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International audience; Mixed plantations have been receiving increasing attention for their documented or supposed potential benefits over monospecific plantations. In particular, the use of neighbouring (or nurse) vegetation around target plants can enhance their performance through limitation of the competing herb layer, and can also improve their morphology. Here we examine the benefits and drawbacks of using neighbour treatments on the response of target trees in open plantations. We set up an experimental plantation in southern France, in which two co-occurring target oak species (the evergreen Quercus ilex and the deciduous Quercus pubescens) were introduced in different neighbour treatments using a tree (Pinus halepensis) and a shrub species (the nitrogen-fixing Coronilla glauca). Oaks were planted with pine neighbours at two densities, with shrub neighbours, in a mixture of pines and shrubs or without neighbours. The ground vegetation was either regularly weeded or left to grow in order to detect any indirect facilitation interactions. Target oak responses (survival, growth, and morphology) were monitored over 7 years. Soil water content and light availability were also measured throughout the experiment. We found competition to be the dominant process driving interactions between neighbours and target tree species. Growth was reduced for both species, but more in weeded than in unweeded treatments, showing an alleviation of competitive interactions by neighbours through limitation of herb layer development. However, in both ground vegetation treatments, growth was severely reduced with Coronilla shrubs. Survival was only significantly impaired for the less shade-tolerant Q. pubescens oak used in combination with shrubs. The negative influence of the neighbour treatments was mainly attributable to light interception, which was particularly high by shrub canopy. Soil moisture was also slightly reduced by shrub neighbours, but it remained high with pines in the unweeded treatment owing to a limited abundance of herbs. However, stem form was improved by the neighbour treatments: oaks developed narrower crowns and greater slenderness with neighbours, whereas oaks in the open showed a bushy morphology. These findings emphasize the need to clearly identify key objectives before implementing mixed plantations (e.g. maximising growth, survival, improving morphology, etc.) and to use active management to control potential undesirable effects of the neighbouring vegetation on target plants.

Ce cahier diffuse des connaissances scientifiques (non exhaustives) sur les effets du changement climatique sur l'agriculture et la forêt en Provence-Alpes-Côte d'Azur. Les cultures (vergers, céréales, riz, maraîchage...) et les forêts (feuillus, conifères) bénéficient des bienfaits du climat méditerranéen qui offre des conditions favorables au développement des plantes sous certaines conditions, mais souffrent aussi des événements extrêmes qui le ponctuent à intervalles irréguliers. Le climat méditerranéen qui sévit en région PACA pourrait se résumer par Toulourenc (« tout ou rien » en provençal), du nom du cours d'eau à caractère torrentiel qui coule dans la vallée étroite située au pied du versant nord du mont Ventoux. Avec le changement climatique actuel, les aspects négatifs de notre climat sont appelés à se renforcer et font déjà peser sur les terroirs agricoles et les forêts emblématiques de la région de nouvelles contraintes auxquelles il est nécessaire de faire face pour éviter des conséquences trop néfastes. Mais la pérennité et le développement des systèmes agricoles et forestiers ne dépendent pas seulement de l'évolution du climat. L'urbanisation, l'occupation des sols, les pollutions locales (sol, air, eau), les incendies, mais aussi les pratiques culturales et la gestion forestière, jouent un rôle fondamental. Il convient donc de privilégier une approche transversale. Cette publication souligne les conséquences du changement climatique sur l'agriculture et la forêt en prenant soin d'identifier les enjeux environnementaux, économiques et sociaux, les risques à l'échelle régionale et locale, mais aussi les solutions susceptibles de réduire les impacts (atténuation, adaptation) et les éventuelles opportunités à saisir. Comme dans le précédent cahier, la contribution des chercheurs et experts, exerçant leur métier en région PACA et dans les territoires limitrophes4, sous forme d'articles et de zooms, apporte des éléments de compréhension afin de mieux cerner les problématiques liées au changement climatique.

XIV World Forestry Congress, Durban, ZAF, 07-/09/2015 - 11/09/2015; International audience; Management strategies, using thinning combined or not combined with underplanting, have been tested in experimental forests of Spain and southern France to promote growth, recruitment and increase their resilience. In dry and fire-prone areas of southern Spain and France, Pinus halepensis forests were thinned at different levels and hardwood species were introduced under pine canopy and in opened areas. Thinning activities increased light, modified microclimate and soil moisture availability. Growth was improved in all sites but survival showed a more contrasted response. In mountainous areas, thinning treatments applied on Abies alba forests of southern France and Pinus nigra forests of central Spain were tested. Thinned silver fir stands showed higher tree growth than the control stands. In black pine forests, maximal seedling emergence was found for high basal area values whereas survival and growth were the highest for lower values. Despite strong site differences, thinning to a basal area around 15-20 m2/ha appeared to be a good compromise to promote growth of the residual stand and recruitment. In fire-prone areas, thinning, alone or combined thinning with the introduction of resprouting hardwood species, is an efficient technique but has to be adapted to site conditions and species under focus. Finally, we discussed usual thinning limitations and options of remediation in the light of the new opportunity opened by the increasing demand in forest biomass.

