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

2. 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

3. 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.

4. 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

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

6. Successional Considerations  In the absence of tending - codominant mixtures of spruce and aspen occur in a mid-successional window  Age 60-140+  <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??)

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

8. 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)

9. 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

10. 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)

11. 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)

12. 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”

13. Lieffers, V.J., R.B. Macmillan, D. MacPherson, K. Branter, and J.D. Stewart. 1996. Semi-natural and intensive silvicultural systems for the boreal mixedwood forest. For. Chron. 72: 286-292

15. 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