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

International audienceAbstractKey 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

Deciphering hybrid larch reaction norms using random regression

The link between phenotypic plasticity and heterosis is a broad fundamental question, with stakes in breeding. We report a case-study evaluating temporal series of wood ring traits of hybrid larch (Larix decidua × L. kaempferi and reciprocal) in relation to soil water availability. Growth rings record the tree plastic responses to past environmental conditions, and we used random regressions to estimate the reaction norms of ring width and wood density with respect to water availability. We investigated the role of phenotypic plasticity on the construction of hybrid larch heterosis and on the expression of its quantitative genetic parameters. The data came from an intra-/interspecific diallel mating design between both parental species. Progenies were grown in two environmentally contrasted sites, in France. Ring width plasticity with respect to water availability was confirmed, as all three taxa produced narrower rings under the lowest water availability. Hybrid larch appeared to be the most plastic taxon as its superiority over its parental species increased with increasing water availability. Despite the low heritabilities of the investigated traits, we found that the expression of a reliable negative correlation between them was conditional to the water availability environment. Finally, by means of a complementary simulation, we demonstrated that random regression can be applied to model the reaction norms of non-repeated records of phenotypic plasticity bound by a family structure. Random regression is a powerful tool for the modeling of reaction norms in various contexts, especially perennial species

Contribution relative de la végétation du sous-bois dans la consommation en eau des placettes forestières soumises aux changements de climat et de pratiques

In the context of ongoing increase of drought in temperate forests, forest managers consider the reduction of stand density to limit soil water depletion. The reduction of tree canopy density increases light below canopy and allows the development of monopolistic understorey vegetation. Our objectives were to characterize the evapotranspiration (ETu) of common understorey plants (Molinia caerulea, Calluna vulgaris, Pteridium aquilinum and Rubus sp.) and to quantify their impacts on soil water content (SWC) in mature oak stands (Quercus petraea). A first experiment was set up in a greenhouse where the 4 understorey species were potted and subjected to 2 levels of light transmittance and 3 levels of SWC. Microclimate and ETu were monitored. A second experiment was carried out on 20 plots (10 with M. caerulea and 10 with P. aquilinum) in oak stands with contrasted LAI. On each plot, two circular areas were set up, one weeded and the other untouched. SWC, microclimate and ETu were monitored. M. caerulea and C. vulgaris are more water spenders, whereas P. aquilinum and Rubus sp. are more water savers under water stress. Soil water depletion was faster with increasing understorey vegetation LAI, which was directly linked to tree canopy opening with a threshold of tree LAI of 2-3 below which the understorey contribution could offset the reduction of tree ET. The experimentations showed that the relative contribution of understory vegetation in the ecosystem water balance is significant, and depends on SWC and on the understorey species identity. These results show the necessity to consider understorey vegetation in forest management when water availability is an issue. Lower tree canopy density could increase the understorey ETu and soil water stress for trees. From a management perspective, thinning should be designed as a compromise between the reduction of tree leaf area to reduce ET, and maintaining sufficient tree canopy to restrict the growth and ETu of understorey vegetation.Avec l'augmentation des sécheresses dans les forêts tempérées, la gestion sylvicole envisage de réduire la densité des peuplements adultes pour limiter le déficit hydrique. Cependant, la réduction de la canopée arborée augmente le rayonnement dans le sous-bois et permet le développement de la strate herbacée monopoliste. Nos objectifs étaient de caractériser l'évapotranspiration (ETu) de 4 herbacées monopolistes (Molinia caerulea, Calluna vulgaris, Pteridium aquilinum and Rubus sp.) et de quantifier leur impact sur la teneur en eau du sol (SWC) dans des peuplements de Quercus petraea. L’ETu des quatre herbacées cultivées en pot a été quantifiée en conditions semicontrôlées caractérisées par deux niveaux d’éclairement relatif et 3 niveaux de SWC. In situ, le SWC, le microclimat et l’ETu de P. aquilinum et M. caerulea ont été mesurés sur 20 dispositifs dans les peuplements forestiers de Q. petraea présentant un gradient croissant de LAI. Chaque dispositif était subdivisé en 2 placettes, l’une était désherbée et l'autre enherbée. M. caerulea et C. vulgaris ont une faible régulation de leur ETu alors que P. aquilinum et Rubus sp. ont une stratégie conservatrice face au stress hydrique. SWC diminue plus rapidement quand le LAI de la strate herbacée augmente, ce qui est directement lié à l’ouverture du couvert arboré avec une valeur seuil de LAI de la strate arborée à 2-3, sous laquelle la contribution de la strate herbacée à l’évapotranspiration du peuplement pourrait compenser la diminution de celle de la strate arborée. Ces résultats montrent la nécessité de considérer la végétation du sous-bois dans la gestion sylvicole notamment lorsque la ressource hydrique est limitante. Ainsi, la réduction de la densité de la strate arborée doit être un compromis entre la réduction de la surface foliaire des arbres pour réduire l’ET, et le maintien d’une densité suffisante de la strate arborée pour limiter la croissance et l'ETu de la végétation du sous-bois

