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Dynamics of boreal mixedwoods: A conceptual model
Dr. Han Chen Office: BB-1009F Phone:
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Definition of mixedwoods (BMW)
MacDonald (1995) Site definition: an area with climatic, topographic, and edaphic conditions that favour the production of closed canopy stands dominated by trembling aspen (Populus tremuloides) or white birch (Betula papyrifera) in early successional stages, black spruce (Picea mariana) or white spruce (Picea glauca) in mid-successional stages, and balsam fir (Abies balsamea) in late successional stage Stand definition: a boreal mixedwood stand is a tree community on a boreal mixedwood site in which no single species comprises 80% or more of the total basal area What would be none- BMW sites? Can 50% SB and 50% PJ be a BMW stand?
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Disturbances in BMWs Fires, windstorms, ice storms, snow breakages, insect outbreaks, and diseases BMWs are less prone to disturbances (resistance and resilience/adjustement) Less susceptible to wildfire than pure pine, but more than lowland black spruce Less prone to blowdown because of their relatively deeper rooting depth than lowland black spruce dominated stands growing on organic soils Suffer less damage from Spruce budworm than balsam fir dominated stands on similar upland sites Less susceptible to Armillaria infections than pure conifer stands
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Resource availability of BMWs
High nutrient availability, a result of a high decomposition rate Mesic soil mositure Higher resource heterogeneity, and plant diversity
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The need to understand dynamics of BMWs
The importance of many species is still unknown, and emulating natural disturbances and processes is considered a safe strategy in forest management The prerequisite for sustainable management is to understand the dynamic structure, composition, process, and driving forces for each forest or stand type “[If] one wants to create a certain kind of forest, the best point of departure is to consider the natural disturbances that bring it into being in nature. Once this is known, it is possible to conjure up ways that simulate the appropriate disturbance”
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Defining BMW stand dynamics
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Stand initiation Stand initiation follows a major disturbance that kills most of the mature trees on a site. Growing space is available for new trees. At the end of this stage, the new cohort of trees forms a horizontally closed canopy Factors influencing stand initiation Type and severity of disturbance Intensity of fire, light vs. intensive on propagules and regeneration substrate Windthrow, insect outbreaks, and others Pre-disturbance stand and site condition Stand composition, stand age, timing in relation to seed crops Site condition, dry, mesic, vs wetter sites
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Stand initiation process
Physical and chemical environments of the site dramatically differ from pre-fire, newly available growing space Within 2-3 years after fire, herbs, shrubs, and trees colonize the site Tree colonization may take only one to several years with vegetative origin (aspen, birch), aerial seed bank (jack pine, and partially black spruce), and forest floor seed bank (pin cherry). Note: white spruce can also have aerial seed bank when fire is not extreme; but balsam fir and cedar seeds are killed in a typical crown fire
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Stem exclusion This stage starts with a fully established tree canopy, ends when shade-tolerant conifers start to penetrate into the canopy layer When pioneer and late successional species reoccupy burns simultaneously, there are two possible outcomes Either the faster growing aspen, birch, and pines create an overstory that suppresses the slower growing spruce and fir to such a degree that they will never reach the main canopy before the next stand-replacing disturbance or There are adequate resources to allow the slower growing species to reach the canopy later
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Canopy transition The intense competition among stems comes to an end
As trees start to decline and die because of longevity or damage from non-stand-replacing disturbances, shade-tolerant coniferous trees from the understory and intermediate canopy now take over the main canopy New individuals can also establish in gaps created by tree mortality Structural characteristics of this stage include canopy gaps as well as a stratified canopy This stage ends when all of the individuals from the initial cohort have died
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Gap dynamics Trees established through self-perpetuation dominate the stand Growing space is available in all strata because of the death of individual or groups of trees This stage is structurally characterized by a mosaic canopy, dominated by shade-tolerant species, usually spruce and fir Gaps are occupied by shade-intolerant and (or) shade-tolerant trees and (or) shrubs such as mountain maple (Acer spicatum), beaked hazel (Corylus cornuta), speckled alder (Alnus incana),raspberries (Rubus spp.), and willows ( Salix spp.), depending on local site conditions.
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Ecological attribute Trembling aspen White birch Black spruce
White spruce Balsam fir White cedar Favoured land forms and soils Fresh to moist Dry to moist All sites Primary reproduction mode after fire Suckers, Seeds Stump sprouts, seeds Seeds Regeneration time after fire Rapid Rapid or gradual Variable (gradual) Reproductive mode in absence of fire Seeding in gaps, suckers Seeding in gaps, sprouts Layering Seeding in understory Seeding in understory, layering Growth habit Fast Medium Slow Shade tolerance Intolerant Very intolerant Tolerant Very tolerant
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Ageing succession-multiple pathways
Sp . spruce budworm + Bw seed . severe spruce budworm d + Shrub field c . windthrow Bw seed a a a d a, d Po Bw a,b , b a Po Sp a Sp Bw Bf Theoretical models a- cyclic b-convergent c-divergent d-parallel e-individualistic Chen & Popadiouk Environmental Reviews 10:
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