On-farm cocoa yields increase with canopy cover of shade trees in two agro-ecological zones in Ghana

Figures & data

Figure 1. Map of Southern Ghana showing four shaded districts consisting of the study sites (•). Forest type boundaries are shown by broken line (-----). Forest-type abbreviations: WE = Wet Evergreen; UE = Upland Evergreen; ME = Moist Evergreen; MSSE = Moist Semi-deciduous; NW = Northwest subtype; SE = Southeast subtype; DS = Dry Semi-deciduous; FZ = Fire Zone subtype; IZ = Inner Zone subtype; SM = Southern Marginal.

Table 1. Selection of random effects by AICc (Second order Akaike Information Criterion) in a model containing all the fixed effects.

Random effects	No. parameters	AICc
District within Year, Year within District, Farm	45	1210.21
District within Year, Farm	35	1183.57
Year within District, Farm	35	1188.47
District nested in Year, Year, Farm	27	1178.24
Year, Farm	26	1211.63

Note: The random effect District within Year means an arbitrary correlation structure between the four districts, which are independent across the four years, and similarly for Year within District. The random effect District nested in Year means a random effect of the 16 combinations of District and Year.

Table 2. Estimates, confidence intervals, p-values and variance components in the selected model.

Effect	Parameter	Estimate	95% Confidence interval	P-value
1. Estimates, confidence intervals, and p-values in the selected model
District	α (Atwima Nwabigya)	6.25	5.18; 7.30	0.0048
	α (Amansie West)	6.03	5.02; 7.11
	α (Wassa Amenfi West)	7.91	6.77; 8.98
	α (Sefwi Wiawso)	7.83	6.73; 9.03
Farmsize	β	−0.17	−0.34; −0.01	0.0305
CC	γ	0.07	0.04; 0.11	<0.0001
LandUse	δ (Long fallow)-δ(Forest)	−0.16	−0.69; 0.40	0.0147
	δ (Short fallow)-δ(Forest)	−0.91	−1.60; −0.19
Training	ε(yes)-ε(no)	0.75	0.18; 1.31	0.0052
Fertilizer	ζ(yes)-ζ(no)	0.64	0.15; 1.13	0.0088
2. Estimates for the variance components in the selected model
Random effect	Variance component	Proportion of total variation
Farm	0.6110	21.3%
District nested in Year	0.6321	22.0%
Year	0.0090	0.3%
error	1.6225	56.4%

Figure 2. Observed yields (points) against canopy cover separated over Fertilizer and Training (in the rows) LandUse (in the columns), together with model predictions (lines) from the selected model with Farmsize.

Figure 3. Observed yields (points) against canopy cover designated by districts together with model predictions (lines) from the selected model.