Agriculture Reference
In-Depth Information
(
L. leucocephala
,
Gliricidia
,
Calliandra
[
Calliandra calothyrsus
], and
S. siamea
),
corresponding to around 5.2-7.1 Mg biomass ha
−1
year
−1
. For eastern Zambia,
Kaonga and Bayliss-Smith (2009) quantified C stocks in improved fallows at 2.9-9.8
Mg ha
−1
, which corresponds to between 5.8 and 21.8 Mg biomass ha
−1
.
11.3.1.3 Rotational Woodlots and Tree Plantations
Much higher biomass accumulation rates are possible in the tree phases of rotational
woodlots. After 7 years of growth, woodlots in Tanzania had between 26.0 and 57.6
Mg biomass ha
−1
, corresponding to mean accumulation rates of 3.7-8.2 Mg ha
−1
year
−1
(Nyadzi et al. 2003). The best performance was obtained from
Acacia lepto-
carpa
, followed by
A. crassicarpa
,
Acacia julifera
,
Senna
, and
Leucaena pallida
.
These data also formed some of the basis for assumptions by Palm et al. (2010), who
estimated that woodlots in Mbola, Tanzania, accumulated 5.3 Mg ha
−1
year
−1
during
a 5-year rotation.
Carbon accumulation in woodlots in Morogoro, Tanzania, ranged between 2.3
Mg ha
−1
year
−1
under
Acacia nilotica
and 5.1 Mg ha
−1
year
−1
under
A. crassicarpa
(Kimaro 2009). These rates correspond to between 4.6 and 11.3 Mg ha
−1
year
−1
of biomass accumulation. Aune et al. (2005) found that the amount of C seques-
tered during a 4-year tree phase of rotational woodlots in Uganda was between
4.9 Mg ha
−1
year
−1
for
Eucalyptus camaldulensis
and 3.9 Mg ha
−1
year
−1
for
Alnus
acuminata
, corresponding to biomass increments by 7.8-10.9 Mg biomass ha
−1
year
−1
. For rotational woodlot systems, biomass accumulation is naturally much
higher during the tree phases, during which all the above-mentioned studies were
conducted, than during the crop phases, before which essentially all tree biomass
is removed.
Even higher biomass accumulation rates than in rotational woodlots are achieved
in tree plantations. Ståhl et al. (2002) found that plantations of
Sesbania
,
Calliandra
,
eucalyptus
(
Eucalyptus saligna
), and grevillea (
Grevillea robusta
) produced 31.5,
24.5, 32.5, and 43.5 Mg aboveground biomass ha
−1
, respectively, during 22 months
in the highlands of eastern Kenya. Palm et al. (2010) used this source to assume a
biomass accumulation rate of 12.2 Mg ha
−1
year
−1
for 5-year-old woodlots in Sauri,
Kenya. A plantation of
Pinus patula
in Tanzania was shown to build up 5.86 Mg C
ha
−1
year
−1
, corresponding to 11.7-13.0 Mg biomass ha
−1
year
−1
.
Thus, while aboveground biomass buildup in the natural miombo forest in this
region ranges from 0.43 to 1 Mg biomass ha
−1
year
−1
, agroforestry species added to
smallholder cropping systems in improved fallows and parklands usually produce
1-5 Mg biomass ha
−1
year
−1
. Rotational woodlots may add up to 8 Mg biomass ha
−1
year
−1
when the figures include the cropping phase. Plantations may produce twice
the biomass of rotational woodlots, but require the land to be removed from cropping
for long periods. Although building up aboveground biomass in farmed landscapes
is an important component of the global biotic C pool, the soil C pool is 4.5 times the
size of the biotic pool (Lal 2004), and thus may serve as an important sink for C, in
addition to offering advantages in soil fertility and cropping system yield described
above. Building up SOC offers important benefits on both the smallholder and eco-
systems scales.
