Causes of Deforestation

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					              FOR417 & FOR1570                                     Shifting Cultivation


   Shifting Cultivation                               One of the oldest forms of agriculture   Random Mixed
                                                      240 to 300 million people
                                                      Need 15 ha per person per year

                                                           therefore need 3.6 billion ha
                                                           the world has 1.5 billion ha of
                                                          arable land (FAO)




    Shifting Cultivation or Slash and Burn




              Slash and Burn                                          Slash and Burn

 Slash                                               Burn
                                                         Kills some stump sprouts and seeds
  Primary, secondary forests or plantations
                                                         Converts wood and leaves to ash, releases nutrients
  Some of the biggest trees may be left
                                                        Makes planting easier
 Slash left to dry for up to a year                    Reduces insect and animal habitat
 Some nutrients lost by drying or removal of logs      Partially sterilizes soil – Good or Bad?
                                                        Some nutrients lost due to volatilization and erosion




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           Shifting Cultivation                                        Slash and Burn

 Why does the farmer shift?                          Ash
   Reduced Yields                                     Contains nutrients

      Soil depletion




                                                       Proportion of nutrient content of aboveground biomass returned to the
                                                             soil as ash following burning (From Giardina et al 2000)


                                                                 Element                        % returned as ash


                                                                     N                                     3

                                                                      P                                   49

                                                                     K                                    57

                                                                     Ca                                   50

                               Giardina et al 2000




              Slash and Burn                                           Slash and Burn

 Ash                                                 Ash
   Contains nutrients                                 Contains nutrients
   Nutrients can be lost through                      Raises pH                                                    Brady 1974
     volatolization
     convection
     wind and water erosion
     leaching




                                                                                                                                  2
                   Slash and Burn                                                  Slash and Burn

 Ash                                                                 Ash
   Contains nutrients                                                  Contains nutrients
   Raises pH                                           Brady 1974      Raises pH                                   Brady 1974


         Makes some nutrients like P more available                        Makes some nutrients like P more available
                                                                            Reduces toxicity of some elements like Al


                                             Giardina et al 2000                                          Giardina et al 2000




                   Slash and Burn                                                  Slash and Burn

 Ash                                                                 Ash
   Contains nutrients                                                  Contains nutrients
   Raises pH                                           Brady 1974      Raises pH
         Makes some nutrients like P more available
                                                                      Raises soil temperature
         Reduces toxicity of some elements like Al
         Improves soil microbial activity
                                             Giardina et al 2000                                          Giardina et al 2000




                   Slash and Burn                                                  Slash and Burn

 Soil heating                                                        Soil heating
   impact on nutrients                                                 impact on nutrients
      nutrients lost through breakdown of
      soil microorganisms, volatilization,                              impact on soil organic matter
      oxidation of biomass.




                                             Giardina et al 2000                                          Giardina et al 2000




                                                                                                                                  3
              Slash and Burn                                                             Slash and Burn

 Soil heating                                                             Soil heating
   impact on nutrients                                                      impact on nutrients
   impact on soil organic matter                                            impact on soil organic matter

   impact on soil bacteria, fungi and macro-fauna                           impact on soil bacteria, fungi and macro-fauna
           “Heating soil for 10 min at 70 C kills some fungi,
           protozoa and bacteria, while temperatures above 127
                                                                             changes in P available to plants
           C almost completely sterilizes soil (Raison, 1979).”                 impacts pH, vegetation and soil microbiology

                                                    Giardina et al 2000                                             Giardina et al 2000




           Shifting Cultivation                                                      Shifting Cultivation

 Why does the farmer shift?                                               Why does the farmer shift?
   Reduced Yields                                                           Reduced Yields

      Soil depletion                                                           Soil depletion
 The effects on soil nutrient levels are complex
                                                                                Increased insect pests
 and will vary from site to site, year to year
 and among slash and burn approaches.




           Shifting Cultivation                                                      Shifting Cultivation

 Why does the farmer shift?                                               Weed Invasions
   Reduced Yields                                                           What is a weed?
      Soil depletion                                                             Any plant growing where you don’t
                                                                                 want it
      Increased insect pests
                                                                             What are some characteristics of
   Weed invasions                                                          weedy plants?




                                                                                                                                          4
             Pioneer vs. Climax Tree Species
                     Pioneer                             Late-successional                          Seeds - the basic tradeoff
Germination          Triggered by heat, light            Germinates below canopy
Shade-tolerance      Cannot survive in shade             Survives in shade
Seeds                Small, many                         Large, few
Seed dormancy        Many form seed bank                 Few spp. have dormancy            Many and small                                  Few and big
                     "orthodox"                          "recalcitrant"
Seed dispersal       Wind, animal                        Diverse
Growth rate          Rapid                               Slower
Physiology           High maximal photosynthesis,        Low maximal photosynthesis,
                     high light compensation point       low light compensation point
Growth               Indeterminate                       Determinate, resting buds
periodicity
Leaf life span       Short                               Long
Herbivory            Little herbivore defense            More herbivore defense, esp.
                                                         generalized defenses (tannins)
Wood                 Low density, often pale             Higher density, gen. darker
Crown                Shallow; casts little shade         Deep; casts deep shade
Bole                 Clear; few knots                    Knotty
Longevity            Short                               Long




                  Seed germination types
     “Orthodox”                              “Recalcitrant”                                                Shifting Cultivation
     • Can survive in dormant                • Germinate or die within weeks
     state                                   • Germination cue is endogenous               Weed Invasions
     • Often can be cold-stored              • Do not form seed banks
     • Form seed banks                       • Most late-successional trees                  Integrated Weed Management
     • Germination triggered by
     light (esp. R:FR ratio), heat,
     and/or moisture
                                                                                                         Prevention
     • Most pioneer trees                                                                                Tolerance
                                                                                                         Control
                                                                                           Gallagher et al 1999. Weed management through short-term improved fallows in
                                                                                           tropical agroecosystems. Agroforestry systems 47:197-221.




