Spatial and temporal land use and carbon stock changes in Uganda: implications for a future REDD strategy

Abstract

Using a map overlay procedure in a Geographical Information System environment, we quantify and map major land use and land cover (LULC) change patterns in Uganda period 1990–2005 and determine whether the transitions were random or systematic. The analysis reveals that the most dominant systematic land use change processes were deforestation (woodland to subsistence farmland—3.32%); forest degradation (woodland to bushland (4.01%) and grassland (4.08%) and bush/grassland conversion to cropland (5.5%) all resulting in a net reduction in forests (6.1%). Applying an inductive approach based on logistic regression and trend analyses of observed changes we analyzed key drivers of LULC change. Significant predictors of forest land use change included protection status, market access, poverty, slope, soil quality and presence/absence of a stream network. Market access, poverty and population all decreased the log odds of retaining forests. In addition, poverty also increased the likelihood of degradation. An increase in slope decreased the likelihood of deforestation. Using the stock change and gain/loss approaches we estimated the change in forest carbon stocks and emissions from deforestation and forest degradation. Results indicate a negligible increase in forest carbon stocks (3,260 t C yr-1) in the period 1990–2005 when compared to the emissions due to deforestation and forest degradation (2.67 million t C yr-1). In light of the dominant forest land use change patterns, the drivers and change in carbon stocks, we discuss options which could be pursued to implement a future national REDD plus strategy which considers livelihood, biodiversity and climate change mitigation objectives.

Appendices

Appendix 1: Detailed description of the model processes

[i] current year, [i-1] refers to previous year, [i+1] is the next year, [0] then refers to the initial-year

1.1 Forests & new plantations

$$ Plant[i] = Plant[i - 1] + \Delta Plant[{\hbox{i}}] $$

(1)

• Plant[i] is the cumulative new-plantation area in year i.

• ΔPlant[i] is new area that has been converted to plantations from grasslands and cropland during year i. This is manually specified every year according to the desired scenario by a user.

$$ \Delta Plant[i] = \Delta PlantC[i] + \Delta PlantG[i] $$

(2)

• ΔPlantC[i] and ΔPlantG[i] are user specified land area transfers from cropland and grassland to new plantations. This may have implications for satisfying cropland demand.

  $$ For[i] = For[i - 1] + \Delta For $$

  (3)

• For[i] is the forest area for year i (the current year)

• ΔFor then refers to the change in forest area

1.2 Grassland

$$ Gras[i] = \max (Gras*[i],Past\min ) - \Delta PlantG[i] $$

(4)

$$ Gras*[i] = Gras[i - 1] + \Delta Gras*[i] $$

(5)

$$ \Delta Gras[i] = Gras[i] - Gras[i - 1] $$

(6)

• ΔGras [i] is the Actual change in grassland

  $$ \Delta GrasDfct[i] = \Delta GrasD[i] - \Delta Gras[i] $$

  (7)

• ΔGrasDfct is the Deficit or unsatisfied grassland demand at current consumption and yield

• Gras[i] is the grassland area for year i (the current year)

• [i-1] then refers to the previous year

• ΔGras then refers to the change in grassland based on demand and constraints such as the user defined minimum value of Pastmin.

• ΔGras* is the change in grassland before correction to ensure that Pastmin is maintained

• ΔGrasD is the change in demand for grassland as driven by livestock and biomass yields.

1.3 Cropland

$$ Crop[i] = if(Gras*[i] \geqslant Past\min, Crop[i - 1] + \Delta Crop*[i],Crop[i - 1] + \Delta Crop*[i] - (Past\min - Gras*[i]) - \Delta PlantC[i] $$

(8)

$$ \Delta Crop[i] = Crop[i] - crop[i - 1] $$

(9)

• Crop[i] = the cropland area in current year

• Gras*[i] = specified in Eq. 5

• Past min = minimum grassland area. (It is specified by the user)

• ΔCrop[i-1] = cropland area in previous year

• Δcrop* = change in cropland before ensuring that the Pastmin is maintained.

• ΔCrop[i] = Actual change in cropland which may be constrained by availability of land and Pastmin.

  $$ \Delta CropDfct[i] = \Delta CropD[i] - \Delta crop[i] $$

  (10)

• ΔCropDfct = Deficit or unsatisfied crop demand at current consumption and yield

1.4 Others

$$ Oth[i] = Oth[i - 1] - \Delta UN[i] $$

(11)

• Crop[i] is the cropland area for year i (the current year)

• UN[i] is the component of the Others’ area that is user specified as available to transfer from Others(if positive) or to return to others (if negative)

• ΔUN[i] is the change in the UN as an effect of transfers to/from Others.

Computing the land demand & supply for each land-use type

2.1 No-intensification process (BiomC = 4.6)

• i.e when For[i-1]-LandFor[i]>Formin[i] i.e, LandFor<dEF)

  $$ {\hbox{dEF[i] = dEF[i - 1] - }}\Delta F{\hbox{or[i - 1] + DEF[i]}} $$

  (12)
  $$ FOR\min {\hbox{[i] }} = For[i - 1] - dEF[i] $$

  (13)

• LandFor[i] is the sum of the changes in demand for cropland and pastures that are not satisfied by UN.i.e P+Q in the diagram.

• FORmin[i] is the minimum area beyond which the forest should not reduce in year i.

• dEF[i] is the actual deforestation that is allowed in a given year that ensures a correction to the deforestation rate for the next year in order to ensure that the cumulative value of DEF[i], the user specified rate, is maintained over a long period.