Sowing acorns from Mediterranean oaks is an age-old practice around the Mediterranean Rim that has been largely replaced by planting seedlings. In this article, we assess the method overall and reconsider it in the light of recent experimentation carried out notably in South-East France and in the north of Spain. First we describe the advantages and drawbacks of sowing compared to planting, followed by the basic principles for collecting, sorting and conserving the acorns. The requirements for effective sowing are then reviewed prior to considering the conditions for success in relation to damage from wildlife (rodents, herbivores, wild boar) and methods for preventing it. Finally, we define the impact of habitat on the success of sowing and the role played by vegetation and “nursing” devices. We conclude by stressing the advantages of the technique for the restoration of habitats and the improvement of their resilience.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; International audience; The regeneration phase is a critical step in plant population dynamics, but is still incompletely understood owing to the intricacy and complexity of the factors concerned. We investigated the role played by pine emergence, survival and growth in the recruitment process of a Mediterranean pine forest under disturbances of various intensities. Disturbances were produced by site treatments including mechanical chopping, soil scarification, and controlled fire of low or high intensity. Pine seedling emergence, survival, dimensions and the relative cover of bare soil, grasses and shrub were monitored on 580 1 m2 plots over 6 years in a mature Aleppo pine forest (Pinus halepensis Mill.). Seedlings did not emerge continuously and regularly with time, but mostly in bursts, which were particularly marked in the rainy autumnal seasons. This specific pattern defined a ‘window' of emergence characterised by the length of time during which seedlings emerged, the number of emerged seedlings and the shape of the window. Survival and growth decreased from the youngest cohort to the oldest cohort. Emergence, survival and growth did not respond in the same way to treatments and soil surface conditions. In particular, scarification treatments were most favourable to emergence, whereas the high-intensity controlled fire treatment was most beneficial to survival. Shrub cover was detrimental to emergence, but positively influenced survival and growth. We conclude that environmental factors controlling survival, emergence and growth processes differ, and that their importance varies over time. A detailed understanding of the processes involved in seedling recruitment can be helpful for selecting the types and intensities of the controlled disturbances to be applied to successfully regenerate forest stands.

International audience; Context: Developing silvicultural methods to help Mediterranean forests adapt to climate change is of high importance. Introducing resprouting hardwood species below pine stands is expected to promote diversity and resilience of these stands, particularly to forest fires. Aims: To examine how the intensity of pine thinning influences understory micro environment and the establishment of various hardwood seedlings in two Mediterranean sites. Methods: Aleppo pine stands were thinned down to three levels of basal area (uncut: 30 m²/ha, moderate thinning: 13-20 m²/ha, heavy thinning: 7-10 m²/ha) at two Mediterranean sites (South-East France and South-East Spain). Seedlings of six hardwood species were introduced in the understory and their survival and growth were monitored and related to changes in micro-environment induced by thinning. Results: At both sites, thinning improved light availability and seedling diameter increment of all target species. Thinning increased extreme temperature and evaporative demand. Heavy thinning increased summer soil moisture in SE Spain but not in SE France. The worst conditions for seedling survival were reached under uncut stands in SE France and low-density stands in SE Spain. Conclusion: Thinning in pine stands accelerated seedling growth but excessive thinning worsened summer drought and affected seedling survival. Moderate thinning (15-20 m²/ha) seems to be the best option in support of the introduction of hardwoods in the understory, which can improve forest diversity and resilience in the future.

