Production of native and hybridized varieties of Populus has received considerable interest in temperate regions as an alternative to agricultural crops and an additional wood source, while acting as a potential carbon (C) sink to offset emissions of fossil fuel-based greenhouse gases. Research of root system dynamics in Populus species is expanding, however, our understanding of the nature and role of fine roots (FR) is incomplete. The study objective, therefore, was to review the literature regarding FR production, mortality and longevity in Populus, and evaluate the magnitude and significance of the FR fraction to C sequestration. FRs, conventionally defined as less than 2 mm in diameter and responsible for water and nutrient uptake, are an essential component of the tree. Populus FRs are relatively short-lived, with reported lifespans ranging from 30 to 300 days, depending on root diameter, tree species and age, and soil environmental factors. Standing FR biomass fluctuates throughout the growing season. Fine root production generally peaks in mid-summer, and ranges between 1.0 and 5.0 mg ha À1 yr À1 , while FR mortality has less seasonal amplitude. Production and mortality dynamics in Populus are highly plastic in response to soil environmental conditions, and although opposing conclusions have been proposed, research suggests soil moisture and nitrogen to be most important. Results from the literature indicate annual FR turnover to the soil C pool may be small (0.2-1.6 mg C ha À1 yr À1 ), but substantial in maintaining or enhancing C levels in natural and managed stands of Populus.

Several indicators have been identified for the conservation and maintenance of soil criterion in the Montreal Protocol. The objective of this study was to use soil compaction and disturbance measures to determine harvesting impacts at a landscape scale in the boreal forest of Saskatchewan. Forest harvesting impacts were studied pre and postharvest for five harvested sites by (i) sampling soil bulk density (Db) at prescribed grid‐points, and (ii) measuring soil disturbance regimes on two 30‐m transects at each grid‐point. Mean soil Db in the harvested area increased significantly (8–11%) from pre to postharvest conditions for the two winter‐harvested sites at both the 10‐ and 20‐cm depths, while two of the three summer harvested sites also showed significant Db increases (7–15%) at the 10‐cm depth. Combining all five sites, showed that after harvest 32% of all the grid‐points had an increased Db of >15%. Mean soil Db at a 10‐cm depth for roadways and landings was significantly higher (8–14%) than postharvest Db for postharvest levels at four of the five harvested sites. Surface soil disturbance regimes were higher for the summer‐harvested sites than that for the winter‐harvested sites. Landscape position showed no significant differences in Db between the shoulder, backslope, and footslope positions; however, within each landscape position, significant differences in Db were found between pre and postharvest conditions. Soil Db and soil disturbance regimes measured on a grid basis provided a simple, but reliable method for monitoring soil compaction and disturbance effects from harvesting at a landscape scale.

. 2009. Nursery stock type, nitrogen fertilization and shoot pruning effects on the growth of juvenile hybrid poplar in Saskatchewan. Can. J. Plant Sci. 89: 289Á301. Increased demand for wood from the forest-products and bio-energy industries in Canada has generated interest in large-scale hybrid poplar production for the prairie-boreal transition region. Because of the harsh climate and short growing season, this region of Canada poses significant challenges for establishment and production of hybrid poplar plantations and currently little silviculture information specific to this region is available. Trials were established at two sites with different prior land use histories: a former alfalfa field and a mixed-grass pasture. Cutting (Ct), rooted cutting (RCt) and rooted plug (RPg) nursery stock of hybrid poplar [P. deltoides)(P. laurifolia)P. nigra) var. Walker] were established with two fertilization rates (0 and 100 kg N ha(1 ) and pruned and unpruned treatments. Despite the alfalfa site having higher amounts of available N than the pasture site and tree growth being superior at the alfalfa site, foliar N concentrations indicated an N deficiency at the alfalfa site in 2003. The difference in growth between sites was attributed to a higher degree of weed competition at the pasture site. Survival of saplings was affected by the nursery stocks. The presence of roots on the initial stock was key to early survival with approximately 92% of rooted stocks surviving over winter the first year of planting, and only 41% of unrooted cuttings surviving the same period. The short growing seasons (115 to 127 frost-free days) provided enough time for RCt to recover from pruning and produce stem volumes larger than unpruned trees. . Effets de la nature du mate´riel de pe´pinie`re, de l'amendement azote´et de l'e´lagage des pousses sur la croissance des jeunes peupliers hybrides en Saskatchewan. Can. J. Plant Sci. 89: 289Á301. La demande accrue de bois par les industries des produits forestiers et de la bioe´nergie au Canada fait qu'on s'inte´resse a`la production a`grande e´chelle de peupliers hybrides dans la zone de transition entre les plaines et la foreˆt bore´ale. À cause du climat rigoureux et de la courte saison ve´ge´tative, l'e´tablissement et la culture du peuplier hybride dans cette re´gion du Canada s'ave`rent cependant plus complique´s, d'autant qu'on connaıˆt mal les conditions sylvicoles locales. Les auteurs ont proce´de´a`des essais a`deux endroits de vocation ante´rieure diffe´rente, en l'occurrence un ancien champ de luzerne et un paˆturage de gramine´es mixtes. Ils y ont plante´des boutures, des boutures enracine´es et des mottes enracine´es de peuplier hybride [P. deltoides x (P. laurifolia ) P. nigra) var. Walker] venant d'une pe´pinie`re a`deux taux de fertilisation (0 et 100 kg de N par hectare), en e´laguant certains plants et pas d'autres. Malgre´une plus forte quantite´de N disponible et une meilleure croissance des arbres, la concentration de N dans les feuilles a re´ve´le´une carence en N dans l...

