Slash And Burn, Natural Gap Dynamics, Sustainable, Silviculture

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basic concepts gap dynamics

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gap dynamics theory- shade intolerant spp can maintain pops by regenerating within gaps in mature or old growth forests. gaps created provide microenvironmental conditions favouring establishment of shade-intolerant spp. seedlings of these spp can become established and may reach to canopy. forest spatial mosaic various compositional and structural phases, change cyclically over time- regeneration complex.

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slash and burn

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o Cut down vegetation on patch of land, then set fire to remainder- ashes serve as viable nutrients for farming on the site for a brief period 3-5 years. According to nutrient dynamics- needs decades to recover.
o Larger trees may survive fire/ palms. Bamboos in some areas take advantage of fires and spread rapidly into open areas.
o Increasing fragmentation, edge effects and penetration of forest areas by humans has increased exposure and vulnerability of RFs to fires- fires set for slash and burn may have much further reaching effects if not controlled.
o Releases 180 metric tons of carbon into atmosphere per acre. Forest fires can rage out of control.
o Can result in significant soil erosion and accompanying landslides, water contamination and/ or dust clouds as without trees and veg, soil washes away during heavy rains.

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Pelletier et al 2012

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at low pop density, SBA isn’t necessarily a threat to natural ecosystems, as low rate of deforestation coupled with a long fallow period permits regeneration of forest areas with limited impacts on biodiversity or emissions (Pelletier et al 2012). SBA dominant in tropical regions in places with low pop density, reduced soil quality and limited access to markets.

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zhang et al 2002

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o However DRC- when population density reaches 20-30 habitants per km2, SBA no longer viable, no longer a fallow period= decreased soil fertility

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Karsenty 2012

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population will double within 22 years, mean population is already nearing 30 individuals per km2 in some areas- could lead to increasing pressure on forests.

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issues w slash and burn

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o Changing socioeconomic conditions, growing populations and reduced land availability have reduced fallow periods, in lowland Bolivia, it has declined from 6-12 years to 4 years. Shorter fallows increase weeds and reduce crop yields, encouraging further clearance.
o Burning negatively effects earthworm density and casting activities. 95% loss.

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gap conditions

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o Dryness, air temperature and photosynthetically active radiation increase towards middle of gap depending on gap size.o Seedlings grow faster in gaps- some spp are faster than others. Small gaps colonised by seedlings that were already there and grow quickly. Large gaps colonised by light demanding pioneer spp not found in more mature rainforest

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formation

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 Can be tens to hundreds of square metres.
 Trees can die standing. More likely with buttressed, sound, dense timbered stems.
 Large multiple gaps common in cyclone prone regions, on steep slopes and among slender top heavy trees that develop on loose fertile soils found in some volcanic regions. Older taller forests larfger gaps.
 Larger gaps- tree crowns extend into space, more unbalanced, more likely to fall into space.

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consequences

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 Creation- damage to seedlings, saplings and young trees.
 Regeneration from pre-existing vegetation, from seeds and occasionally epiphytic seedlings in crowns of fallen trees. Different tree spp develop in distinct parts.
 Larger gaps- higher light levels, saplings and seedlings- surge of growth, rapidly compete for space. towards shadier edges, growth can be slower. different sizes and short lives can lead to localised succession. Can fill with liana thickets that suppress seedlings. Palms, heliconias, bamboos etc thrive and shade out seedlings.
 Lianas increase prevalence of tree falls and size of resulting gaps.
 Climber more abundant in gap areas- positive feedback.

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pioneer

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o Pioneer: seedling can’t survive below canopy. Seeds small copious, produced continuously and early on. Many spp in soil seed bank due to perpetual rain. Dispersal- wind/ animals- long distance. Growth rate- high carbon fixation rate, unit leaf rate and relative growth rate. Leaf life is short. Susceptible to herbivory. Short longevity.

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climax

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seedlings can survive below canopy, grow slowly; large seeds not frequently produced, only by mature trees (60 years to mature); few spp in soil seed bank; seeds dispersed by gravity over short distances or fruits; lower growth rates- invest more in defence; leaves long lived; resistant to herbivory; can live for a long time.

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pioneer species

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o Pioneer spp grow first, generate microclimate and shade- S America= cercropia, Africa musanga, SE Asia Macaranga- all possess larger leaves, so they can grow as fast as possible to set seed again.