Understorey plant contribution in water balance along a gradient of oak stand density

International audienceIn the context of climate change, water resources of temperate forest ecosystems may become limiting with risk of stand decline. Forest managers advice to reduce stand density to limit stand water consumption, but this often leads to the development of monopolistic understorey plants. Our objective was to quantify the relative water uptake of two common understorey plants (Molinia cerulea Moench and Pteridium aquilinum Kuhn) in mature Quercus petraea Liebl. stands close to Orléans (47°49’29”N, 2°29’02”E), France. Twenty plots (10 plots per species) were established in spring 2012 along a gradient of stand basal area (from 10 to 30 m².ha-1). On each plot, two circular areas (40 m-diameter) were set up, one weeded and the other untouched. Soil water content (SWC) and tree growth were monitored every two weeks. Understorey had a significant impact on stand water balance in 2012. The SWC minimal values were higher in weeded plots, and under stand with bracken SWC decreased faster than with purple moor-grass. In 2013 we will pay attention to the phenology of both species and consequences on the water balance dynamic. These results will be discussed in relation forest management in the perspective of increasing droughts in the next decades

Fougère aigle et jeunes arbres : une cohabitation quasi impossible

National audienceL’ensemble des propriétés de la fougère aigle, sa très forte interception de la lumière, sa tolérance à la sécheresse, sa forte captation des éléments minéraux, sa forte épaisseur de litière, sa toxicité probable pour les autres plantes et les herbivores, en font une redoutable inhibitrice de l’installation des semis et des jeunes arbres. Même sous couvert forestier, son fort taux de développement à l’ombre ne permet pas d’envisager une régénération naturelle du peuplement en sa présence

Biologie et écologie de la fougère aigle

National audienceBracken is very common in France, his only weakness being a relative difficulty to sexual reproduction and his failure to grow in too wet or too skeletal soils. However, its vegetative reproduction capabilities and its extraordinary lifespan allow it to reach a size comparable as those of largest living organisms. A low light level is sufficient to ensure its survival in the undergrowth before a tree fall or a fire allows it to regain full development. Its biological features induce a high level of interference against other plant species and insure its sustainability.La fougère aigle est très fréquente en France, ses seules faiblesses étant une relative difficulté à se reproduire par voie sexuée et une intolérance aux sols trop humides ou trop squelettiques. Elle bénéficie par contre d’excellentes capacités de reproduction végétative et d’une durée de vie extraordinaire lui permettant d’atteindre des dimensions dignes des plus grands organismes vivants. Elle se contente de peu de lumière pour survivre en sous-bois avant de bénéficier d’une coupe, d’un chablis ou d’un incendie pour retrouver un plein développement. Ses qualités biologiques induisent un niveau d’interférence élevé vis-à-vis des autres espèces végétales et assurent sa pérennité

The contribution of understorey vegetation to ecosystem evapotranspiration in boreal and temperate forests: a literature review and analysis

International audienceIn the context of increasing heat periods and recurrence of droughts, and thus higher soil water depletion, we explored and quantified the role of understorey vegetation in ecosystem evapotranspiration in boreal and temperate forests. We reviewed and analysed about 200 papers that explicitly gave figures of understorey vegetation evapotranspiration relative to different stand features and traits. Understorey vegetation accounted on average for one-third of total ecosystem evapotranspiration during the growing season. Overstorey leaf area index (LAI) is the main variable that drives understorey evapotranspiration through radiation interception. Most data show that below an overstorey LAI of 2–3, the contribution of the understorey vegetation to ecosystem evapotranspiration increases exponentially, following the exponential increase of the climatic demand, i.e. potential evapotranspiration. Different factors have the potential to modulate this effect such as species composition and phenology, root distribution, and interaction with droughts. Consequently, managers must be aware that depending on understorey species present on site and stand structure, understorey vegetation can contribute significantly to a negative stand water balance

