Regeneration of Populus nine years after variable retention harvest in boreal mixedwood forests Tomasz Gradowski a , Victor J. Lieffers a, *, Simon M. Landha ¨ usser a , Derek Sidders b , Jan Volney b , John R. Spence a a Department of Renewable Resources, University of Alberta, 442 ESB, Edmonton, AB, Canada T6G 2H1 b Canadian Forestry Service, Northern Forestry Centre, 5320 122 St., Edmonton, AB, Canada T6H 3S5 1. Introduction Across the boreal mixedwood forest retention of mature trees at harvest has become commonly employed to maintain stand structure and therefore sustain biodiversity (Puettmann and Ek, 1999; Work et al., 2003; Klenner and Sullivan, 2003; Brais et al., 2004; Gotmark et al., 2005; Harrison et al., 2005; Hedenas and Hedstrom, 2007). These harvesting systems retain live mature trees in a range of densities, often in dispersed patterns, after forest logging. While retaining mature trees has long been a strategy for promoting regeneration of shade tolerant species (e.g., shelter- wood systems) (Lieffers et al., 1996; Groot, 2002; Groot et al., 2005), retention of mature trees has usually been considered to be detrimental to regeneration of intolerant species such as aspen (Populus tremuloides Michx.) (Perala, 1990). Boreal mixedwood forests are mostly dominated by mixtures of aspen, balsam poplar (Populus balsamifera L.) and white spruce (Picea glauca (Moench) Voss) at various scales, with individual stands ranging from pure aspen and spruce to a variety of mixtures of both species. Aspen and balsam poplar are clonal species, which regenerate following disturbance primarily by suckering along shallow roots. The initiation and density of root suckers relates to the size of the bud bank (amount of root mass) of the stand prior to disturbance (Frey et al., 2003). It is generally assumed that the basal area of aspen prior to logging is directly related to the potential for production of suckers following harvest (Perala, 1990). Although this is a rational conjecture, sucker density did not vary with pre-harvest basal area across a number of stands in Quebec (Lavertu et al., 1994). Nonetheless, few published accounts describe the effect of basal area of aspen pre-harvest, on aspen regeneration after harvest. Root suckering most commonly occurs only after the above ground portion of the tree or clone is killed or stressed by defoliation, stopping the flow of auxin from the stem to the roots (Schier and Smith, 1979). Leaving residual mature aspen or balsam poplar stems sustains some supply of auxins to the root system, consequently suppressing initiation of suckers. Further, aspen and balsam poplar are shade intolerant species and their growth is negatively affected by shade (Frey et al., 2003). Indeed, previous studies from juvenile aspen stands (Mulak et al., 2006), mature aspen stands (e.g., Palik et al., 2003; Man et al., 2008) and mature Forest Ecology and Management 259 (2010) 383–389 ARTICLE INFO Article history: Received 21 June 2009 Received in revised form 14 October 2009 Accepted 18 October 2009 Keywords: Aspen suckering Variable retention harvest Forest regeneration ABSTRACT Aspen and balsam poplar regeneration from root suckers were assessed in boreal mixedwood forests nine years after logging in a variable retention experiment (EMEND Project—Ecosystem Management Emulating Natural Disturbance) located north of Peace River, Alberta, Canada. Five levels of retention of mature trees (2%, 10%, 20%, 50% or 75% of the original basal area) were applied in stands dominated by aspen, white spruce or mixtures of the two species. Basal area of aspen (or that of aspen plus balsam poplar combined) prior to logging strongly influenced sucker density of aspen (or aspen + balsam poplar combined) and in some cases their growth. Nine years after harvest there was a decline in sucker density and volume ha 1 with increasing retention levels of aspen (or both poplars combined); sucker density declined by 50% when only 20% of the original basal area was left in the stand. Retaining mature spruce trees in the stand had little influence on the number of suckers but did affect their total volume ha 1 . Thus, we suggest that by knowing stand aspen and balsam poplar density prior to logging and varying levels of retention of aspen and balsam poplar or conifers at harvest, the density of Populus regeneration can be predicted by managers, thereby allowing them to create a range of mixedwood conditions. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author at: Stantec, 4370 Dominion St., Burnaby BC Canada, V5G 4L7. Tel.: +1 780 492 2852; fax: +1 780 492 1767. E-mail address: Victor.Lieffers@ualberta.ca (V.J. Lieffers). Contents lists available at ScienceDirect Forest Ecology and Management journal homepage: www.elsevier.com/locate/foreco 0378-1127/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2009.10.033