Shoot and root responses of hybrid poplars to interspecific competition and soil fertility

Un zonage forestier incluant une sylviculture intensive d'arbres à croissance rapide sur une portion restreinte du territoire permettrait de répondre à la demande pour les produits du bois, réduire la pression sur les forêts naturelles, et augmenter la superficie d'aires protégées. Le peuplier hybride constitue un excellent candidat à la sylviculture intensive puisque sa croissance permet une récolte après 20 ans. Cette étude avait pour objectif d'évaluer les conditions environnementales qui optimisent la croissance du peuplier hybride en sol forestier boréal. Trois techniques d'aménagement sylvicole (la préparation mécanique du sol, le contrôle de la compétition végétale, et la fertilisation) ont été testées sur 40 hectares de plantations dans la région du Saguenay-Lac-Saint-Jean. Des excavations de systèmes racinaires de peupliers, accompagnées d'incubations de sol à différentes profondeurs, ont été effectuées pour expliquer la réponse aux techniques d'aménagement. La compréhension du comportement des racines du peuplier a été approfondie par une expérience en pots, où des peupliers ont été plantés en présence de graminées et avec une distribution spatiale des nutriments homogène ou hétérogène. Les résultats suggèrent de favoriser la préparation mécanique du sol avant les autres techniques d'aménagement sylvicole puisqu'elle offre les meilleurs gains en croissance. La préparation par monticules s'avère être la meilleure méthode pour assurer la survie et la croissance des peupliers. Le développement du système racinaire est en effet grandement favorisé dans les monticules grâce au sol plus chaud et plus meuble, à une minéralisation accrue de l'azote, et à une faible compétition racinaire, ce qui donne accès à un large volume de sol et à un important réservoir de nutriments. Les racines de peupliers évitent effectivement la compétition, et ce, même à leur détriment, quand les zones les plus fertiles du sol sont occupées par la compétition. Pourtant, les résultats de l'expérience en pots démontrent que le peuplier peut malgré tout pénétrer dans la zone d'un compétiteur, si cette dernière est au moins aussi fertile que la sienne. Les racines de peuplier ont exploré davantage le sol et accédé ainsi à plus de ressources, malgré la présence de compétition, quand la distribution des nutriments était homogène. La croissance de la tige est dans ce cas semblable aux traitements sans compétition. Le peuplier réagit à la compétition et à la distribution spatiale des nutriments en modifiant son architecture racinaire, ce qui témoigne de sa grande plasticité. Cette étude favorisera le succès des plantations de peupliers hybrides, une condition gagnante du zonage forestier, en plus de générer des connaissances nouvelles pour mieux comprendre le comportement des plantes. \ud ______________________________________________________________________________ \ud MOTS-CLÉS DE L’AUTEUR : aménagement sylvicole, azote, compétition végétale, distribution spatiale des nutriments, fertilisation, peuplier hybride, plantation d'arbres à croissance rapide, préparation mécanique du sol, racines, sols forestiers

How Best Should We Manage Hybrid Poplar Plantations? Interactions of Site Preparation, Vegetation Control and Fertilization

The silviculture of hybrid poplars is a promising solution to reduce the pressure on natural forests while maintaining the wood supply to industries. However, hybrid poplars are sensitive to competing vegetation and to inadequate soil conditions and fertility. Possible management tools include mechanical site preparation (MSP), vegetation control, and fertilization. We present here the results after five years of growth for eight formerly forested sites (40 hectares total) on Podzols in the province of Quebec, Canada. The experimental design combines four MSP treatments (harrowing, scarifying, mounding, and no preparation) with four frequencies of plant competition control by brushing (from never to once a year) and N or N+P fertilization. The best growth was found in the following MSP treatments: mounding \u3e harrowing \u3e scarifying \u3e none. The effect of competition control is more evident on the more productive sites and in the less severe preparation treatments, which brings insight into the Grime-Tilman debate over the importance and intensity of competition. Fertilization can provide substantial improvement in immediate growth, although it does not seem to compensate for deficient root systems. We will discuss the causes and implications of our results for future management strategies of hybrid poplar plantations

Extractability of elements in sugar maple xylem along a gradient of soil acidity

Dendrochemistry has been used for the historical dating of pollution. Its reliability is questionable due primarily to the radial mobility of elements in sapwood. In the present study, the extractability of seven elements was characterized to assess their suitability for the monitoring of environmental conditions. Nine mature sugar maple trees (Acer saccharum Marsh.), a wide-ranging species in eastern North America that has suffered decline in past decades, were sampled in three Quebec watersheds along a soil acidity gradient. Five-year groups of annual tree rings were treated by sequential chemical extractions using extractants of varying strength (deionized H2O, 0.05 M HCl, and concentrated HNO3) to selectively solubilize the elements into three fractions (water-soluble, acid-soluble, and residual). Monovalent K; divalent Ba, Ca, Cd, Mg, Mn; and trivalent Al cations were found mostly in the water-soluble, acid-soluble, and residual fractions, respectively. Forms more likely to be mobile within the tree (water-soluble and acid-soluble) do not seem to be suitable for temporal monitoring because of potential lateral redistribution in sapwood rings. However, certain elements (Cd, Mn) were responsive to current soil acidity and could be used in spatial variation monitoring. Extractability of Al varied according to soil acidity; at less acidic sites, up to 90% of Al was contained in the residual form, whereas on very acidic soils, as much as 45% was found in the water-soluble and acid-soluble fractions. Sequential extractions can be useful for determining specific forms of metals as key indicators of soil acidification

