Silvicultural Systems for Mixedwood Management Phil Comeau Dept. of Renewable Resources University of Alberta

Outline  Ecology  Aspen and spruce silvics  Successional considerations (TIME)  Spatial considerations  Examples of silvicultural systems  Clearcutting  Advance regeneration or planting  “Continuous cover”  Shelterwood – uniform and group  Retention  Selection

Tolerance and Limiting factors to regeneration and growth of aspen Aspen  an intolerant species  Regeneration requirements  Removal of apical dominance  Light, soil temperature, nutrients  Aspen basal area >5 m 2 /ha can inhibit aspen regeneration; little aspen regeneration observed when basal area above 15 m 2 /ha in healthy stands  Good aspen regeneration has been reported in openings 40 m diameter in 30 m tall stands (Kabzems 2001). Openings less than 0.5 tree height in diameter may be a problem.  Poor aspen regeneration observed in narrow strips through dense patches of spruce  Growth  Rapid initial growth reaching near full height by age 70.

Tolerance and Major Limiting factors to regeneration and growth of white spruce White Spruce  Intermediate shade tolerance (survives fine in shade, but grows “best” in nearly open conditions)  Regeneration  Seedbed (mineral soil, dw, or humus)  Seed supply (variable production and heavy predation)  Competition – shrubs, grass, aspen, …  Hare  Chinook and frost injury  Growth  Slower early growth than aspen, catching up at age 60 or greater

Regenerating spruce in the understory  Reduced frost and winter injury  Reduced vigour of Calamagrostis canadensis EMEND – Stewart et al.

Successional Considerations  In the absence of tending - codominant mixtures of spruce and aspen occur in a mid-successional window  Age  <60 – Aspen dominated  >140 – increasing conifer dominance or reversion to “young” aspen  Fire likely to be less common as the major stand initiating disturbance (replaced by harvesting??)

Space  Within mixedwood stands – spatial heterogeneity – intimate and patchy  Forest/Landscape – desired forest composition (Aspen, mixed and conifer) needs to be defined and considered

Silvicultural Systems  Clearcutting  Avoidance/protection of advance regeneration  Shelterwood or “Retention” system to reduce windthrow of tall spruce  Underplant to provide advance regeneration  Plant  Tend (herbicide, brush, pct) – to accelerate spruce growth, alter composition, alter within stand spatial patterns (Doug Pitt)

Understory Protection Harvest (aka Natural Shelterwood; commercial thin)  Remove overstory aspen at ages 60 to 80 while leaving advance regeneration of white spruce (commercial thinning) Underplant stands at age 40 to provide advance regeneration where desired

Shelterwood Systems  Planted spruce seedlings grow well under shelterwoods with removal of 25% or more of the basal area (e.g. Man and Lieffers 1999, EMEND (Stewart et al. unpubl)  Uniform shelterwood can create a problem for aspen regeneration if much canopy is retained – but lead to Conifer or CD stands  Group or strip shelterwoods  openings wider than 50% of the tree height can abundant aspen regeneration  Studies so far indicate that spruce natural regeneration is unreliable in Alberta (Kabzems 2001)

Retention Systems  Widely practiced  Uniform and group retention  Biodiversity and structural objectives  15% of aspen volume left to provide wind protection (Understory protection harvesting)  Issues  impacts on aspen and spruce regeneration and growth ?? (EMEND and Adaptive Mixedwoods studies)

Tending/Silviculture -- Age shifts  Establishment and tending practices will change:  stand composition at different stages  rate at which component species reach merchantable sizes or volumes  rate of volume development of component species  age at which stands reach different “stages”

Lieffers, V.J., R.B. Macmillan, D. MacPherson, K. Branter, and J.D. Stewart Semi-natural and intensive silvicultural systems for the boreal mixedwood forest. For. Chron. 72:

Conclusions  Selection of an appropriate system – based on starting stand characteristics, stand and forest level objectives  Need to keep TIME and SUCCESSION in mind  Mixedwood stands are mid-successional (take time to develop and have a natural tendancy to move on to spruce in time) (it may take 2 or 3 cyles of aspen regeneration before spruce eventually dominates)  Aspen regeneration requires reasonable size gaps or very open canopies  Sw natural regeneration unreliable - Requirements to achieve successful natural Sw regeneration not well understood (C. canadensis likely one of the key factors).  Can create a range of mixedwood compositions – in time and space – with some planning – (expect some surprises!)  Range of options available  Use of advance regeneration can save cost and accelerate development of spruce component of the stand  Clearcutting and group/strip shelterwood, and group/strip selection systems have potential application  Traditional shelterwood systems a problem economically (returning for small aspen volumes)  Windthrow is an issue with any types of canopy retention,  risks are lower if retained trees are in groups  Harvest planning can help reduce windthrow problems  Tending will cause age/time shifts in stand development