                                                                                                           Shifting Cultivation

                                                                                           Shifting Cultivation as a weed break
                                                                                                 • Two Phases

                                                                                                        • Canopy establishment – woody
                                                                                                        plants out-compete weeds

                                                                                                         • Continuous cover – weed seed
      Gallagher et al 1999. Weed management through short-term improved fallows in                       bank depletion
      tropical agroecosystems. Agroforestry Systems 47:197-221.




                                                                                                                                                                          5
          Shifting Cultivation                                     Shifting Cultivation

 Shifting Cultivation as a weed break                     Shifting Cultivation as a weed break
    • Seed bank depletion                                     • Seed bank depletion

  “…in Nigeria, seeds of 11 out of 15 common                The roles of nitrate and ethylene in seed
  tropical weeds lost at least 50% viability after          bank depletion
  one year of burial…”

                                   Gallagher et al 1999                                   Gallagher et al 1999




          Shifting Cultivation                                     Shifting Cultivation

 Shifting Cultivation as a weed break                     Shifting Cultivation as a weed break
    • Seed bank depletion                                     • Weed Recruitment
                                                                • Quality of light
  Seed decay and seed predation
                                                                   • R (600-700nm)
                                                                      • promotes germination
                                                                   • FR (700-800 nm)
                                                                      • promotes dormancy
                                   Gallagher et al 1999
                                                                                          Gallagher et al 1999




          Shifting Cultivation                                     Shifting Cultivation

 Shifting Cultivation as a weed break                     Shifting Cultivation as a weed break
    • Weed Recruitment                                        • Weed Recruitment
      • Quality of light                                        • Allelopathy of fallow species
         • Lower R:FR ratio under a
         canopy
            • Lower weed recruitment

                                   Gallagher et al 1999                                   Gallagher et al 1999




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             Shifting Cultivation

 Shifting Cultivation as a weed break
      • Weed Tolerance




                                               Gallagher et al 1999                                                 Gallagher et al 1999




                                                      de Rouw, A. 1995                                                     de Rouw, A. 1995




             Shifting Cultivation                                                     Shifting Cultivation

 Shifting Cultivation as a weed break                                    Shifting Cultivation as a weed break
  Cropping        Fallow        Closed                                        • Impact of intensity of clearing
               Establishment    Canopy           Slash & Burn

    Weed      Existing weeds    Weed seed         Some weeds                              Fallow        Closed
Establishment out-competed     bank depleted       eliminated              Cropping                                     Slash & Burn
                                                                                       Establishment    Canopy
  1-2 yrs       2-5 yrs         10-15 yrs            1 yr
                                                                                      Existing weeds    Weed seed       Some weeds
                                                                                       out-competed    bank depleted     eliminated
                                                                           1-2 yrs      2-5 yrs         10-15 yrs           1 yr




                                                                                                                                              7
              Shifting Cultivation                                                     Shifting Cultivation

 Shifting Cultivation as a weed break                                 Shifting Cultivation as a weed break
     • Impact of intensity of clearing
     • Impact of fallow length                                              In successive short fallow cycles the
  Cropping          Fallow        Closed                                    ratio of weeds to forest plants
                 Establishment    Canopy         Slash & Burn
                                                                            increases so that the establishment of
    Weed      Existing weeds      Weed seed      Some weeds                 overhead cover is impaired.
Establishment out-competed       bank depleted    eliminated
  1-2 yrs        2-5 yrs          10-15 yrs         1 yr




              Shifting Cultivation                                                     Shifting Cultivation

 Shifting Cultivation as a weed break                                              Increased pressure on land

Factors that improve weed control:
                                                                                    Shortened fallow period
1. tree seedling present after the cropping phase
2. Forest plants have to survive the cultivation phase
3. Number of arable weeds should be less than the
   number of forest plants.
                                                   de Rouw, A. 1995




              Shifting Cultivation                                                     Shifting Cultivation

             Increased pressure on land                              Long Fallow                 Short Fallow              Forest Pioneer
                                                                      •long fallow rotation       •short fallow rotation    •no rotation
                                                                      •traditional                •semi-traditional         •modern
                                                                      •mainly subsistence crops   •mixed subsistence &      •mainly cash crops
             Shortened fallow period                                 •mainly self-generated      cash crops                •mainly outside capital
                                                                      capital                     •mixed capital sources    •close to urban areas
                                                                      •far from urban areas       •intermediate distance
                                                                                                  to urban areas
             Shifting vs. Shifted Cultivators

                                                                                                                           Brown & Schrekenberg 1998




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         Shifting Cultivation

Is shifting cultivation REALLY an
agroforestry practice?


What roles do trees play in the re-
establishment of productivity during the
fallow phase?




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