèces). Les deux espèces répondent à la lumière, selon une courbe en cloche avec un maximum à 40–50 % pour R. idaeus et une augmentation quasi linéaire jusqu’à 80 % (maximum mesuré dans l’expérimentation) pour C. scoparius. Ces résultats sont discutés en terme de gestion de la végétation forestière pour favoriser la régénération.

A double-digitising method for building 3D virtual trees with non-planar leaves: application to the morphology and light-capture properties of young beech trees (Fagus sylvatica)

We developed a double-digitising method combining a hand-held electromagnetic digitizer and a non-contact 3D laser scanner. The former was used to record the positions of all leaves in a tree and the orientation angles of their lamina. The latter served to obtain the morphology of the leaves sampled in the tree. As the scanner outputs a cloud of points, software was developed to reconstruct non-planar (NP) leaves composed of triangles, and to compute numerical shape parameters: midrib curvature, torsion and transversal curvature of the lamina. The combination of both methods allowed construction of 3D virtual trees with NP leaves. The method was applied to young beech trees (Fagus sylvatica L.) from different sunlight environments (from 1 to 100% incident light) in a forest in central France. Leaf morphology responded to light availability, with a more bent shape in well-lit leaves. Light interception at the leaf scale by NP leaves decreased from 4 to 10% for shaded and sunlit leaves compared with planar leaves. At the tree scale, light interception by trees made of NP leaves decreased by 1 to 3% for 100% to 1% light, respectively.

Modeling of light transmission under heterogeneous forest canopy: an appraisal of the effect of the precision level of crown description

Abstract• ContextLight availability in forest understory is essential for many processes; it is, therefore, a valuable information regarding forest management. However, its estimation is often difficult and direct measurements are tedious. Models can be used to compute understory light, but they often require a lot of field data to accurately predict light distribution, particularly in the case of heterogeneous canopies.• AimsThe influence of the precision level of crown description was studied with a model (MμSLIM) that can be used with both detailed and coarse parameterization with the aim of reducing field data requirements to a minimum.• MethodsWe analyzed the deterioration of the prediction quality of light distribution to the reduction of inputs by comparing simulations to transmitted light measurements in forests of increasing complexity in three different locations.• ResultsWith a full set of parameters to describe the tree crown (i.e., crown extension in at least eight directions, crown height and length), the model accurately simulated the light distribution. Simplifying crown description by a geometric shape with a mean radius of crown extension led to deteriorated but acceptable light distributions. Allometric relationships used to calculate crown extension from trunk diameter at breast height seriously reduced light distribution accuracy.

Compared effects of competition by grasses (Graminoids) and broom (Cytisus scoparius) on growth and functional traits of beech saplings (Fagus sylvatica)

Abstract• The effect of two weeds (grasses and broom, Cytisus scoparius) competition on the growth and functional traits of European beech saplings (Fagus sylvatica) was investigated in an experimental plantation in the French Massif central.• We hypothesized that grasses would have a much more harmful effect than broom on beech growth through strong competition for soil water and nitrogen.• A randomized block design was used with three separate blocks, each possessing three types of vegetation; grasses (mainly Agrostis capillaris, Holcus mollis and H. lanatus), broom, and bare soil as a control.• Two years after plantation beech sapling growth was significantly greater in bare soil than with competition from grasses, and was intermediate on broom. Growth was positively correlated with beech leaf nitrogen content (Nm) and xylem water potential (Ψx). N and Ψx were dependent, with nitrogen uptake probably decreasing with increasing drought. There was no clear correlation between growth and light transmitted 10 cm below the apex of the saplings.• Beech leaf mass on an area basis (LMA) was correlated with the three resource variables (water, nitrogen and light) and with growth, confirming that LMA is a sensitive functional trait integrating the degree of stress experienced by the saplings.• The results suggest that broom is less competitive than grasses and, if properly managed on a sufficiently well-watered site, can generate satisfactory conditions for beech establishment and growth.Résumé• L’effet de la compétition de deux adventices (graminées et genêt, Cytisus scoparius) sur la croissance et les traits fonctionnels de plants de hêtre européen (Fagus sylvatica) a été expérimenté dans une plantation du Massif central.• Nous avons testé l’hypothèse selon laquelle les graminées ont un effet plus délétère que le genêt sur la croissance du hêtre à cause d’une très forte compétition pour l’eau du sol et l’azote.• Un plan d’expérience en trois blocs complets a été utilisé, chacun possédant trois types de végétation : des graminées (principalement Agrostis capillaris, Holcus mollis and H. lanatus), du genêt et un sol nu comme témoin.• Deux ans après la plantation, les plants de hêtre étaient significativement plus gros en sol nu par rapport à ceux subissant la compétition des graminées, ceux en présence du genêt étant intermédiaires. La croissance a été positivement reliée à la teneur en azote des feuilles (Nm) ainsi qu’au potentiel hydrique de xylème (Ψx). Nm et Ψx semblent dépendant, l’absorption d’azote diminuant avec l’augmentation de la sécheresse. Aucune corrélation significative n’a pu être mise en évidence entre la croissance des plants et la lumière transmise à 10 cm sous leur apex.• La masse foliaire rapportée à sa surface (LMA) a été corrélée aux trois variables correspondant aux ressources (azote, eau et lumière) et avec la croissance, confirmant que le LMA est un trait fonctionnel pertinent intégrant le degré de stress subi par les plants.• Les résultats suggèrent que le genêt est moins compétitif que les graminées. Sur un site suffisamment pourvu en eau, il peut produire des conditions favorables à la croissance des hêtres s’il est géré correctement.