Oak coppices are the main component of the hardwood stands in Mediterranean southern France. However natural regeneration is difficult to obtain, especially in ageing coppices. In order to favour seedling establishment and to better understand oak coppice growth, an experiment was set up in 1985 in a downy oak coppice (Quercus pubescens) located in the Lure massif (south-eastern France) at an altitude of 1200m. The coppice was thinned more or less severely (= treatments): partial cuts with removal of 10, 20, 40% of the basal area, clear cut (all trees removed), traditional cut (only 100 trees left) and control (no thinning). Pre-existing and newly established seedlings were measured in 1986. The experiment also included partial thinning in 1998 and the stands were again measured in 2012. Results showed that the seedlings that were present in the years following the treatments did not survive as no regeneration was observed in 2012, whatever the treatment. In contrast, European beech seedlings successfully established and were more abundant in partial cuts (10, 20 or 40%) than in clear or traditional cuts. In such latter treatment, the density of Scots pine seedlings was higher. Beech expansion was also fostered by the species' fast growth whereas oak growth was reduced and symptoms of decay were frequently observed in the canopy of 80-90 year-old oaks. This experiment, as well as previous trials, showed that oak self-seeding was possible after thinning but seedlings did not manage to develop and surviveover the longer term. We conclude that there is a need for further experiments in order to elucidate this lack of seedling survival and to propose the most suitable silvicultural operations.; Les taillis de chêne sont la composante dominante des peuplements feuillus en région méditerranéenne française. Cependant, leur régénération est incertaine en particulier pour les taillis les plus âgés. Pour favoriser cette régénération et mieux connaître leur croissance, un dispositif expérimental avait été installé en 1985 dans un taillis avec réserves de chêne blanc en forêt domaniale de Lure à 1200m d'altitude (Alpes-de-Haute-Provence). Le dispositif testait différentes coupes d'éclaircie du taillis : éclaircies partielles (témoin ou prélèvement de 10, 20 ou 40% de la surface terrière) ou éclaircies intenses (coupe classique à 100 baliveaux/ha et coupe à blanc). Le dispositif, bien que perturbé en 1998 par une éclaircie, a été remesuré pour partie en 2012. Les résultats montrent que les semis de chêne, bien que présents dans les premières années suivant les coupes, ne se sont pas installés durablement puisque qu'aucune régénération n'a été observée en 2012 dans les différents traitements. En revanche, on note une régénération en hêtre abondante dans les éclaircies partielles et plus modeste dans les éclaircies intenses avec dans ce dernier cas une installation plus marquée du pin sylvestre. Le dynamisme du hêtre s'affirme aussi par une croissance soutenue alors que celle des réserves de chêne (âge 80-90 ans) est faible et que les arbres montrent de très fréquents signes de dépérissement de leur houppier. L'expérimentation de Lure et de précédents travaux montrent que même si l'installation initiale des semis est possible, leur développement et leur survie sur le long terme sont compromis. Nous concluons à la nécessité d'engager de nouvelles expérimentations pour pouvoir d'une part cerner les facteurs de cet échec et d'autre part proposer des itinéraires de gestion susceptibles de favoriser la régénération des taillis.

[Departement_IRSTEA]Territoires [TR1_IRSTEA]SEDYVIN; Pinus halepensis Mill. is a Mediterranean pioneer forest species with shade intolerance features. The purpose of this study is to better understand how stand fertility and allelopathic properties of adult trees influence shade acclimation of saplings. Crown growth and morphological plasticity were studied under different light, fertilization, and allelopathic conditions in a nursery experiment. We tested whether shade-acclimation capacity increases with fertilization, and is affected by autotoxicity due to pine leachates. We examined stem diameter, and crown characteristics (length, width, shape and density) in a factorial experiment with two levels for each tested factor: light (full and 20 % reduced light), fertilization (low and high rate of NPK fertilizer) and allelopathy (control and allelopathic leachates uptake). In our study, shading induced a significantly higher crown length, width and surface. Fertilization strongly increased crown length and vertical expended crown shape (the ratio crown length/crown width). Leachates uptake reduced crown length and density, highlighting an autotoxicity phenomenon. We concluded that P. halepensis saplings presented a shade avoiding syndrome and that the crown shade-acclimation response increased with fertilization but was severely compromised by autotoxicity. We finally discuss the role of fertilization and allelopathy in early P. halepensis acclimation ability.

/ Le mémoire présente les travaux effectués depuis 1999 au Cemagref de Clermont-Ferrand (jusqu'en 2004) puis au Cemagref d'Aix-en-Provence concernant l'étude des boisements spontanés. Trois thématiques sont abordées. La première traite de l'étude de la dynamique ligneuse après abandon cultural ou pastoral. Après l'analyse des différents processus de l'afforestation spontanée, un simulateur a été produit permettant de modéliser la dynamique de colonisation par le pin sylvestre et mettant en exergue le rôle des fenêtres de régénération. La deuxième thématique aborde les conséquences de la mise en place des boisements spontanés sur la végétation et les propriétés des sols. Les travaux ont souligné en particulier le rôle des héritages liés aux pratiques culturales anciennes sur les compositions de flore et les caractéristiques des sols. Enfin la troisième thématique concerne la gestion des boisements spontanés notamment pour les pinèdes à pin d'Alep en région méditerranéenne. Les travaux font appel à l'ingénierie écologique et portent sur le pilotage des dynamiques naturelles de succession et l'obtention de peuplements mélangés pin/chêne.