Understorey protection harvest expedites recolonisation of boreal forest stands by North American red squirrels Jason T. Fisher * , S.M. Bradbury 1 Alberta Research Council, Sustainable Ecosystems Unit, Bag 4000, Vegreville, Alberta T9C 1T4, Canada Received 21 February 2006; received in revised form 12 June 2006; accepted 13 June 2006 Abstract Clearcut harvesting removes old-forest structure from stands, rendering these habitats unsuitable for tree-dependent mammal species such as North American red squirrels (Tamiasciurus hudsonicus). Unsuitability of boreal mixedwood forest clearcuts may persist for almost a century, longer than most harvest rotations in this region. In these systems, one alternative to clearcutting is mixedwood understorey protection (MUP) harvesting. MUP harvesting selectively removes mature deciduous canopy trees, retaining some mature and immature trees in the overstorey and understorey, and promoting release of understorey conifers. Live tree retention may also serve to promote early recolonisation by tree squirrels. We compared red squirrel abundance and demography before and after MUP harvest, and between different stand types, in the boreal forest of northern Alberta, Canada. Red squirrels were mark-recaptured in MUP, mixedwood reserve, deciduous, and conifer stands June–July 2001–2003. Red squirrel abundance and masses differed between stand types within years. Abundance and masses did not differ between pre- and post-harvest mixedwood stands across years. Across all stand types, red squirrel abundance was predicted by abundance of spruce trees and fungi. Our results indicate that MUP harvesting retains forest structure required by red squirrels, thus allowing persistence of red squirrel populations; it thus holds promise as a component of the natural disturbance model for maintaining arboreal sciurid populations in managed landscapes. # 2006 Elsevier B.V. All rights reserved. Keywords: Alberta; Red squirrel; Tamiasciurus; Boreal forest; Understorey protection; Sustainable harvest 1. Introduction The human requirement for timber and pulp often clashes with forest-dwelling species’ requirements for trees in which to forage, den, and reproduce. Clearcut harvesting removes all trees and shrubs, and disturbs most ground vegetation in a forested stand. In boreal mixedwood stands typical of northern Alberta, Canada – stands usually dominated by a canopy of trembling aspen (Populus tremuloides) with a subcanopy of white spruce (Picea glauca) – clearcutting results in a loss of future harvesting opportunity for spruce (Navratil et al., 1994). In the short-term, clearcutting often initiates regrowth along an aspen trajectory (Peterson and Peterson, 1992), thus ‘unmixing’ the mixedwood forest. The clearcutting of forests also has the effect of re-initialising forest succession to an earlier seral stage, eliminating the in-stand old-forest structure required by many forest-dwelling animal species. Recovery of the old-forest mammalian species assemblage to pre-harvest states can take over a century in boreal mixedwood stands (Fisher and Wilkinson, 2005). This time span is often much greater than rotation age in Canada’s boreal landscape, particularly in the aspen-dominated mixed- wood forests of northern Alberta. The natural disturbance model, wherein forest harvesting is implemented to spatially mimic forest fire at the landscape level, has been suggested as a tool to mitigate loss of species through forest harvest. 1.1. Sustainable harvest alternatives Hunter (1993) suggested harvesting could be made to emulate fire by implementing harvest patterns that match (1) fire frequency; (2) fire size (mean and variance); and (3) retention rates of snags, live trees, and downed woody material of harvest. Although burn rates can be highly variable (Armstrong, 1999), the first measure would generally allow the ‘overmaturation’ of forests into old growth phases where growth and yield curves have reached a plateau. The third www.elsevier.com/locate/foreco Forest Ecology and Management 234 (2006) 40–47 * Corresponding author. Tel.: +1 250 389 0184; fax: +1 780 632 8379. E-mail addresses: Jason.Fisher@arc.ab.ca (J.T. Fisher), bradbury@arc.ab.ca (S.M. Bradbury). 1 Tel.: +1 780 632 8305; fax: +1 780 632 8379. 0378-1127/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2006.06.030