Le renouvellement des peuplements forestiers irréguliers est envisagé par la création de trouées, afin d’augmenter localement l’éclairement en sous-bois et favoriser la croissance de la régénération d’arbres.Cependant, cette augmentation de lumière favorise aussi la colonisation des trouées par des espèces herbacées et semi-ligneuses compétitrices pour les ressources. Cette thèse s’est focalisée sur les interactions pour la lumière entre de jeunes pins sylvestres (Pinus sylvestris) et une végétation interférente représentée par la callune( Calluna vulgaris), la molinie (Molinia caerulea) et la fougère (Pteridium aquilinum) dans le sous-bois de peuplements mélangés chêne sessile (Quercus petraea)-pin sylvestre.La lumière est le facteur principal limitant la croissance des semis (hauteur<0,30m) et gaules(0,30m<hauteur<6m) de pin sylvestre. Cependant, alors que les semis supportent un ombrage conséquent, les besoins en lumière augmentent avec la dimension des individus.Le développement des trois espèces interférentes a été quantifié en fonction de l’éclairement. A recouvrement égal, la fougère intercepte plus de lumière que la molinie et la callune. La fougère affecte donc plus négativement la croissance des semis de pin, notamment parce qu’elle est aussi capable de développer un couvert dense même pour de très faibles éclairements. Pour les trois espèces, des facteurs autres que la lumière sont cependant impliqués qui pourraient être des phénomènes d’allélopathie, de compétition souterraine ou d’interférence mécanique. Les différents résultats sont intégrés dans un modèle conceptuel de dynamique forestière (RReShar, Regeneration and Resource Sharing). / Irregular forest stands regeneration is considered using gap creation that provokes a light increase in the understorey. The light increase leads to the recruitment of new trees as well as the potential colonization by competitive herbaceous and shrubby species. This study focused on light interactions between Scots pine (Pinussylvestris) seedlings and saplings and competitive vegetation made up of Calluna vulgaris, Molinia caerulea andPteridium aquilinum growing in mixed sessile oak (Quercus petraea)-Scots pine stands in acidic temperateforests in France.The light was the key limiting factor controlling Scots pine seedling and sapling growth. Seedlings are able totolerate a consequent shading, whereas sapling light requirement increases with their size. The growth of C.vulgaris, M. caerulea and P. aquilinum was measured according to light availability. For a given cover,P. aquilinum intercepted more light than M. caerulea and C. vulgaris. P. aquilinum involved a more pronounceddecrease of pine seedling growth as this species was able to reach high cover even in very shaded conditions. For all the three species, processes like allelopathy, belowground competition or mechanical interference were assumed to also impede with pine seedling growth. All those results were integrated in a conceptual forest dynamics model named RReShar (Resource and Regeneration Sharing).

The current silvicultural regimes of Pinus radiata plantations in New Zealand with wider initial tree spacings have created the potential for increased growth of understorey vegetation. A consequence of this is that the response of P. radiata to P fertiliser is expected to be more influenced by the interaction between the P fertiliser, the tree and the understorey vegetation than was the case in the past. The objectives of this study were to investigate the influence of different rates of a soluble and a sparingly-soluble P fertiliser (Triple superphosphate and Ben-Geurier phosphate rock) and weed control, and their interactions, on soil P chemistry and the growth and P uptake of 4-5-year-old second-rotation P. radiata on an Allophanic Soil (Kaweka forest) and a Pumice Soil (Kinleith forest). The results showed that the application of P fertilisers had no effect on P. radiata growth at both field trial sites two years after this treatment, although it increased radiata needle P concentration. However, at both sites, the understorey vegetation removal treatment increased tree diameter at breast height and basal area. At the highly P-deficient (Bray-2 P 4 µg g-1) Kaweka forest, the presence of understorey (bracken fern and some manuka) reduced resin-Pi and Olsen P concentrations, but at the moderate P fertility (Bray-2 P 13 µg g-1) Kinleith forest, the understorey (Himalayan honeysuckle, buddleia and some toetoe) increased Bray-2 P, resin-Pi, and Olsen P concentrations. A glasshouse study on P. radiata seedlings was conducted to test the hypothesis that when ryegrass (Lolium multiflorum) is grown with P. radiata, it increases radiata needle P concentration, while when broom (Cytisus scoparius L.) is grown with P. radiata, it has no effect. The acid phosphatase activity in the rhizosphere of P. radiata was higher when radiata was grown with broom than that when it was grown with ryegrass. This is consistent with the higher P concentration in needles of radiata grown with broom than that of radiata grown with ryegrass, in the absence of P fertiliser addition. However, when P fertiliser was added (50 and 100 µg P g-1 soil) the needle P concentration of radiata grown with broom was lower than that when radiata was grown with ryegrass.

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 semi-contrôlées 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. / 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.

No description available.