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Variable Retention Harvesting in Great Lakes Mixed-Pine Forests: Emulating a Natural Model in Managed Ecosystems Brian J

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Variable Retention Harvesting in Great Lakes Mixed-Pine Forests: Emulating a Natural Model in Managed Ecosystems Brian J Palik and D’Amato Ecological Processes (2019) 8:16 https://doi.org/10.1186/s13717-019-0171-y REVIEW Open Access Variable retention harvesting in Great Lakes mixed-pine forests: emulating a natural model in managed ecosystems Brian J. Palik1* and Anthony W. D’Amato2 Abstract Variable retention harvesting (VRH) systems have gained wide use in many different forest types across the globe, but largely have been implemented in forests characterized by severe, infrequent disturbance regimes. There has been less attention given to developing VRH approaches in forests that are characterized as having a mixed-severity disturbance regime that often results in only partial mortality of canopy trees in spatially heterogeneous patterns. One example of such a forest type is red pine (Pinus resinosa Ait.)-dominated ecosystem of the western Great Lakes region of North America. The purpose of this review is to provide a conceptual foundation for developing VRH approaches in red pine ecosystems that are based on a mixed-severity disturbance regime. Our contention is that red pine forests managed following a natural model are more resilient to disturbances and external threats such as climate change. For the red pine ecosystem, VRH application should reflect the often severe, but partial canopy removal from natural disturbance that is characteristic of this ecosystem and that results in more than trivial numbers of surviving overstory trees across a range of spatial configurations in regenerating stands. Retained live trees should span a range of diameters, but favor the larger end of the diameter distribution, as this reflects the likely pattern of survival after natural disturbance and is often a key structural element lacking from managed areas. VRH should be applied in ways that vary the spatial pattern of legacy trees in and among stands, but largely in ways that reflect the pattern of spatially patchy canopy structure, with large openings surrounded by a less disturbed matrix, as occurs with a natural disturbance regime. Legacy trees and deadwood structures should reflect the composition of the pre-disturbance forest, including species in addition to dominant red pine. Finally, retained structures should be viewed as dynamic entities that grow, die, and decay and that need to be documented and accounted for over time. While more organizations are incorporating some form of VRH into policy and practice for red pine-dominated ecosystems, this application is not always based on a comprehensive understanding of the actual natural model of development, which reflects a mixed-severity disturbance regime. Our goal is to review the ecological evidence for this disturbance regime and interpret the structural and compositional outcomes of the disturbance model, so as to advance VRH approaches that better emulate the actual disturbance and development model for this regionally important ecosystem. Keywords: Red pine, Mixed-severity, Variable retention, Natural disturbance * Correspondence: [email protected] 1USDA Forest Service, Northern Research Station, Grand Rapids, MN 55744, USA Full list of author information is available at the end of the article © The Author(s). 2019 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Palik and D’Amato Ecological Processes (2019) 8:16 Page 2 of 15 Introduction that fits the mixed-severity disturbance archetype is the Variable retention harvesting (VRH) systems have gained mixed-pine forests and woodlands that occur on xeric to wide use in many different forest types across the globe dry-mesic sites in the western Great Lakes region of (Gustafsson et al. 2012). The rationale for VRH is that North America. These ecosystems are dominated by red even the most severe natural disturbances are often not pine (Pinus resinosa Ait.), often with shared importance stand replacing and in fact varying amounts of overstory of eastern white pine (Pinus strobus L.), and with varying trees escape mortality and significant amounts of large amounts of other conifers and hardwood species. Ma- deadwood is created by and remains after the disturb- ture and old-growth stands often have complex age ance (Franklin et al. 2000, 2018). VRH applied in this structures, abundant large deadwood, and heterogeneous context can better emulate the structural and compos- canopy conditions. The characteristic disturbance agent itional outcomes of natural disturbance by leaving vary- in these ecosystems is