Calorific value of Prosopis africana and Balanites aegyptiaca wood: relationships with tree growth, wood density and rainfall gradients in the West African Sahel
Calorific value of Prosopis
Calorific value of Prosopis africana and Balanites aegyptiaca
wood: relationships with tree growth, wood density and rainfall
gradients in the West African Sahel
Carmen Sotelo Montes1, Dimas Agostinho da Silva2, Rosilei A. Garcia3,
Graciela Inês Bolzón de Muñiz2, John C. Weber1
1World Agroforestry Centre (Bamako, Mali), 2Universidade Federal do Paraná (Curitiba, Brazil),
3Universidade Federal Rural do Rio de Janeiro (Rio de Janeiro, Brazil)
Prosopis africana (Guill., Perrott. and Rich.) Taub. and Balanites aegyptiaca (L.) Delile are native tree species in the West African Sahel and provide wood for fuel, construction and other essential products. A
provenance/progeny test of each species was established at one relatively dry site in Niger, and evaluated at 13 years. Results from the tests at 13 years indicated that there was significant genetic variation in tree growth and
wood density in P. africana but not B. aegyptiaca; that provenance means for tree growth of both species, and for wood density and survival of P. africana increased from the more humid to the drier parts of the sample region;
and that larger trees of both species tended to have denser wood. This paper presents additional results from the provenance/progeny tests of P. africana and B. aegyptiaca at 13 years The major objectives were to determine
(a) if tree growth and wood density were correlated with calorific value of the wood, (b) if correlations among growth and wood variables differed in strength between the drier and more humid parts of the sample regions, and
(c) if calorific value was related to rainfall gradients in the sample regions.
Materials and Methods
The sample region for P. africana extends from central Burkina Faso to central Niger, covering an area ~1200 km from west to east, and 50–200 km from south to north. Mean annual rainfall in this sample region decreases from
west to east (~750–350 mm) and from south to north (~750–650 mm in the west, ~450–350 mm in the east). The sample region for B. aegyptiaca extends ~625 km from west to east, and ~325 km from south to north in central
Niger. Mean annual rainfall in this sample region decreases from south to north and from west to east (latitudinal decrease ~550–350 mm in the west and ~450–350 mm in the east). Seeds were collected in 1993 from P.
africana (275 trees from 28 provenances) and B. aegyptiaca (108 trees from 12 provenances). Latitude, longitude and elevation of each sampled tree was recorded using a GPS receiver. Progeny plants were established in
provenance/progeny tests in 1994 at the ICRISAT Sahelian Centre in Niger (mean annual rainfall ~540 mm). Each test had eight replications. A subsample of the trees in each of the eight replications in each test was randomly
selected in 2007 for this study: ~20 trees per replication for P. africana and 10 trees per replication for B. aegyptiaca. The following variables were measured on each sampled tree: tree height (cm), stem diameter (cm) over
bark at 1.3 m (DBH) for P. africana and at 30 cm (Dia30) for B. aegyptiaca, basic density of a disk and air‐dry density of an increment core of wood (BDen and ADen, respectively, kg m‐3), and calorific value (CV) of a sawdust
sample. Samples for BDen, ADen and CV were obtained close to where diameter was measured. Three estimates of CV were calculated: gross and net CV (MJ kg‐1) and gross CV m‐3 (gross CVm3). Gross CV is the maximum
energy available from an oven‐dry sample, whereas net net CV is the energy available from an air‐dry sample. Mean percent ash content (AC) and moisture content (MC) were determined from the residue for a random sub‐
sample of the sawdust samples. These were used to calculate a fuel value index (FVI) for each species, using the following formula: FVI = [(BDen)(net CV)]/[(AC)(MC)]. Pearson correlations among tree growth and wood
variables were calculated using data from all sampled trees and separately for trees originatiing from provenances in the drier and more humid zones in the sample region (below and above 550 mm rainfall, respectively).
Multiple linear regression was used to investigate whether provenance means for gross CV and CVm3 varied significantly with provenance latitude, longitude and/or elevation and by implication the rainfall gradients in the
FVI: FVI was higher for P. africana than for B. aegyptiaca (3278 versus 1114) because AC and MC were lower for P. africana (0.33% and 10%, respectively) than for B. aegyptiaca (0.80% and 12%, respectively).
Correlations (Table 1): Correlations among all trees indicated that larger trees of both species tended to have greater BDen, ADen and gross CVm3, but correlations with gross CV were not significant. Correlations differed in
magnitude between the drier and more humid zones: for P. africana, growth variables had stronger correlations with wood density in the more humid zone, and with gross CVm3 in the drier zone. For B. aegyptiaca, all
correlations between growth and wood variables were stronger in the drier zone. Gross CV was weakly correlated with growth of P. africana only in the drier zone, but the correlation was not significant for B. aegyptiaca in
either zone. Wood density was not significantly correlated with gross CV (not tabled), but ADen and especially BDen were strongly correlated with gross CVm3 (e.g., r with BDen = 0.534 for P. africana and 0.608 for B.
aegyptiaca, P < 0.001) because basic density was used to calculate gross CVm3, and the two density measurements were strongly correlated (r = 0.607 for P. africana and 0.759 for B. aegyptiaca, P < 0.001).
Regressions with provenance location (Table 2): Mean gross CV of wood of P. africana and B. aegyptiaca provenances tended to decrease in general from the more humid to the drier parts of their sample regions. For P.
africana, it decreased from west to east and increased with elevation (Figure 1). For B. aegyptiaca, it decreased with latitude in the southern part of its sample region, but increased with latitude in the northern part of the
sample region (Figure 2). Regressions involving gross CVm3 were not significant.
