FINAL1shiftingcultivation Flashcards
Shifting cultivation
Widespread throughout world as traditional form of agriculture •Europe •Asia •Latin America •Africa •USA - early colonists
What is meant by a successional agroecosystem?
●Mimics process of succession found in many ecosystems following a disturbance
•Forest succession => open grassland => mixed grassland => shrubs and small trees => long-lived perennials
●Generalized description of succession, but stages vary with environment/climate
Shifting cultivation
Three main phases:
- Clearing and burning
- Cropping (short)
- Fallow (long)
•range 1-12 yrs, usually 2-4 yrs
●Cropping cycle: vary with site fertility
Fallow: abandonment & forest regeneration
●traditional fallow: 20+ years
Clearing methods
Traditional hand clearance shown to be optimal
Hand clearance leaves root systems intact
including dense surface root mat found in tropical forests on infertile soils
Large or selected trees often left intact
Trials using machinery to uproot trees => cause soil structure problems, increased erosion and nutrient loss
Why burn?.
Efficient and often only realistic way to deal with slash
Most of cut biomass ash
Release nutrients for crops
Reduce weeds? - not clearly demonstrated
Reduce soil pests? - not clearly demonstrated
Problem with burning
Some nutrients lost to atmosphere
volatilization
•convective loss
Ash on ground also vulnerable to leaching loss if rains before crops established
burning-Effects of increased pH
●Very important, especially in acid-infertile soils typical of much of humid tropics (often pH <4)
●Most crops grow poorly at low pH
●pH of soil increases due to more base cations (K, Ca, Mg) added from ash increased P availability
●Reduces solubility of Al and Mn, both toxic to most crops
burning-Effects on other nutrients
●Nonvolatile elements like Ca, K, Mg are concentrated in ash
●Heat kills soil microbes making more C and N in readily available forms
●pH changes make P more available – P is often limiting nutrient
●Microbial/fungal activity stimulated increased P availability
burning-Cropping cycle
●Duration varies widely with location
●Often follows a pattern:
•nutrient demanding annual crops
•annuals + short-lived perennials (eg cassava)
•perennials - still harvested as forest species recolonize
Importance of fallow
●Replenish above ground biomass and nutrient stores
●Suppress weedy species
●Protect soil and rebuild biodiversity
●Provide other resources:
•fruit, nuts, game, medicinal products, fuelwood, timber etc.
●Shifting cultivation (swidden agriculture, milpa) vs. slash-and-burn
Managed fallow
●By deliberately managing the fallow period, farmers have been able to meet multiple needs
●Plant specific trees at different times during cycle
●Some intensive managed fallows evolved due to land/population pressures (e.g. bamboo fallow in Indonesia)
●Note: short cycle possible in young fertile volcanic soils
Is shifting agriculture sustainable?
sustainable because…
●Works in sites with nutrient limitations
●Can be adapted to different climates
●Provides variety of crops
●Maintains biodiversity, ecosystem processes and forest succession
●Promotes biological control
●Requires low capital and labor
●Requires knowledge and expertise
●Can be maintained over long periods of time
trends to unsustainability
progressive shift to shorter fallows and larger areas of clearance
Drivers of increased forest clearance
●Increasing population pressure
●Poverty and food insecurity
●Unsustainable cropping practices
●Policies
•colonization and road building policies (Brazil, Indonesia)
•tax and credit that favor land clearing (Brazil)
•resource tenure policies (all countries)
•negative international terms of trade in humid forest zones
•market price fluctuations for agricultural commodities, timber and non-timber products (all countries)
•lack of involvement of local communities in management of their natural resources (all countries)
Trends to unsustainability
Problems with shorter fallow:
Spiral of degradation: less biomass accumulated during follow period = less nutrients available for subsequent crop/fallow phase
Increased weed pressure: infestations by aggressive weedy plants