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stand table

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pioneers have fewer in seedling phase whereas climax has lots- they wait for seedling release upon gap formation, so disturbance is key

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silviculture

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 Timber properties correlate to growth rate therefore the extent to which the tree is light demanding.
 Manipulate gap size to promote certain spp, thin forest, promote tree of interest.
 Polcyclic- more diverse, repeated removal of certain trees favours climax trees.
 Monocyclic- mostly this. Remove all saleable trees- favours pioneers.
 Failure follows from working beyond limits of inherent dynamic capabilities of forest ecosystem

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Eastern Tropics

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• Dipterocarp
o Monocyclic
o Mast fruiting every few years- then remove big trees and next generation will grow, felling after seeding.
o Adapted to strong weather.
o Malaysian uniform system- spp rich but similar wood density with many light hardwoods- fast growing.

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Neotropics

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• Fewer light demanding species
• Less disturbed
• No hurricanes- difficult to open canopy for hardwood
• Mahogany= valuable- little known about regeneration. Unlikely to be stimulated by small gap formation
• Many spp difficult to stimulate one to grow

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West Africa

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• Polycyclic system- prescribed system
• No LT incentive to use this
• Mahoganies here- equivalent to eastern dipterocarps- thin forest.

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slash and burn - Tschakert et al 2007

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richer families had more land in fallow, fallow periods longer, large shares in old fallow. those driven by food needs and timely returns to agricultural efforts have shorter fallow systems. insufficient or inadequate food supply- three most prevalent sources of risks, risk associated w health and living conditions as well as labour constraints- limited their incentive and ability to increase fallow length or diversity land use types and sources of income.

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gap mosaic model

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gap phase following canopy opening event, building phase= regen and growth, mature phase- high forest canopy established.

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gap formation

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can be large, can be minimal. standing death- smaller space, more likely buttressed spp. uprooting- large ground= large space. brings down other tees with it. common in cyclone prone regions- top heavy trees on loose fertile soils in volcanic regions. older taller forests larger gaps.

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gaps

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There will be more light, heat, and wind on the forest floor where a gap forms. The forest floor of the gap will become hotter and drier than previously, although more rain will reach the ground (but it will be dried quickly by the sun). The temperature here can be as much as 10o C higher than under the canopy. Even the wave lengths of light reaching the forest floor are altered. Normally canopy trees absorb “red” (long) wavelengths and the forest floor receives only about 2% of the photosynthetically-active wavelengths (440-700 nm) in small spots or flecks. But in a gap, more of the light reaching the forest floor will be in the red wavelength range (660-770 nm). Under these circumstances, the slower-growing and shade-tolerant seedlings in the understory cannot survive, and are gradually replaced by seedlings of fast-growing and light-tolerant species (the so-called pioneer species).

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gaps

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Pioneer species are characterized most importantly by a requirement for strong light for seed germination and seedling establishment, and also, in general, aggressiveness, tolerance to dessication, rapid growth, early reproduction, efficient seed dispersal mechanisms, and small seeds with long dormancy periods

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gaps

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When a gap forms and light strikes the forest floor, their seeds, which have been dormant for a long time, are able to sprout and grow rapidly to fill in the gap. At this early successional stage, the extent of seedling sprouting and survival is determined mainly by factors in the environment – competition, nutrient availability, temperature, degree of shade, etc. Then those seedlings which survive begin to influence their own environment as they grow – by producing shade, using soil organic matter, and producing new types of habitats. At the same time, epiphytes and climbing plants begin to colonize the growing young trees.

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gaps

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The early pioneer trees are often low and short-lived, and may be replaced by longer-lived, taller pioneer species which form a higher canopy forest. Meanwhile, the seedlings of climax species remain undeveloped in the shade of the pioneer trees, as they do not do well under gap conditions of high temperatures and high light intensities. Moreover, specimens of climax species in gaps appear to be susceptible to attack by boring insects. The pioneer species will eventually, however, be replaced by climax species since, as pioneer seedlings require light, they cannot survive and reproduce under the newly-formed (pioneer) canopy. Climax species in general have large seeds with substantial nutrient reserves (so they can wait), have shade-tolerant seedlings, are slow-growing relative to the pioneer species, and are self-perpetuating, since once the pioneer trees form a canopy, the climax species’ shade-tolerant seedlings can grow in their shade.

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gaps

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As large pioneer trees die, conditions are conducive for the small trees of the climax species to grow rapidly and take their place. (While climax species’ seedlings are shade-requiring, older specimens actively seek light.) Once established, a climax forest can reproduce itself endlessly, since it provides shade for its seedlings, and the large trees have attained the canopy. This series of events is what Whitmore (1998) calls a “shifting mosaic steady state.”