Fig. 1. Location of Laba and Tiogo forests in isohyet map of Burkina Faso.

Table 1. Descriptive statistics (mean ± standard deviation, minimum and maximum) of the studied tree species: circumference 0.2 m above ground level (C0.2), circumference 1.30 m above ground level (C1.3) and height (H).
  C0.2 (cm) C1.3 (cm) H (cm)
N Mean ± Sd Min. Max. Mean ± Sd Min. Max. Mean ±vSd Min. Max.
Anogeissus leiocarpa 30 27.5±2.7 8 61 18.1±1.8 5 41 573±50 247 1110
Detarium microcarpum 30 37.6±3.5 12 86 25.7±3.1 7 75 455±30 195 800
Piliostigma thonningii 30 26.5±1.7 12 40 15.7±1.4 6 30 334±18 171 540
Vitellaria paradoxa 10 29.0±9.1 14 40 19.5±8.5 8 34 426±100 150 650
All species 100 31.9±1.6 8 86 21.6±1.4 5 75 456±19 150 1110
Table 2. The allometric equations for the below-ground biomass (BGB) (kg) of the focal species based on indicated explanatory variables: basal area at 0.2 m above ground level (BA0.2), basal area (m2) at 1.3 m above ground level (BA1.3) and height (H) (cm). SE = Standard error. Bolded rows represent parameters of best specific models.
  Variable a SEa b SEb R2 SE p-value
Anogeissus leiocarpa BA0.2 150.6 69.4 0.82 0.08 0.82 0.40 <0.0001
BA0.2×H 17.8 4.2 0.61 0.06 0.84 0.37 <0.0001
BA1.3 225.4 125.7 0.77 0.09 0.78 0.44 <0.0001
BA1.3×H 25.9 7.9 0.58 0.06 0.81 0.41 <0.0001
H 0.1 0.03 1.87 0.24 0.72 0.49 <0.0001
Detarium microcarpum BA0.2 414.9 296.2 0.72 0.14 0.54 0.67 <0.0001
BA0.2×H 75.9 30.7 0.52 0.10 0.54 0.68 <0.0001
BA1.3 628.7 518.3 0.68 0.14 0.52 0.69 <0.0001
BA1.3×H 108.9 53.0 0.50 0.10 0.52 0.69 <0.0001
Piliostigma thonningii BA0.2 532.9 414.3 0.91 0.14 0.64 0.60 <0.0001
BA0.2×H 94.1 46.7 0.74 0.11 0.66 0.58 <0.0001
BA1.3 519.6 456.4 0.75 0.13 0.58 0.64 <0.0001
BA1.3×H 112.8 67.2 0.63 0.10 0.60 0.63 <0.0001
Vitellaria paradoxa BA0.2 38057.0 27231.4 1.72 0.14 0.95 0.33 <0.0001
BA0.2×H 483.1 236.9 1.20 0.13 0.92 0.43 <0.0001
BA1.3 5759.0 6001.4 1.15 0.17 0.85 0.58 <0.0001
BA1.3×H 385.6 230.8 0.91 0.12 0.87 0.53 <0.0001
H 0.1 0.2 2.62 0.81 0.57 0.99 <0.0001
All species BA0.2 912.2 462.3 1.01 0.09 0.59 0.82 <0.0001
BA0.2×H 67.2 21.9 0.70 0.08 0.50 0.90 <0.0001
BA1.3 1084.9 603.0 0.88 0.09 0.56 0.85 <0.0001

Fig. 2. The best equations for predicting belowground biomass (BGB) based on basal area at 0.2 m above ground level (BA0.2) and it combination with height (H) (BA0.2 × H) for: a) Anogeissus leiocarpa, (b) Detarium microcarpum, (c) Piliostigma thonningii, (d) Vitellaria paradoxa and (e) all species.

Table 3. Means and 95% confidence intervals (CI 95%) of below-ground biomass (BGB), above-ground biomass (AGB) and root:shoot ratio (RS) of the sampled tree species.
  BGB (kg) ABG (kg) RS
Mean CI 95% Mean CI 95% Mean CI 95%
Anogeissus leiocarpa 3.0 a 6.4–12.5 4.4 a 32.2–41.1 0.6 a 0.1–1.0
Detarium microcarpum 26.9 c 17.3–36.6 7.4 a 29.9–44.8 2.9 b 2.5–3.4
Piliostigma thonningii 5.8 ab 3.8–15.4 2.2 a 35.1–39.5 2.5 b 2.0–3.0
Vitellaria paradoxa 22.7 bc 13.3–32.2 97.3 b 60.7–134.0 0.3 a 0.1–0.8
Mean values with the same letters in the same column are not significantly different (p < 0.05)

Fig. 3. Plots of measured and predicted values of published models and Eq. 3 of this study.

Fig. 4. Mean errors (%) for the best below-ground biomass equations for the three diameter classes of the four focal species obtained in this study (Table 3), and presented by Kuyah at al. (2012) (BGB = 0.048 × DBH2.303), Mokany et al. (2006) (BGB = 0.27 × AGB) and Cairns et al. (1997) (BGB = –1.085 + 0.9256 × ln(ABG)).