. Do trees use reserve or newly assimilated carbon for their defense reactions? A 13 C labeling approach with young Scots pines inoculated with a bark-beetle-associated fungus (Ophiostoma brunneo ciliatum). Annals of Forest Science, Springer Verlag/EDP Sciences, 2007, 64 (6), pp.601-608.
-The aggressiveness towards Scots pine (Pinus sylvestris L.) of the association between a bark beetle (Ips acuminatus Gyll) and an Ophiostomatale fungus (Ophiostoma brunneo-ciliatum Math.) was investigated by estimating experimentally with young trees, the critical threshold of attack or inoculation densities. Records of the relationship between natural attack densities by the beetles and survival of trees in a pine stand yielded a critical attack density threshold of about 900 m -2 . Experimental mass inoculations of young pines with the fungus, in a forest stand in Central France, demonstrated a weak pathogenicity of this fungal species towards Scots pine. Inoculation densities varying from 200 to 1000 m -2 were used. Damage in the bark or in the sapwood recorded three months after the inoculations, remained rather limited. The length of the induced reaction zones in the bark was small as compared to those obtained with more aggressive fungi, and did not increase with inoculation density. Damage in the sapwood, estimated either visually from the observed sapwood drying, and from resinosis, remained limited, but increased significantly with the inoculation density. The impairment of hydraulic conductivity of inoculated trunk segments was rather large with, at highest densities, a loss of conductivity estimated to about 60%. Nevertheless, due to the fact that the resistance to water transfer in the trunk is much smaller than in other organs (like roots or needles), this increase probably only had a small impact on water relations at whole tree level. It may be concluded that the association I. acuminatus -O. brunneo-ciliatum displays only a weak aggressiveness towards Scots pine, and that high densities of attacks or inoculations (above 1000 m -2 ) are required in order to reach the critical threshold able to kill trees. Pinus sylvestris
1 The present study investigated the influence of tree size and bole position on selection of ponderosa pines ( Pinus ponderosa Dougl. ex Laws.) by Ips pini Say (Coleoptera: Curculionidae: Scolytinae) in a northern Arizona forest. Ips pini were attracted to lower and upper bole positions with pheromone lures, and the number of attacks counted. In addition, tree phloem and bark thickness, resin flow in response to wounding and characteristics of tree water and carbon relations were all measured. 2 Bark thickness was the most strongly related tree characteristic to number of I. pini attacks. Thin bark and a high number of attacks occurred at the lower bole position of 10-cm diameter trees and upper bole positions of 23-, 35-, and 50-cm diameter trees. 3 Phloem thickness increased directly with tree size, was greater at the upper bole compared with the lower bole, but was not associated with number of attacks. 4 Resin flow did not differ strongly over tree sizes or bole positions, and was not related to number of I. pini attacks. Attacks were not related to characteristics of tree carbon and water relations measured during I. pini flights.
Résumé -De jeunes Chênes rouges d'Amérique plantés sur un sol d'origine agricole ont été soumis pendant quatre ans à deux niveaux extrêmes de compétition herbacée croisés avec deux niveaux de fertilisation initiale. Un suivi de la croissance et une analyse comparée du développement des arbres ont été réalisés sur les diverses composantes de la croissance en hauteur des arbres. La compétition herbacée se traduit par une diminution globale de la hauteur totale des arbres de 70 %, du nombre d'unités de croissance émises chaque année (polycyclisme) de 18 à 86 %, ainsi que du nombre d'entre-noeuds et de feuilles (-50 %) et de la longueur moyenne des entrenoeuds (-30 à 60 %) des unités de croissance. La fertilisation initiale n'a favorisé que la croissance en diamètre des plants désherbés. Par ailleurs, les plants enherbés montrent un nombre de branches par unité de croissance plus faible (-20 à -80 %), mais cette tendance s'annule ou s'inverse si ce nombre est rapporté à l'unité de longueur. Enfin, la mortalité des branches les plus anciennes est augmentée par la compétition pour la lumière. D'une manière générale, la compétition herbacée limite l'expansion des arbres et semble bloquer leur sé-quence de différenciation architecturale.architecture / Quercus rubra / croissance / morphologie / compétition herbacée Abstract -Influence of herbaceous competition on growth and architecture of young red oak trees (Quercus rubra L.) in plantation. Young red oak trees planted on a former agricultural soil have been submitted to two extreme levels of herbaceous competition combined with two levels of fertilization. The herbaceous competition is associated with a decrease of 70% of the mean total height and with a decrease of 18% to 86% of the number of growth units produced each year. It induced also a decrease of 50% of the growth units mean number of internodes and leaves and a decrease of 30% up to 60% in its mean internode length. Initial fertilization stimulated only diameter growth of weeded trees. The number of branches per growth unit is lower for non-weeded trees (between -20% and -80%), but not different or upper per length unit, and oldest branch mortality is higher in relation to light competition. In a general way, weeds competition tends to limit tree crown expansion and the expression of the architectural sequence of differentiation.architecture / Quercus rubra / growth / morphology / herbaceous competition Ann. For. Sci. 58 (2001)
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