Elevated mortality of residual trees following structural retention harvesting in boreal mixedwoods

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.

Retention forestry and intensified biomass harvest: epiphytic lichen assemblages under opposing ecological effects in pine-dominated boreal forests

“…On all sites, the risk of tree fall varied among tree species (II), as has also been observed in previous studies (e.g. Bladon et al, 2008;. Pines were most likely to remain as snags, while spruces were three times more likely and deciduous trees two times more likely to fall (II).…”

“…Tree-level factors, such as tree species, diameter or height may also affect mortality rates (e.g. Bladon et al, 2008;Lavoie et al, 2012); for example, trees with larger height-diameter ratios tend to be more susceptible to windthrow (Scott and Mitchell, 2005). However, a majority of the studies that have addressed the mortality and dynamics of retention trees have been conducted in North American boreal and temperate forests (e.g.…”

“…In general, tree mortality increases after harvest in comparison to uncut stands (Bladon et al, 2008;Lavoie et al, 2012). It can be affected by both stand-and tree-level factors: tree mortality is usually lower when the retention level is high (Scott and Mitchell, 2005;Busby et al, 2006;Solarik et al, 2012) or when the trees are retained in groups rather than dispersed (Scott and Mitchell, 2005).…”

“…However, the outcome of retention forestry depends on the post-harvest dynamics of the retained trees, as a continuity of living trees and dead wood needs to be maintained to sustain lichen species richness. The "life-boating" function of retention forestry, which is very relevant for epiphytic lichens, is compromised by high tree mortality (Bladon et al, 2008). Furthermore, high mortality reduces the habitats of species associated with living trees and disrupts dead wood continuity.…”

“…However, a majority of the studies that have addressed the mortality and dynamics of retention trees have been conducted in North American boreal and temperate forests (e.g. Busby et al, 2006;Bladon et al, 2008;Lavoie et al, 2012), and the results may not be applicable elsewhere due to differences, for example, in tree species composition and climatic conditions (Lavoie et al, 2012) or in the applied harvesting methods, such as the volume of retained trees . Furthermore, tree mortality following fire has been assessed only in closed forests (e.g.…”