2.2 Grassland

$$ pas{t_d}[i] = f(BiomC = 4.6,Liv[i],BiomPY[i]) $$

(14)

$$ BiomPY[i]) = k*Rain[i] $$

(15)

• K may vary from 0.001 to 0.004. This value has to be calibrated. Rain[i] is the average rainfall (mm) in particular year i.

• Pastd is the demand for pastures by livestock

• BiomC is the livestock Biomass consumption by an equivalent livestock unit

• Liv is the number of livestock equivalent

• BiomPY is the Biomass Yield in the grasslands

  $$ GrasD[i] = \max (Gras[i - 1],pas{t_d}[i]) $$

  (16)

• GrasD is the required grassland area which may be equal to the existing area or the demanded pasture area, whichever is greater.

$$ \Delta GrasD[i] = \max (0,GrasD[i] - GrasD[i - 1]) $$

(17)

• ΔGrasD is the change is GrasD. If it is negative, it not assigned, hence a value of zero is taken to mean there was no real change demanded. The negative value can be due to reduction in livestock numbers and or increase in biomass yields.

2.3 Cropland

$$ cro{p_d}[i] = f(FoodC,Popn[i],cropY[i],net{\rm Im} p[i]) $$

(18)

• Cropd is the cropland area demand for crops intended for human consumption

• FoodC is the food consumption per capita

• Popn is the population

• cropY is the crop yield (equivalent yield of a composite crop)

• netImp is the difference between agricultural imports and exports

  $$ \Delta cro{p_d}[i] = \max (0,cro{p_d}[i] - cro{p_d}[i - 1]) $$

  (19)

• Δcropd is the difference between cropd for the current year and crop d for the previous year. If it is negative, it not assigned, hence a value of zero is taken to mean there was no real change demanded. The negative value can be due to a large increase in crop yield or net-agricultural imports netImp.

  $$ Fal[0] = crop[0] - cro{p_d}[0] $$

  (20)

• Fal is the the Fallow area in year i. We calculate the fallow for [i-1] as the fallow for [i] is unknown until Δfal, the change in fallow, has been calculated

  $$ cf[i] = \frac{{Fal[i]}}{{cro{p_d}[i]}} $$

  (21)

• cf1 is the reference fallow for a year in the past. Usually the first year in the simulation. [i = 0] indicates the starting year.

  $$ \Delta Fal[i] = (\Delta cro{p_d}[i]*cf1) - (\Delta cropd[i] + \Delta GrasD[i] - UN[i])*\left( {1 - \frac{{past[i - 1]}}{{crop[i - 1]}}} \right) $$

  (22)

(Stéphenne and Lambin 2001)

$$ Fal[i] = Fal[i - 1] + \Delta Fal[i] $$

(23)

$$ \Delta cropD[i] = \Delta cro{p_d}[i - 1] + \Delta Fal[i] $$

(24)

• ΔcropD is the change in total cropland demand including the change in Fallow.

2.4 Transfers

$$ R[i] = \min (\Delta cropD[i],UN[i]) $$

(25)

• R[i] is the transfer of area between Others to Cropland

  $$ S[i] = \min ((UN[i] - R[i]),\Delta GrasD[i]). $$

  (26)

• S[i] is the transfer of area between Others and Grasslands

  $$ P[i] = \max (\Delta cropD[i] - R[i],0) $$

  (27)

• P[i] is the transfer from Forests to Croplands

  $$ Q[i] = \max (\Delta GrasD[i] - S[i],0) $$

  (28)

• Q[i] is the transfer from Forests to Grasslands

  $$ LandFor[i] = P[i] + Q[i] $$

  (29)

  Check for intensification

  $$ if(For[i - 1] - LandFor > FOR\min [i]...then...\Delta For = LandFor $$

  (30)

If this condition is true then the changes in Land-use in Eqs. 2,4,5 and 6 are calculated as follows for year I (No intensification);

$$ \begin{array}{*{20}{c}} {\Delta For[i] = LandFor[i],} \hfill \\{\Delta Gras*[i] = \Delta GrasD[i],} \hfill \\{\Delta Crop*[i] = \Delta CropD[i],} \hfill \\{\Delta UN[i] = (R[i] + S[i])} \hfill \\\end{array} $$

(31a,b,c,d)

If this condition is False, then all above calculations from Eq. 8 are repeated as explained below.

2.5 Intensification process (BiomC = 2.3) In Eq. 8, BiomC becomes 2.3

Also recalculate Eqs. 9 to 15 which remain unchanged

2.6 Transfers

Recalculate Eqs. 25 and 26 which also remain unchanged. P& Q change slightly, but to avoid confusion we now call them U & V respectively and renumber to 23 and 24.

$$ U[i] = \min (\Delta cropD[i] - R[i],dEF[i]) $$

(32)

• U[i] is the transfer from Forests to Croplands

  $$ V[i] = \min (dEF[i] - U[i],\Delta GrasD[i] - S[i]) $$

  (33)

• Q[i] is the transfer from Forests to Grasslands

A new term T[i] to represent transfers from Grasslands to cropland. These transfers are possible during the intensification phase as livestock is increasingly fed on crop residues.

$$ T[i] = \Delta cropD - (R + U) $$

(34)

It then follows that the changes in Land-use in Eqs. 2,4,5 and 6 are calculated as follows for year i (under-intensification);

$$ \begin{array}{*{20}{c}} {\Delta For[i] = U[i] + V[i],} \hfill \\{\Delta Gras*[i] = S[i] + V[i] - T[i],} \hfill \\{\Delta Crop*[i] = \Delta cropD[i],} \hfill \\{\Delta UN[i] = (R[i] + S[i])} \hfill \\\end{array} $$

(35)