Mixedwood Forests – A discussion (see Smith et al. 1997 Chapter 16 Stands of Mixed Species) A general overview of the benefits of mixedwood management, vs monoculture management is available at: http://www.for.gov.bc.ca/research/mixdwood/ Additional discussion on this topic is provided by Man and Lieffers (1999). Lieffers et al. (1996) provide an overview of silvicultural systems applicable to different mixedwood stands, and their potential outcomes. Mixedwoods A mixedwood forest is a forest that is comprised of recognizable amounts of 2 or more species – since we rely on traditional forest inventory methods – in application – a stand is recognized as a mixedwood stand when less than 80% (on a volume or basal area basis) of the main canopy is a single species. Around the world – naturally occurring monocultures are rare and are typically found on harsh sites, in early successional stages following major (stand destroying) disturbances, or in late successional stages when aggressive shad tolerant species become dominant. Mixedwood forests are an important part of the boreal forest. Most upland sites in the boreal have some deciduous. The amount of deciduous tends to decline with time since the last major stand replacing disturbance – Why? Because more tolerant tree species grow up in the understory and eventually take over the stand in the absence of stand destroying disturbances. In boreal forests we can have even-aged as well as uneven aged mixtures. Spruce regeneration can occur in some stands at the same time as aspen regeneration, however, since the aspen grow much more rapidly than the spruce they typically tend to be taller for the first 40 to 60 years. What factors would favour establishment of both spruce and aspen at the same time? [spruce seed source, favourable ground surface conditions – bare mineral soil, lack of seed predation, disturbance was severe enough to inhibit the grass and other competition so that trees could establish]. Young aspen stands tend to close canopy somewhere between ages 5 and 15, and from this time, up until about age 40 – it can be very dark in the understory and very difficult for much regeneration of spruce to occur. As aspen stands pass age 40 and beyond – we may start to see new spruce regeneration establishing and growing in the understory. In reality, we probably see some combination of initial and subsequent establishment of spruce.
Why mixedwoods? With plantation forestry there is a tendency to become focused on individual crops species and to try to simplify the forest by promoting single species and even-aged stands. But, there are a number of reasons why we should be cautious in application of this model. 1) Mixedwoods are a natural part of the landscape 2) Visual resource – aesthetic (they look nice) 3) More diverse and generally support a greater diversity of other organisms 4) Health – mixedwood stands often (but not always) suffer lower impacts from insects than monoclultures – white pine weevil damage to white spruce and sitka spruce is often lower when spruce are grown under shade from overtopping hardwoods.; - mixtures reduce changes of contact between roots; - overall – risk of losing the entire stand is lower 5) Nurse Crop a. Reducing vigor of grasses, herbs and shrubs b. Carbon transfer between roots that share the same mycorrhizae c. Frost protection 6) Nutrient availability and cycling a. Red alder – symbiotic N fixation can add up to 125 kg N/ha/y b. N fixation by free-living bacteria (pseudomonads) is increased in the presence of birch 7) Yield a. Red alder – Douglas-fir mixtures – i. 200 red alder + 1000 Fd gives maximum yield of Fd plus some alder; >300 alder leads to reduced yield of Fd. ii. Varying proportions – maximum Fd yield is at 20% red alder and maximum total stand yield is at 40% Dr. b. Birch – Douglas-fir mixtures i. Simulations do not show a dramatic response, results suggest that the presence of birch could result in some reduction in Fd yield – the kind of response you might expect when growing two
intolerants as codominants in a mixture – basically involving splitting the yield between two species in relation to their abundance in the stand – a similar thing happens between aspen and lodgepole pine on some sites (but not on all sites) c. In general – growing a mid-tolerant or tolerant understory species together with an intololerant overstory species as a stratified mixture – should result in increased total yield – but at the same time – there may be some yield reductions of the understory 8) Economics – cost-benefit analyses often suggest that best returns are typically achieved by keeping initial investments to a minimum (due to discounting – the cost of carrying the interest on money invested). Another economic aspect is the fact that mixed-species stands may produce greater diversity of end products. Options for mixedwood management Natural (leave them alone entirely) - Total yield may be high, but conifer yield may be reduced - Tending might be used to increase spruce yield - Aspen plus pine – would see serious reductions in pine yield without tending - Advantage – cost savings; In some systems (e.g. birch-Douglas fir) cutting birch may increase virulence of Armillaria Intimate Mixtures Stratified or co-dominant mixtures Advantage: nursing effects are evenly distributed Disadvantages: - Have to use selective (single tree) treatments - Most conifers (in a mixedwood) suffer some growth loss - Quality of widely spaced broadleaves may be low,
- Harvesting broadleaf component and protecting understory conifers may increase expense Patchy Mixtures Interspersed patches of broadleaf (with conifer understory), and conifer Somewhat artificial – but they do occur to some extent naturally Conifers near the edge of the broadleaf patch may benefit from nutrient inputs in litterfall - red alder influence can extend 15 m into an adjacent Fd stand due to litterfall Shading influence of alder and aspen does not typically extend very far into a conifer block (<5 m) Tending of these stands is much easier, and growth of conifers in treated patch may be substantially higher. Temporal mixtures (mixing in time) (One-cut shelterwood or natural shelterwood systems) Slower growing Sw in the understory is typically not ready to harvest until 100 years, while aspen might be harvested between 50 and 80 years. Advantages: - Can telescope the two species, so that one is established before the other one is harvested, effectively shortening the rotation for the second species. - Maintain forest cover - May reduce problems with grass, shrub, and aspen competition during spruce establishment Disadvantages: - additional cost of understory protection
Some challenges in mixedwood management (Lieffers and Beck 1994) 1) Unless the understory species is exceptionally shade tolerant – its establishment and survival may be poor during early stages of stand development 2) Mixing species that mature at different times creates challenges to harvesting – “understory protection” 3) Yield prediction is very difficult (virtually impossible) 4) Estimating future economic values of different species is a challenge. [Establishment costs differ for different species – should we always accept the cheapest? What will we get if we allow industry to use the least expensive option – without restrictions on species?] 5) Different operations and mills have different demands for different species. Overlapping tenures create management challenges and can create conflict. 6) It is far easier to plan and manage for single species than for mixtures, but mixtures might be more FUN! 7) Mixedwoods comprise a wide range of species structures and compositions – complexity and confusion 8) Regulations promote monocultures.
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