The retention system:reconciling variable retention with the principles of silvicultural systems

In the past 10 to 15 years, alternative silvicultural treatments involving partial harvesting have been developed for boreal forests, with the goal of achieving a balance between biodiversity maintenance and continued timber production. Most prior research has focussed on the impacts of partial harvesting on biological diversity, while stand dynamic responses remain little studied. In this paper we explore partial stand harvesting in the Canadian boreal-its rationale, current extent, and impact on stand dynamic patterns. Empirical studies from the boreal and elsewhere indicate that residual trees of many species respond to partial harvesting with enhanced growth, commonly showing a lagged response after which peak growth occurs five to 25 years following harvest. Post-harvest mortality is also prevalent but much more variable, with losses of residual trees ranging from nearly zero to more than 50% above background mortality rates in the initial years following harvest. With the exception of strip cutting in parts of northern Ontario and Quebec (HARP/CPPTM), operational partial harvesting in the Canadian boreal currently involves very low levels of retention. Available data suggest that such low retention levels, particularly when spatially dispersed, generally result in unacceptably high rates of post-harvest mortality, which are unlikely to be offset by increases in residual tree growth. There is an urgent need for development of spatially explicit stand simulation models that will allow accurate yield predictions for partial harvest systems to assess their feasibility in boreal forest management.Key words: ecosystem management, natural disturbance emulation, boreal forest, partial cut, structural retention, growth response, windthrow, post-harvest mortality RÉSUMÉ Au cours des dix à quinze dernières années, des traitements sylvicoles alternatifs comprenant une coupe partielle ont été élaborés en forêt boréale, dans le but d'obtenir un équilibre entre le maintien de la biodiversité et la production continue de matière ligneuse. La majeure partie de la recherche réalisée à ce jour a porté sur les impacts de la coupe partielle sur la diversité biologique, tandis que les réactions au niveau de la dynamique du peuplement recevaient peu d'attention. Dans cet article, nous explorons la coupe partielle des peuplements dans les forêts boréales du Canada : le rationnel, le niveau actuel d'utilisation et les conséquences sur les patrons de dynamique des peuplements. Les études empiriques en provenance des forêts boréales et des autres forêts indiquent que les arbres résiduels de plusieurs espèces réagissent à la coupe partielle par un accroissement de la croissance, démontrant généralement une réaction déphasée suivie d'un pic de croissance survenant entre 5 et 25 ans après la récolte. La mortalité après coupe est également présente quoique plus variable, avec des pertes parmi les arbres résiduels avec des taux de zéro à plus de 50 % de mortalité par rapport au milieu naturel au cours des premières années suivant...

Section: Post-harvest Tree Mortalitymentioning

confidence: 99%

Section: A Note On Terminologymentioning

confidence: 99%

Partial harvesting in the Canadian boreal: Success will depend on stand dynamic responses

Thorpe¹,

Thomas²

2007

“…This silvicultural approach has been inspired by the patterns of natural disturbance (i.e., fire, wind, or disease) in unmanaged forests, which generally leave some standing structure. The stand-level objectives of partial harvesting are to leave a high degree of structural heterogeneity with varying amounts, types and spatial patterns of living and dead trees to address a broad array of forest management goals (Mitchell and Beese 2002). It is assumed that the retention of later seral conditions will contribute to sustaining ecosystem functions and biological diversity at the stand level (Franklin et al 1997, Bergeron et al 2002.…”

Section: Introductionmentioning

confidence: 99%

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

Bladon¹,

Lieffers²,

Silins³

et al. 2008

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.

“…These include both traditional silvicultural and variable retention systems (Mitchell and Beese 2002), in which some trees are left across the harvested site as aggregated (e.g., strip cut, patch cut) or dispersed (e.g., shelterwood, green tree) retention. The Montane Alternative Silvicultural Systems (MASS) partnership was formed in 1992 to test several alternative silvicultural systems in an old-growth coastal montane forest on Vancouver Island (Arnott and Beese 1997).…”

Section: Introductionmentioning

confidence: 99%

Post-harvest nitrogen cycling in clearcut and alternative silvicultural systems in a montane forest in coastal British Columbia

Titus¹,

Prescott²,

Maynard³

et al. 2006

The MASS (Montane Alternative Silvicultural Systems) trial was established in the coastal mountains of British Columbia to compare clearcut, patch cut, green tree and shelterwood systems. A number of studies were carried out at the MASS trial to determine the extent to which these variable levels of stand retention retained old-growth attributes of N cycling and associated ecological processes. Harvesting led to increases in N mineralization in the forest floor (2ϫ) and mineral soil (10ϫ), and fluxes of N through the upper 25 cm of mineral soil (2ϫ to 3ϫ). However, fluxes of N were not large (< 0.35 kg ha -1 per growing season). Nitrogen mineralized was predominantly ammonium and not nitrate in the forest floor (> 95% in all but clearcut, > 75% in clearcut) and upper mineral soil horizon (42-86%). The nitrate component came from heterotrophic decomposition of organic matter, not conversion of ammonium to nitrate by autotrophs, and nitrate increases resulted from decreased gross nitrate consumption with harvesting, rather than increased nitrate production. The increases in soil N availability resulting from harvesting were reflected in only slight increases in seedling foliar N concentrations for two to four years after logging (peak of~ 2% for western hemlock and ~ 1.6% for amabilis fir) before decreasing to below deficiency thresholds for both species. Overall, estimated losses of N from the rooting zone after harvesting (1 kg ha -1 yr -1 ) were minimal in comparison to estimated N inputs (4 kg ha -1 yr -1 ), N exports in logs at harvesting (250 kg ha -1 ) and soil reserves (11 400 kg ha -1 ). Although unlikely to affect future site productivity, losses of N could be reduced somewhat through the use of shelterwood harvesting.