fire. Importantly, fires that kill ing amounts of large live trees, by retaining existing overstory trees are patchy within stands, and thus, they large deadwood including logs and snags, or by creating can be less than stand replacing and certainly vary in deadwood from live trees at the time of harvest. Increas- their severity across a landscape. ingly, VRH used in this way is one component of a more Historically, red pine forests have been managed in comprehensive ecological approach to silviculture ways similar to the severe, infrequent disturbance forest (D’Amato et al. 2016; Palik and D’Amato 2017). type by using even-aged regeneration systems with little The ecological principle addressed by VRH is that of to no meaningful retention. More recently, when VRH is continuity of structure, composition, and function be- applied in these forests, it mirrors its application in for- tween the pre- and post-disturbance forest (Palik and est characterized by severe, infrequent disturbances, by D’Amato 2017; Franklin et al. 2018). Use of VRH to using largely even-aged regeneration systems with the emulate natural disturbance in forests managed for tim- addition of some low level of retention (e. g., Minnesota ber better ensures some degree of continuity in microcli- Forest Resources Council 2013). The distinct difference mates, habitat features, and resource environments, in the disturbance regime and resultant structural and compared to forests managed without significant legacy compositional features of red pine forests, compared to retention. As a consequence, such forests are positioned forests characterized by a severe, infrequent disturbance to sustain the full complement of native species and eco- regime, suggests a need for a different approach to VRH system processes that are found in the early in the former, if emulation of a natural model is the post-disturbance environment after natural disturbance goal. and may be more resilient to external threats such as cli- The purpose of this paper is to provide a conceptual mate change, invasive species, and novel disturbances. foundation for use of VRH in red pine-dominated eco- VRH as described is readily applied in forests that ex- systems in ways that emulate the natural model of dis- perience severe, infrequent canopy disturbances that turbance and development. Specific objectives include: leave legacy structures behind, such as Douglas-fir (i) reviewing the evidence for a mixed-severity disturb- (Pseudotsuga menziesii (Mirb.) Franco)-dominated for- ance regime in red pine-dominated ecosystems, (ii) de- ests in the Pacific Northwest or mountain ash (Eucalyp- veloping a framework for VRH approaches that emulate tus regnans F. Muell.) forests of southeastern Australia the natural model, and (iii) examining real world exam- (Franklin et al. 2018). Often, these and similar types of ples of VRH in red pine ecosystems that are based on forests have been managed historically using even-aged the natural model. regeneration systems, typically with little retention. A justification for use of the latter management approach Red pine ecosystems of the Western Great Lakes has been that it in fact emulates the natural model of region, North America stand-replacement disturbance, but ample research in Historically, red pine-dominated ecosystems were an im- forests characterized by severe, infrequent disturbances portant component of the landscape on dryer sites in demonstrates that VRH approaches better emulate the the western Great Lakes region of North America, in- natural model (e.g., Hansen et al. 1991; Bergeron et al. cluding northern Wisconsin, Michigan, and Minnesota 1999; Kuuluvainen and Siitonen 2013). in the United States (US) and southern parts of Ontario There has been less attention given to developing a ra- and Manitoba in Canada. These forests occupied over 4 tionale and approach for VRH in forests that are better million ha in the US alone prior to significant characterized as having a mixed-severity disturbance re- Euro-American settlement (Frelich 1995). Red pine, gime, where disturbances can be more frequent, may along with eastern white pine, is the dominant tree spe- range in severity within and among stands, but often re- cies on these sites, with varying amounts of other spe- sult in only partial mortality of canopy trees in spatially cies, including balsam fir (Abies balsamea (L.) Mill), heterogeneous patterns. An example of an ecosystem white spruce (Picea glauca (Moench) Voss), jack pine Palik and D’Amato Ecological Processes (2019) 8:16 Page 3 of 15 (Pinus banksiana Lamb.), trembling and bigtooth aspen activity, with shorter return intervals associated with (Populus tremuloides Michx., Populus grandidentata more active use areas.
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