Species-specific and generic biomass equations for seedlings and saplings of European tree species

AbstractBiomass equations are a helpful tool to estimate the tree and stand biomass production and standing stock. Such estimations are of great interest for science but also of great importance for global reports on the carbon cycle and the global climate system. Even though there are various collections and generic meta-analyses available with biomass equations for mature trees, reports on biomass equations for juvenile trees (seedlings and saplings) are mainly missing. Against the background of an increasing amount of reforestation and afforestation projects and forests in young successional stages, such equations are required. In this study we have collected data from various studies on the aboveground woody biomass of 19 common tree species growing in Europe. The aim of this paper was to calculate species-specific biomass equations for the aboveground woody biomass of single trees in dependence of root-collar-diameter (RCD), height (H) and the combination of the two (RCD2 H). Next to calculating species-specific biomass equations for the species available in the dataset, we also calculated generic biomass equations for all broadleaved species and all conifer species. The biomass equations should be a contribution to the pool of published biomass equations, whereas the novelty is here that the equations were exclusively derived for young trees.

Forest management adaptation to climate change: a Cornelian dilemma between drought resistance and soil macro-detritivore functional diversity

Global warming induces new constraints on forest ecosystems and requires forest management adaptation. The reduction in stand density is currently debated as a potential tool to face increasing summer drought risk by improving forest resistance to climate change-induced tree mortality. However, few studies have yet assessed the impacts of this management change on soil biodiversity. We conducted a large-scale, multi-site assessment of the response of soil macro-detritivore assemblages and soil functioning to experimental manipulations of stand density. A total of 33 stands were studied covering a wide gradient of stand density, that is stand basal area from 25 to 437m(2)ha(-1), stand age, that is 18-171years old, and local abiotic context. We observed contrasting responses as a function of both taxonomic and functional groupings. Exploratory analysis using causal diagrams, that is path analysis, highlights that these changes were mainly related to alterations in understorey vegetation, microclimatic and soil pH conditions. The response of soil macro-detritivore assemblages to stand density manipulation was consistent over the gradient of stand ages. Among the litter-dwelling macro-detritivores, millipede abundance and diversity decreased with stand density reduction, while woodlice and epigeic earthworms were unaffected. Further, a shift in soil-dwelling earthworm community composition was observed in mull stands. Endogeic earthworm abundance showed a sharp increase with stand density reduction, which translated into an increase in soil respiration. In contrast, anecic earthworm abundance decreased and was strongly associated with a decline of the rate of forest floor turnover.Synthesis and applications. Our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. Managing stand density of oak forests at an intermediate level, that is 25m(2)ha(-1), appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability. Our study provides strong evidence that reductions of stand density will have substantial impacts on soil macro-detritivore assemblages and cascading effects on soil functioning, particularly in mull stands. Managing stand density of oak forests at an intermediate level, that is 25m(2)ha(-1), appears to be best to optimize the trade-off between improving forest resistance to climate change and ensuring the conservation of functional diversity to preserve forest ecosystem functioning and stability.

Transpiration of four common understorey plant species according to drought intensity in temperate forests

Key messageWater use patterns of understorey vegetation are species-dependent.Calluna vulgarisshowed little or no regulation of transpiration in response to soil water depletion or air vapour pressure deficit, unlike Pteridium aquilinum, Rubus sp. and Molinia caerulea.ContextEvapotranspiration at forest stand scale is the sum of three components: overstorey and understorey transpiration, and evaporation from soil. During periods of soil water shortage, evapotranspiration of trees declines significantly, but the response of understorey vegetation is less well known. Some reports suggest that understorey vegetation can sometimes be the main source of water depletion in a forest stand during drought episodes.AimsWe assessed transpiration in response to decreased soil water content (SWC) and increased vapour pressure deficit (VPD) in the atmosphere for four understorey species with contrasting patterns of resource capture.MethodsPotted plants of Pteridium aquilinum, Molinia caerulea, Calluna vulgaris and Rubus sect. Fruticosi were grown under two radiation levels combined with three levels of SWC. Temperature, radiation, VPD and transpiration were monitored.ResultsCalluna vulgaris displayed a water spender behaviour with little or no regulation of transpiration during soil water depletion and increased VPD, whereas Pteridium aquilinum showed a low transpiration rate whatever the conditions. Rubus sect. Fruticosi gradually decreased transpiration during soil water depletion and increased VPD, whereas Molinia caerulea responded strongly to soil water depletion but only moderately to VPD.ConclusionThis study highlights the importance of adding identity and water use strategy of understorey species to the tree canopy component to establish a reliable forest water balance.