Contributions of a global network of tree diversity experiments to sustainable forest plantations

status: publishe

Conservation et restauration de la forêt tropicale du Panama : enjeux et pistes de solutions

Le Panama, pays davantage connu pour son célèbre canal, est aussi le berceau d’une biodiversité extraordinaire. Toutefois, à l’instar d’un nombre grandissant de pays situés dans les Tropiques, le Panama fait face à une déforestation extensive depuis le milieu des années 1950. Or pour contrer cette tendance, la partie s’annonce difficile. En effet, l’agriculture itinérante qui est la principale cause de la disparition des forêts panaméennes tend à augmenter en superficie avec l’accroissement de la population. Les petits propriétaires terriens, en grande partie responsable de cette situation, sont loin d’être de mauvaise foi. Avec plus de 60% de la population rurale vivant sous le seuil de la pauvreté, il s’agit ici davantage de survie que d’exploitation à des fins mercantiles. Toutefois, la partie n’est pas encore jouée : 25% du territoire est déjà en aires protégées, et plusieurs projets de reforestation (plantations) et d’agroforesterie ont été mis en place au courant des dernières décennies afin d’offrir une alternative durable aux agriculteurs. Malgré le potentiel énorme de ces alternatives, plusieurs contraintes limitent encore leur utilisation à grande échelle. D’abord, le coût d’implantation est souvent au-delà des moyens financiers des agriculteurs et les besoins des populations locales sont trop peu considérés à priori. De ce fait, nous avons tenté d'apporter, non pas des solutions car elles existent déjà, mais une évaluation des pistes possibles à suivre pour favoriser le succès de tels projets.Although Panama is better known for its famous transoceanic canal, it is also the cradle of extraordinary biodiversity. However, as is the case in a growing number of tropical countries, it has also been subjected to extensive deforestation since the mid-1950s. Counteracting this trend will not likely be an easy task. For one, a growing population is increasing the adoption of shifting cultivation practices, a major cause of Panamanian forest destruction. Small landholders, while mainly responsible for this situation, are certainly not acting with bad intent. In considering that 60% of the rural population is living in poverty, it is clear that motivations for such actions relate to survival rather than profit. Nonetheless, not all is lost: 25% of the territory is protected, and several reforestation (plantation) and agroforestry projects have been undertaken in the last decades in order to offer a sustainable alternative to farmers. Despite the enormous potential of these alternatives, many obstacles still challenge and limit their widespread use. The starting costs are often beyond the farmers' financial means, and the local populations' needs are rarely consulted beforehand. Since solutions to the deforestation issue already exist, we have focused our attention on evaluating the different ways of increasing the success of the aforementioned alternative projects

Root production of hybrid poplars and nitrogen mineralization improve following mounding of boreal Podzols

Successful establishment of fast-growing trees could depend on early root development and the access to belowground resources. Boreal podzolic soils present a distinctive vertical zonation wherein nutrient availability and the presence of plant roots decline sharply with depth. Mechanical soil preparation that modifies the vertical arrangement of soil layers creates microsites with improved physical conditions but potentially lower nutrient availability. We compared the vertical distribution of proximal roots of young hybrid poplars in soil layers of mechanically prepared (by mounding) and unprepared microsites. We also evaluated the relationship between root distribution and the availability and mineralization of soil nitrogen. Hybrid poplar roots were less abundant in the surface organic layer of unprepared soils, whereas they proliferated in the buried organic layer of mounds. Total mineralized N was highest in the upper mineral layer of mounds, whereas it was similar between the buried organic layer of mounds and the unprepared organic layer. Altogether, mounding created conditions conducive to greater soil N mineralization and greater production and vertical distribution of proximal roots. This possibly provided access to a larger soil volume and greater soil nutrient pools, which may explain the success of mounding in terms of aboveground growth of hybrid poplars

Conservation et restauration de la forêt tropicale du Panama : enjeux et pistes de solutions

Although Panama is better known for its famous transoceanic canal, it is also the cradle of extraordinary biodiversity. However, as is the case in a growing number of tropical countries, it has also been subjected to extensive deforestation since the mid-1950s. Counteracting this trend will not likely be an easy task. For one, a growing population is increasing the adoption of shifting cultivation practices, a major cause of Panamanian forest destruction. Small landholders, while mainly responsible for this situation, are certainly not acting with bad intent. In considering that 60% of the rural population is living in poverty, it is clear that motivations for such actions relate to survival rather than profit. Nonetheless, not all is lost: 25% of the territory is protected, and several reforestation (plantation) and agroforestry projects have been undertaken in the last decades in order to offer a sustainable alternative to farmers. Despite the enormous potential of these alternatives, many obstacles still challenge and limit their widespread use. The starting costs are often beyond the farmers' financial means, and the local populations' needs are rarely consulted beforehand. Since solutions to the deforestation issue already exist, we have focused our attention on evaluating the different ways of increasing the success of the aforementioned alternative projects