Table 1. Correlations between tree growth and wood variables of P. africana and B. aegyptiaca trees Table 2. Multiple linear regressions predicting gross calorific value of wood of P. africana and 20.6
at 13 years in provenance/progeny tests in Niger.
Variable P. africana B. aegypticaca B. aegyptiaca provenances at 13 years from provenance latitude, longitude and elevation. 20.4
Height DBH Height Dia30 Regression equation predicting gross CV R2 P SE 20.2
All ttrees 19.8
BDen 0.396 *** 0.489 *** 0.494 *** 0.571 *** P. africana 18.2794 + 0.0064 (Ele) – 0.0003 (LonEle) 0.284 Ele: ** Ele: 0.0020
Gross CV (MJ kg-1)
Gross CV (MJ kg-1)
ADen NS 0.187 * 0.261 * 0.270 * LonEle: * LonEle: 0.0001 250 m
Gross CV NS NS NS NS
0.320 *** 0.371 *** 0.356 ** 0.378 ***
1. Drier zone B. aegyptiaca 176.7536 – 23.6097 (Lat) + 0.8835 (Lat ) 0.545 Lat: ** Lat: 7.1910 19.6 19.2
BDen 0.371 *** 0.462 *** 0.623 *** 0.591 *** Lat : ** Lat2: 0.2689
Aden NS NS 0.490 *** 0.383 * 19.4 19
Gross CV 0.177 * 0.185 * NS NS Dependent variable: gross CV = calorific value in MJ kg‐1. Independent variables: Lat =
Gross CVm 0.339 *** 0.396 *** 0.461 ** 0.391 ** latitude from south to north, and Lon = longitude from west to east (decimal degrees); Ele =
0 1 2 3 4 5 6 7 8 9
12 12.5 13 13.5 14 14.5
2. More humid zone Longitude (°E) Latitude (°N)
BDen 0.487 *** 0.558 *** 0.389 * 0.550 *** elevation (m); Lat2 = quadratic term for Lat; LonEle = Lon x Ele interaction. Sample size: 28
Aden NS NS NS NS for P. africana and 12 for B. aegyptiaca. R2 = coefficient of determination of model. P = Figure 1. Geographical variation in gross CV Figure 1. Geographical variation in gross CV of
Gross CV NS NS NS NS
Gross CVm 3
NS NS NS 0.369 * probability of F: *** P < 0.001, ** P < 0.01, * P < 0.05. SE = standard error of regression of P. africana wood (equation in Table 2). B.aegyptiaca wood (equation in Table 2).
Variables: height = tree height, DBH and Dia30 = stem diameter over bark measured at 1.3 m and at coefficient.
30 cm, BDen = basic density of disk, ADen = air-dry density of increment core, gross CV and CVm =
gross calorific values in MJ kg and MJ m . Sample size: P. africana all trees = 167 for BDen and
gross CVm , 169 for others; zone 1 = 124 for BDen and gross CVm , 126 for others; zone 2 = 43; B.
aegypticaca all trees = 80, zone 1 = 43, zone 2 = 37. Significance: *** P < 0.001, ** P < 0.01, * P <
0 05 NS P > 0 05
Correlations between tree growth and wood density may be an adaptation to reduce bending stress produced by wind. In general, larger trees require greater strength at the base of the stem in order to reduce bending stress,
and strength can be increased by producing denser wood. In general, stand densities are higher in more humid zones, and root systems and tree crowns may become interwoven to varying degrees over time, thereby allowing
the stand to “collectively” reduce the bending stress. In drier zone, this “collective” response would be less apparent because stand densities are lower. Following this argument, one would expect a stronger correlation
between tree growth and wood density in drier zones. Results of this study are inconclusive with respect to this hypothesis: growth‐density correlations were stronger for B. aegyptiaca trees originating from the drier zone, but
for P. africana trees originating from the more humid zone.
Larger trees of both species tended to have greater gross CVm3. The positive correlation partially reflects the fact that gross CVm3 is the product of gross CV and BDen, and BDen is positively correlated with growth. The growth‐
density correlations were stronger for B. aegyptiaca trees originating from the drier zone, but for P. africana trees originating from the more humid zone. If the correlation between growth and gross CVm3 was simply due to the
formula for gross CVm3 and the growth‐density relationship, then one would expect the growth‐gross CVm3 and growth‐density correlations to exhibit similar differences between zones. This was not observed: the growth‐
gross CVm3 correlation for both species was higher for trees originating from the drier zone.
Wood with higher lignin content has greater density and calorific value, and denser wood tends to have more lignin, so one would expect a positive correlation between wood density and calorific value. In this study, gross CV
of both species was not significantly correlated with wood density. These results illustrate that there is no simple and general relationship between wood density and calorific value.
In general, gross CV of provenances of both species was greater in the more humid parts of their sample regions. In contrast, tree growth of both species, and wood density of P. africana were greater in the drier parts of their
sample regions The lack of correspondence between the geographical variation in gross CV tree growth and wood density reflects the fact that gross CV was not significantly correlated with growth and density in the analysis
of all trees.
A biological interpretation of the clines requires an understanding of the chemical composition of the wood in the drier and more humid zones, and also the relative proportions of protective chemicals in the wood. In theory,
one would expect that natural selection would increase the amount of protective chemicals in the wood in regions where there are higher population levels of wood pests and pathogens. Perhaps pest and pathogen pressures
are higher in the more humid zones of this study, but we have no evidence to support this statement.
Research funded by the International Fund for Agricultural Development. Research article citation: Biomass and
Bioenergy (2010) in press. Related research articles: Forest Annals of Forest Science (2009) 66:713, New Forests (2010)
39:39‐49. Contact email@example.com or firstname.lastname@example.org for further details.