Evaluation et homologation de variétés issues des vergers à graines de l'Etat - programme 2018

This report describes the work done in 2018 by Irstea, INRA, ONF and CNPF in the frame of a joint research program dedicated to seed orchard testing. Two species or groups of species were involved: Douglas-fir and larch. With regard to Douglas-fir, the data collected 5 years after planting in 24 trials have been synthesized, which represents an important step forward. The 8 French seed orchards do not differ for survival but some differ for height growth. It is interesting to notice that the ranking varies with the type of site: a seed orchard characterized by a poor growth in cool and moist environments performs better in hot and dry sites. In addition, analysis of data from 18 trials allowed us to improve our knowledge about the performance and ecological requirements of French orchards. For larch, the main result concerns the estimation of the performance of a seed orchard composed of F1-hybrids. This work resulted in the admission of this orchard to the national register of basic materials, in 'qualified' category, and in a first cone collection.Ce rapport présente les actions entreprises en 2018 par Irstea, l'INRA, l'ONF et le CNPF dans le cadre du programme d'évaluation des variétés issues des vergers à graines de l'Etat. Elles concernent essentiellement le Douglas et les mélèzes. Pour le Douglas, le bilan des résultats obtenus 5 ans après plantation dans 24 dispositifs du réseau national d'évaluation représente une étape importante. Si les 8 vergers à graines français ne diffèrent pas pour la survie, certains d'entre eux se différencient pour la croissance en hauteur. D'autre part, le classement varie en fonction du type de milieu : un verger, qui se distingue par des performances médiocres dans les environnements froids et humides, voit son classement amélioré en milieux chauds et secs. Par ailleurs, les connaissances sur les exigences écologiques et le comportement des vergers ont progressé grâce à l'analyse de données issues de 18 tests âgés de 2 à 8 ans et un essai en conditions contrôlées destiné à étudier la résistance à la sécheresse. Pour les mélèzes, la principale avancée concerne l'évaluation des performances, en termes de survie, de croissance et de forme d'un verger constitué d'hybrides F1. Ce travail a permis d'admettre ce verger au registre national des matériels de base, en catégorie qualifiée, et d'y réaliser la première récolte commerciale

Evaluation et homologation de variétés issues des vergers à graines de l'Etat - programme 2017. Rapport final

This report describes the work done in 2017 by Irstea, INRA, ONF and CNPF in the frame of a joint research programme dedicated to seed orchard testing. Two species or groups of species were involved: Douglas-fir and larch. With regard to Douglas-fir, the common network of comparative trials has recently been completed. Actions now aim to improve our knowledge on the performance of the 8 French seed orchards considering adaptive traits, vigor and stem form. In 2017, first results were obtained for three additional traits: growth in diameter, branch angle and branch density. For larch, the 'calling card' of the hybrid variety Rêve-vert enlarged with new results for important wood traits. In addition, other studies were continued: artificial inoculation of larch canker to assess clone vulnerability, identification of hybrid materials using near infrared spectrometry (NIRS).Ce rapport présente les actions entreprises en 2017 par Irstea, l'INRA, l'ONF et le CNPF dans le cadre de l'évaluation des variétés issues des vergers à graines de l'Etat. Elles concernent le Douglas et les mélèzes. Pour le Douglas, le réseau national d'évaluation des vergers à graines de l'Etat se trouve à présent dans sa configuration définitive. Les principales actions visent à comparer les performances des variétés pour des caractères adaptatifs, la croissance et la forme de l'arbre. En 2017, de premiers résultats ont été obtenus pour trois nouveaux caractères : la croissance en diamètre, la densité de branches et leur angle d'insertion. Pour le mélèze, la connaissance des performances de la variété hybride Rêve-vert a progressé grâce à la caractérisation de caractères déterminant la qualité du bois (densité et duraminisation). D'autre part, plusieurs études se sont poursuivies : estimation de la vulnérabilité au chancre des clones des vergers de mélèze d'Europe, identification des hybrides par spectrométrie infrarouge

Le bilan hydrique des forêts est-il influencé par la composition et la densité des peuplements ?

International audienceResults suggest that mixed oak-Scots pine stands have a lower water consumption and that water deficit stress could be reduced compared to pure oak stands. However, the mixture does not seem to help to reduce the period of water flooding, which is a limiting factor to oak growth in spring