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Block, R. M. A. and Van Rees, K. C. J. 2006. Wheat root dynamics as affected by landscape position. Can. J. Soil Sci. 86: 523-531. The effects of landscape position on root production and mortality were assessed in a 90-cm-deep soil profile at a shoulder and footslope landscape position seeded to spring wheat (Triticum aestivum L.). Root length was measured over eight sampling dates using a minirhizotron system, and soil water content and temperature were recorded at various depths at each landscape position. The shoulder position was drier than the footslope position in the upper 30 cm due to a greater frequency and duration of soil temperatures > 20°C, and at depth (> 75 cm). Mean root length was greatest at the footslope position and was concentrated in the upper 20 cm of the profile, while the shoulder position had the greatest root length at the 40-to 60-cm depth. Mean daily root production peaked at 5.0 to 6.0 m m -2 d -1 at the 43rd day after planting (DAP) for both landscape positions, which corresponded to the time of booting. Daily rates for root mortality ranged from 0.5 to 2.5 m m -2 d -1 . Soil water content and daily root production at the 10-cm depth were positively correlated at both landscape positions. Information on landscape position differences in root productivity and mortality could help to improve placement of inorganic fertilizers, and estimation of below-ground carbon sequestration. Ils ont mesuré la longueur des racines huit fois après échantillonnage au moyen d'un mini rhizotron puis établi la teneur en eau et la température du sol à diverses profondeurs, à chaque endroit. Le sol des 30 premiers centimètres et des profondeurs supérieures à 75 cm était plus sec à l'épaulement qu'au pied de la pente en raison de la plus grande fréquence et durée des températures audessus de 20°C. La longueur moyenne des racines est plus grande au pied de la pente et les racines les plus longues se concentrent dans les 20 premiers centimètres du profil, alors qu'à l'épaulement, elles s'observent à la profondeur de 40 à 60 cm. La production quotidienne moyenne de racines atteint un pic de 5,0 à 6,0 m par mètre carré et par jour le 43 e jour suivant la plantation aux deux endroits, ce qui correspond à l'épiaison. Le taux de mortalité quotidien des racines varie de 0,5 à 2,5 m par mètre carré et par jour. La teneur en eau du sol présente une corrélation positive avec la production quotidienne de racines à la profondeur de 10 cm aux deux emplacements. Ces renseignements sur la variation de la productivité et de la mortalité des racines selon l'emplacement dans le relief pourraient avoir leur utilité lors de l'application des engrais inorganiques et de l'estimation du carbone séquestré dans le sol.Mots clés: Blé, racines, mini rhizotron, emplacement dans le relief Landscapes can assert a considerable influence on plant productivity and biomass in managed agroecosystems. Productivity is affected by landscape position because of resource (water, nutrients) redistribution; typically, soil moisture and fertility ...