The impacts of partial cut systems on nutrient availability and understory vegetation are poorly understood. To examine these responses, white spruce dominated stands in the boreal mixedwood of Alberta were clear-cut or partial-cut and the forest floor treated by slash burning, mixing, mounding, or scalping in a split-plot design. Soil nutrient availability (ion exchange resin), net N mineralization (in situ incubations), and vegetation (density and cover) responses were assessed. With the exception of higher Mg availability in the clearcuts, differences in nutrient availability were driven by forest floor disturbance and not harvest method. Relative to controls, burning increased availability of NH4+, NO3–, and P, and scalping increased Ca and Mg but diminished K. Controls had low levels of NO3–. The mixing treatment substantially reduced net N mineralization. In terms of vegetation, partial cuts reduced root suckering by Populus spp. (Populus tremuloides Michx., Populus balsamifera L.) relative to clearcuts. Burning and mounding stimulated fireweed (Epilobium angustifolium L.) cover, while scalping increased Populus spp. sucker density. In contrast, mixing largely reduced vegetation establishment, likely because of the destruction of roots and rhizomes and reduced N supply. Nutrient availability and vegetation establishment were more strongly controlled by forest floor disturbance than by partial canopy retention.

In northeastern Alberta, the current biomass and periodic annual biomass increment (PAI) was measured in 29 stands of maturing aspen (Populus trernu1oides)-white spruce (Picea glauca), aged 48 to 105 years. Plots in pure aspen were paired with nearby plots of aspen growing on a similar landform but with a spruce understory. Biomass was estimated by diameter at breast height and allometric equations. Totalled over both species, there was 10.5 % greater PA1 and 10.0 % greater biomass in the mixed plots than in the pure aspen plots. Pure aspen plots, however, had 12.9% greater aspen biomass and 25.2% greater aspen PA1 than the aspen component of mixed plots. The apparent decline in productivity of aspen in the mixed stands, however, could not be related to the variation in spruce abundance in these mixed stands. Le dCclin apparent de la productivitC du tremble dans les peuplements mBlangCs, cependant, ne pouvait pas &tre relie 2 la variation en abondance de 1'Cpinette dans ces peuplements mBlangCs.

Gremmeniella abietina infects shoots of red and Scots pines through stomata on bracts th.it subtend short shoots. The gertn tulic penetrates between guard cells and sparsely colonizes bmct tissue by late summer or fall. Only after about late Januaiy or early Fcbruaiy of the following year does the fungus extend from the bract and begin eolonization of the short shoot and surrounding cortical tissue.~j

No abstract

In recent years boreal forests have been harvested to retain a portion of the original canopy, thereby providing forest structure, mostly for biodiversity reasons. Boreal mixedwood cutovers were surveyed at one and five years after harvesting with approximately 10% structural retention, to quantify the mean annual mortality rates of the residual trembling aspen, balsam poplar, paper birch and white spruce trees. For comparison, "natural" mortality rates by species were estimated from permanent sample plots in stands of similar composition. Species ranking of the annual mortality rates of residuals in areas harvested with structural retention were: poplar (10.2%) > birch (8.7%) > aspen (6.1%) > spruce (2.9%). Annual mortality rates were 2.5 to 4 times greater than in the reference stands. The majority of broadleaved species died as snags (~70%-90%), while most spruce died due to windthrow (80%). Mortality rates increased with slenderness coefficient for codominant and understory poplar and for understory birch. For aspen, codominants were most likely to die, while in spruce, dominant trees and trees with the greatest damage to the bole from harvesting operations had the highest mortality.Key words: Alberta, Betula papyrifera, dieback, harvesting damage, mixedwood forests, variable retention, Picea glauca, Populus balsamifera, Populus tremuloides, structural retention, sustainable forest management RÉSUMÉ Au cours des dernières années, les forêts boréales ont été récoltées pour retenir une portion de la canopée originale, fournissant ainsi une structure forestière, surtout pour des raisons de biodiversité. À la suite de la récolte, on a vérifié le parterre de la coupe après un an et après cinq ans en ayant conservé environ 10 % de la structure, afin de quantifier la moyenne des taux annuels de mortalité des peupliers faux-trembles, des peupliers baumiers, des bouleaux à papier et des épinettes blanches. Aux fins de comparaison, les taux de mortalité « naturelle » par les espèces ont été estimés à partir des lotissements permanents représentatifs dans des peuplements de composition semblable. Le classement par espèce des taux de mortalité annuelle des arbres rémanents dans les zones récoltées avec une rétention structurelle étaient : le peuplier (10,2 %) > le bouleau (8,7 %) > le tremble (6,1 %) > l'épinette (2,9 %). Les taux de mortalité annuelle étaient de 2,5 à 4 fois plus élevés que les taux dans les peuplements de référence. La majorité des espèces caducifoliées est morte comme chicot (~70 % -90 %), alors que la plupart des épinettes sont mortes à cause du déracinement par le vent (80 %). Les taux de mortalité ont augmenté avec le coefficient de sveltesse pour le peuplier codominant et dominé et pour le bouleau dominé. Pour le tremble, il était plus probable que les codominants meurent tandis que pour l'épinette, les arbres dominants et les arbres ayant le plus de dommages sur le tronc à cause des opérations de récolte étaient ceux qui avaient le taux le plus élevé de mortalité.