INDIGENOUS CROPS OR COLONIST CATTLE? SOIL ORGANIC CARBON OF FOREST-TO-CROP

VERSUS FOREST-TO-PASTURE LAND USE CHANGES IN EASTERN PANAMA

Martin P. HegerLondon School of Economics

Land and Poverty conferenceThe World Bank, March 24, 2015

Wounaan vs Colonist land use

• Amerindian crop farmers vs smallholding cattle ranchers

• Both do slash and burn• Colonists permanently convert, grow pasture,

and actively weed-control cattle ranchingForest-to-pasture conversion

• Wounaan practice shifting agriculture (fallow periods) grow crops (mostly rice and maize).

Forest-to-crop conversion

Study site

Ecologically adverse impacts of land use: Wounaan versus colonists

• Extensive measure: Wounaan deforest about 35 times less per household than colonist cattle ranchers (Heger, 2009)– Cattle ranching is more land extensive

• Intensive measure: Per selected area (e.g. ha) – Is cattle ranching also more land intensive? Does it

degrade soil quality (SOC – soil organic carbon) more? YES

Literature: forest-to-pasture conversion no change (increase) in SOC

• The preponderance of papers found that SOC was either higher or the same (e.g. Huth et al., 2012).

• Meta-analyses in the tropics: – Average increase in SOC stock by almost 10 percent

(Powers et al., 2011)– Overall SOC levels rose by 8 percent (Guo and Gifford,

2002)

• Average results. Depend heavily on precipitation, soil type, mineralogy, topography, etc…

Literature: forest-to-crop conversion decrease in SOC

• Despite the nutrients captured in the soil because of combustion of the vegetation in the slash-and-burn process, soils under crop-cultivation contain significantly less SOC than forests.

• Most severe SOC losses follow a conversion of primary forest to cropland, by about 1/4th (Don, Schumacher and Freibauer, 2011).

• On average forest-to-crop conversion leads to a decrease in SOC of more than 20 percent (Powers et al., 2011).

• Shifting cultivation is found to reduce SOC by a bit less so than crop cultivation without fallow periods

average decrease of 10 percent (ranging from -23 to +7).

Literature- Forest-to-crop conversion: lowers SOC- Forest-to-pasture conversion: neutral (positive) for

SOC

HYPOTHESIS:Forest = Pasture > Crop-cultivation

Body of literature does not look at indigenous agricultural techniques and the few that do do not contrast it to alternative land uses!

Schematic of soil sampling strategy

Testing with comparison of means (ANOVA) & Tukey’s Honestly Significant Difference Test.

SOC by land use (vegetation) class

Forest-to-pasture conversion implications for SOC also depend on topography

Forest-to-crop conversion implications for SOC depend on topography

Soil Organic Content in primary versus secondary forest

Soil organic content drops over time since forest-to-pasture conversion

Cronosequence from deforestation to reforestation

Main finding

Forest = Pasture > Crop-cultivation

Forest = Crop-cultivation > Pasture

Conclusions

• Indigenous smallholding crop-cultivation different from previously studied commercial agriculture. Not sign different from forest land.

• Only forest-to-pasture conversion sign reduced SOC– Particularly if pasture plots are older.– Keeps getting worse over time. First 10-year period

almost as much decreased in SOC than second 10 year period.

• Restoring of SOC possible with reforestation

Discussion

• Sustainable agricultural practices applied by indigenous community– no-tillage farming– long fallow periods

• Unsustainable practices of the colonist cattle ranchers – ranching cattle beyond the carrying capacity of the

land– no-fallow management of pastures

THE END

Thank you for your attention

Deforestation

Panama, Central America, deforestation is mostly driven by disorderly and unsanctioned settlements on forested land and the conversion of forests to agricultural systems, for either animal grazing or crop cultivation (Tschakert, Coomes and Potvin (2007). Ranching cattle is the primary reason for deforestation and LULCC. In fact 80 percent of current deforestation is because of cattle ranching (Nepstad et al, 2008). Of all deforestation activities, cattle ranching is widely regarded to cause the most environmental damage.

Study Objectives

• Investigate differences in Carbon between the land use strategies:1. Old-growth forest2. Secondary (re-growth) forest3. Crop-cultivation (indigenous community)4. Cattle ranching (colonists)

• Investigate whether soil quality drops over time in pasture land

Deforestation

Impacts on soil degradation & climate change:• Above-ground Carbon • Soil-stored Carbon – Soils contain more Carbon than both living plant

biomass and atmospheric CO2 combined (Jobbagy &

Jackson, 2000 & Laganiere et al., 2009). – Tropical soils: 36-60 percent of ecosystem Carbon

is being stored in the soil (Dixon et al., 1994, FAO,

2006, Malhi et al., 1999).

Hypotheses on Soil-Stored Carbon

Hyp 1:Primary forest = Secondary forest = Pasture

> Crop-cultivation

Hyp 2:Pasture 2 years > Pasture 10 years >

Pasture 20 year

Testing with comparison of means (ANOVA) & Tukey’s Honestly Significant Difference Test.

Literature: SOC recovery after reforestation? yes!

• Secondary forest SOC pools can recover to levels similar to those in old-growth forests (Guo & Gifford, 2002)

• They may even exceed SOC of primary forest (Sayes et al., 2005) … but it takes time (Fonseca et al. (2011)) • It takes anywhere from 20 to 100 years for full recovery to kick in

(Rhoades et al., 2000; & Neumann-Cosel et al., 2010). – Biomass continuously increases in secondary forests with age, reaching

comparable levels at about 30 years of age, and even surpassing old growth biomass beyond that, at 30 to 42 years of age (Letcher and Chazdon, 2009). who applied a chronosequence for aboveground biomass by looking at different forest age groups found that

– The slow recovery is attributed to the sluggish uptake of carbon in soils (Gamboa et al., 2008; McGrath et al., 2001; Robert, 2002; Saynes et al., 2005; Singh et al., 2007; Turner et al., 2005).